This list can be downloaded as a BibTeX file. You can also see a list of group theses, and group poster presentations.

2022

  • Y. E. Khatchenko, M. V. Yakushev, C. Seibel, H. Bentmann, M. Orlita, V. Golyashov, Y. S. Ponosov, N. P. Stepina, A. V. Mudryi, K. A. Kokh, O. E. Tereshchenko, F. Reinert, R. W. Martin, and T. V. Kuznetsova, “Structural, optical and electronic properties of the wide bandgap topological insulator Bi1.1Sb0.9Te2S,” Journal of Alloys and Compounds, vol. 890, 2022. doi:10.1016/j.jallcom.2021.161824
    [BibTeX] [Abstract] [Download PDF]

    Successful applications of a topological insulator (TI) in spintronics require its bandgap to be wider then in a typical TI and the energy position of the Dirac point in the dispersion relations to be away from the valence and conduction bands. In this study we grew Bi1.1Sb0.9Te2S crystals and examined their elemental composition, structural, optical and electronic properties as well as the electronic band structure. The high structural quality of the grown crystals was established by X-ray diffraction and Raman spectroscopy. Angular resolved photoelectron spectroscopy demonstrated a near parabolic character of the valence and conduction bands and a direct bandgap of 0.36 eV. The dispersion relations also revealed a Dirac cone, confirming the topological insulator nature of this material, with the position of the Dirac point being 100 meV above the valence band maximum. Far infrared reflectivity spectra revealed a plasma edge and two phonon dips. Fitting these spectra with theoretical functions based on the Drude-Lorentz model allows determination of the high frequency dielectric constant (41.3), plasma frequency (936 cm?1) and the frequencies of two infrared phonons (177.7 cm?1 and 77.4 cm?1).

    @article{strathprints77902,
    volume = {890},
    month = {January},
    title = {Structural, optical and electronic properties of the wide bandgap topological insulator Bi1.1Sb0.9Te2S},
    year = {2022},
    doi = {10.1016/j.jallcom.2021.161824},
    journal = {Journal of Alloys and Compounds},
    keywords = {topological insulator, Bi1.1Sb0.9Te2S, electronic structure, ARPES, far infrared, optical reflectivity, Physics, Physical and theoretical chemistry, Materials Chemistry, Mechanics of Materials, Metals and Alloys, Mechanical Engineering, Atomic and Molecular Physics, and Optics},
    url = {https://doi.org/10.1016/j.jallcom.2021.161824},
    issn = {0925-8388},
    abstract = {Successful applications of a topological insulator (TI) in spintronics require its bandgap to be wider then in a typical TI and the energy position of the Dirac point in the dispersion relations to be away from the valence and conduction bands. In this study we grew Bi1.1Sb0.9Te2S crystals and examined their elemental composition, structural, optical and electronic properties as well as the electronic band structure. The high structural quality of the grown crystals was established by X-ray diffraction and Raman spectroscopy. Angular resolved photoelectron spectroscopy demonstrated a near parabolic character of the valence and conduction bands and a direct bandgap of 0.36 eV. The dispersion relations also revealed a Dirac cone, confirming the topological insulator nature of this material, with the position of the Dirac point being 100 meV above the valence band maximum. Far infrared reflectivity spectra revealed a plasma edge and two phonon dips. Fitting these spectra with theoretical functions based on the Drude-Lorentz model allows determination of the high frequency dielectric constant (41.3), plasma frequency (936 cm?1) and the frequencies of two infrared phonons (177.7 cm?1 and 77.4 cm?1).},
    author = {Khatchenko, Yu E. and Yakushev, M. V. and Seibel, C. and Bentmann, H. and Orlita, M. and Golyashov, V. and Ponosov, Y. S. and Stepina, N. P. and Mudryi, A. V. and Kokh, K. A. and Tereshchenko, O. E. and Reinert, F. and Martin, R. W. and Kuznetsova, T. V.}
    }

2021

  • L. Spasevski, G. Kusch, P. Pampili, V. Z. Zubialevich, D. V. Dinh, J. Bruckbauer, P. R. Edwards, P. J. Parbrook, and R. W. Martin, “A systematic comparison of polar and semipolar Si-doped AlGaN alloys with high AlN content,” Journal of Physics D: Applied Physics, vol. 54, iss. 3, 2021.
    [BibTeX] [Abstract] [Download PDF]

    With a view to supporting the development of ultra-violet light-emitting diodes and related devices, the compositional, emission and morphology properties of Si-doped n-type Al x Ga1-x N alloys are extensively compared. This study has been designed to determine how the different Al x Ga1-x N crystal orientations (polar (0001) and semipolar (11-22)) affect group-III composition and Si incorporation. Wavelength dispersive x-ray (WDX) spectroscopy was used to determine the AlN mole fraction (x {$\approx$} 0.57-0.85) and dopant concentration (3 1018-1 1019 cm-3) in various series of Al x Ga1-x N layers grown on (0001) and (11-22) AlN/sapphire templates by metalorganic chemical vapor deposition. The polar samples exhibit hexagonal surface features with Ga-rich boundaries confirmed by WDX mapping. Surface morphology was examined by atomic force microscopy for samples grown with different disilane flow rates and the semipolar samples were shown to have smoother surfaces than their polar counterparts, with an approximate 15\% reduction in roughness. Optical characterization using cathodoluminescence (CL) spectroscopy allowed analysis of near-band edge emission in the range 4.0-5.4 eV as well as various deep impurity transition peaks in the range 2.7-4.8 eV. The combination of spatially-resolved characterization techniques, including CL and WDX, has provided detailed information on how the crystal growth direction affects the alloy and dopant concentrations.

    @Article{strathprints74054,
    author = {Lucia Spasevski and Gunnar Kusch and Pietro Pampili and Vitaly Z. Zubialevich and Duc V. Dinh and Jochen Bruckbauer and Paul R. Edwards and Peter J. Parbrook and Robert W. Martin},
    journal = {Journal of Physics D: Applied Physics},
    title = {A systematic comparison of polar and semipolar {Si-}doped {AlGaN} alloys with high {AlN} content},
    year = {2021},
    month = {January},
    number = {3},
    volume = {54},
    abstract = {With a view to supporting the development of ultra-violet light-emitting diodes and related devices, the compositional, emission and morphology properties of Si-doped n-type Al x Ga1-x N alloys are extensively compared. This study has been designed to determine how the different Al x Ga1-x N crystal orientations (polar (0001) and semipolar (11-22)) affect group-III composition and Si incorporation. Wavelength dispersive x-ray (WDX) spectroscopy was used to determine the AlN mole fraction (x {$\approx$} 0.57-0.85) and dopant concentration (3 1018-1 1019 cm-3) in various series of Al x Ga1-x N layers grown on (0001) and (11-22) AlN/sapphire templates by metalorganic chemical vapor deposition. The polar samples exhibit hexagonal surface features with Ga-rich boundaries confirmed by WDX mapping. Surface morphology was examined by atomic force microscopy for samples grown with different disilane flow rates and the semipolar samples were shown to have smoother surfaces than their polar counterparts, with an approximate 15\% reduction in roughness. Optical characterization using cathodoluminescence (CL) spectroscopy allowed analysis of near-band edge emission in the range 4.0-5.4 eV as well as various deep impurity transition peaks in the range 2.7-4.8 eV. The combination of spatially-resolved characterization techniques, including CL and WDX, has provided detailed information on how the crystal growth direction affects the alloy and dopant concentrations.},
    keywords = {AlGaN, crystal orientation, alloy composition, III-nitride semiconductors, Si doping, cathodoluminescence, X-ray microanalysis, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/74054/},
    }

  • M. A. Sulimov, M. N. Sarychev, M. V. Yakushev, J. Márquez-Prieto, I. Forbes, Y. V. Ivanov, P. R. Edwards, A. V. Mudryi, J. Krustok, and R. W. Martin, “Effects of irradiation of ZnO/CdS/Cu₂ZnSnSe₄/Mo/glass solar cells by 10 MeV electrons on photoluminescence spectra,,” Materials Science in Semiconductor Processing, vol. 121, p. 105301, 2021.
    [BibTeX] [Abstract] [Download PDF]

    Solar cells with the structure ZnO/CdS/Cu₂ZnSnSe₄/Mo were studied by photoluminescence (PL) before and after irradiation with a dose of 1.8 × 10¹⁵ cm⁻² and then 5.4 × 10¹⁵ cm⁻² of 10 MeV electrons carried out at 77 K in liquid nitrogen bath. The low temperature PL spectra before irradiation revealed two bands, a broad and asymmetrical dominant FB band at 0.94 eV from the CZTSe layer and a lower intensity band HEB at 1.3 eV, generated by defects in the CdS buffer layer. Analysis of the excitation intensity and temperature dependencies suggested that the dominant band is the recombination of free electrons with holes localised at acceptors whose energy levels are affected by potential fluctuations of the valence band due to high concentrations of randomly distributed charged defects. Irradiation did not induce any new bands in the examined spectral range (from 0.5 µm to 1.65 µm) but reduced the intensity of both bands in the PL spectra measured at 77 K without warming the cells. The higher the dose the greater was the reduction. After this the cells were warmed to 300 K and moved to a variable temperature cryostat to measure excitation intensity and temperature dependencies of the PL spectra. After irradiation the rate of red shift of the FB band with temperature rise was found to increase. Electrons displace atoms in the lattice creating primary defects: interstitials and vacancies. These defects recombine during and shortly after irradiation forming secondary defect complexes which work as deep non-radiative traps of charge carriers reducing the PL intensity and increasing the rate of the temperature red shift. Irradiation did not affect the mean depth of the band tails estimated from the shape of the low energy side of the dominant PL band

    @Article{strathprints73346,
    author = {M. A. Sulimov and M. N. Sarychev and M. V. Yakushev and J. M{\'a}rquez-Prieto and I. Forbes and V. Yu. Ivanov and P. R. Edwards and A. V. Mudryi and J. Krustok and R. W. Martin},
    journal = {Materials Science in Semiconductor Processing},
    title = {Effects of irradiation of {ZnO/CdS/Cu₂ZnSnSe₄/Mo}/glass solar cells by 10 {MeV} electrons on photoluminescence spectra,},
    year = {2021},
    month = {July},
    pages = {105301},
    volume = {121},
    abstract = {Solar cells with the structure ZnO/CdS/Cu₂ZnSnSe₄/Mo were studied by photoluminescence (PL) before and after irradiation with a dose of 1.8 × 10¹⁵ cm⁻² and then 5.4 × 10¹⁵ cm⁻² of 10 MeV electrons carried out at 77 K in liquid nitrogen bath. The low temperature PL spectra before irradiation revealed two bands, a broad and asymmetrical dominant FB band at 0.94 eV from the CZTSe layer and a lower intensity band HEB at 1.3 eV, generated by defects in the CdS buffer layer. Analysis of the excitation intensity and temperature dependencies suggested that the dominant band is the recombination of free electrons with holes localised at acceptors whose energy levels are affected by potential fluctuations of the valence band due to high concentrations of randomly distributed charged defects. Irradiation did not induce any new bands in the examined spectral range (from 0.5 µm to 1.65 µm) but reduced the intensity of both bands in the PL spectra measured at 77 K without warming the cells. The higher the dose the greater was the reduction. After this the cells were warmed to 300 K and moved to a variable temperature cryostat to measure excitation intensity and temperature dependencies of the PL spectra. After irradiation the rate of red shift of the FB band with temperature rise was found to increase. Electrons displace atoms in the lattice creating primary defects: interstitials and vacancies. These defects recombine during and shortly after irradiation forming secondary defect complexes which work as deep non-radiative traps of charge carriers reducing the PL intensity and increasing the rate of the temperature red shift. Irradiation did not affect the mean depth of the band tails estimated from the shape of the low energy side of the dominant PL band},
    url = {https://strathprints.strath.ac.uk/73346/},
    }

  • F. C. -P. Massabuau, J. W. Roberts, D. Nicol, P. R. Edwards, M. McLelland, G. L. Dallas, D. A. Hunter, E. A. Nicolson, J. C. Jarman, A. Kovács, R. W. Martin, R. A. Oliver, and P. R. Chalker, “Progress in atomic layer deposited α-Ga₂O₃ materials and solar-blind detectors,” in Proceedings Volume 11687, Oxide-based Materials and Devices, D. J. Rogers, D. C. Look, and F. H. Teherani, Eds., Bellingham, WA, United States: Society of Photo-Optical Instrumentation Engineers, 2021. doi:10.1117/12.2588729
    [BibTeX] [Abstract] [Download PDF]

    Atomic layer deposition (ALD) offers a low thermal budget method for producing {\ensuremath{\alpha}}-Ga2O3 films on sapphire substrate. In this paper we review the recent progress on plasma-enhanced ALD growth of {\ensuremath{\alpha}}-Ga2O3 and present the optical and photoconductive properties of the deposited films. We show that the deposited material exhibits an epitaxial relationship with the sapphire substrate, and with an atomically sharp film-substrate interface. The {\ensuremath{\alpha}}-Ga2O3 films had an optical bandgap energy measured at 5.11 eV, and exhibited a broad luminescence spectrum dominated by ultraviolet, blue and green bands, in line with current literature. We finally demonstrate the suitability of the material for solar-blind photodetection.

    @InCollection{strathprints75759,
    author = {Massabuau, F. C.-P. and Roberts, J. W. and Nicol, D. and Edwards, P. R. and McLelland, M. and Dallas, G. L. and Hunter, D. A. and Nicolson, E. A. and Jarman, J. C. and Kov{\'a}cs, A. and Martin, R. W. and Oliver, R. A. and Chalker, P. R.},
    booktitle = {Proceedings Volume 11687, Oxide-based Materials and Devices},
    publisher = {Society of Photo-Optical Instrumentation Engineers},
    title = {Progress in atomic layer deposited {α-Ga₂O₃} materials and solar-blind detectors},
    year = {2021},
    address = {Bellingham, WA, United States},
    editor = {David J. Rogers and David C. Look and Ferechteh H. Teherani},
    isbn = {9781510642096},
    month = {March},
    note = {{\copyright} 2021 Society of Photo Optical Instrumentation Engineers (SPIE). One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this publication for a fee or for commercial purposes, and modification of the contents of the publication are prohibited.},
    series = {Proceedings of SPIE - The International Society for Optical Engineering},
    abstract = {Atomic layer deposition (ALD) offers a low thermal budget method for producing {\ensuremath{\alpha}}-Ga2O3 films on sapphire substrate. In this paper we review the recent progress on plasma-enhanced ALD growth of {\ensuremath{\alpha}}-Ga2O3 and present the optical and photoconductive properties of the deposited films. We show that the deposited material exhibits an epitaxial relationship with the sapphire substrate, and with an atomically sharp film-substrate interface. The {\ensuremath{\alpha}}-Ga2O3 films had an optical bandgap energy measured at 5.11 eV, and exhibited a broad luminescence spectrum dominated by ultraviolet, blue and green bands, in line with current literature. We finally demonstrate the suitability of the material for solar-blind photodetection.},
    doi = {10.1117/12.2588729},
    keywords = {gallium oxide, corundum phase, atomic layer deposition, solar-blind detection, Optics. Light, Atomic and Molecular Physics, and Optics},
    url = {https://doi.org/10.1117/12.2588729},
    }

  • A. Rossi, N. W. Hendrickx, A. Sammak, M. Veldhorst, G. Scappucci, and M. Kataoka, “Single-hole pump in germanium,” Journal of Physics D: Applied Physics, vol. 54, iss. 43, 2021. doi:10.1088/1361-6463/ac181d
    [BibTeX] [Abstract] [Download PDF]

    Single-charge pumps are the main candidates for quantum-based standards of the unit ampere because they can generate accurate and quantized electric currents. In order to approach the metrological requirements in terms of both accuracy and speed of operation, in the past decade there has been a focus on semiconductor-based devices. The use of a variety of semiconductor materials enables the universality of charge pump devices to be tested, a highly desirable demonstration for metrology, with GaAs and Si pumps at the forefront of these tests. Here, we show that pumping can be achieved in a yet unexplored semiconductor, i.e. germanium. We realise a single-hole pump with a tunable-barrier quantum dot electrostatically de?ned at a Ge/SiGe heterostructure interface. We observe quantized current plateaux by driving the system with a single sinusoidal drive up to a frequency of 100 MHz. The operation of the prototype was a?ected by accidental formation of multiple dots, probably due to disorder potential, and random charge ?uctuations. We suggest straightforward re?nements of the fabrication process to improve pump characteristics in future experiments.

    @Article{strathprints77296,
    author = {Rossi, Alessandro and Hendrickx, Nico W. and Sammak, Amir and Veldhorst, Menno and Scappucci, Giordano and Kataoka, Masaya},
    journal = {Journal of Physics D: Applied Physics},
    title = {Single-hole pump in germanium},
    year = {2021},
    issn = {0022-3727},
    month = {October},
    number = {43},
    volume = {54},
    abstract = {Single-charge pumps are the main candidates for quantum-based standards of the unit ampere because they can generate accurate and quantized electric currents. In order to approach the metrological requirements in terms of both accuracy and speed of operation, in the past decade there has been a focus on semiconductor-based devices. The use of a variety of semiconductor materials enables the universality of charge pump devices to be tested, a highly desirable demonstration for metrology, with GaAs and Si pumps at the forefront of these tests. Here, we show that pumping can be achieved in a yet unexplored semiconductor, i.e. germanium. We realise a single-hole pump with a tunable-barrier quantum dot electrostatically de?ned at a Ge/SiGe heterostructure interface. We observe quantized current plateaux by driving the system with a single sinusoidal drive up to a frequency of 100 MHz. The operation of the prototype was a?ected by accidental formation of multiple dots, probably due to disorder potential, and random charge ?uctuations. We suggest straightforward re?nements of the fabrication process to improve pump characteristics in future experiments.},
    doi = {10.1088/1361-6463/ac181d},
    keywords = {single charge pumps, single hole pumps, germanium, Physics, Surfaces, Coatings and Films, Acoustics and Ultrasonics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {https://doi.org/10.1088/1361-6463/ac181d},
    }

  • F. Massabuau, D. Nicol, F. Adams, J. Jarman, J. Roberts, A. Kovács, P. Chalker, and R. Oliver, “Study of Ti contacts to corundum α-Ga₂O₃,” Journal of Physics D: Applied Physics, vol. 54, iss. 38, 2021. doi:10.1088/1361-6463/ac0d28
    [BibTeX] [Abstract] [Download PDF]

    We present a study of the electrical, structural and chemical properties of Ti contacts on atomic layer deposited {\ensuremath{\alpha}}-Ga2O3 film. Ti forms an ohmic contact with {\ensuremath{\alpha}}-Ga2O3. The contact performance is highly dependent on the post-evaporation annealing temperature, where an improved conductivity is obtained when annealing at 450 oC, and a strong degradation when annealing at higher temperatures. Structural and chemical characterisation by transmission electron microscopy techniques reveal that the electrical improvement or degradation of the contact upon annealing can be attributed to oxidation of the Ti metallic layer by the Ga2O3 film in combination with the possibility for Ti diffusion into the Au layer. The results highlight that the grain boundaries and inclusions in the Ga2O3 film provide fast diffusion pathways for this reaction, leaving the {\ensuremath{\alpha}}-Ga2O3 crystallites relatively unaffected – this result differs from previous reports conducted on {\ensuremath{\beta}}-Ga2O3. This study underlines the necessity for a phase-specific and growth method-specific study of contacts on Ga2O3 devices.

    @Article{strathprints76944,
    author = {Massabuau, F. and Nicol, D. and Adams, F. and Jarman, J. and Roberts, J. and Kov{\'a}cs, A. and Chalker, P. and Oliver, R.},
    journal = {Journal of Physics D: Applied Physics},
    title = {Study of {Ti} contacts to corundum {α-Ga₂O₃}},
    year = {2021},
    issn = {0022-3727},
    month = {September},
    number = {38},
    volume = {54},
    abstract = {We present a study of the electrical, structural and chemical properties of Ti contacts on atomic layer deposited {\ensuremath{\alpha}}-Ga2O3 film. Ti forms an ohmic contact with {\ensuremath{\alpha}}-Ga2O3. The contact performance is highly dependent on the post-evaporation annealing temperature, where an improved conductivity is obtained when annealing at 450 oC, and a strong degradation when annealing at higher temperatures. Structural and chemical characterisation by transmission electron microscopy techniques reveal that the electrical improvement or degradation of the contact upon annealing can be attributed to oxidation of the Ti metallic layer by the Ga2O3 film in combination with the possibility for Ti diffusion into the Au layer. The results highlight that the grain boundaries and inclusions in the Ga2O3 film provide fast diffusion pathways for this reaction, leaving the {\ensuremath{\alpha}}-Ga2O3 crystallites relatively unaffected - this result differs from previous reports conducted on {\ensuremath{\beta}}-Ga2O3. This study underlines the necessity for a phase-specific and growth method-specific study of contacts on Ga2O3 devices.},
    doi = {10.1088/1361-6463/ac0d28},
    keywords = {Ti alloys, {\ensuremath{\alpha}}-Ga2O3, atomic layers, Physics, Surfaces, Coatings and Films, Acoustics and Ultrasonics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {https://doi.org/10.1088/1361-6463/ac0d28},
    }

  • M. J. Holmes, T. Zhu, F. C. -P. Massabuau, J. Jarman, R. A. Oliver, and Y. Arakawa, “Pure single photon emission from an InGaN/GaN quantum dot,” APL Materials, vol. 9, iss. 6, 2021. doi:10.1063/5.0049488
    [BibTeX] [Abstract] [Download PDF]

    Single-photon emitters with high degrees of purity are required for photonic-based quantum technologies. InGaN/GaN quantum dots are promising candidates for the development of single-photon emitters but have typically exhibited emission with insufficient purity. Here, pulsed single-photon emission with high purity is measured from an InGaN quantum dot. A raw g(2)(0) value of 0.043 {$\pm$} 0.009 with no corrections whatsoever is achieved under quasi-resonant pulsed excitation. Such a low value is, in principle, sufficient for use in quantum key distribution systems.

    @Article{strathprints76861,
    author = {Holmes, M. J. and Zhu, T. and Massabuau, F. C.-P. and Jarman, J. and Oliver, R. A. and Arakawa, Y.},
    journal = {APL Materials},
    title = {Pure single photon emission from an {InGaN/GaN} quantum dot},
    year = {2021},
    issn = {2166-532X},
    month = {June},
    number = {6},
    volume = {9},
    abstract = {Single-photon emitters with high degrees of purity are required for photonic-based quantum technologies. InGaN/GaN quantum dots are promising candidates for the development of single-photon emitters but have typically exhibited emission with insufficient purity. Here, pulsed single-photon emission with high purity is measured from an InGaN quantum dot. A raw g(2)(0) value of 0.043 {$\pm$} 0.009 with no corrections whatsoever is achieved under quasi-resonant pulsed excitation. Such a low value is, in principle, sufficient for use in quantum key distribution systems.},
    doi = {10.1063/5.0049488},
    keywords = {pure, single-photon emission, InGaN/GaN quantum dot, Physics, Physics and Astronomy(all)},
    url = {https://doi.org/10.1063/5.0049488},
    }

  • P. Vacek, M. Frentrup, L. Y. Lee, F. C. -P. Massabuau, M. J. Kappers, D. J. Wallis, R. Groger, and R. A. Oliver, “Defect structures in (001) zincblende GaN/3C-SiC nucleation layers,” Journal of Applied Physics, vol. 129, iss. 15, 2021. doi:10.1063/5.0036366
    [BibTeX] [Abstract] [Download PDF]

    The defect structure of zincblende GaN nucleation layers grown by metalorganic vapor-phase epitaxy on 3C-SiC/Si (001) was investigated by high-resolution scanning transmission electron microscopy. Perfect dislocations, partial dislocations, and stacking faults are present in the layers. Perfect dislocations are identified as 60o mixed-type and act as misfit dislocations to relieve the compressive lattice mismatch strain in GaN. Stacking faults are mainly bounded by 30o Shockley partial dislocations and rarely by Lomer-Cottrell partial dislocations, both of which are able to relieve the compressive lattice mismatch strain in the layer. We propose that the stacking faults and their partial dislocations originate from the dissociation of perfect dislocations present in the zincblende GaN layer and by direct nucleation of partial dislocations loops from the surface. These are the two main mechanisms that lead to the final defect structure of the zincblende GaN nucleation layers.

    @Article{strathprints76277,
    author = {Vacek, Petr and Frentrup, Martin and Lee, Lok Yi and Massabuau, Fabien C.-P. and Kappers, Menno J. and Wallis, David J. and Groger, Roman and Oliver, Rachel A.},
    journal = {Journal of Applied Physics},
    title = {Defect structures in (001) zincblende {GaN/3C-SiC} nucleation layers},
    year = {2021},
    issn = {0021-8979},
    month = {April},
    number = {15},
    volume = {129},
    abstract = {The defect structure of zincblende GaN nucleation layers grown by metalorganic vapor-phase epitaxy on 3C-SiC/Si (001) was investigated by high-resolution scanning transmission electron microscopy. Perfect dislocations, partial dislocations, and stacking faults are present in the layers. Perfect dislocations are identified as 60o mixed-type and act as misfit dislocations to relieve the compressive lattice mismatch strain in GaN. Stacking faults are mainly bounded by 30o Shockley partial dislocations and rarely by Lomer-Cottrell partial dislocations, both of which are able to relieve the compressive lattice mismatch strain in the layer. We propose that the stacking faults and their partial dislocations originate from the dissociation of perfect dislocations present in the zincblende GaN layer and by direct nucleation of partial dislocations loops from the surface. These are the two main mechanisms that lead to the final defect structure of the zincblende GaN nucleation layers.},
    doi = {10.1063/5.0036366},
    keywords = {defect structures, nucleation, zincblende GaN, electron microscopy, stacking faults, Physics, Optics. Light, Physics and Astronomy(all)},
    url = {https://doi.org/10.1063/5.0036366},
    }

  • R. J. Turner, P. Bots, A. Richardson, P. A. Bingham, A. Scrimshire, A. Brown, M. S’Ari, J. Harrington, S. A. Cumberland, J. C. Renshaw, M. J. Baker, P. R. Edwards, C. Jenkins, and A. Hamilton, “(Hydroxy)apatite on cement : insights into a new surface treatment,” Materials Advances, 2021. doi:10.1039/d1ma00320h
    [BibTeX] [Abstract] [Download PDF]

    (Hydroxy)apatite (HAp) [Ca10(PO4)6(OH)2], has emerging potential as a cement coating material, with applications in environmental remediation, nuclear waste storage and architectural preservation. In these low temperature environments and when precipitating from aqueous solution on to a porous substrate, the crystal size, nucleation sites and modified surface properties created are key to designing the most effective coating. In this study we show that bacterial (biogenic) or chemical (abiotic) syntheses on to Portland cement alter these critical performance parameters. We identify that the most significant difference between these two methods is the rate of pH change of the solution during synthesis, as this alters the surface properties and layer structure of HAp formed on cement. We show that iron present in Portland cement is not incorporated into the HAp structure; that formation of nanoparticulate/nanocrystalline HAp begins in the top 20-50 {\ensuremath{\mu}}m of the cement pore structure; and that a slow pH rise in the deposition solution controlled by bacteria metabolic activity leads to a rougher and more hydrophilic HAp coating compared to the abiotic synthesis. The results present the possibility of tailoring the surface topography and hydrophilicity of (hydroxy)apatite coated cement.

    @article{strathprints77861,
    month = {July},
    title = {(Hydroxy)apatite on cement : insights into a new surface treatment},
    year = {2021},
    doi = {10.1039/d1ma00320h},
    journal = {Materials Advances},
    keywords = {environmental remediation, nuclear waste, cement, materials, (Hydroxy)apatite (HAp) [Ca10(PO4)6(OH)2], Environmental engineering, Environmental Engineering},
    url = {https://doi.org/10.1039/d1ma00320h},
    abstract = {(Hydroxy)apatite (HAp) [Ca10(PO4)6(OH)2], has emerging potential as a cement coating material, with applications in environmental remediation, nuclear waste storage and architectural preservation. In these low temperature environments and when precipitating from aqueous solution on to a porous substrate, the crystal size, nucleation sites and modified surface properties created are key to designing the most effective coating. In this study we show that bacterial (biogenic) or chemical (abiotic) syntheses on to Portland cement alter these critical performance parameters. We identify that the most significant difference between these two methods is the rate of pH change of the solution during synthesis, as this alters the surface properties and layer structure of HAp formed on cement. We show that iron present in Portland cement is not incorporated into the HAp structure; that formation of nanoparticulate/nanocrystalline HAp begins in the top 20-50 {\ensuremath{\mu}}m of the cement pore structure; and that a slow pH rise in the deposition solution controlled by bacteria metabolic activity leads to a rougher and more hydrophilic HAp coating compared to the abiotic synthesis. The results present the possibility of tailoring the surface topography and hydrophilicity of (hydroxy)apatite coated cement.},
    author = {Turner, Ronald J. and Bots, Pieter and Richardson, Alan and Bingham, Paul A. and Scrimshire, Alex and Brown, Andrew and S'Ari, Mark and Harrington, John and Cumberland, Susan A. and Renshaw, Joanna C. and Baker, Matthew J. and Edwards, Paul R. and Jenkins, Cerys and Hamilton, Andrea}
    }

  • L. K. Jagadamma, P. R. Edwards, R. W. Martin, A. Ruseckas, and I. D. W. Samuel, “Nanoscale heterogeneity in CsPbBr₃ and CsPbBr₃:KI perovskite films revealed by cathodoluminescence hyperspectral imaging,” ACS Applied Energy Materials, vol. 4, iss. 3, p. 2707–2715, 2021. doi:10.1021/acsaem.0c03154
    [BibTeX] [Abstract] [Download PDF]

    The nanoscale morphology of solar cell materials strongly affects their performance. We report direct evidence for the existence of multiple length scales of heterogeneity in halide perovskites such as CsPbBr3 and CsPbBr3:KI. Contrary to the general notion of two distinct phases, our study suggests the presence of multiple phases in mixed halide perovskites. Highly spatially resolved ({$\approx$}50 nm) cathodoluminescence maps reveal that the length scale of heterogeneity is composition-dependent: smaller ({$\approx$}200 nm) for CsPbBr3 and larger ({$\approx$}500?1000 nm) forCsPbBr3:KI. Moreover, these nano-/micro-scale heterogeneities exist both laterally and vertically in mixed halides and correlate with high densities of carrier traps and fast trap-assisted recombination. The observed heterogeneities also lead to reduced power conversion efficiency of solar cells, higher hysteresis loss, and faster degradation. These insights argue for advanced nanoscale characterization of halide perovskites to guide reduction of heterogeneity and so improve device performance and stability.

    @Article{strathprints75740,
    author = {Jagadamma, Lethy Krishnan and Edwards, Paul R. and Martin, Robert W. and Ruseckas, Arvydas and Samuel, Ifor D. W.},
    journal = {ACS Applied Energy Materials},
    title = {Nanoscale heterogeneity in {CsPbBr₃} and {CsPbBr₃:KI} perovskite films revealed by cathodoluminescence hyperspectral imaging},
    year = {2021},
    issn = {2574-0962},
    month = {March},
    number = {3},
    pages = {2707--2715},
    volume = {4},
    abstract = {The nanoscale morphology of solar cell materials strongly affects their performance. We report direct evidence for the existence of multiple length scales of heterogeneity in halide perovskites such as CsPbBr3 and CsPbBr3:KI. Contrary to the general notion of two distinct phases, our study suggests the presence of multiple phases in mixed halide perovskites. Highly spatially resolved ({$\approx$}50 nm) cathodoluminescence maps reveal that the length scale of heterogeneity is composition-dependent: smaller ({$\approx$}200 nm) for CsPbBr3 and larger ({$\approx$}500?1000 nm) forCsPbBr3:KI. Moreover, these nano-/micro-scale heterogeneities exist both laterally and vertically in mixed halides and correlate with high densities of carrier traps and fast trap-assisted recombination. The observed heterogeneities also lead to reduced power conversion efficiency of solar cells, higher hysteresis loss, and faster degradation. These insights argue for advanced nanoscale characterization of halide perovskites to guide reduction of heterogeneity and so improve device performance and stability.},
    doi = {10.1021/acsaem.0c03154},
    keywords = {solar cell materials, cathodoluminescence, power conversion efficiency, Physics, Physics and Astronomy(all)},
    url = {https://doi.org/10.1021/acsaem.0c03154},
    }

  • L. Spasevski, B. Buse, P. R. Edwards, D. A. Hunter, J. Enslin, H. M. Foronda, T. Wernicke, F. Mehnke, P. J. Parbrook, M. Kneissl, and R. W. Martin, “Quantification of trace-level silicon doping in AlₓGa₁₋ₓN films using wavelength-dispersive X-ray microanalysis,” Microscopy and Microanalysis, vol. 27, p. 696–704, 2021. doi:10.1017/S1431927621000568
    [BibTeX] [Abstract] [Download PDF]

    Wavelength dispersive X-ray (WDX) spectroscopy was used to measure silicon atom concentrations in the range 35-100 ppm (corresponding to (3-9) {$\times$}1018 cm-3) in doped AlₓGa₁₋ₓN films using an electron probe microanalyser also equipped with a cathodoluminescence (CL) spectrometer. Doping with Si is the usual way to produce the n-type conducting layers that are critical in GaN and AlₓGa₁₋ₓN-based devices such as LEDs and laser diodes. Previously we have shown excellent agreement for Mg dopant concentrations in p-GaN measured by WDX with values from the more widely used technique of secondary ion mass spectrometry (SIMS). However, a discrepancy between these methods has been reported when quantifying the n-type dopant, silicon. We identify the cause of discrepancy as inherent sample contamination and propose a way to correct this using a calibration relation. This new approach, using a method combining data derived from SIMS measurements on both GaN and AlₓGa₁₋ₓN samples, provides the means to measure the Si content in these samples with account taken of variations in the ZAF corrections. This method presents a cost effective and time saving way to measure the Si doping and can also benefit from simultaneously measuring other signals, such as CL and electron channeling contrast imaging.

    @Article{strathprints76920,
    author = {Spasevski, Lucia and Buse, Ben and Edwards, Paul R. and Hunter, Daniel A. and Enslin, Johannes and Foronda, Humberto M. and Wernicke, Tim and Mehnke, Frank and Parbrook, Peter J. and Kneissl, Michael and Martin, Robert W.},
    journal = {Microscopy and Microanalysis},
    title = {Quantification of trace-level silicon doping in {AlₓGa₁₋ₓN} films using wavelength-dispersive {X}-ray microanalysis},
    year = {2021},
    issn = {1431-9276},
    month = {August},
    pages = {696--704},
    volume = {27},
    abstract = {Wavelength dispersive X-ray (WDX) spectroscopy was used to measure silicon atom concentrations in the range 35-100 ppm (corresponding to (3-9) {$\times$}1018 cm-3) in doped AlₓGa₁₋ₓN films using an electron probe microanalyser also equipped with a cathodoluminescence (CL) spectrometer. Doping with Si is the usual way to produce the n-type conducting layers that are critical in GaN and AlₓGa₁₋ₓN-based devices such as LEDs and laser diodes. Previously we have shown excellent agreement for Mg dopant concentrations in p-GaN measured by WDX with values from the more widely used technique of secondary ion mass spectrometry (SIMS). However, a discrepancy between these methods has been reported when quantifying the n-type dopant, silicon. We identify the cause of discrepancy as inherent sample contamination and propose a way to correct this using a calibration relation. This new approach, using a method combining data derived from SIMS measurements on both GaN and AlₓGa₁₋ₓN samples, provides the means to measure the Si content in these samples with account taken of variations in the ZAF corrections. This method presents a cost effective and time saving way to measure the Si doping and can also benefit from simultaneously measuring other signals, such as CL and electron channeling contrast imaging.},
    doi = {10.1017/S1431927621000568},
    keywords = {wavelength dispersive x-ray, cathodoluminescence, Wide band-gap semiconductors, Physics, Instrumentation},
    url = {https://doi.org/10.1017/S1431927621000568},
    }

  • T. J. O’Hanlon, F. C. -P. Massabuau, A. Bao, M. J. Kappers, and R. A. Oliver, “Directly correlated microscopy of trench defects in InGaN quantum wells,” Ultramicroscopy, 2021. doi:10.1016/j.ultramic.2021.113255
    [BibTeX] [Abstract] [Download PDF]

    Directly correlated measurements of the surface morphology, light emission and subsurface structure and composition were carried out on the exact same nanoscale trench defects in InGaN quantum well (QW) structures. Multiple scanning probe, scanning electron and transmission electron microscopy techniques were used to explain the origin of their unusual emission behaviour and the relationship between surface morphology and cathodoluminescence (CL) redshift. Trench defects comprise of an open trench partially or fully enclosing material in InGaN QWs with different CL emission properties to their surroundings. The CL redshift was shown to typically vary with the width of the trench and the prominence of the material enclosed by the trench above its surroundings. Three defects, encompassing typical and atypical features, were prepared into lamellae for transmission electron microscopy (TEM). A cross marker technique was used in the focused ion beam-scanning electron microscope (FIB-SEM) to centre the previously characterised defects in each lamella for further analysis. The defects with wider trenches and strong redshifts in CL emission had their initiating basal-plane stacking fault (BSF) towards the bottom of the QW stack, while the BSF formed near the top of the QW stack for a defect with a narrow trench and minimal redshift. The raised-centre, prominent defect showed a slight increase in QW thickness moving up the QW stack while QW widths in the level-centred defect remained broadly constant. The indium content of the enclosed QWs increased above the BSF positions up to a maximum, with an increase of approximately 4\% relative to the surroundings seen for one defect examined. Gross fluctuations in QW width (GWWFs) were present in the surrounding material in this sample but were not seen in QWs enclosed by the defect volumes. These GWWFs have been linked with indium loss from surface step edges two or more monolayers high, and many surface step edges appear pinned by the open trenches, suggesting another reason for the higher indium content seen in QWs enclosed by trench defects.

    @Article{strathprints76067,
    author = {O'Hanlon, T. J. and Massabuau, F. C.-P. and Bao, A. and Kappers, M. J. and Oliver, R. A.},
    journal = {Ultramicroscopy},
    title = {Directly correlated microscopy of trench defects in {InGaN} quantum wells},
    year = {2021},
    issn = {0304-3991},
    month = {March},
    abstract = {Directly correlated measurements of the surface morphology, light emission and subsurface structure and composition were carried out on the exact same nanoscale trench defects in InGaN quantum well (QW) structures. Multiple scanning probe, scanning electron and transmission electron microscopy techniques were used to explain the origin of their unusual emission behaviour and the relationship between surface morphology and cathodoluminescence (CL) redshift. Trench defects comprise of an open trench partially or fully enclosing material in InGaN QWs with different CL emission properties to their surroundings. The CL redshift was shown to typically vary with the width of the trench and the prominence of the material enclosed by the trench above its surroundings. Three defects, encompassing typical and atypical features, were prepared into lamellae for transmission electron microscopy (TEM). A cross marker technique was used in the focused ion beam-scanning electron microscope (FIB-SEM) to centre the previously characterised defects in each lamella for further analysis. The defects with wider trenches and strong redshifts in CL emission had their initiating basal-plane stacking fault (BSF) towards the bottom of the QW stack, while the BSF formed near the top of the QW stack for a defect with a narrow trench and minimal redshift. The raised-centre, prominent defect showed a slight increase in QW thickness moving up the QW stack while QW widths in the level-centred defect remained broadly constant. The indium content of the enclosed QWs increased above the BSF positions up to a maximum, with an increase of approximately 4\% relative to the surroundings seen for one defect examined. Gross fluctuations in QW width (GWWFs) were present in the surrounding material in this sample but were not seen in QWs enclosed by the defect volumes. These GWWFs have been linked with indium loss from surface step edges two or more monolayers high, and many surface step edges appear pinned by the open trenches, suggesting another reason for the higher indium content seen in QWs enclosed by trench defects.},
    doi = {10.1016/j.ultramic.2021.113255},
    keywords = {Gallium nitride, trench defect, quantum well, multi-microscopy, sample preparation, properties correlation, Physics, Instrumentation, Atomic and Molecular Physics, and Optics, Electronic, Optical and Magnetic Materials},
    url = {https://doi.org/10.1016/j.ultramic.2021.113255},
    }

  • A. Rossi, P. G. Baity, V. M. Schäfer, and M. Weides, “Quantum computing hardware in the cloud: should a computational chemist care?,” International Journal of Quantum Chemistry, vol. 121, iss. 14, 2021. doi:10.1002/qua.26688
    [BibTeX] [Abstract] [Download PDF]

    Within the last decade much progress has been made in the experimental realization of quantum computing hardware based on a variety of physical systems. Rapid progress has been fuelled by the conviction that sufficiently powerful quantum machines will herald enormous computational advantages in many fields, including chemical research. A quantum computer capable of simulating the electronic structures of complex molecules would be a game changer for the design of new drugs and materials. Given the potential implications of this technology, there is a need within the chemistry community to keep abreast with the latest developments as well as becoming involved in experimentation with quantum prototypes. To facilitate this, here we review the types of quantum computing hardware that have been made available to the public through cloud services. We focus on three architectures, namely superconductors, trapped ions and semiconductors. For each one we summarize the basic physical operations, requirements and performance. We discuss to what extent each system has been used for molecular chemistry problems and highlight the most pressing hardware issues to be solved for a chemistry-relevant quantum advantage to eventually emerge.

    @Article{strathprints76316,
    author = {Rossi, Alessandro and Baity, Paul G. and Sch{\"a}fer, Vera M. and Weides, Martin},
    journal = {International Journal of Quantum Chemistry},
    title = {Quantum computing hardware in the cloud: should a computational chemist care?},
    year = {2021},
    issn = {0020-7608},
    month = {July},
    number = {14},
    volume = {121},
    abstract = {Within the last decade much progress has been made in the experimental realization of quantum computing hardware based on a variety of physical systems. Rapid progress has been fuelled by the conviction that sufficiently powerful quantum machines will herald enormous computational advantages in many fields, including chemical research. A quantum computer capable of simulating the electronic structures of complex molecules would be a game changer for the design of new drugs and materials. Given the potential implications of this technology, there is a need within the chemistry community to keep abreast with the latest developments as well as becoming involved in experimentation with quantum prototypes. To facilitate this, here we review the types of quantum computing hardware that have been made available to the public through cloud services. We focus on three architectures, namely superconductors, trapped ions and semiconductors. For each one we summarize the basic physical operations, requirements and performance. We discuss to what extent each system has been used for molecular chemistry problems and highlight the most pressing hardware issues to be solved for a chemistry-relevant quantum advantage to eventually emerge.},
    doi = {10.1002/qua.26688},
    keywords = {quantum computing hardware, quantum computing, computational chemistry, Physics, Physics and Astronomy(all)},
    url = {https://doi.org/10.1002/qua.26688},
    }

  • P. Mukhopadhyay, I. Hatipoglu, T. S. Sakthivel, D. A. Hunter, N. K. Gunasekar, P. R. Edwards, R. W. Martin, S. Seal, and W. V. Schoenfeld, “High figure-of-merit gallium oxide UV photodetector on silicon by MBE: a path toward monolithic integration,” Advanced Photonics Research, vol. 2, iss. 4, 2021. doi:10.1002/adpr.202000067
    [BibTeX] [Abstract] [Download PDF]

    We demonstrate a high figure-of-merit UV-C solar-blind photodetectors fabricated from polycrystalline beta-gallium oxide ({\ensuremath{\beta}}-Ga2O3) grown on n-Si substrates by plasma assisted molecular beam epitaxy (PAMBE). We developed film growth sequences for nucleation of Ga2O3 on (100) and (111) oriented Si substrates, and systematically investigate the influence of crucial growth parameters, namely substrate temperature, oxygen flow rate and plasma power on the functional properties of the photodetectors. The photodetectors show ultra-high responsivity of 837 A/W and fast ON/OFF time below 4ms observed under 248 nm illumination at -5V. In addition, they display strong rectifying properties and sharp cut-off below 280 nm with average responsivities between 10-80 A/W, detectivity on the order of 1010 Jones, and rise/fall times between 4 to 500 ms at -5V. High photoconductive gain is shown to likely be due to the mid-bandgap donor/acceptor defect levels, including oxygen vacancies in the form of self-trapped holes. We demonstrate that these defect levels can be modified by controlling the growth conditions, thereby, allowing for tailoring of the photodetector characteristics for specific applications. Our methodology represents a cost-effective solution over homoepitaxial approaches, with characteristics that meet or exceed those reported previously, offering new possibilities for on-wafer integration with Si electronics.

    @Article{strathprints74787,
    author = {Mukhopadhyay, Partha and Hatipoglu, Isa and Sakthivel, Tamil Selvan and Hunter, Daniel A. and Gunasekar, Naresh Kumar and Edwards, Paul R. and Martin, Robert W. and Seal, Sudipta and Schoenfeld, Winston Vaughan},
    journal = {Advanced Photonics Research},
    title = {High figure-of-merit gallium oxide {UV} photodetector on silicon by {MBE}: a path toward monolithic integration},
    year = {2021},
    month = {April},
    number = {4},
    volume = {2},
    abstract = {We demonstrate a high figure-of-merit UV-C solar-blind photodetectors fabricated from polycrystalline beta-gallium oxide ({\ensuremath{\beta}}-Ga2O3) grown on n-Si substrates by plasma assisted molecular beam epitaxy (PAMBE). We developed film growth sequences for nucleation of Ga2O3 on (100) and (111) oriented Si substrates, and systematically investigate the influence of crucial growth parameters, namely substrate temperature, oxygen flow rate and plasma power on the functional properties of the photodetectors. The photodetectors show ultra-high responsivity of 837 A/W and fast ON/OFF time below 4ms observed under 248 nm illumination at -5V. In addition, they display strong rectifying properties and sharp cut-off below 280 nm with average responsivities between 10-80 A/W, detectivity on the order of 1010 Jones, and rise/fall times between 4 to 500 ms at -5V. High photoconductive gain is shown to likely be due to the mid-bandgap donor/acceptor defect levels, including oxygen vacancies in the form of self-trapped holes. We demonstrate that these defect levels can be modified by controlling the growth conditions, thereby, allowing for tailoring of the photodetector characteristics for specific applications. Our methodology represents a cost-effective solution over homoepitaxial approaches, with characteristics that meet or exceed those reported previously, offering new possibilities for on-wafer integration with Si electronics.},
    doi = {10.1002/adpr.202000067},
    keywords = {UV-C photodetector, gallium oxide, molecular beam epitaxy, heterostructure, heterointegration, Physics, Physics and Astronomy(all)},
    url = {https://doi.org/10.1002/adpr.202000067},
    }

2020

  • G. Naresh-Kumar, H. Macintyre, S. Shanthi, P. R. Edwards, R. W. Martin, K. Daivasigamani, K. Sasaki, and A. Kuramata, “Origin of red emission in β-Ga₂O₃ analysed by cathodoluminescence and photoluminescence spectroscopy,” Physica Status Solidi B, 2020.
    [BibTeX] [Abstract] [Download PDF]

    The spectroscopic techniques of cathodoluminescence and photoluminescence are used to study the origin of red emission in β-Ga₂O₃ grown using the edge-defined film-fed grown (EFG) method and hydride vapor phase epitaxy. Room temperature cathodoluminescence shows red emission peaks from samples doped with Fe, Sn, and Si and from unintentionally doped samples. Narrow emission lines around 690 nm are seen strongly in the Fe and unintentionally doped samples. Temperature-dependent photoluminescence analysis of the two prominent red emission lines reveals properties similar to the R lines in sapphire for all the samples, but with different level of existence. These lines are attributed to Cr³⁺ ionic transitions rather than to Fe³⁺, as reported previously. The most likely origin of the unintentional Cr incorporation is the source material used in the EFG method.

    @Article{strathprints74145,
    author = {Gunasekar Naresh-Kumar and Hazel Macintyre and Shanthi Shanthi and Paul R. Edwards and Robert W. Martin and Krishnamurthy Daivasigamani and Kohei Sasaki and Akito Kuramata},
    journal = {Physica Status Solidi B},
    title = {Origin of red emission in {β-Ga₂O₃} analysed by cathodoluminescence and photoluminescence spectroscopy},
    year = {2020},
    month = {October},
    abstract = {The spectroscopic techniques of cathodoluminescence and photoluminescence are used to study the origin of red emission in β-Ga₂O₃ grown using the edge-defined film-fed grown (EFG) method and hydride vapor phase epitaxy. Room temperature cathodoluminescence shows red emission peaks from samples doped with Fe, Sn, and Si and from unintentionally doped samples. Narrow emission lines around 690 nm are seen strongly in the Fe and unintentionally doped samples. Temperature-dependent photoluminescence analysis of the two prominent red emission lines reveals properties similar to the R lines in sapphire for all the samples, but with different level of existence. These lines are attributed to Cr³⁺ ionic transitions rather than to Fe³⁺, as reported previously. The most likely origin of the unintentional Cr incorporation is the source material used in the EFG method.},
    keywords = {spectroscopy methods, gallium oxide, scanning electron microscope (SEM), cathodoluminescence , photoluminescence, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {https://strathprints.strath.ac.uk/74145/},
    }

  • D. F. Luca, H. Zhang, K. Mingard, M. Stewart, B. M. Jablon, C. Trager-Cowan, and M. G. Gee, “Nanomechanical behaviour of individual phases in WC-Co cemented carbides, from ambient to high temperature,” Materialia, vol. 12, 2020.
    [BibTeX] [Abstract] [Download PDF]

    The dependence of the mechanical behaviour of individual phases in WC-Co on microstructural parameters such as grain size and orientation were investigated by means of nanoindentation and electron microscopy. A broad range of WC grain dimensions, from about 1 to 1000 µm², were selected and subsequently indented to investigate any size effect. A decrease in hardness as a function of grain dimensions was observed, due to an increase in dislocation mobility in larger grains. Whilst the binder phase only exhibits a hardness of about 11 GPa, the hardness of WC grains was measured about 29 and 53 GPa for the prismatic and basal orientations, respectively, in ambient conditions. All WC orientations exhibited a similar decrease in hardness with temperature, up to 700 °C. Damage mechanisms occurring in WC-Co during nanoindentation were investigated for the different grain orientations at various temperatures. The damage was visualised using electron microscopy near the residual indent coupled with Focused Ion Beam (FIB) sectioning across the indent. The three-dimensional distribution of plastic deformation across multiple grains in the vicinity of an indent was examined using Electron Channelling Contrast Imaging (ECCI). ECCI micrographs enabled the observation of crystal defects, especially dislocations, and slip lines as well as the entire plastic zone. The defect density and spatial distribution in the deformed WC grains were compared to that of an untested WC grain to identify the type of deformation originating from spherical indentation. The work provides important information on the relationship between WC-Co microstructure and performance at operating temperatures.

    @Article{strathprints73784,
    author = {F. De Luca and H. Zhang and K. Mingard and M. Stewart and B. M. Jablon and C. Trager-Cowan and M. G. Gee},
    journal = {Materialia},
    title = {Nanomechanical behaviour of individual phases in {WC-Co} cemented carbides, from ambient to high temperature},
    year = {2020},
    month = {August},
    volume = {12},
    abstract = {The dependence of the mechanical behaviour of individual phases in WC-Co on microstructural parameters such as grain size and orientation were investigated by means of nanoindentation and electron microscopy. A broad range of WC grain dimensions, from about 1 to 1000 µm², were selected and subsequently indented to investigate any size effect. A decrease in hardness as a function of grain dimensions was observed, due to an increase in dislocation mobility in larger grains. Whilst the binder phase only exhibits a hardness of about 11 GPa, the hardness of WC grains was measured about 29 and 53 GPa for the prismatic and basal orientations, respectively, in ambient conditions. All WC orientations exhibited a similar decrease in hardness with temperature, up to 700 °C. Damage mechanisms occurring in WC-Co during nanoindentation were investigated for the different grain orientations at various temperatures. The damage was visualised using electron microscopy near the residual indent coupled with Focused Ion Beam (FIB) sectioning across the indent. The three-dimensional distribution of plastic deformation across multiple grains in the vicinity of an indent was examined using Electron Channelling Contrast Imaging (ECCI). ECCI micrographs enabled the observation of crystal defects, especially dislocations, and slip lines as well as the entire plastic zone. The defect density and spatial distribution in the deformed WC grains were compared to that of an untested WC grain to identify the type of deformation originating from spherical indentation. The work provides important information on the relationship between WC-Co microstructure and performance at operating temperatures.},
    keywords = {electron microscopy, high temperature deformation, mechanical properties, microstructure, nanoindentation, WC-Co, Physics, Materials Science(all)},
    url = {https://strathprints.strath.ac.uk/73784/},
    }

  • X. Zhao, K. Huang, J. Bruckbauer, S. Shen, C. Zhu, P. Fletcher, P. Feng, Y. Cai, J. Bai, C. Trager-Cowan, R. W. Martin, and T. Wang, “Influence of an InGaN superlattice pre-layer on the performance of semi-polar (11-22) green LEDs grown on silicon,” Scientific Reports, vol. 10, 2020.
    [BibTeX] [Abstract] [Download PDF]

    It is well-known that it is crucial to insert either a single InGaN underlayer or an InGaN superlattice (SLS) structure (both with low InN content) as a pre-layer prior to the growth of InGaN/GaN multiple quantum wells (MQWs) served as an active region for a light-emitting diode (LED). So far, this growth scheme has achieved a great success in the growth of III-nitride LEDs on c-plane substrates, but has not yet been applied in the growth of any other orientated III-nitride LEDs. In this paper, we have applied this growth scheme in the growth of semi-polar (11-22) green LEDs, and have investigated the impact of the SLS pre-layer on the optical performance of semi-polar (11-22) green LEDs grown on patterned (113) silicon substrates. Our results demonstrate that the semi-polar LEDs with the SLS pre-layer exhibit an improvement in both internal quantum efficiency and light output, which is similar to their c-plane counterparts. However, the performance improvement is not so significant as in the c-plane case. This is because the SLS pre-layer also introduces extra misfit dislocations for the semi-polar, but not the c-plane case, which act as non-radiative recombination centres.

    @Article{strathprints73474,
    author = {X. Zhao and K. Huang and J. Bruckbauer and S. Shen and C. Zhu and P. Fletcher and P. Feng and Y. Cai and J. Bai and C. Trager-Cowan and R. W. Martin and T. Wang},
    journal = {Scientific Reports},
    title = {Influence of an {InGaN} superlattice pre-layer on the performance of semi-polar (11-22) green LEDs grown on silicon},
    year = {2020},
    month = {July},
    volume = {10},
    abstract = {It is well-known that it is crucial to insert either a single InGaN underlayer or an InGaN superlattice (SLS) structure (both with low InN content) as a pre-layer prior to the growth of InGaN/GaN multiple quantum wells (MQWs) served as an active region for a light-emitting diode (LED). So far, this growth scheme has achieved a great success in the growth of III-nitride LEDs on c-plane substrates, but has not yet been applied in the growth of any other orientated III-nitride LEDs. In this paper, we have applied this growth scheme in the growth of semi-polar (11-22) green LEDs, and have investigated the impact of the SLS pre-layer on the optical performance of semi-polar (11-22) green LEDs grown on patterned (113) silicon substrates. Our results demonstrate that the semi-polar LEDs with the SLS pre-layer exhibit an improvement in both internal quantum efficiency and light output, which is similar to their c-plane counterparts. However, the performance improvement is not so significant as in the c-plane case. This is because the SLS pre-layer also introduces extra misfit dislocations for the semi-polar, but not the c-plane case, which act as non-radiative recombination centres.},
    keywords = {light-emitting diode (LED), III-nitride LEDs, semi-polar LEDs, performance, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/73474/},
    }

  • F. P. Bonafé, B. Aradi, B. Hourahine, C. R. Medrano, F. J. Hernández, T. Frauenheim, and C. G. Sánchez, “A real-time time-dependent density functional tight-binding implementation for semiclassical excited state electron-nuclear dynamics and pump-probe spectroscopy simulations,” Journal of Chemical Theory and Computation, vol. 16, p. 4454–4469, 2020. doi:10.1021/acs.jctc.9b01217
    [BibTeX] [Abstract] [Download PDF]

    The increasing need to simulate the dynamics of photoexcited molecular and nanosystems in the sub-picosecond regime demands new efficient tools able to describe the quantum nature of matter at a low computational cost. By combining the power of the approximate DFTB method with the semiclassical Ehrenfest method for nuclear-electron dynamics we have achieved a real-time time-dependent DFTB (TD-DFTB) implementation that fits such requierements. In addition to enabling the study of nuclear motion effects in photoinduced charge transfer processes, our code adds novel features to the realm of static and time-resolved computational spectroscopies. In particular, the optical properties of periodic materials such as graphene nanoribbons or the use of corrections such as the “LDA+U” and “pseudo SIC” methods to improve the optical properties in some systems, can now be handled at the TD-DFTB level. Moreover, the simulation of fully-atomistic time-resolved transient absorption spectra and impulsive vibrational spectra can now be achieved within reasonable computing time, owing to the good performance of the implementation and a parallel simulation protocol. Its application to the study of UV/visible light-induced vibrational coherences in molecules is demonstrated and opens a new door into the mechanisms of non-equilibrium ultrafast phenomena in countless materials with relevant applications.

    @Article{strathprints72639,
    author = {Franco P. Bonaf{\'e} and B{\'a}lint Aradi and Ben Hourahine and Carlos R. Medrano and Federico J. Hern{\'a}ndez and Thomas Frauenheim and Cristi{\'a}n G. S{\'a}nchez},
    journal = {Journal of Chemical Theory and Computation},
    title = {A real-time time-dependent density functional tight-binding implementation for semiclassical excited state electron-nuclear dynamics and pump-probe spectroscopy simulations},
    year = {2020},
    month = {June},
    note = {Manuscript includes supplementary information.},
    pages = {4454--4469},
    volume = {16},
    abstract = {The increasing need to simulate the dynamics of photoexcited molecular and nanosystems in the sub-picosecond regime demands new efficient tools able to describe the quantum nature of matter at a low computational cost. By combining the power of the approximate DFTB method with the semiclassical Ehrenfest method for nuclear-electron dynamics we have achieved a real-time time-dependent DFTB (TD-DFTB) implementation that fits such requierements. In addition to enabling the study of nuclear motion effects in photoinduced charge transfer processes, our code adds novel features to the realm of static and time-resolved computational spectroscopies. In particular, the optical properties of periodic materials such as graphene nanoribbons or the use of corrections such as the "LDA+U" and "pseudo SIC" methods to improve the optical properties in some systems, can now be handled at the TD-DFTB level. Moreover, the simulation of fully-atomistic time-resolved transient absorption spectra and impulsive vibrational spectra can now be achieved within reasonable computing time, owing to the good performance of the implementation and a parallel simulation protocol. Its application to the study of UV/visible light-induced vibrational coherences in molecules is demonstrated and opens a new door into the mechanisms of non-equilibrium ultrafast phenomena in countless materials with relevant applications.},
    doi = {10.1021/acs.jctc.9b01217},
    keywords = {nanosystems, photoexcited molecular systems, DFTB, graphene nanoribbons, time-resolved computational spectroscopies, Solid state physics. Nanoscience, Physical and Theoretical Chemistry, Computer Science Applications},
    url = {https://strathprints.strath.ac.uk/72639/},
    }

  • G. Naresh-Kumar, A. Alasamari, G. Kusch, P. R. Edwards, R. W. Martin, K. P. Mingard, and C. Trager-Cowan, “Metrology of crystal defects through intensity variations in secondary electrons from the diffraction of primary electrons in a scanning electron microscope,” Ultramicroscopy, vol. 213, p. 112977, 2020.
    [BibTeX] [Abstract] [Download PDF]

    Understanding defects and their roles in plastic deformation and device reliability is important for the development of a wide range of novel materials for the next generation of electronic and optoelectronic devices. We introduce the use of gaseous secondary electron detectors in a variable pressure scanning electron microscope for non-destructive imaging of extended defects using electron channelling contrast imaging. We demonstrate that all scattered electrons, including the secondary electrons, can provide diffraction contrast as long as the sample is positioned appropriately with respect to the incident electron beam. Extracting diffraction information through monitoring the modulation of the intensity of secondary electrons as a result of diffraction of the incident electron beam, opens up the possibility of performing low energy electron channelling contrast imaging to characterise low atomic weight and ultra-thin film materials. Our methodology can be adopted for large area, nanoscale structural characterisation of a wide range of crystalline materials including metals and semiconductors, and we illustrate this using the examples of aluminium nitride and gallium nitride. The capability of performing electron channelling contrast imaging, using the variable pressure mode, extends the application of this technique to insulators, which usually require conducting coatings on the sample surface for traditional scanning electron microscope based microstructural characterisation.

    @Article{strathprints72016,
    author = {G. Naresh-Kumar and A. Alasamari and G. Kusch and P. R. Edwards and R. W. Martin and K. P. Mingard and C. Trager-Cowan},
    journal = {Ultramicroscopy},
    title = {Metrology of crystal defects through intensity variations in secondary electrons from the diffraction of primary electrons in a scanning electron microscope},
    year = {2020},
    month = {March},
    pages = {112977},
    volume = {213},
    abstract = {Understanding defects and their roles in plastic deformation and device reliability is important for the development of a wide range of novel materials for the next generation of electronic and optoelectronic devices. We introduce the use of gaseous secondary electron detectors in a variable pressure scanning electron microscope for non-destructive imaging of extended defects using electron channelling contrast imaging. We demonstrate that all scattered electrons, including the secondary electrons, can provide diffraction contrast as long as the sample is positioned appropriately with respect to the incident electron beam. Extracting diffraction information through monitoring the modulation of the intensity of secondary electrons as a result of diffraction of the incident electron beam, opens up the possibility of performing low energy electron channelling contrast imaging to characterise low atomic weight and ultra-thin film materials. Our methodology can be adopted for large area, nanoscale structural characterisation of a wide range of crystalline materials including metals and semiconductors, and we illustrate this using the examples of aluminium nitride and gallium nitride. The capability of performing electron channelling contrast imaging, using the variable pressure mode, extends the application of this technique to insulators, which usually require conducting coatings on the sample surface for traditional scanning electron microscope based microstructural characterisation.},
    keywords = {electron channelling, secondary electrons, semiconductors, extended defects, SEM, Physics, Atomic and Molecular Physics, and Optics},
    url = {https://strathprints.strath.ac.uk/72016/},
    }

  • B. Hourahine, B. Aradi, V. Blum, F. Bonafé, A. Buccheri, C. Camacho, C. Cevallos, M. Y. Deshaye, T. Dumitriča, A. Dominguez, S. Ehlert, M. Elstner, T. van der Heide, J. Hermann, S. Irle, J. J. Kranz, C. Kohler, T. Kowalczyk, T. Kubař, I. S. Lee, V. Lutsker, R. J. Maurer, S. K. Min, I. Mitchell, C. Negre, T. A. Niehaus, A. M. N. Niklasson, A. J. Page, A. Pecchia, G. Penazzi, M. P. Persson, J. Řezáč, C. G. Sánchez, M. Sternberg, M. Stöhr, F. Stuckenberg, A. Tkatchenko, V. W. -z. Yu, and T. Frauenheim, “DFTB+, a software package for efficient approximate density functional theory based atomistic simulations,” Journal of Chemical Physics, vol. 152, iss. 12, p. 124101, 2020.
    [BibTeX] [Abstract] [Download PDF]

    DFTB+ is a versatile community developed open source software package offering fast and efficient methods for carrying out atomistic quantum mechanical simulations. By implementing various methods approximating density functional theory (DFT), like the density functional based tight binding (DFTB) and the extended tight binding (xTB) method, it enables simulations of large systems and long timescales with reasonable accuracy while being considerably faster for typical simulations than respective ab initio methods. Based on the DFTB framework it additionally offers approximated versions of various DFT extensions including hybrid functionals, time dependent formalism for treating excited systems, electron transport using non-equilibrium Green?s functions and many more. DFTB+ can be used as a user-friendly standalone application as well as being embedded into other software packages as a library or acting as a calculation-server accessed by socket communication. We give an overview of the recently developed capabilities of the DFTB+ code, demonstrating with a few use case examples, discuss the strengths and weaknesses of the various features and discuss on-going developments and possible future perspectives.

    @Article{strathprints71868,
    author = {B. Hourahine and B. Aradi and V. Blum and F. Bonaf{\'e} and A. Buccheri and C. Camacho and C. Cevallos and M.Y. Deshaye and T. Dumitri{\v c}a and A. Dominguez and S. Ehlert and M. Elstner and van der Heide, T. and J. Hermann and S. Irle and J. J. Kranz and C. Kohler and T. Kowalczyk and T. Kuba{\v r} and I. S. Lee and V. Lutsker and R. J. Maurer and S. K. Min and I. Mitchell and C. Negre and T. A. Niehaus and A. M. N. Niklasson and A. J. Page and A. Pecchia and G. Penazzi and M. P. Persson and J. {\v R}ez{\'a}{\v c} and C. G. S{\'a}nchez and M. Sternberg and M. St{\"o}hr and F. Stuckenberg and Alexandre Tkatchenko and V. W.-z. Yu and T. Frauenheim},
    journal = {Journal of Chemical Physics},
    title = {{DFTB+}, a software package for efficient approximate density functional theory based atomistic simulations},
    year = {2020},
    month = {March},
    number = {12},
    pages = {124101},
    volume = {152},
    abstract = {DFTB+ is a versatile community developed open source software package offering fast and efficient methods for carrying out atomistic quantum mechanical simulations. By implementing various methods approximating density functional theory (DFT), like the density functional based tight binding (DFTB) and the extended tight binding (xTB) method, it enables simulations of large systems and long timescales with reasonable accuracy while being considerably faster for typical simulations than respective ab initio methods. Based on the DFTB framework it additionally offers approximated versions of various DFT extensions including hybrid functionals, time dependent formalism for treating excited systems, electron transport using non-equilibrium Green?s functions and many more. DFTB+ can be used as a user-friendly standalone application as well as being embedded into other software packages as a library or acting as a calculation-server accessed by socket communication. We give an overview of the recently developed capabilities of the DFTB+ code, demonstrating with a few use case examples, discuss the strengths and weaknesses of the various features and discuss on-going developments and possible future perspectives.},
    keywords = {electronic structure theory, software engineering, open quantum systems, excited states, dispersion interactions, nanotube modeling, molecular dynamics method, correlated systems, GPU acceleration, parallel algorithms, Physics, Physics and Astronomy(all), Physical and Theoretical Chemistry},
    url = {https://strathprints.strath.ac.uk/71868/},
    }

  • D. Cameron, K. P. O’Donnell, P. R. Edwards, M. Peres, K. Lorenz, M. J. Kappers, and M. Boćkowski, “Acceptor state anchoring in gallium nitride,” Applied Physics Letters, vol. 116, p. 102105, 2020.
    [BibTeX] [Abstract] [Download PDF]

    The dual nature of the magnesium acceptor in gallium nitride results in dynamic defect complexes. Europium spectator ions reveal switching between two spectrally unique metastable centres, each corresponding to a particular acceptor state. By ion co-implantation of europium and oxygen into GaN(Mg), we produce, in addition, an anchored state system. In doing so we create an abundance of previously unidentified stable centres which we denote as “Eu0(Ox)”. We introduce a microscopic model for these centres with oxygen substituting for nitrogen in the bridging site.

    @Article{strathprints71643,
    author = {D. Cameron and K. P. O'Donnell and P. R. Edwards and M. Peres and K. Lorenz and M. J. Kappers and M. Bo{\'c}kowski},
    journal = {Applied Physics Letters},
    title = {Acceptor state anchoring in gallium nitride},
    year = {2020},
    month = {February},
    pages = {102105},
    volume = {116},
    abstract = {The dual nature of the magnesium acceptor in gallium nitride results in dynamic defect complexes. Europium spectator ions reveal switching between two spectrally unique metastable centres, each corresponding to a particular acceptor state. By ion co-implantation of europium and oxygen into GaN(Mg), we produce, in addition, an anchored state system. In doing so we create an abundance of previously unidentified stable centres which we denote as "Eu0(Ox)". We introduce a microscopic model for these centres with oxygen substituting for nitrogen in the bridging site.},
    keywords = {rare earth (RE) ions, europium, gallium nitride, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {https://strathprints.strath.ac.uk/71643/},
    }

  • M. V. Yakushev, M. A. Sulimov, C. Faugeras, A. V. Mudryi, and R. W. Martin, “The g-factor of CuGaSe₂ studied by circularly polarised magneto-reflectance,” Journal of Physics D: Applied Physics, vol. 53, p. 17LT02, 2020.
    [BibTeX] [Abstract] [Download PDF]

    High structural quality single crystals of CuGaSe2 were studied using photoluminescence (PL), optical reflectivity (OR) and circularly polarised magneto-reflectance (MR) at 4.2K in magnetic fields B up to 14 T. At B = 0 T both the PL and OR spectra exhibited the A free exciton, associated with the uppermost sub-band of the valence band of CuGaSe2 split by the tetragonal distortion in the lattice. The magnetic field induced a blue shift of the exciton in the MR spectra. Analysis of the dependence of the spectral position of the A exciton on B in the MR spectra, measured for both right and left circular polarisations of light, enabled the magnitude of its effective g-factor to be determined g = 0.46.

    @Article{strathprints71602,
    author = {M. V. Yakushev and M. A. Sulimov and C. Faugeras and A. V. Mudryi and R. W. Martin},
    journal = {Journal of Physics D: Applied Physics},
    title = {The g-factor of {CuGaSe₂} studied by circularly polarised magneto-reflectance},
    year = {2020},
    month = {February},
    pages = {17LT02},
    volume = {53},
    abstract = {High structural quality single crystals of CuGaSe2 were studied using photoluminescence (PL), optical reflectivity (OR) and circularly polarised magneto-reflectance (MR) at 4.2K in magnetic fields B up to 14 T. At B = 0 T both the PL and OR spectra exhibited the A free exciton, associated with the uppermost sub-band of the valence band of CuGaSe2 split by the tetragonal distortion in the lattice. The magnetic field induced a blue shift of the exciton in the MR spectra. Analysis of the dependence of the spectral position of the A exciton on B in the MR spectra, measured for both right and left circular polarisations of light, enabled the magnitude of its effective g-factor to be determined g = 0.46.},
    keywords = {CuGaSe2, photoluminescence, PL, optical reflectivity, semiconductors, optical spectroscopy, magnetic fields, Physics, Atomic and Molecular Physics, and Optics},
    url = {https://strathprints.strath.ac.uk/71602/},
    }

  • C. Trager-Cowan, A. Alasmari, W. Avis, J. Bruckbauer, P. R. Edwards, G. Ferenczi, B. Hourahine, A. Kotzai, S. Kraeusel, G. Kusch, R. W. Martin, R. McDermott, N. Gunasekar, M. Nouf-Allehiani, E. Pascal, D. Thomson, S. Vespucci, M. D. Smith, P. J. Parbrook, J. Enslin, F. Mehnke, C. Kuhn, T. Wernicke, M. Kneissl, S. Hagedorn, A. Knauer, S. Walde, M. Weyers, P. Coulon, P. Shields, J. Bai, Y. Gong, L. Jiu, Y. Zhang, R. Smith, T. Wang, and A. Winkelmann, “Structural and luminescence imaging and characterisation of semiconductors in the scanning electron microscope,” Semiconductor Science and Technology, vol. 35, p. 54001, 2020.
    [BibTeX] [Abstract] [Download PDF]

    The scanning electron microscopy techniques of electron backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI) and hyperspectral cathodoluminescence imaging (CL) provide complementary information on the structural and luminescence properties of materials rapidly and non-destructively, with a spatial resolution of tens of nanometres. EBSD provides crystal orientation, crystal phase and strain analysis, whilst ECCI is used to determine the planar distribution of extended defects over a large area of a given sample. CL reveals the influence of crystal structure, composition and strain on intrinsic luminescence and/or reveals defect-related luminescence. Dark features are also observed in CL images where carrier recombination at defects is non-radiative. The combination of these techniques is a powerful approach to clarifying the role of crystallography and extended defects on a materials’ light emission properties. Here we describe the EBSD, ECCI and CL techniques and illustrate their use for investigating the structural and light emitting properties of UV-emitting nitride semiconductor structures. We discuss our investigations of the type, density and distribution of defects in GaN, AlN and AlGaN thin films and also discuss the determination of the polarity of GaN nanowires.

    @Article{strathprints71512,
    author = {Carol Trager-Cowan and Aeshah Alasmari and William Avis and Jochen Bruckbauer and Paul R. Edwards and Gergely Ferenczi and Benjamin Hourahine and Almpes Kotzai and Simon Kraeusel and Gunnar Kusch and Robert W. Martin and Ryan McDermott and Naresh Gunasekar and M. Nouf-Allehiani and Elena Pascal and David Thomson and Stefano Vespucci and Matthew David Smith and Peter J. Parbrook and Johannes Enslin and Frank Mehnke and Christian Kuhn and Tim Wernicke and Michael Kneissl and Sylvia Hagedorn and Arne Knauer and Sebastian Walde and Markus Weyers and Pierre-Marie Coulon and Philip Shields and J. Bai and Y. Gong and Ling Jiu and Y. Zhang and Richard Smith and Tao Wang and Aimo Winkelmann},
    journal = {Semiconductor Science and Technology},
    title = {Structural and luminescence imaging and characterisation of semiconductors in the scanning electron microscope},
    year = {2020},
    month = {February},
    pages = {054001},
    volume = {35},
    abstract = {The scanning electron microscopy techniques of electron backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI) and hyperspectral cathodoluminescence imaging (CL) provide complementary information on the structural and luminescence properties of materials rapidly and non-destructively, with a spatial resolution of tens of nanometres. EBSD provides crystal orientation, crystal phase and strain analysis, whilst ECCI is used to determine the planar distribution of extended defects over a large area of a given sample. CL reveals the influence of crystal structure, composition and strain on intrinsic luminescence and/or reveals defect-related luminescence. Dark features are also observed in CL images where carrier recombination at defects is non-radiative. The combination of these techniques is a powerful approach to clarifying the role of crystallography and extended defects on a materials' light emission properties. Here we describe the EBSD, ECCI and CL techniques and illustrate their use for investigating the structural and light emitting properties of UV-emitting nitride semiconductor structures. We discuss our investigations of the type, density and distribution of defects in GaN, AlN and AlGaN thin films and also discuss the determination of the polarity of GaN nanowires.},
    keywords = {EBSD, nitride, scanning electron microscopy, Physics, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
    url = {https://strathprints.strath.ac.uk/71512/},
    }

  • A. Winkelmann, M. B. Jablon, V. Tong, C. Trager-Cowan, and K. Mingard, “Improving EBSD precision by orientation refinement with full pattern matching,” Journal of Microscopy, vol. 277, iss. 2, p. 79–92, 2020. doi:10.1111/jmi.12870
    [BibTeX] [Abstract] [Download PDF]

    We present a comparison of the precision of different approaches for orientation imaging using electron backscatter diffraction (EBSD) in the scanning electron microscope. We have used EBSD to image the internal structure of WC grains, which contain features due to dislocations and subgrains. We compare the conventional, Hough-transform based orientation results from the EBSD system software with results of a high-precision orientation refinement using simulated pattern matching at the full available detector resolution of 640 × 480 pixels. Electron channeling contrast imaging (ECCI) is used to verify the correspondence of qualitative ECCI features with the quantitative orientation data from pattern matching. For the investigated sample, this leads to an estimated pattern matching sensitivity of about 0.5mrad (0.03°) and a spatial feature resolution of about 100nm. In order to investigate the alternative approach of post-processing noisy orientation data, we analyse the effects of two different types of orientation filters. Using reference features in the high-precision pattern matching results for comparison, we find that denoising of orientation data can reduce the spatial resolution, and can lead to the creation of orientation artefacts for crystallographic features near the spatial and orientational resolution limits of EBSD.

    @Article{strathprints71326,
    author = {Aimo Winkelmann and B. Matat Jablon and Vivian Tong and Carol Trager-Cowan and Ken Mingard},
    journal = {Journal of Microscopy},
    title = {Improving {EBSD} precision by orientation refinement with full pattern matching},
    year = {2020},
    month = {January},
    number = {2},
    pages = {79--92},
    volume = {277},
    abstract = {We present a comparison of the precision of different approaches for orientation imaging using electron backscatter diffraction (EBSD) in the scanning electron microscope. We have used EBSD to image the internal structure of WC grains, which contain features due to dislocations and subgrains. We compare the conventional, Hough-transform based orientation results from the EBSD system software with results of a high-precision orientation refinement using simulated pattern matching at the full available detector resolution of 640 × 480 pixels. Electron channeling contrast imaging (ECCI) is used to verify the correspondence of qualitative ECCI features with the quantitative orientation data from pattern matching. For the investigated sample, this leads to an estimated pattern matching sensitivity of about 0.5mrad (0.03°) and a spatial feature resolution of about 100nm. In order to investigate the alternative approach of post-processing noisy orientation data, we analyse the effects of two different types of orientation filters. Using reference features in the high-precision pattern matching results for comparison, we find that denoising of orientation data can reduce the spatial resolution, and can lead to the creation of orientation artefacts for crystallographic features near the spatial and orientational resolution limits of EBSD.},
    doi = {10.1111/jmi.12870},
    keywords = {EBSD analysis, orientation imaging, pattern matching, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/71326/},
    }

  • A. A. Wiles, J. Bruckbauer, N. Mohammed, M. Cariello, J. Cameron, N. J. Findlay, E. Taylor-Shaw, D. J. Wallis, R. W. Martin, P. J. Skabara, and G. Cooke, “A poly(urethane)-encapsulated benzo[2,3-d:6,7-d’]diimidazole organic down-converter for green hybrid LEDs,” Materials Chemistry Frontiers, vol. 4, p. 1006–1012, 2020.
    [BibTeX] [Abstract] [Download PDF]

    The development of organic down-converting materials continues to attract attention in hybrid LED technology by obviating the need for non-sustainable rare-earth elements. In this work, a benzodiimidazole-based system (TPA-BDI) has been employed as a down-converting layer in a hybrid organic-inorganic LED device. A commercially available poly(urethane)-based resin is used as the encapsulating material, providing a dilute layer of TPA-BDI that is deposited on top of the GaN-based LED. Crucially, the solution-state emissive performance is generally maintained when encapsulated at dilute concentrations within this resin. A maximum luminous efficacy of 87 lm/W was demonstrated using a 1.0 mg/ml concentration of TPA-BDI in the resin. The suitability of using organic down-converters to produce green light from hybrid devices was demonstrated by the excellent repeatability of the device characteristics across a series of encapsulated LEDs.

    @Article{strathprints71224,
    author = {Alan A. Wiles and Jochen Bruckbauer and Nabeel Mohammed and Michele Cariello and Joseph Cameron and Neil J. Findlay and Elaine Taylor-Shaw and David J. Wallis and Robert W. Martin and Peter J. Skabara and Graeme Cooke},
    journal = {Materials Chemistry Frontiers},
    title = {A poly(urethane)-encapsulated benzo[2,3-d:6,7-d']diimidazole organic down-converter for green hybrid {LED}s},
    year = {2020},
    pages = {1006--1012},
    volume = {4},
    abstract = {The development of organic down-converting materials continues to attract attention in hybrid LED technology by obviating the need for non-sustainable rare-earth elements. In this work, a benzodiimidazole-based system (TPA-BDI) has been employed as a down-converting layer in a hybrid organic-inorganic LED device. A commercially available poly(urethane)-based resin is used as the encapsulating material, providing a dilute layer of TPA-BDI that is deposited on top of the GaN-based LED. Crucially, the solution-state emissive performance is generally maintained when encapsulated at dilute concentrations within this resin. A maximum luminous efficacy of 87 lm/W was demonstrated using a 1.0 mg/ml concentration of TPA-BDI in the resin. The suitability of using organic down-converters to produce green light from hybrid devices was demonstrated by the excellent repeatability of the device characteristics across a series of encapsulated LEDs.},
    keywords = {down-converter, LEDs, TPA-BDI, Chemistry, Chemistry(all)},
    url = {https://strathprints.strath.ac.uk/71224/},
    }

  • V. W. Yu, C. Campos, W. Dawson, A. García, V. Havu, B. Hourahine, W. P. Huhn, M. Jacquelin, W. Jia, M. Keçeli, R. Laasner, Y. Li, L. Lin, J. Lu, J. Moussa, J. E. Roman, Á. Vázquez-Mayagoitia, C. Yang, and V. Blum, “ELSI – An open infrastructure for electronic structure solvers,” Computer Physics Communications, vol. 256, p. 107459, 2020. doi:10.1016/j.mssp.2020.105301
    [BibTeX] [Abstract] [Download PDF]

    Routine applications of electronic structure theory to molecules and periodic systems need to compute the electron density from given Hamiltonian and, in case of non-orthogonal basis sets, overlap matrices. System sizes can range from few to thousands or, in some examples, millions of atoms. Different discretization schemes (basis sets) and different system geometries (finite non-periodic vs. infinite periodic boundary conditions) yield matrices with different structures. The ELectronic Structure Infrastructure (ELSI) project provides an open-source software interface to facilitate the implementation and optimal use of high-performance solver libraries covering cubic scaling eigensolvers, linear scaling density-matrix-based algorithms, and other reduced scaling methods in between. In this paper, we present recent improvements and developments inside ELSI, mainly covering (1) new solvers connected to the interface, (2) matrix layout and communication adapted for parallel calculations of periodic and/or spin-polarized systems, (3) routines for density matrix extrapolation in geometry optimization and molecular dynamics calculations, and (4) general utilities such as parallel matrix I/O and JSON output. The ELSI interface has been integrated into four electronic structure code projects (DFTB+, DGDFT, FHI-aims, SIESTA), allowing us to rigorously benchmark the performance of the solvers on an equal footing. Based on results of a systematic set of large-scale benchmarks performed with Kohn-Sham density-functional theory and density-functional tight-binding theory, we identify factors that strongly affect the efficiency of the solvers, and propose a decision layer that assists with the solver selection process. Finally, we describe a reverse communication interface encoding matrix-free iterative solver strategies that are amenable, e.g., for use with planewave basis sets.

    @Article{strathprints71182,
    author = {Victor Wen-zhe Yu and Carmen Campos and William Dawson and Alberto Garc{\'i}a and Ville Havu and Ben Hourahine and William P. Huhn and Mathias Jacquelin and Weile Jia and Murat Ke{\c c}eli and Raul Laasner and Yingzhou Li and Lin Lin and Jianfeng Lu and Jonathan Moussa and Jose E. Roman and {\'A}lvaro V{\'a}zquez-Mayagoitia and Chao Yang and Volker Blum},
    journal = {Computer Physics Communications},
    title = {{ELSI} -- {A}n open infrastructure for electronic structure solvers},
    year = {2020},
    month = {December},
    pages = {107459},
    volume = {256},
    abstract = {Routine applications of electronic structure theory to molecules and periodic systems need to compute the electron density from given Hamiltonian and, in case of non-orthogonal basis sets, overlap matrices. System sizes can range from few to thousands or, in some examples, millions of atoms. Different discretization schemes (basis sets) and different system geometries (finite non-periodic vs. infinite periodic boundary conditions) yield matrices with different structures. The ELectronic Structure Infrastructure (ELSI) project provides an open-source software interface to facilitate the implementation and optimal use of high-performance solver libraries covering cubic scaling eigensolvers, linear scaling density-matrix-based algorithms, and other reduced scaling methods in between. In this paper, we present recent improvements and developments inside ELSI, mainly covering (1) new solvers connected to the interface, (2) matrix layout and communication adapted for parallel calculations of periodic and/or spin-polarized systems, (3) routines for density matrix extrapolation in geometry optimization and molecular dynamics calculations, and (4) general utilities such as parallel matrix I/O and JSON output. The ELSI interface has been integrated into four electronic structure code projects (DFTB+, DGDFT, FHI-aims, SIESTA), allowing us to rigorously benchmark the performance of the solvers on an equal footing. Based on results of a systematic set of large-scale benchmarks performed with Kohn-Sham density-functional theory and density-functional tight-binding theory, we identify factors that strongly affect the efficiency of the solvers, and propose a decision layer that assists with the solver selection process. Finally, we describe a reverse communication interface encoding matrix-free iterative solver strategies that are amenable, e.g., for use with planewave basis sets.},
    doi = {10.1016/j.mssp.2020.105301},
    keywords = {electronic structure theory, density-functional theory, density-functional tight binding, parallel computing, eigensolver, density matrix, Physics, Hardware and Architecture, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/71182/},
    }

  • J. Bruckbauer, C. Trager-Cowan, B. Hourahine, A. Winkelmann, P. Vennegues, A. Ipsen, X. Yu, X. Zhao, M. J. Wallace, P. R. Edwards, G. Naresh-Kumar, M. Hocker, S. Bauer, R. Müller, J. Bai, K. Thonke, T. Wang, and R. W. Martin, “Luminescence behaviour of semi-polar (10-11) InGaN/GaN “bow-tie” structures on patterned Si substrates,” Journal of Applied Physics, vol. 127, p. 35705, 2020.
    [BibTeX] [Abstract] [Download PDF]

    In this work, we report on the innovative growth of semi-polar ‘bow-tie’-shaped GaN structures containing InGaN/GaN multiple quantum wells (MQWs), and on their structural and luminescence characterisation. We investigate the impact of growth on patterned (113) Si substrates which results in the bow-tie cross-section with upper surfaces having the (10-11) orientation. Room temperature cathodoluminescence (CL) hyperspectral imaging reveals two types of extended defects: black spots appearing in intensity images of the GaN near band edge emission; and dark lines running parallel in the direction of the Si stripes in MQW intensity images. Electron channelling contrast imaging (ECCI) identifies the black spots as threading dislocations (TDs) propagating to the inclined (10-11) surfaces. Line defects in ECCI, propagating in the [1-210] direction parallel to the Si stripes, are attributed to misfit dislocations (MDs) introduced by glide in the basal (0001) planes at the interfaces of the MQW structure. Identification of these line defects as MDs within the MQWs is only possible because they are revealed as dark lines in the MQW CL intensity images, but not in the GaN intensity images. Low temperature CL spectra exhibit additional emission lines at energies below the GaN bound exciton emission line. These emission lines only appear at the edge or the centre of the structures where two (0001) growth fronts meet and coalesce (join of the bow-tie). They are most likely related to basal-plane or prismatic stacking faults or partial dislocations at the GaN/Si interface and the coalescence region.

    @Article{strathprints70977,
    author = {Jochen Bruckbauer and Carol Trager-Cowan and Ben Hourahine and Aimo Winkelmann and Philippe Vennegues and Anja Ipsen and Xiang Yu and Xunming Zhao and Michael J. Wallace and Paul R. Edwards and G. Naresh-Kumar and Matthias Hocker and Sebastian Bauer and Raphael M{\"u}ller and Jie Bai and Klaus Thonke and Tao Wang and Robert W. Martin},
    title = {Luminescence behaviour of semi-polar (10-11) {InGaN/GaN} ``bow-tie'' structures on patterned {Si} substrates},
    journal = {Journal of Applied Physics},
    year = {2020},
    volume = {127},
    pages = {035705},
    month = {December},
    abstract = {In this work, we report on the innovative growth of semi-polar 'bow-tie'-shaped GaN structures containing InGaN/GaN multiple quantum wells (MQWs), and on their structural and luminescence characterisation. We investigate the impact of growth on patterned (113) Si substrates which results in the bow-tie cross-section with upper surfaces having the (10-11) orientation. Room temperature cathodoluminescence (CL) hyperspectral imaging reveals two types of extended defects: black spots appearing in intensity images of the GaN near band edge emission; and dark lines running parallel in the direction of the Si stripes in MQW intensity images. Electron channelling contrast imaging (ECCI) identifies the black spots as threading dislocations (TDs) propagating to the inclined (10-11) surfaces. Line defects in ECCI, propagating in the [1-210] direction parallel to the Si stripes, are attributed to misfit dislocations (MDs) introduced by glide in the basal (0001) planes at the interfaces of the MQW structure. Identification of these line defects as MDs within the MQWs is only possible because they are revealed as dark lines in the MQW CL intensity images, but not in the GaN intensity images. Low temperature CL spectra exhibit additional emission lines at energies below the GaN bound exciton emission line. These emission lines only appear at the edge or the centre of the structures where two (0001) growth fronts meet and coalesce (join of the bow-tie). They are most likely related to basal-plane or prismatic stacking faults or partial dislocations at the GaN/Si interface and the coalescence region.},
    keywords = {semi-polar, GaN structures, luminescence, hyperspectral imaging, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/70977/},
    }

  • B. M. Jablon, K. Mingard, A. Winkelmann, G. Naresh-Kumar, B. Hourahine, and C. Trager-Cowan, “Subgrain structure and dislocations in WC-Co hard metals revealed by electron channelling contrast imaging,” International Journal of Refractory Metals and Hard Materials, vol. 87, p. 105159, 2020.
    [BibTeX] [Abstract] [Download PDF]

    In this study, electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) have been used to examine the substructure and dislocations in tungsten carbide (WC) grains in tungsten carbide-cobalt (WC-Co) hardmetals. These complimentary scanning electron microscopy (SEM) diffraction techniques provide quantifiable information of the substructure without the difficulty of transmission electron microscopy (TEM) sample preparation and examination. Subgrain structures in WC grains have rarely been reported previously because of the sample preparation difficulty, but this study has found they can occur frequently and may provide information on grain growth during sintering. ECCI has also shown for the first time complex dislocation networks across large grains, indicating accumulation of stress in as-sintered materials. To identify the defects revealed by ECCI more precisely, WC grains with surface normals [0001], [1-100] and [11-20], were identified using inverse pole figure orientation maps generated from EBSD data. ECC images from these grains reveal defects intersecting the surface and subgrains bound by dislocations. The combination of ECCI and EBSD allows for new insights into dislocation networks in a WC-Co hardmetal sample over a large, in this case 75 µm × 75 µm, field of view.

    @Article{strathprints70604,
    author = {B. M. Jablon and K. Mingard and A. Winkelmann and G. Naresh-Kumar and B. Hourahine and C. Trager-Cowan},
    journal = {International Journal of Refractory Metals and Hard Materials},
    title = {Subgrain structure and dislocations in {WC-Co} hard metals revealed by electron channelling contrast imaging},
    year = {2020},
    month = {November},
    pages = {105159},
    volume = {87},
    abstract = {In this study, electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) have been used to examine the substructure and dislocations in tungsten carbide (WC) grains in tungsten carbide-cobalt (WC-Co) hardmetals. These complimentary scanning electron microscopy (SEM) diffraction techniques provide quantifiable information of the substructure without the difficulty of transmission electron microscopy (TEM) sample preparation and examination. Subgrain structures in WC grains have rarely been reported previously because of the sample preparation difficulty, but this study has found they can occur frequently and may provide information on grain growth during sintering. ECCI has also shown for the first time complex dislocation networks across large grains, indicating accumulation of stress in as-sintered materials. To identify the defects revealed by ECCI more precisely, WC grains with surface normals [0001], [1-100] and [11-20], were identified using inverse pole figure orientation maps generated from EBSD data. ECC images from these grains reveal defects intersecting the surface and subgrains bound by dislocations. The combination of ECCI and EBSD allows for new insights into dislocation networks in a WC-Co hardmetal sample over a large, in this case 75 µm × 75 µm, field of view.},
    keywords = {hardmetal, subgrain, dislocations, ECCI, EBSD, WC, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/70604/},
    }

  • S. Walde, S. Hagedorn, P. -M. Coulon, A. Mogilatenko, C. Netzel, J. Weinrich, N. Susilo, E. Ziffer, L. Matiwe, C. Hartmann, G. Kusch, A. Alasmari, G. Naresh-Kumar, C. Trager-Cowan, T. Wernicke, T. Straubinger, M. Bickermann, R. W. Martin, P. A. Shields, M. Kneissl, and M. Weyers, “AlN overgrowth of nano-pillar-patterned sapphire with different offcut angle by metalorganic vapor phase epitaxy,” Journal of Crystal Growth, vol. 531, p. 125343, 2020.
    [BibTeX] [Abstract] [Download PDF]

    We present overgrowth of nano-patterned sapphire with different offcut angles by metalorganic vapor phase epitaxy. Hexagonal arrays of nano-pillars were prepared via Displacement Talbot Lithography and dry-etching. 6.6 µm crack-free and fully coalesced AlN was grown on such substrates. Extended defect analysis comparing X-ray diffraction, electron channeling contrast imaging and selective defect etching revealed a threading dislocation density of about 10⁹ cm⁻². However, for c-plane sapphire offcut of 0.2° towards m direction the AlN surface shows step bunches with a height of 10 nm. The detrimental impact of these step bunches on subsequently grown AlGaN multi-quantum-wells is investigated by cathodoluminescence and transmission electron microscopy. By reducing the sapphire offcut to 0.1° the formation of step bunches is successfully suppressed. On top of such a sample an AlGaN-based UVC LED heterostructure is realized emitting at 265 nm and showing an emission power of 0.81 mW at 20 mA (corresponds to an external quantum efficiency of 0.86 %).

    @Article{strathprints70583,
    author = {S. Walde and S. Hagedorn and P.-M. Coulon and A. Mogilatenko and C. Netzel and J. Weinrich and N. Susilo and E. Ziffer and L. Matiwe and C. Hartmann and G. Kusch and A. Alasmari and G. Naresh-Kumar and C. Trager-Cowan and T. Wernicke and T. Straubinger and M. Bickermann and R. W. Martin and P. A. Shields and M. Kneissl and M. Weyers},
    journal = {Journal of Crystal Growth},
    title = {{AlN} overgrowth of nano-pillar-patterned sapphire with different offcut angle by metalorganic vapor phase epitaxy},
    year = {2020},
    month = {November},
    pages = {125343},
    volume = {531},
    abstract = {We present overgrowth of nano-patterned sapphire with different offcut angles by metalorganic vapor phase epitaxy. Hexagonal arrays of nano-pillars were prepared via Displacement Talbot Lithography and dry-etching. 6.6 µm crack-free and fully coalesced AlN was grown on such substrates. Extended defect analysis comparing X-ray diffraction, electron channeling contrast imaging and selective defect etching revealed a threading dislocation density of about 10⁹ cm⁻². However, for c-plane sapphire offcut of 0.2° towards m direction the AlN surface shows step bunches with a height of 10 nm. The detrimental impact of these step bunches on subsequently grown AlGaN multi-quantum-wells is investigated by cathodoluminescence and transmission electron microscopy. By reducing the sapphire offcut to 0.1° the formation of step bunches is successfully suppressed. On top of such a sample an AlGaN-based UVC LED heterostructure is realized emitting at 265 nm and showing an emission power of 0.81 mW at 20 mA (corresponds to an external quantum efficiency of 0.86 %).},
    keywords = {metal organic vapor phase epitaxy, nitrides, sapphire, light emitting diodes, Physics, Materials Chemistry, Condensed Matter Physics},
    url = {https://strathprints.strath.ac.uk/70583/},
    }

  • A. Barthel, J. Roberts, M. Napari, M. Frentrup, T. Huq, A. Kovács, R. Oliver, P. Chalker, T. Sajavaara, and F. Massabuau, “Ti alloyed \ensuremath\alpha-Ga2O3 : route towards wide band gap engineering,” Micromachines, vol. 11, iss. 12, 2020. doi:10.3390/mi11121128
    [BibTeX] [Abstract] [Download PDF]

    The suitability of Ti as a band gap modifier for {\ensuremath{\alpha}}-Ga2O3 was investigated, taking advantage of the isostructural {\ensuremath{\alpha}} phases and high band gap difference between Ti2O3 and Ga2O3. Films of (Ti,Ga)2O3 were synthesized by atomic layer deposition on sapphire substrates, and characterized to determine how crystallinity and band gap vary with composition for this alloy. We report the deposition of high quality {\ensuremath{\alpha}}-(TixGa1-x)2O3 films with x = 3.7\%. For greater compositions the crystalline quality of the films degrades rapidly, where the corundum phase is maintained in films up to x = 5.3\%, and films containing greater Ti fractions being amorphous. Over the range of achieved corundum phase films, that is 0\% {$\leq$} x {$\leq$} 5.3\%, the band gap energy varies by {$\sim$} 270 meV. The ability to maintain a crystalline phase at low fractions of Ti, accompanied by a modification in band gap, shows promising prospects for band gap engineering and the development of wavelength specific solar-blind photodetectors based on {\ensuremath{\alpha}}-Ga2O3.

    @article{strathprints74941,
    volume = {11},
    number = {12},
    month = {December},
    title = {Ti alloyed {\ensuremath{\alpha}}-Ga2O3 : route towards wide band gap engineering},
    year = {2020},
    doi = {10.3390/mi11121128},
    journal = {Micromachines},
    keywords = {gallium oxide, wide band gap semiconductors, solar-blind detection, atomic layer deposition, thin films, alloying, bandgap, Physics, Physics and Astronomy(all)},
    url = {https://doi.org/10.3390/mi11121128},
    issn = {2072-666X},
    abstract = {The suitability of Ti as a band gap modifier for {\ensuremath{\alpha}}-Ga2O3 was investigated, taking advantage of the isostructural {\ensuremath{\alpha}} phases and high band gap difference between Ti2O3 and Ga2O3. Films of (Ti,Ga)2O3 were synthesized by atomic layer deposition on sapphire substrates, and characterized to determine how crystallinity and band gap vary with composition for this alloy. We report the deposition of high quality {\ensuremath{\alpha}}-(TixGa1-x)2O3 films with x = 3.7\%. For greater compositions the crystalline quality of the films degrades rapidly, where the corundum phase is maintained in films up to x = 5.3\%, and films containing greater Ti fractions being amorphous. Over the range of achieved corundum phase films, that is 0\% {$\leq$} x {$\leq$} 5.3\%, the band gap energy varies by {$\sim$} 270 meV. The ability to maintain a crystalline phase at low fractions of Ti, accompanied by a modification in band gap, shows promising prospects for band gap engineering and the development of wavelength specific solar-blind photodetectors based on {\ensuremath{\alpha}}-Ga2O3.},
    author = {Barthel, Armin and Roberts, Joseph and Napari, Mari and Frentrup, Martin and Huq, Tahmida and Kov{\'a}cs, Andr{\'a}s and Oliver, Rachel and Chalker, Paul and Sajavaara, Timo and Massabuau, Fabien}
    }

  • A. J. Ramsay and A. Rossi, “Relaxation dynamics of spin-3/2 silicon vacancies in 4H-SiC,” Physical Review B: Condensed Matter and Materials Physics, vol. 101, iss. 16, 2020. doi:10.1103/PhysRevB.101.165307
    [BibTeX] [Abstract] [Download PDF]

    Room-temperature optically detected magnetic resonance experiments on spin- 3 2 silicon vacancies in 4H-SiC are reported. The ms = +1 2 {$\leftrightarrow$} ?1 2 transition is accessed using a two-microwave-frequency excitation protocol. The ratio of the Rabi frequencies of the +3 2 {$\leftrightarrow$} +1 2 and +1 2 {$\leftrightarrow$} ?1 2 transitions is measured to be (0.901 {$\pm$} 0.013). The deviation from {$\sqrt$}3/2 is attributed to small difference in g factor for different magnetic dipole transitions. Whereas a spin- 1 2 system is characterized by a single-spin lifetime T1, we experimentally demonstrate that the spin- 3 2 system has three distinct relaxation modes that can be preferentially excited and detected. The measured relaxation times are (0.41 {$\pm$} 0.02)Tslow = Td = (3.3 {$\pm$} 0.5)Tfast. This differs from the values of Tp/3 = Td = 2Tf expected for pure dipole (Tp), quadrupole (Td ), and octupole (Tf ) relaxation modes, respectively, and implies admixing of the slow dipole and fast octupole relaxation modes.

    @article{strathprints73022,
    volume = {101},
    number = {16},
    month = {April},
    title = {Relaxation dynamics of spin-3/2 silicon vacancies in 4H-SiC},
    year = {2020},
    doi = {10.1103/PhysRevB.101.165307},
    journal = {Physical Review B: Condensed Matter and Materials Physics},
    keywords = {magnetic resonance experiments, spin, relaxation modes, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {https://doi.org/10.1103/PhysRevB.101.165307},
    issn = {1098-0121},
    abstract = {Room-temperature optically detected magnetic resonance experiments on spin- 3 2 silicon vacancies in 4H-SiC are reported. The ms = +1 2 {$\leftrightarrow$} ?1 2 transition is accessed using a two-microwave-frequency excitation protocol. The ratio of the Rabi frequencies of the +3 2 {$\leftrightarrow$} +1 2 and +1 2 {$\leftrightarrow$} ?1 2 transitions is measured to be (0.901 {$\pm$} 0.013). The deviation from {$\sqrt$}3/2 is attributed to small difference in g factor for different magnetic dipole transitions. Whereas a spin- 1 2 system is characterized by a single-spin lifetime T1, we experimentally demonstrate that the spin- 3 2 system has three distinct relaxation modes that can be preferentially excited and detected. The measured relaxation times are (0.41 {$\pm$} 0.02)Tslow = Td = (3.3 {$\pm$} 0.5)Tfast. This differs from the values of Tp/3 = Td = 2Tf expected for pure dipole (Tp), quadrupole (Td ), and octupole (Tf ) relaxation modes, respectively, and implies admixing of the slow dipole and fast octupole relaxation modes.},
    author = {Ramsay, A. J. and Rossi, A.}
    }

  • C. Trager-Cowan, A. Alasmari, W. Avis, J. Bruckbauer, P. R. Edwards, B. Hourahine, S. Kraeusel, G. Kusch, B. M. Jablon, R. Johnston, R. W. Martin, R. McDermott, G. Naresh-Kumar, M. Nouf-Allehiani, E. Pascal, D. Thomson, S. Vespucci, K. Mingard, P. J. Parbrook, M. D. Smith, J. Enslin, F. Mehnke, M. Kneissl, C. Kuhn, T. Wernicke, A. Knauer, S. Hagedorn, S. Walde, M. Weyers, P-M. Coulon, P. A. Shields, Y. Zhang, L. Jiu, Y. Gong, R. M. Smith, T. Wang, and A. Winkelmann, “Advances in electron channelling contrast imaging and electron backscatter diffraction for imaging and analysis of structural defects in the scanning electron microscope,” IOP Conference Series: Materials Science and Engineering, vol. 891, iss. 1, 2020. doi:10.1088/1757-899X/891/1/012023
    [BibTeX] [Abstract] [Download PDF]

    In this article we describe the scanning electron microscopy (SEM) techniques of electron channelling contrast imaging and electron backscatter diffraction. These techniques provide information on crystal structure, crystal misorientation, grain boundaries, strain and structural defects on length scales from tens of nanometres to tens of micrometres. Here we report on the imaging and analysis of dislocations and sub-grains in nitride semiconductor thin films (GaN and AlN) and tungsten carbide-cobalt (WC-Co) hard metals. Our aim is to illustrate the capability of these techniques for investigating structural defects in the SEM and the benefits of combining these diffraction-based imaging techniques.

    @article{strathprints74728,
    volume = {891},
    number = {1},
    month = {August},
    title = {Advances in electron channelling contrast imaging and electron backscatter diffraction for imaging and analysis of structural defects in the scanning electron microscope},
    year = {2020},
    doi = {10.1088/1757-899X/891/1/012023},
    journal = {IOP Conference Series: Materials Science and Engineering},
    keywords = {scanning electron microscopy, contrast imaging, imaging techniques, Physics, Materials Science(all), Engineering(all), Physics and Astronomy(all)},
    url = {https://doi.org/10.1088/1757-899X/891/1/012023},
    issn = {1757-899X},
    abstract = {In this article we describe the scanning electron microscopy (SEM) techniques of electron channelling contrast imaging and electron backscatter diffraction. These techniques provide information on crystal structure, crystal misorientation, grain boundaries, strain and structural defects on length scales from tens of nanometres to tens of micrometres. Here we report on the imaging and analysis of dislocations and sub-grains in nitride semiconductor thin films (GaN and AlN) and tungsten carbide-cobalt (WC-Co) hard metals. Our aim is to illustrate the capability of these techniques for investigating structural defects in the SEM and the benefits of combining these diffraction-based imaging techniques.},
    author = {Trager-Cowan, C. and Alasmari, A. and Avis, W. and Bruckbauer, J. and Edwards, P. R. and Hourahine, B. and Kraeusel, S. and Kusch, G. and Jablon, B. M. and Johnston, R. and Martin, R. W. and McDermott, R. and Naresh-Kumar, G. and Nouf-Allehiani, M. and Pascal, E. and Thomson, D. and Vespucci, S. and Mingard, K. and Parbrook, P. J. and Smith, M. D. and Enslin, J. and Mehnke, F. and Kneissl, M. and Kuhn, C. and Wernicke, T. and Knauer, A. and Hagedorn, S. and Walde, S. and Weyers, M. and Coulon, P-M and Shields, P. A. and Zhang, Y. and Jiu, L. and Gong, Y. and Smith, R. M. and Wang, T. and Winkelmann, A.}
    }

  • J. Denholm and B. Hourahine, “Anomalous Ising freezing times,” Journal of Statistical Mechanics: Theory and Experiment, vol. 2020, 2020. doi:10.1088/1742-5468/abb0df
    [BibTeX] [Abstract] [Download PDF]

    We measure the relaxation time of a square lattice Ising ferromagnet that is quenched to zero-temperature from supercritical initial conditions. We reveal an anomalous and seemingly overlooked timescale associated with the relaxation to ‘frozen’ two-stripe states. While close to a power law of the form {$\sim$} L{\^{ }}{\ensuremath{\nu}} , we argue this timescale actually grows as L{\^{ }}2 ln L, with L the linear dimension of the system. We uncover the mechanism behind this scaling form by using a synthetic initial condition that replicates the late time ordering of two-stripe states, and subsequently explain it heuristically.

    @article{strathprints74119,
    volume = {2020},
    month = {September},
    title = {Anomalous Ising freezing times},
    year = {2020},
    doi = {10.1088/1742-5468/abb0df},
    journal = {Journal of Statistical Mechanics: Theory and Experiment},
    keywords = {cond-mat.stat-mech, square lattice Ising ferromagnet, freezing times, two-stripe states, Physics, Statistical and Nonlinear Physics, Statistics and Probability, Statistics, Probability and Uncertainty},
    url = {https://doi.org/10.1088/1742-5468/abb0df},
    issn = {1742-5468},
    abstract = {We measure the relaxation time of a square lattice Ising ferromagnet that is quenched to zero-temperature from supercritical initial conditions. We reveal an anomalous and seemingly overlooked timescale associated with the relaxation to 'frozen' two-stripe states. While close to a power law of the form {$\sim$} L{\^{ }}{\ensuremath{\nu}} , we argue this timescale actually grows as L{\^{ }}2 ln L, with L the linear dimension of the system. We uncover the mechanism behind this scaling form by using a synthetic initial condition that replicates the late time ordering of two-stripe states, and subsequently explain it heuristically.},
    author = {Denholm, J and Hourahine, B}
    }

  • T. Walther, Y. Calahorra, and F. Massabuau, “Preface for the special issue on Microscopy of Semiconducting Materials 2019,” Semiconductor Science and Technology, vol. 35, iss. 12, 2020. doi:10.1088/1361-6641/abb6b9
    [BibTeX] [Abstract] [Download PDF]

    This issue contains selected invited and contributed presentations from the 21st international conference on ‘Microscopy of Semiconducting Materials’ held at Fitzwilliam College, University of Cambridge, on 9-12 April 2019. The meeting was organised by the Institute of Physics, supported by the Royal Microscopical Society, the European Microscopy Society, attolight (Platinum sponsor), JEOL (Gold sponsor) and ThermoFisher Scientific (Silver sponsor).

    @article{strathprints75146,
    volume = {35},
    number = {12},
    month = {October},
    title = {Preface for the special issue on Microscopy of Semiconducting Materials 2019},
    year = {2020},
    doi = {10.1088/1361-6641/abb6b9},
    journal = {Semiconductor Science and Technology},
    keywords = {preface, special issue, microscopy, semiconducting materials, Electrical engineering. Electronics Nuclear engineering, Electronic, Optical and Magnetic Materials, Condensed Matter Physics, Electrical and Electronic Engineering, Materials Chemistry},
    url = {https://doi.org/10.1088/1361-6641/abb6b9},
    issn = {0268-1242},
    abstract = {This issue contains selected invited and contributed presentations from the 21st international conference on 'Microscopy of Semiconducting Materials' held at Fitzwilliam College, University of Cambridge, on 9-12 April 2019. The meeting was organised by the Institute of Physics, supported by the Royal Microscopical Society, the European Microscopy Society, attolight (Platinum sponsor), JEOL (Gold sponsor) and ThermoFisher Scientific (Silver sponsor).},
    author = {Walther, Thomas and Calahorra, Yonatan and Massabuau, Fabien}
    }

  • J. Bruckbauer, Y. Gong, L. Jiu, M. J. Wallace, A. Ipsen, S. Bauer, R. Müller, J. Bai, K. Thonke, T. Wang, C. Trager-Cowan, and R. W. Martin, “Influence of micro-patterning of the growth template on defect reduction and optical properties of non-polar (11-20) GaN,” Journal of Physics D: Applied Physics, vol. 54, iss. 2, 2020. doi:10.1088/1361-6463/abbc37
    [BibTeX] [Abstract] [Download PDF]

    We investigate the influence of different types of template micro-patterning on defect reduction and optical properties of non-polar GaN using detailed luminescence studies. Non-polar (11-20) (or a-plane) GaN exhibits a range of different extended defects compared with its more commonly used c-plane counterpart. In order to reduce the number of defects and investigate their impact on luminescence uniformity, non-polar GaN was overgrown on four different GaN microstructures. The micro-patterned structures consist of a regular microrod array; a microrod array where the -c-side of the microrods has been etched to suppress defect generation; etched periodic stripes and finally a subsequent combination of etched stripes and etched microrods (double overgrowth). Overall the presence of extended defects, namely threading dislocations and stacking faults (SFs) is greatly reduced for the two samples containing stripes compared with the two microrod samples. This is evidenced by more uniform emission and reduction in dark regions of non-radiative recombination in room temperature cathodoluminescence imaging as well as a reduction of the SF emission line in low temperature photoluminescence. The observed energy shifts of the GaN near band edge emission are related to anisotropic strain relaxation occurring during the overgrowth on these microstructures. A combination of stripes and microrods is a promising approach for defect reduction and emission uniformity in non-polar GaN for applications in light-emitting devices as well as power electronics.

    @article{strathprints73997,
    volume = {54},
    number = {2},
    month = {October},
    title = {Influence of micro-patterning of the growth template on defect reduction and optical properties of non-polar (11-20) GaN},
    year = {2020},
    doi = {10.1088/1361-6463/abbc37},
    journal = {Journal of Physics D: Applied Physics},
    keywords = {non-polar GaN, GaN microstructures, light emitting devices, Physics, Surfaces, Coatings and Films, Acoustics and Ultrasonics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {https://doi.org/10.1088/1361-6463/abbc37},
    issn = {0022-3727},
    abstract = {We investigate the influence of different types of template micro-patterning on defect reduction and optical properties of non-polar GaN using detailed luminescence studies. Non-polar (11-20) (or a-plane) GaN exhibits a range of different extended defects compared with its more commonly used c-plane counterpart. In order to reduce the number of defects and investigate their impact on luminescence uniformity, non-polar GaN was overgrown on four different GaN microstructures. The micro-patterned structures consist of a regular microrod array; a microrod array where the -c-side of the microrods has been etched to suppress defect generation; etched periodic stripes and finally a subsequent combination of etched stripes and etched microrods (double overgrowth). Overall the presence of extended defects, namely threading dislocations and stacking faults (SFs) is greatly reduced for the two samples containing stripes compared with the two microrod samples. This is evidenced by more uniform emission and reduction in dark regions of non-radiative recombination in room temperature cathodoluminescence imaging as well as a reduction of the SF emission line in low temperature photoluminescence. The observed energy shifts of the GaN near band edge emission are related to anisotropic strain relaxation occurring during the overgrowth on these microstructures. A combination of stripes and microrods is a promising approach for defect reduction and emission uniformity in non-polar GaN for applications in light-emitting devices as well as power electronics.},
    author = {Bruckbauer, Jochen and Gong, Yipin and Jiu, Ling and Wallace, Michael J and Ipsen, Anja and Bauer, Sebastian and M{\"u}ller, Raphael and Bai, Jie and Thonke, Klaus and Wang, Tao and Trager-Cowan, Carol and Martin, Robert W}
    }

  • H. Amano, R. Collazo, C. De Santi, S. Einfeldt, M. Funato, J. Glaab, S. Hagedorn, A. Hirano, H. Hirayama, R. Ishii, Y. Kashima, Y. Kawakami, R. Kirste, M. Kneissl, R. Martin, F. Mehnke, M. Meneghini, A. Ougazzaden, P. J. Parbrook, S. Rajan, P. Reddy, F. Römer, J. Ruschel, B. Sarkar, F. Scholz, L. J. Schowalter, P. Shields, Z. Sitar, L. Sulmoni, T. Wang, T. Wernicke, M. Weyers, B. Witzigmann, Y. Wu, T. Wunderer, and Y. Zhang, “The 2020 UV emitter roadmap,” Journal of Physics D: Applied Physics, vol. 53, iss. 50, 2020. doi:10.1088/1361-6463/aba64c
    [BibTeX] [Abstract] [Download PDF]

    Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm-due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments.

    @article{strathprints74344,
    volume = {53},
    number = {50},
    month = {September},
    title = {The 2020 UV emitter roadmap},
    year = {2020},
    doi = {10.1088/1361-6463/aba64c},
    journal = {Journal of Physics D: Applied Physics},
    keywords = {AlGaN, InGaN, UV-LED, light emitting diodes, ultraviolet, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics, Acoustics and Ultrasonics, Surfaces, Coatings and Films},
    url = {https://doi.org/10.1088/1361-6463/aba64c},
    issn = {0022-3727},
    abstract = {Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm-due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments.},
    author = {Amano, Hiroshi and Collazo, Ram{\'o}n and De Santi, Carlo and Einfeldt, Sven and Funato, Mitsuru and Glaab, Johannes and Hagedorn, Sylvia and Hirano, Akira and Hirayama, Hideki and Ishii, Ryota and Kashima, Yukio and Kawakami, Yoichi and Kirste, Ronny and Kneissl, Michael and Martin, Robert and Mehnke, Frank and Meneghini, Matteo and Ougazzaden, Abdallah and Parbrook, Peter J and Rajan, Siddharth and Reddy, Pramod and R{\"o}mer, Friedhard and Ruschel, Jan and Sarkar, Biplab and Scholz, Ferdinand and Schowalter, Leo J and Shields, Philip and Sitar, Zlatko and Sulmoni, Luca and Wang, Tao and Wernicke, Tim and Weyers, Markus and Witzigmann, Bernd and Wu, Yuh-Renn and Wunderer, Thomas and Zhang, Yuewei}
    }

  • H. M. Foronda, D. A. Hunter, M. Pietsch, L. Sulmoni, A. Muhin, S. Graupeter, N. Susilo, M. Schilling, J. Enslin, K. Irmscher, R. W. Martin, T. Wernicke, and M. Kneissl, “Electrical properties of (11-22) Si:AlGaN layers at high Al contents grown by metal-organic vapor phase epitaxy,” Applied Physics Letters, vol. 117, iss. 22, 2020. doi:10.1063/5.0031468
    [BibTeX] [Abstract] [Download PDF]

    In this work, the growth and conductivity of semipolar AlxGa1?xN:Si with (11-22) orientation are investigated. AlxGa1?xN:Si (x = 0.60 {$\pm$} 0.03 and x = 0.80 {$\pm$} 0.02) layers were grown with different SiH4 partial pressures, and the electrical properties were determined using Hall measurements at room temperature. The aluminum mole fraction was measured by wavelength dispersive x-ray spectroscopy and x-ray diffraction, and the Si-concentration was measured by wavelength dispersive x-ray spectroscopy and secondary ion mass spectroscopy. Layer resistivities as low as 0.024 ? cm for x = 0.6 and 0.042 ? cm for x = 0.8 were achieved. For both aluminum mole fractions, the resistivity exhibits a minimum with the increasing Si concentration, which can be explained by compensation due to the formation of cation vacancy complexes at high doping levels. The onset of self-compensation occurs at larger estimated Si concentrations for larger Al contents.

    @article{strathprints74889,
    volume = {117},
    number = {22},
    month = {November},
    title = {Electrical properties of (11-22) Si:AlGaN layers at high Al contents grown by metal-organic vapor phase epitaxy},
    year = {2020},
    doi = {10.1063/5.0031468},
    journal = {Applied Physics Letters},
    keywords = {Hall measurements, electrical properties, partial pressures, Physics, Physics and Astronomy(all)},
    url = {https://doi.org/10.1063/5.0031468},
    issn = {0003-6951},
    abstract = {In this work, the growth and conductivity of semipolar AlxGa1?xN:Si with (11-22) orientation are investigated. AlxGa1?xN:Si (x = 0.60 {$\pm$} 0.03 and x = 0.80 {$\pm$} 0.02) layers were grown with different SiH4 partial pressures, and the electrical properties were determined using Hall measurements at room temperature. The aluminum mole fraction was measured by wavelength dispersive x-ray spectroscopy and x-ray diffraction, and the Si-concentration was measured by wavelength dispersive x-ray spectroscopy and secondary ion mass spectroscopy. Layer resistivities as low as 0.024 ? cm for x = 0.6 and 0.042 ? cm for x = 0.8 were achieved. For both aluminum mole fractions, the resistivity exhibits a minimum with the increasing Si concentration, which can be explained by compensation due to the formation of cation vacancy complexes at high doping levels. The onset of self-compensation occurs at larger estimated Si concentrations for larger Al contents.},
    author = {Foronda, Humberto M. and Hunter, Daniel A. and Pietsch, Mike and Sulmoni, Luca and Muhin, Anton and Graupeter, Sarina and Susilo, Norman and Schilling, Marcel and Enslin, Johannes and Irmscher, Klaus and Martin, Robert W. and Wernicke, Tim and Kneissl, Michael}
    }

  • F. C. -P. Massabuau, P. H. Griffin, H. P. Springbett, Y. Liu, V. R. Kumar, T. Zhu, and R. A. Oliver, “Dislocations as channels for the fabrication of sub-surface porous GaN by electrochemical etching,” APL Materials, vol. 8, iss. 3, 2020. doi:10.1063/1.5142491
    [BibTeX] [Abstract] [Download PDF]

    Porosification of nitride semiconductors provides a new paradigm for advanced engineering of the properties of optoelectronic materials. Electrochemical etching creates porosity in doped layers whilst leaving undoped layers undamaged, allowing the realisation of complex three-dimensional porous nanostructures, potentially offering a wide range of functionalities, such as in distributed Bragg reflectors. Porous/non-porous multilayers can be formed by etching whole, as-grown wafers uniformly in one simple process, without any additional processing steps. The etch penetrates from the top down, through the undoped layers, leaving them almost untouched. Here, atomic-resolution electron microscopy is used to show that the etchant accesses the doped layers via nanometre-scale channels that form at dislocation cores and transport the etchant and etch products to and from the doped layer respectively. Results on AlGaN and non-polar GaN multilayers indicate the same mechanism is operating, suggesting this approach may be applicable in a range of materials.

    @article{strathprints71762,
    volume = {8},
    number = {3},
    month = {March},
    title = {Dislocations as channels for the fabrication of sub-surface porous GaN by electrochemical etching},
    year = {2020},
    doi = {10.1063/1.5142491},
    journal = {APL Materials},
    keywords = {electrochemical etching, porous nanostructures, electron microscopy, Physics, Physics and Astronomy(all)},
    url = {https://doi.org/10.1063/1.5142491},
    issn = {2166-532X},
    abstract = {Porosification of nitride semiconductors provides a new paradigm for advanced engineering of the properties of optoelectronic materials. Electrochemical etching creates porosity in doped layers whilst leaving undoped layers undamaged, allowing the realisation of complex three-dimensional porous nanostructures, potentially offering a wide range of functionalities, such as in distributed Bragg reflectors. Porous/non-porous multilayers can be formed by etching whole, as-grown wafers uniformly in one simple process, without any additional processing steps. The etch penetrates from the top down, through the undoped layers, leaving them almost untouched. Here, atomic-resolution electron microscopy is used to show that the etchant accesses the doped layers via nanometre-scale channels that form at dislocation cores and transport the etchant and etch products to and from the doped layer respectively. Results on AlGaN and non-polar GaN multilayers indicate the same mechanism is operating, suggesting this approach may be applicable in a range of materials.},
    author = {Massabuau, Fabien C.-P. and Griffin, Peter H. and Springbett, Helen P. and Liu, Yingjun and Kumar, R. Vasant and Zhu, Tongtong and Oliver, Rachel A.}
    }

  • M. D. Smith, J. A. Cuenca, D. E. Field, Y. Fu, C. Yuan, F. Massabuau, S. Mandal, J. W. Pomeroy, R. A. Oliver, M. J. Uren, K. Elgaid, O. A. Williams, I. Thayne, and M. Kuball, “GaN-on-diamond technology platform : bonding-free membrane manufacturing process,” AIP Advances, vol. 10, iss. 3, 2020. doi:10.1063/1.5129229
    [BibTeX] [Abstract] [Download PDF]

    GaN-on-diamond samples were demonstrated using a membrane-based technology. This was achieved by selective area Si substrate removal of areas of up to 1 cm {$\times$} 1 cm from a GaN-on-Si wafer, followed by direct growth of a polycrystalline diamond using microwave plasma chemical vapor deposition on etch exposed N-polar AlN epitaxial nucleation layers. Atomic force microscopy and transmission electron microscopy were used to confirm the formation of high quality, void-free AlN/diamond interfaces. The bond between the III-nitride layers and the diamond was validated by strain measurements of the GaN buffer layer. Demonstration of this technology platform is an important step forward for the creation of next generation high power electronic devices.

    @article{strathprints71547,
    volume = {10},
    number = {3},
    month = {March},
    title = {GaN-on-diamond technology platform : bonding-free membrane manufacturing process},
    year = {2020},
    doi = {10.1063/1.5129229},
    journal = {AIP Advances},
    keywords = {GaN-on-diamond, atomic force microscopy, membrane manufacturing, Physics, Materials Science(all)},
    url = {https://doi.org/10.1063/1.5129229},
    issn = {2158-3226},
    abstract = {GaN-on-diamond samples were demonstrated using a membrane-based technology. This was achieved by selective area Si substrate removal of areas of up to 1 cm {$\times$} 1 cm from a GaN-on-Si wafer, followed by direct growth of a polycrystalline diamond using microwave plasma chemical vapor deposition on etch exposed N-polar AlN epitaxial nucleation layers. Atomic force microscopy and transmission electron microscopy were used to confirm the formation of high quality, void-free AlN/diamond interfaces. The bond between the III-nitride layers and the diamond was validated by strain measurements of the GaN buffer layer. Demonstration of this technology platform is an important step forward for the creation of next generation high power electronic devices.},
    author = {Smith, Matthew D. and Cuenca, Jerome A. and Field, Daniel E. and Fu, Yen-chun and Yuan, Chao and Massabuau, Fabien and Mandal, Soumen and Pomeroy, James W. and Oliver, Rachel A. and Uren, Michael J. and Elgaid, Khaled and Williams, Oliver A. and Thayne, Iain and Kuball, Martin}
    }

  • C. M. Palomares Garcia, A. Di Bernardo, G. Kimbell, M. E. Vickers, F. C-P. Massabuau, S. Komori, G. Divitini, Y. Yasui, H. Gyeol Lee, J. Kim, B. Kim, M. G. Blamire, A. Vecchione, R. Fittipaldi, Y. Maeno, T. Won Noh, and J. W. A. Robinson, “Pair suppression caused by mosaic-twist defects in superconducting Sr2RuO4 thin-films prepared using pulsed laser deposition,” Communications Materials, vol. 1, 2020. doi:10.1038/s43246-020-0026-1
    [BibTeX] [Abstract] [Download PDF]

    Sr2RuO4 is a prototypical unconventional superconductor, but the superconducting symmetries of the bulk and surface states in single crystals remains controversial. Solving this problem is impeded by the challenge of producing thin-films of Sr2RuO4 free of defects and impurities which annihilate the superconductivity. Here, we report the reliable growth of superconducting Sr2RuO4 thin-films by pulsed laser deposition and identify the universal material properties that are destructive to the superconducting state. We demonstrate that careful control of the starting material is essential to achieve superconductivity as well as the use of a single crystal target of Sr3Ru2O7. By systematically varying the Sr2RuO4 film thickness, we identify mosaic twist as the key in-plane defect that suppresses superconductivity. These results are central to the development of our understanding of unconventional superconductivity.

    @article{strathprints71930,
    volume = {1},
    month = {May},
    title = {Pair suppression caused by mosaic-twist defects in superconducting Sr2RuO4 thin-films prepared using pulsed laser deposition},
    year = {2020},
    doi = {10.1038/s43246-020-0026-1},
    journal = {Communications Materials},
    keywords = {superconductor, Sr2RuO4, thin film, Physics, Atomic and Molecular Physics, and Optics},
    url = {https://doi.org/10.1038/s43246-020-0026-1},
    issn = {2662-4443},
    abstract = {Sr2RuO4 is a prototypical unconventional superconductor, but the superconducting symmetries of the bulk and surface states in single crystals remains controversial. Solving this problem is impeded by the challenge of producing thin-films of Sr2RuO4 free of defects and impurities which annihilate the superconductivity. Here, we report the reliable growth of superconducting Sr2RuO4 thin-films by pulsed laser deposition and identify the universal material properties that are destructive to the superconducting state. We demonstrate that careful control of the starting material is essential to achieve superconductivity as well as the use of a single crystal target of Sr3Ru2O7. By systematically varying the Sr2RuO4 film thickness, we identify mosaic twist as the key in-plane defect that suppresses superconductivity. These results are central to the development of our understanding of unconventional superconductivity.},
    author = {Palomares Garcia, Carla Maria and Di Bernardo, Angelo and Kimbell, Graham and Vickers, Mary E. and Massabuau, Fabien C-P. and Komori, Sachio and Divitini, Giorgio and Yasui, Yuuki and Gyeol Lee, Han and Kim, Jinkwon and Kim, Bongju and Blamire, Mark G. and Vecchione, Antonio and Fittipaldi, Rosalba and Maeno, Yoshiteru and Won Noh, Tae and Robinson, Jason W. A.}
    }

  • O. J. Burton, F. C-P. Massabuau, V. Veigang-Radulescu, B. Brennen, A. J. Pollard, and S. Hofmann, “Integrated wafer scale growth of single crystal metal films and high quality graphene,” ACS Nano, vol. 14, iss. 10, p. 13593–13601, 2020. doi:10.1021/acsnano.0c05685
    [BibTeX] [Abstract] [Download PDF]

    We report on an approach to bring together single crystal metal catalyst preparation and graphene growth in a combined process flow using a standard cold-wall chemical vapor deposition (CVD) reactor. We employ a sandwich arrangement between a commercial polycrystalline Cu foil and c-plane sapphire wafer and show that close-spaced vacuum sublimation across the confined gap can result in an epitaxial, single-crystal Cu(111) film at high growth rate. The arrangement is scalable (we demonstrate 2? wafer scale) and suppresses reactor contamination with Cu. While starting with an impure Cu foil, the freshly prepared Cu film is of high purity as measured by time-of-flight secondary ion mass spectrometry. We seamlessly connect the initial metallization with subsequent graphene growth via the introduction of hydrogen and gaseous carbon precursors, thereby eliminating contamination due to substrate transfer and common lengthy catalyst pretreatments. We show that the sandwich approach also enables for a Cu surface with nanometer scale roughness during graphene growth and thus results in high quality graphene similar to previously demonstrated Cu enclosure approaches. We systematically explore the parameter space and discuss the opportunities, including subsequent dry transfer, generality, and versatility of our approach particularly regarding the cost-efficient preparation of different single crystal film orientations and expansion to other material systems.

    @article{strathprints74112,
    volume = {14},
    number = {10},
    month = {October},
    title = {Integrated wafer scale growth of single crystal metal films and high quality graphene},
    journal = {ACS Nano},
    doi = {10.1021/acsnano.0c05685},
    pages = {13593--13601},
    year = {2020},
    keywords = {single crystal, Cu, copper, epitaxial metal film, graphene, 2D material, chemical vapor deposition, Physics, Physics and Astronomy(all)},
    url = {https://doi.org/10.1021/acsnano.0c05685},
    issn = {1936-0851},
    abstract = {We report on an approach to bring together single crystal metal catalyst preparation and graphene growth in a combined process flow using a standard cold-wall chemical vapor deposition (CVD) reactor. We employ a sandwich arrangement between a commercial polycrystalline Cu foil and c-plane sapphire wafer and show that close-spaced vacuum sublimation across the confined gap can result in an epitaxial, single-crystal Cu(111) film at high growth rate. The arrangement is scalable (we demonstrate 2? wafer scale) and suppresses reactor contamination with Cu. While starting with an impure Cu foil, the freshly prepared Cu film is of high purity as measured by time-of-flight secondary ion mass spectrometry. We seamlessly connect the initial metallization with subsequent graphene growth via the introduction of hydrogen and gaseous carbon precursors, thereby eliminating contamination due to substrate transfer and common lengthy catalyst pretreatments. We show that the sandwich approach also enables for a Cu surface with nanometer scale roughness during graphene growth and thus results in high quality graphene similar to previously demonstrated Cu enclosure approaches. We systematically explore the parameter space and discuss the opportunities, including subsequent dry transfer, generality, and versatility of our approach particularly regarding the cost-efficient preparation of different single crystal film orientations and expansion to other material systems.},
    author = {Burton, Oliver J. and Massabuau, Fabien C-P. and Veigang-Radulescu, Vlad-Petru and Brennen, Barry and Pollard, Andrew J. and Hofmann, Stephan}
    }

  • D. E. Field, J. A. Cuenca, M. Smith, S. M. Fairclough, F. C-P. Massabuau, J. W. Pomeroy, O. Williams, R. A. Oliver, I. Thayne, and M. Kuball, “Crystalline interlayers for reducing the effective thermal boundary resistance in GaN-on-diamond,” ACS Applied Materials and Interfaces, vol. 12, iss. 48, p. 54138–54145, 2020. doi:10.1021/acsami.0c10129
    [BibTeX] [Abstract] [Download PDF]

    Integrating diamond with GaN high electron mobility transistors (HEMTs) improves thermal management, ultimately increasing the reliability and performance of high-power high-frequency radio frequency amplifiers. Conventionally, an amorphous interlayer is used before growing polycrystalline diamond onto GaN in these devices. This layer contributes significantly to the effective thermal boundary resistance (TBReff) between the GaN HEMT and the diamond, reducing the benefit of the diamond heat spreader. Replacing the amorphous interlayer with a higher thermal conductivity crystalline material would reduce TBReff and help to enable the full potential of GaN-on-diamond devices. In this work, a crystalline Al0.32Ga0.68N interlayer has been integrated into a GaN/AlGaN HEMT device epitaxy. Two samples were studied, one with diamond grown directly on the AlGaN interlayer and another incorporating a thin crystalline SiC layer between AlGaN and diamond. The TBReff, measured using transient thermoreflectance, was improved for the sample with SiC (30 {$\pm$} 5 m2 K GW-1) compared to the sample without (107 {$\pm$} 44 m2 K GW-1). The reduced TBReff is thought to arise from improved adhesion between SiC and the diamond compared to the diamond directly on AlGaN because of an increased propensity for carbide bond formation between SiC and the diamond. The stronger carbide bonds aid transmission of phonons across the interface, improving heat transport.

    @article{strathprints74634,
    volume = {12},
    number = {48},
    month = {December},
    title = {Crystalline interlayers for reducing the effective thermal boundary resistance in GaN-on-diamond},
    journal = {ACS Applied Materials and Interfaces},
    doi = {10.1021/acsami.0c10129},
    pages = {54138--54145},
    year = {2020},
    keywords = {GaN-on-diamond, thermal boundary resistance, thermal management, GaN, diamond, Physics, Physics and Astronomy(all)},
    url = {https://doi.org/10.1021/acsami.0c10129},
    issn = {1944-8244},
    abstract = {Integrating diamond with GaN high electron mobility transistors (HEMTs) improves thermal management, ultimately increasing the reliability and performance of high-power high-frequency radio frequency amplifiers. Conventionally, an amorphous interlayer is used before growing polycrystalline diamond onto GaN in these devices. This layer contributes significantly to the effective thermal boundary resistance (TBReff) between the GaN HEMT and the diamond, reducing the benefit of the diamond heat spreader. Replacing the amorphous interlayer with a higher thermal conductivity crystalline material would reduce TBReff and help to enable the full potential of GaN-on-diamond devices. In this work, a crystalline Al0.32Ga0.68N interlayer has been integrated into a GaN/AlGaN HEMT device epitaxy. Two samples were studied, one with diamond grown directly on the AlGaN interlayer and another incorporating a thin crystalline SiC layer between AlGaN and diamond. The TBReff, measured using transient thermoreflectance, was improved for the sample with SiC (30 {$\pm$} 5 m2 K GW-1) compared to the sample without (107 {$\pm$} 44 m2 K GW-1). The reduced TBReff is thought to arise from improved adhesion between SiC and the diamond compared to the diamond directly on AlGaN because of an increased propensity for carbide bond formation between SiC and the diamond. The stronger carbide bonds aid transmission of phonons across the interface, improving heat transport.},
    author = {Field, Daniel E. and Cuenca, Jerome A. and Smith, Matthew and Fairclough, Simon M. and Massabuau, Fabien C-P and Pomeroy, James W. and Williams, Oliver and Oliver, Rachel A. and Thayne, Iain and Kuball, Martin}
    }

  • T. J. O’Hanlon, A. Bao, F. C. -P. Massabuau, M. J. Kappers, and R. A. Oliver, “Cross-shaped markers for the preparation of site-specific transmission electron microscopy lamellae using focused ion beam techniques,” Ultramicroscopy, vol. 212, 2020. doi:10.1016/j.ultramic.2020.112970
    [BibTeX] [Abstract] [Download PDF]

    We describe the use of a cross-shaped platinum marker deposited using electron-beam-induced deposition (EBID) in a focused ion beam – scanning electron microscope (FIB-SEM) system to facilitate site-specific preparation of a TEM foil containing a trench defect in an InGaN/GaN multiple quantum well structure. The defect feature is less than 100 nm wide at the surface. The marker is deposited prior to the deposition of a protective platinum strap (also by EBID) with the centre of the cross indicating the location of the feature of interest, while the arms of the square cross make an acute angle of 45o with the strap’s long axis. During the ion-beam thinning process, the marker may be viewed in cross-section from both sides of the sample alternately, and the coming together of the features relating to the arms of the cross indicates increasing proximity to the feature of interest. Although this approach does allow increased precision in locating the region of interest during thinning, it also increases the time required to complete the sample preparation. Hence, this method is particularly well suited to directly correlated multi-microscopy investigations in previously characterised material where high yield and the precise location are more important than preparation time. In addition to TEM lamella preparation, this method could equally be useful for preparing site-specific atom probe tomography (APT) samples.

    @article{strathprints71665,
    volume = {212},
    month = {May},
    title = {Cross-shaped markers for the preparation of site-specific transmission electron microscopy lamellae using focused ion beam techniques},
    year = {2020},
    doi = {10.1016/j.ultramic.2020.112970},
    journal = {Ultramicroscopy},
    keywords = {cross-shaped markers, electron microscopy, electron-beam-induced deposition (EBID), FIB-SEM, Physics, Instrumentation, Atomic and Molecular Physics, and Optics, Electronic, Optical and Magnetic Materials},
    url = {https://doi.org/10.1016/j.ultramic.2020.112970},
    issn = {0304-3991},
    abstract = {We describe the use of a cross-shaped platinum marker deposited using electron-beam-induced deposition (EBID) in a focused ion beam - scanning electron microscope (FIB-SEM) system to facilitate site-specific preparation of a TEM foil containing a trench defect in an InGaN/GaN multiple quantum well structure. The defect feature is less than 100 nm wide at the surface. The marker is deposited prior to the deposition of a protective platinum strap (also by EBID) with the centre of the cross indicating the location of the feature of interest, while the arms of the square cross make an acute angle of 45o with the strap's long axis. During the ion-beam thinning process, the marker may be viewed in cross-section from both sides of the sample alternately, and the coming together of the features relating to the arms of the cross indicates increasing proximity to the feature of interest. Although this approach does allow increased precision in locating the region of interest during thinning, it also increases the time required to complete the sample preparation. Hence, this method is particularly well suited to directly correlated multi-microscopy investigations in previously characterised material where high yield and the precise location are more important than preparation time. In addition to TEM lamella preparation, this method could equally be useful for preparing site-specific atom probe tomography (APT) samples.},
    author = {O'Hanlon, T. J. and Bao, A. and Massabuau, F. C.-P. and Kappers, M. J. and Oliver, R. A.}
    }

  • F. C-P. Massabuau, H. P. Springbett, G. Divitini, P. H. Griffin, T. Zhu, and R. A. Oliver, “Sequential plan-view imaging of sub-surface structures in the transmission electron microscope,” Materialia, vol. 12, 2020. doi:10.1016/j.mtla.2020.100798
    [BibTeX] [Abstract] [Download PDF]

    Transmission electron microscopy (TEM) is a central technique for the characterisation of materials at the atomic scale. However, it requires the sample to be thin enough to be electron transparent, imposing strict limitations when studying thick structures in plan-view. Here we present a method for sequential plan-view TEM that allows one to image complex structures at various depths. The approach consists of performing an iterative series of front-side ion milling followed by TEM imaging. We show it is possible to image how the sample properties vary with depth up to several microns below the surface, with no degradation of the sample and imaging conditions throughout the experiment. We apply this approach to 3D cavities in mesoporous GaN distributed Bragg reflectors, demonstrating the ability to characterise the morphology of the pores, local crystal features and chemical composition through the multilayer structure. The same workflow can be applied to a variety of complex micron-scale systems which are by nature too thick for standard TEM analysis, and can also be adapted for profiling samples in cross-section.

    @article{strathprints72869,
    volume = {12},
    month = {August},
    title = {Sequential plan-view imaging of sub-surface structures in the transmission electron microscope},
    year = {2020},
    doi = {10.1016/j.mtla.2020.100798},
    note = {Manuscript includes supplementary information.},
    journal = {Materialia},
    keywords = {transmission electron microscopy (TEM), sample preparation, layered structures, Gallium Nitride, dislocations, Physics, Electronic, Optical and Magnetic Materials},
    url = {https://doi.org/10.1016/j.mtla.2020.100798},
    issn = {2589-1529},
    abstract = {Transmission electron microscopy (TEM) is a central technique for the characterisation of materials at the atomic scale. However, it requires the sample to be thin enough to be electron transparent, imposing strict limitations when studying thick structures in plan-view. Here we present a method for sequential plan-view TEM that allows one to image complex structures at various depths. The approach consists of performing an iterative series of front-side ion milling followed by TEM imaging. We show it is possible to image how the sample properties vary with depth up to several microns below the surface, with no degradation of the sample and imaging conditions throughout the experiment. We apply this approach to 3D cavities in mesoporous GaN distributed Bragg reflectors, demonstrating the ability to characterise the morphology of the pores, local crystal features and chemical composition through the multilayer structure. The same workflow can be applied to a variety of complex micron-scale systems which are by nature too thick for standard TEM analysis, and can also be adapted for profiling samples in cross-section.},
    author = {Massabuau, F. C-P. and Springbett, H. P. and Divitini, G. and Griffin, P. H. and Zhu, T. and Oliver, R. A.}
    }

  • R. Armstrong, P-M. Coulon, P. Bozinakis, R. W. Martin, and P. A. Shields, “Creation of regular arrays of faceted AlN nanostructures via a combined topdown, bottom-up approach,” Journal of Crystal Growth, vol. 548, 2020. doi:10.1016/j.jcrysgro.2020.125824
    [BibTeX] [Abstract] [Download PDF]

    The realisation of spatially-determined, uniform arrays of faceted aluminium nitride (AlN) nanostructures has had limited exploration, largely due to the fact that selective area growth of AlN via MOVPE (Metal Organic Vapour Phase Epitaxy) has not been realised. Instead, this paper reports the use of a combined top-down, bottom-up approach to realise well-faceted, highly-uniform, periodic nanotextured AlN surfaces. MOVPE regrowth is performed upon dry-etched AlN nanorods and nanoholes, and we present a study into the effect of the growth conditions on the resulting faceting and morphology. Specifically, growth temperature, V/III ratio and growth time are investigated and analysed via scanning-electron and atomic-force microscopy. The V/III ratio was found to influence the nanostructure morphology most whilst the growth temperature was found to have much less of an impact within the temperature range studied. Experiments with a longer growth time are performed to create nanostructures for potential use in applications, such as for AlGaN-based quantum-well or quantum-dot emitters.

    @article{strathprints74337,
    volume = {548},
    month = {October},
    title = {Creation of regular arrays of faceted AlN nanostructures via a combined topdown, bottom-up approach},
    year = {2020},
    doi = {10.1016/j.jcrysgro.2020.125824},
    journal = {Journal of Crystal Growth},
    keywords = {semiconducting aluminum compounds, surface structure, metalorganic chemical vapor deposition, metalorganic vapor phase epitaxy, nanostructures, Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
    url = {https://doi.org/10.1016/j.jcrysgro.2020.125824},
    issn = {0022-0248},
    abstract = {The realisation of spatially-determined, uniform arrays of faceted aluminium nitride (AlN) nanostructures has had limited exploration, largely due to the fact that selective area growth of AlN via MOVPE (Metal Organic Vapour Phase Epitaxy) has not been realised. Instead, this paper reports the use of a combined top-down, bottom-up approach to realise well-faceted, highly-uniform, periodic nanotextured AlN surfaces. MOVPE regrowth is performed upon dry-etched AlN nanorods and nanoholes, and we present a study into the effect of the growth conditions on the resulting faceting and morphology. Specifically, growth temperature, V/III ratio and growth time are investigated and analysed via scanning-electron and atomic-force microscopy. The V/III ratio was found to influence the nanostructure morphology most whilst the growth temperature was found to have much less of an impact within the temperature range studied. Experiments with a longer growth time are performed to create nanostructures for potential use in applications, such as for AlGaN-based quantum-well or quantum-dot emitters.},
    author = {Armstrong, R. and Coulon, P-M. and Bozinakis, P. and Martin, R. W. and Shields, P. A.}
    }

  • J. A. Cuenca, M. D. Smith, D. E. Field, F. C-P. Massabuau, S. Mandal, J. Pomeroy, D. J. Wallis, R. A. Oliver, I. Thayne, M. Kuball, and O. A. Williams, “Thermal stress modelling of diamond on GaN/III-Nitride membranes,” Carbon, 2020. doi:10.1016/j.carbon.2020.11.067
    [BibTeX] [Abstract] [Download PDF]

    Diamond heat-spreaders for gallium nitride (GaN) devices currently depend upon a robust wafer bonding process. Bonding-free membrane methods demonstrate potential, however, chemical vapour deposition (CVD) of diamond directly onto a III-nitride (III-N) heterostructure membrane induces significant thermal stresses. In this work, these thermal stresses are investigated using an analytical approach, a numerical model and experimental validation. The thermal stresses are caused by the mismatch in the coefficient of thermal expansion (CTE) between the GaN/III-N stack, silicon (Si) and the diamond from room temperature to CVD growth temperatures. Simplified analytical wafer bow models underestimate the membrane bow for small sizes while numerical models replicate the stresses and bows with increased accuracy using temperature gradients. The largest tensile stress measured using Raman spectroscopy at room temperature was approximately 1.0 {$\pm$}0.2GPa while surface profilometry shows membrane bows as large as 58{\ensuremath{\mu}}m. This large bow is caused by additional stresses from the Si frame in the initial heating phase which are held in place by the diamond and highlights challenges for any device fabrication using contact lithography. However, the bow can be reduced if the membrane is pre-stressed to become flat at CVD temperatures. In this way, a sufficient platform to grow diamond on GaN/III-N structures without wafer bonding can be realised.

    @article{strathprints74786,
    month = {November},
    title = {Thermal stress modelling of diamond on GaN/III-Nitride membranes},
    year = {2020},
    doi = {10.1016/j.carbon.2020.11.067},
    journal = {Carbon},
    keywords = {cvd diamond, allium nitride, membranes, thermal stress, finite element modelling, Physics, Chemistry(all)},
    url = {https://doi.org/10.1016/j.carbon.2020.11.067},
    issn = {0008-6223},
    abstract = {Diamond heat-spreaders for gallium nitride (GaN) devices currently depend upon a robust wafer bonding process. Bonding-free membrane methods demonstrate potential, however, chemical vapour deposition (CVD) of diamond directly onto a III-nitride (III-N) heterostructure membrane induces significant thermal stresses. In this work, these thermal stresses are investigated using an analytical approach, a numerical model and experimental validation. The thermal stresses are caused by the mismatch in the coefficient of thermal expansion (CTE) between the GaN/III-N stack, silicon (Si) and the diamond from room temperature to CVD growth temperatures. Simplified analytical wafer bow models underestimate the membrane bow for small sizes while numerical models replicate the stresses and bows with increased accuracy using temperature gradients. The largest tensile stress measured using Raman spectroscopy at room temperature was approximately 1.0 {$\pm$}0.2GPa while surface profilometry shows membrane bows as large as 58{\ensuremath{\mu}}m. This large bow is caused by additional stresses from the Si frame in the initial heating phase which are held in place by the diamond and highlights challenges for any device fabrication using contact lithography. However, the bow can be reduced if the membrane is pre-stressed to become flat at CVD temperatures. In this way, a sufficient platform to grow diamond on GaN/III-N structures without wafer bonding can be realised.},
    author = {Cuenca, Jerome A. and Smith, Matthew D. and Field, Daniel E. and Massabuau, Fabien C-P. and Mandal, Soumen and Pomeroy, James and Wallis, David J. and Oliver, Rachel A. and Thayne, Iain and Kuball, Martin and Williams, Oliver A.}
    }

  • E. J. W. Smith, A. H. Piracha, D. Fields, J. W. Pomeroy, G. R. Mackenzie, Z. Abdallah, F. C-P. Massabuau, A. M. Hinz, D. J. Wallis, R. A. Oliver, M. Kuball, and P. W. May, “Mixed-size diamond seeding for low-thermal-barrier growth of CVD diamond onto GaN and AlN,” Carbon, vol. 167, p. 620–626, 2020. doi:10.1016/j.carbon.2020.05.050
    [BibTeX] [Abstract] [Download PDF]

    We report a method of growing a diamond layer via chemical vapour deposition (CVD) utilizing a mixture of microdiamond and nanodiamond seeding to give a low effective thermal boundary resistance (TBR eff) for heat-spreading applications in high-frequency, high-power electronic devices. CVD diamond was deposited onto thin layers of both GaN and AlN on Si substrates, comparing conventional nanodiamond seeding with a two-step process involving sequential seeding with microdiamond then nanodiamond. Thermal properties were determined using transient thermoreflectance (TTR), and the samples were also analysed with SEM and X-ray tomography. While diamond growth directly onto GaN proved to be unsuccessful due to poor adhesion, films grown on AlN were adherent and robust. The two-step mixed-seeding method gave TBR eff values {\ensuremath{<}} 6 m 2 K GW ?1 that were 30 times smaller than for films grown under identical conditions but using nanodiamond seeding alone. Such remarkably low thermal barriers obtained with the mixed-seeding process offer a promising route for fabrication of high-power GaN HEMTs using diamond as a heat spreader with an AlN interlayer.

    @article{strathprints72481,
    volume = {167},
    month = {October},
    title = {Mixed-size diamond seeding for low-thermal-barrier growth of CVD diamond onto GaN and AlN},
    year = {2020},
    pages = {620--626},
    doi = {10.1016/j.carbon.2020.05.050},
    journal = {Carbon},
    keywords = {chemical vapour deposition (CVD), thermal boundary resistance, transient thermoreflectance (TTR), diamond seeding, Chemistry, Physics, Chemistry(all)},
    url = {https://doi.org/10.1016/j.carbon.2020.05.050},
    issn = {0008-6223},
    abstract = {We report a method of growing a diamond layer via chemical vapour deposition (CVD) utilizing a mixture of microdiamond and nanodiamond seeding to give a low effective thermal boundary resistance (TBR eff) for heat-spreading applications in high-frequency, high-power electronic devices. CVD diamond was deposited onto thin layers of both GaN and AlN on Si substrates, comparing conventional nanodiamond seeding with a two-step process involving sequential seeding with microdiamond then nanodiamond. Thermal properties were determined using transient thermoreflectance (TTR), and the samples were also analysed with SEM and X-ray tomography. While diamond growth directly onto GaN proved to be unsuccessful due to poor adhesion, films grown on AlN were adherent and robust. The two-step mixed-seeding method gave TBR eff values {\ensuremath{<}} 6 m 2 K GW ?1 that were 30 times smaller than for films grown under identical conditions but using nanodiamond seeding alone. Such remarkably low thermal barriers obtained with the mixed-seeding process offer a promising route for fabrication of high-power GaN HEMTs using diamond as a heat spreader with an AlN interlayer.},
    author = {Smith, E. J. W. and Piracha, A. H. and Fields, D. and Pomeroy, J. W. and Mackenzie, G. R. and Abdallah, Z. and Massabuau, F. C-P. and Hinz, A. M. and Wallis, D. J. and Oliver, R. A. and Kuball, M. and May, P. W.}
    }

2019

  • E. Pascal, B. Hourahine, C. Trager-Cowan, and M. D. Graef, "Two beam toy model for dislocation contrast in ECCI," Microscopy and Microanalysis, vol. 25, iss. S2, p. 1968–1969, 2019.
    [BibTeX] [Abstract] [Download PDF]

    Dislocation contrast in the SEM, as observed though electron channelling contrast imaging (ECCI), is commonly treated analogously to the contrast in the TEM. This perception is based on early studies done for dislocations parallel with the surface where the surface relaxation is negligible. However, for threading dislocations (TD) that interact with the surface (normal or inclined), as is the case for nitrides materials, g b type invisibility criteria are no longer fully applicable to ECCI, especially in forward geometry [1]. Dislocations change locally the lattice curvature and Bragg diffraction conditions in the crystal, affecting the form and diffracting behaviour of the electron wavefunction in that region. More explicitly, Howie and Whelan [2] had shown that dislocation contrast is the result of interband transitions between Bloch waves states which, in turn, are caused by the change in the displacement field, u(r), around the dislocation or local ?strain?. Dynamical models have been used successfully to both predict and characterise dislocations in ECCI [3]. Nevertheless, the behaviour of dislocation contrast in ECCI in particular and diffraction contrast in the SEM in general remains somewhat opaque. In the work we investigate the behaviour of contrast causing strain as a means of insight into this problem.

    @article{strathprints73805,
    volume = {25},
    number = {S2},
    month = {August},
    author = {Elena Pascal and Ben Hourahine and Carol Trager-Cowan and Marc De Graef},
    title = {Two beam toy model for dislocation contrast in ECCI},
    journal = {Microscopy and Microanalysis},
    pages = {1968--1969},
    year = {2019},
    keywords = {dislocation contrast, electron wave function, electron channelling contrast imaging, Physics, Instrumentation},
    url = {https://strathprints.strath.ac.uk/73805/},
    abstract = {Dislocation contrast in the SEM, as observed though electron channelling contrast imaging (ECCI), is commonly treated analogously to the contrast in the TEM. This perception is based on early studies done for dislocations parallel with the surface where the surface relaxation is negligible. However, for threading dislocations (TD) that interact with the surface (normal or inclined), as is the case for nitrides materials, g b type invisibility criteria are no longer fully applicable to ECCI, especially in forward geometry [1]. Dislocations change locally the lattice curvature and Bragg diffraction conditions in the crystal, affecting the form and diffracting behaviour of the electron wavefunction in that region. More explicitly, Howie and Whelan [2] had shown that dislocation contrast is the result of interband transitions between Bloch waves states which, in turn, are caused by the change in the displacement field, u(r), around the dislocation or local ?strain?. Dynamical models have been used successfully to both predict and characterise dislocations in ECCI [3]. Nevertheless, the behaviour of dislocation contrast in ECCI in particular and diffraction contrast in the SEM in general remains somewhat opaque. In the work we investigate the behaviour of contrast causing strain as a means of insight into this problem.}
    }

  • J. Enslin, T. Wernicke, A. Lobanova, G. Kusch, L. Spasevski, T. Teke, B. Belde, R. W. Martin, R. Talalaev, and M. Kneissl, "Indium incorporation in quaternary Inx Aly Ga1-x-y N for UVB-LEDs," Japanese Journal of Applied Physics, vol. 58, iss. SC, p. SC1004, 2019.
    [BibTeX] [Abstract] [Download PDF]

    Consistent studies of the quaternary composition are rare as it is impossible to fully determine the quaternary composition by X-ray diffraction or deduce it from that of ternary alloys. In this paper we determined the quaternary composition by wavelength dispersive X-ray spectroscopy of Inx Aly layers grown by metal organic vapor phase epitaxy. Further insights explaining the peculiarities of Inx Aly Ga1-x-yN growth in a showerhead reactor were gained by simulations of the precursor decomposition, gas phase adduct formation and indium incorporation including desorption. The measurements and simulations agree very well showing that the indium incorporation in a range from 0\% to 2\% is limited by desorption which is enhanced by the compressive strain to the relaxed Al0.5Ga0.5N buffer layer as well as indium incorporation into AlN particles forming in the gas phase. Utilizing Inx Aly Ga1-x-yN layers containing 2\% of indium for multiple quantum wells (MQWs), it was possible to show an almost five times higher photoluminescence intensity of InAlGaN MQWs in comparison to AlGaN MQWs.

    @Article{strathprints71494,
    author = {Johannes Enslin and Tim Wernicke and Anna Lobanova and Gunnar Kusch and Lucia Spasevski and Tolga Teke and Bettina Belde and Robert W. Martin and Roman Talalaev and Michael Kneissl},
    title = {Indium incorporation in quaternary Inx Aly Ga1-x-y N for UVB-LEDs},
    journal = {Japanese Journal of Applied Physics},
    year = {2019},
    volume = {58},
    number = {SC},
    pages = {SC1004},
    month = {April},
    abstract = {Consistent studies of the quaternary composition are rare as it is impossible to fully determine the quaternary composition by X-ray diffraction or deduce it from that of ternary alloys. In this paper we determined the quaternary composition by wavelength dispersive X-ray spectroscopy of Inx Aly layers grown by metal organic vapor phase epitaxy. Further insights explaining the peculiarities of Inx Aly Ga1-x-yN growth in a showerhead reactor were gained by simulations of the precursor decomposition, gas phase adduct formation and indium incorporation including desorption. The measurements and simulations agree very well showing that the indium incorporation in a range from 0\% to 2\% is limited by desorption which is enhanced by the compressive strain to the relaxed Al0.5Ga0.5N buffer layer as well as indium incorporation into AlN particles forming in the gas phase. Utilizing Inx Aly Ga1-x-yN layers containing 2\% of indium for multiple quantum wells (MQWs), it was possible to show an almost five times higher photoluminescence intensity of InAlGaN MQWs in comparison to AlGaN MQWs.},
    keywords = {quaternary composition, X-ray spectroscopy, X-ray diffraction, UVB, ultraviolet light, light emitting diodes, LEDs, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/71494/},
    }

  • A. M. Shaltout, K. G. Lagoudakis, J. van de Groep, S. J. Kim, J. Vučković, V. M. Shalaev, and M. L. Brongersma, "Spatiotemporal light control with frequency-gradient metasurfaces," Science (New York, N.Y.), vol. 365, iss. 6451, p. 374–377, 2019.
    [BibTeX] [Abstract] [Download PDF]

    The capability of on-chip wavefront modulation has the potential to revolutionize many optical device technologies. However, the realization of power-efficient phasegradient metasurfaces that offer full-phase modulation (0 to 2p) and high operation speeds remains elusive. We present an approach to continuously steer light that is based on creating a virtual frequency-gradient metasurface by combining a passive metasurface with an advanced frequency-comb source. Spatiotemporal redirection of light naturally occurs as optical phase-fronts reorient at a speed controlled by the frequency gradient across the virtual metasurface. An experimental realization of laser beam steering with a continuously changing steering angle is demonstrated with a single metasurface over an angle of 25? in just 8 picoseconds. This work can support integrated-on-chip solutions for spatiotemporal optical control, directly affecting emerging applications such as solid-state light detection and ranging (LIDAR), threedimensional imaging, and augmented or virtual systems.

    @Article{strathprints71332,
    author = {Amr M. Shaltout and Konstantinos G. Lagoudakis and Jorik van de Groep and Soo Jin Kim and Jelena Vu{\v c}kovi{\'c} and Vladimir M. Shalaev and Mark L. Brongersma},
    title = {Spatiotemporal light control with frequency-gradient metasurfaces},
    journal = {Science (New York, N.Y.)},
    year = {2019},
    volume = {365},
    number = {6451},
    pages = {374--377},
    month = {July},
    abstract = {The capability of on-chip wavefront modulation has the potential to revolutionize many optical device technologies. However, the realization of power-efficient phasegradient metasurfaces that offer full-phase modulation (0 to 2p) and high operation speeds remains elusive. We present an approach to continuously steer light that is based on creating a virtual frequency-gradient metasurface by combining a passive metasurface with an advanced frequency-comb source. Spatiotemporal redirection of light naturally occurs as optical phase-fronts reorient at a speed controlled by the frequency gradient across the virtual metasurface. An experimental realization of laser beam steering with a continuously changing steering angle is demonstrated with a single metasurface over an angle of 25? in just 8 picoseconds. This work can support integrated-on-chip solutions for spatiotemporal optical control, directly affecting emerging applications such as solid-state light detection and ranging (LIDAR), threedimensional imaging, and augmented or virtual systems.},
    keywords = {laser beam steering, spatiotemporal light control, metasurfaces, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/71332/},
    }

  • M. V. Yakushev, A. V. Rodina, R. P. Seisyan, Y. E. Kitaev, S. A. Vaganov, M. A. Abdullaev, A. V. Mudryi, T. V. Kuznetsova, C. Faugeras, and R. W. Martin, "Electronic energy band parameters of CuInSe₂ : Landau levels in magnetotransmission spectra," Physical Review B: Condensed Matter and Materials Physics, vol. 100, iss. 23, p. 235202, 2019.
    [BibTeX] [Abstract] [Download PDF]

    Magnetotransmission (MT) at magnetic fields up to 29 T was used to study the electronic structure of CuInSe2 in thin polycrystalline films. The zero field absorption spectra exhibited resolved A, B, and C free excitons. Quantum oscillations, due to diamagnetic excitons comprising electrons and holes from Landau levels quantized in the conduction and valence band, respectively, appeared in the MT spectra at fields over 5 T. Spectral energies of Landau levels and binding energies of the corresponding diamagnetic excitons, theoretically calculated assuming a quasicubic approximation of the CuInSe2 tetragonal lattice structure, helped to identify the character of the experimentally observed diamagnetic excitons. Spectral energies of diamagnetic excitons in the MT spectra with different circular polarizations were used to determine the electron and light hole effective masses, whereas heavy hole masses as well as the {\ensuremath{\gamma}} and {\ensuremath{\gamma}}1 Luttinger parameters, Ep Kane energy, and F parameter of the influence of remote bands, as well as their polaron values, were calculated using the Luttinger theory.

    @Article{strathprints71034,
    author = {M. V. Yakushev and A. V. Rodina and R. P. Seisyan and Yu. E. Kitaev and S. A. Vaganov and M. A. Abdullaev and A. V. Mudryi and T. V. Kuznetsova and C. Faugeras and R. W. Martin},
    title = {Electronic energy band parameters of {CuInSe₂} : {L}andau levels in magnetotransmission spectra},
    journal = {Physical Review B: Condensed Matter and Materials Physics},
    year = {2019},
    volume = {100},
    number = {23},
    pages = {235202},
    month = dec,
    abstract = {Magnetotransmission (MT) at magnetic fields up to 29 T was used to study the electronic structure of CuInSe2 in thin polycrystalline films. The zero field absorption spectra exhibited resolved A, B, and C free excitons. Quantum oscillations, due to diamagnetic excitons comprising electrons and holes from Landau levels quantized in the conduction and valence band, respectively, appeared in the MT spectra at fields over 5 T. Spectral energies of Landau levels and binding energies of the corresponding diamagnetic excitons, theoretically calculated assuming a quasicubic approximation of the CuInSe2 tetragonal lattice structure, helped to identify the character of the experimentally observed diamagnetic excitons. Spectral energies of diamagnetic excitons in the MT spectra with different circular polarizations were used to determine the electron and light hole effective masses, whereas heavy hole masses as well as the {\ensuremath{\gamma}} and {\ensuremath{\gamma}}1 Luttinger parameters, Ep Kane energy, and F parameter of the influence of remote bands, as well as their polaron values, were calculated using the Luttinger theory.},
    keywords = {electronic energy band parameters, CuInSe2, magnetotransmission spectra, MT, Landau levels, energies, Physics, Nuclear and High Energy Physics},
    url = {https://strathprints.strath.ac.uk/71034/},
    }

  • F. C. -P. Massabuau, J. Bruckbauer, C. Trager-Cowan, and R. A. Oliver, "Microscopy of defects in semiconductors," in Characaterisation and Control of Defects in Semiconductors, F. Tuomisto, Ed., [S.I.]: IET, 2019.
    [BibTeX] [Abstract] [Download PDF]

    In this chapter, the authors discuss microscopy techniques that can be useful in addressing defects in semiconductors. They focus on three main families: scanning probe microscopy, scanning electron microscopy and transmission electron microscopy. They first address the basic principles of the selected microscopy techniques In discussions of image formation, they elucidate the mechanisms by which defects are typically imaged in each technique. Then, in the latter part of the chapter, they describe some key examples of the application of microscopy to semiconductor materials, addressing both point and extended defects and both two-dimensional (2D) and three-dimensional (3D) materials.

    @InCollection{strathprints70802,
    author = {Fabien C.-P. Massabuau and Jochen Bruckbauer and Carol Trager-Cowan and Rachel A. Oliver},
    title = {Microscopy of defects in semiconductors},
    booktitle = {Characaterisation and Control of Defects in Semiconductors},
    publisher = {IET},
    year = {2019},
    editor = {Filip Tuomisto},
    series = {Materials, Circuits and Devices},
    address = {[S.I.]},
    month = {September},
    abstract = {In this chapter, the authors discuss microscopy techniques that can be useful in addressing defects in semiconductors. They focus on three main families: scanning probe microscopy, scanning electron microscopy and transmission electron microscopy. They first address the basic principles of the selected microscopy techniques In discussions of image formation, they elucidate the mechanisms by which defects are typically imaged in each technique. Then, in the latter part of the chapter, they describe some key examples of the application of microscopy to semiconductor materials, addressing both point and extended defects and both two-dimensional (2D) and three-dimensional (3D) materials.},
    keywords = {microscopy, cathodoluminescence, atomic force microscopy, image formation, point defects, scanning electron microscopy, Physics, Atomic and Molecular Physics, and Optics},
    url = {https://strathprints.strath.ac.uk/70802/},
    }

  • A. A. Roble, S. K. Patra, F. Massabuau, M. Frentrup, M. A. Leontiadou, P. Dawson, M. J. Kappers, R. A. Oliver, D. M. Graham, and S. Schulz, "Impact of alloy fluctuations and Coulomb effects on the electronic and optical properties of c-plane GaN/AlGaN quantum wells," Scientific Reports, 2019.
    [BibTeX] [Abstract] [Download PDF]

    We report on a combined theoretical and experimental study of the impact of alloy fluctuations and Coulomb effects on the electronic and optical properties ofc-plane GaN/AlGaN multi-quantum well systems. The presence of carrier localization effects in this system was demonstrated by experimental observations, such as the "S-shape" temperature dependence of the photoluminescence (PL) peak energy, and non-exponential PL decay curves that varied across the PL spectra at 10 K. A three-dimensional modified continuum model, coupled with a self-consistent Hartree scheme, was employed to gain insight into the electronic and optical properties of the experimentally studied c-plane GaN/AlGaN quantum wells. This model confirmedthe existence of strong hole localization arising from the combined effects of the built-in polarization field along the growth direction and the alloy fluctuations at the quantum well/barrier interface. However, for electrons these localization effects are less pronounced in comparison to the holes. Furthermore, our calculations show that the attractive Coulomb interaction between electron and hole results in exciton localization. This behavior is in contrast to the picture of independently localized electrons and holes, often used to explain the radiative recombination process in c-plane InGaN/GaN quantum well systems

    @article{strathprints70116,
    month = {October},
    title = {Impact of alloy fluctuations and Coulomb effects on the electronic and optical properties of c-plane GaN/AlGaN quantum wells},
    year = {2019},
    journal = {Scientific Reports},
    keywords = {photoluminescence, GaN/AlGaN quantum wells, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/70116/},
    issn = {2045-2322},
    abstract = {We report on a combined theoretical and experimental study of the impact of alloy fluctuations and Coulomb effects on the electronic and optical properties ofc-plane GaN/AlGaN multi-quantum well systems. The presence of carrier localization effects in this system was demonstrated by experimental observations, such as the "S-shape" temperature dependence of the photoluminescence (PL) peak energy, and non-exponential PL decay curves that varied across the PL spectra at 10 K. A three-dimensional modified continuum model, coupled with a self-consistent Hartree scheme, was employed to gain insight into the electronic and optical properties of the experimentally studied c-plane GaN/AlGaN quantum wells. This model confirmedthe existence of strong hole localization arising from the combined effects of the built-in polarization field along the growth direction and the alloy fluctuations at the quantum well/barrier interface. However, for electrons these localization effects are less pronounced in comparison to the holes. Furthermore, our calculations show that the attractive Coulomb interaction between electron and hole results in exciton localization. This behavior is in contrast to the picture of independently localized electrons and holes, often used to explain the radiative recombination process in c-plane InGaN/GaN quantum well systems},
    author = {Roble, A. A. and Patra, S. K. and Massabuau, F. and Frentrup, M. and Leontiadou, M. A. and Dawson, P. and Kappers, M. J. and Oliver, R. A. and Graham, D. M. and Schulz, S.}
    }

  • C. Trager-Cowan, A. Alasmari, W. Avis, J. Bruckbauer, P. R. Edwards, B. Hourahine, S. Kraeusel, G. Kusch, R. Johnston, G. Naresh-Kumar, R. W. Martin, M. Nouf-Allehiani, E. Pascal, L. Spasevski, D. Thomson, S. Vespucci, P. J. Parbrook, M. D. Smith, J. Enslin, F. Mehnke, M. Kneissl, C. Kuhn, T. Wernicke, S. Hagedorn, S. Walde, M. Weyers, P. -M. Coulon, P. A. Shields, Y. Zhang, L. Jiu, Y. P. Gong, R. M. Smith, T. Wang, and A. Winkelmann, "The scanning electron microscope as a flexible tool for investigating the properties of UV-emitting nitride semiconductor thin films," Photonics Research, vol. 7, iss. 11, p. B73–B82, 2019.
    [BibTeX] [Abstract] [Download PDF]

    In this article we describe the scanning electron microscopy techniques of electron backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI), wavelength dispersive X-ray spectroscopy (WDX) and cathodoluminescence (CL) hyperspectral imaging. We present our recent results on the use of these non-destructive techniques to obtain information on the topography, crystal misorientation, defect distributions, composition, doping and light emission from a range of UV emitting nitride semiconductor structures. We aim to illustrate the developing capability of each of these techniques for understanding the properties of UV emitting nitride semiconductors, and the benefits were appropriate, in combining the techniques.

    @Article{strathprints69913,
    author = {C. Trager-Cowan and A. Alasmari and W. Avis and Jochen Bruckbauer and P. R. Edwards and B. Hourahine and S. Kraeusel and G. Kusch and R. Johnston and G. Naresh-Kumar and R. W. Martin and M. Nouf-Allehiani and E. Pascal and L. Spasevski and D. Thomson and S. Vespucci and P. J. Parbrook and M. D. Smith and J. Enslin and F. Mehnke and M. Kneissl and C. Kuhn and T. Wernicke and S. Hagedorn and S. Walde and M. Weyers and P.-M. Coulon and P. A. Shields and Y. Zhang and L. Jiu and Y. P. Gong and R. M. Smith and T. Wang and A. Winkelmann},
    title = {The scanning electron microscope as a flexible tool for investigating the properties of {UV}-emitting nitride semiconductor thin films},
    journal = {Photonics Research},
    year = {2019},
    volume = {7},
    number = {11},
    pages = {B73--B82},
    month = {September},
    abstract = {In this article we describe the scanning electron microscopy techniques of electron backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI), wavelength dispersive X-ray spectroscopy (WDX) and cathodoluminescence (CL) hyperspectral imaging. We present our recent results on the use of these non-destructive techniques to obtain information on the topography, crystal misorientation, defect distributions, composition, doping and light emission from a range of UV emitting nitride semiconductor structures. We aim to illustrate the developing capability of each of these techniques for understanding the properties of UV emitting nitride semiconductors, and the benefits were appropriate, in combining the techniques.},
    keywords = {scanning electron microscopy, electron backscatter diffraction, non-destructive techniques, UV emitting nitride semiconductors, Physics, Atomic and Molecular Physics, and Optics},
    url = {https://strathprints.strath.ac.uk/69913/},
    }

  • C. Dory, D. Vercruysse, K. Y. Yang, N. V. Sapra, A. E. Rugar, S. Sun, D. M. Lukin, A. Y. Piggott, J. L. Zhang, M. Radulaski, K. G. Lagoudakis, L. Su, and J. Vučković, "Inverse-designed diamond photonics," Nature Communications, vol. 10, iss. 1, p. 3309, 2019.
    [BibTeX] [Abstract] [Download PDF]

    Diamond hosts optically active color centers with great promise in quantum computation, networking, and sensing. Realization of such applications is contingent upon the integration of color centers into photonic circuits. However, current diamond quantum optics experiments are restricted to single devices and few quantum emitters because fabrication constraints limit device functionalities, thus precluding color center integrated photonic circuits. In this work, we utilize inverse design methods to overcome constraints of cutting-edge diamond nanofabrication methods and fabricate compact and robust diamond devices with unique specifications. Our design method leverages advanced optimization techniques to search the full parameter space for fabricable device designs. We experimentally demonstrate inverse-designed photonic free-space interfaces as well as their scalable integration with two vastly different devices: classical photonic crystal cavities and inverse-designed waveguide-splitters. The multi-device integration capability and performance of our inverse-designed diamond platform represents a critical advancement toward integrated diamond quantum optical circuits.

    @Article{strathprints69678,
    author = {Constantin Dory and Dries Vercruysse and Ki Youl Yang and Neil V. Sapra and Alison E. Rugar and Shuo Sun and Daniil M. Lukin and Alexander Y. Piggott and Jingyuan L. Zhang and Marina Radulaski and Konstantinos G. Lagoudakis and Logan Su and Jelena Vu{\v c}kovi{\'c}},
    title = {Inverse-designed diamond photonics},
    journal = {Nature Communications},
    year = {2019},
    volume = {10},
    number = {1},
    pages = {3309},
    month = {July},
    abstract = {Diamond hosts optically active color centers with great promise in quantum computation, networking, and sensing. Realization of such applications is contingent upon the integration of color centers into photonic circuits. However, current diamond quantum optics experiments are restricted to single devices and few quantum emitters because fabrication constraints limit device functionalities, thus precluding color center integrated photonic circuits. In this work, we utilize inverse design methods to overcome constraints of cutting-edge diamond nanofabrication methods and fabricate compact and robust diamond devices with unique specifications. Our design method leverages advanced optimization techniques to search the full parameter space for fabricable device designs. We experimentally demonstrate inverse-designed photonic free-space interfaces as well as their scalable integration with two vastly different devices: classical photonic crystal cavities and inverse-designed waveguide-splitters. The multi-device integration capability and performance of our inverse-designed diamond platform represents a critical advancement toward integrated diamond quantum optical circuits.},
    keywords = {diamond photonics, quantum computation, diamond quantum optics, Optics. Light, Biochemistry, Genetics and Molecular Biology(all), Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/69678/},
    }

  • M. V. Yakushev, C. Faugeras, A. V. Mudryi, and R. W. Martin, "A magneto-reflectivity study of CuInTe₂ single crystals," Physica Status Solidi B, vol. 257, p. 1900464, 2019.
    [BibTeX] [Abstract] [Download PDF]

    CuInTe2 single crystals were studied using optical magneto-reflectance (MR) in magnetic fields B up to 20 T at 4.2 K. The spectra exhibited the A and B free excitons blue shifting at increasing magnetic fields. Fitting quadratic functions to the experimental dependencies of the exciton spectral energy on B assuming a lowfield limit allowed determination of diamagnetic shift rates of 8.2?10-5 eV/T2 and 8.5?10-5 eV/T2 for the A and B free excitons, respectively. Exciton reduced masses of 0.0575m0 and 0.0568m0 (m0 is the free electron mass), Rydbergs of 6.2 meV and 6.1 meV, and Bohr radii 10.4 nm and 10.5 nm were then estimated. An electron effective mass of 0.062m0 and B sub-band effective hole mass of 0.70m0 were determined using a literature value of the A valence sub-band hole of 0.78m0 .

    @Article{strathprints69479,
    author = {Michael V. Yakushev and Clement Faugeras and Alexander V. Mudryi and Robert W. Martin},
    title = {A magneto-reflectivity study of CuInTe₂ single crystals},
    journal = {Physica Status Solidi B},
    year = {2019},
    volume = {257},
    pages = {1900464},
    month = {August},
    abstract = {CuInTe2 single crystals were studied using optical magneto-reflectance (MR) in magnetic fields B up to 20 T at 4.2 K. The spectra exhibited the A and B free excitons blue shifting at increasing magnetic fields. Fitting quadratic functions to the experimental dependencies of the exciton spectral energy on B assuming a lowfield limit allowed determination of diamagnetic shift rates of 8.2?10-5 eV/T2 and 8.5?10-5 eV/T2 for the A and B free excitons, respectively. Exciton reduced masses of 0.0575m0 and 0.0568m0 (m0 is the free electron mass), Rydbergs of 6.2 meV and 6.1 meV, and Bohr radii 10.4 nm and 10.5 nm were then estimated. An electron effective mass of 0.062m0 and B sub-band effective hole mass of 0.70m0 were determined using a literature value of the A valence sub-band hole of 0.78m0 .},
    keywords = {CuInTe2, excitons, magnetic field, effective masses, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/69479/},
    }

  • Z. S. Mbalaha, P. R. Edwards, D. J. S. Birch, and Y. Chen, "Synthesis of small gold nanorods and their subsequent functionalization with hairpin single stranded DNA," ACS Omega, vol. 4, iss. 9, p. 13740–13746, 2019.
    [BibTeX] [Abstract] [Download PDF]

    Small gold nanorods have a significantly large absorption/scattering ratio and are especially beneficial in exploiting photothermal effects, for example in photothermal therapy and remote drug release. This work systematically investigates the influence of growth conditions on the size, growth yield and stability of small gold nanorods. The silver-assisted seed-mediated growth method was optimised to synthesize stable small gold nanorods with a high growth yield ({\ensuremath{>}}85\%). Further study on the influence of silver ions on the growth facilitates the growth of small gold nanorods with tuneable longitudinal surface plasmon resonance from 613 nm to 912 nm, with average dimensions of 13-25 nm in length and 5-6 nm in diameter. Moreover, the small gold nanorods were successfully functionalized with thiol- modified hairpin oligonucleotides (hpDNA) labelled with Cy5. Fluorescence intensity measurements show an increase in the presence of target DNA and an enhanced signal/background ratio when the longitudinal surface plasmon resonance of small gold nanorods overlaps with the excitation and emission wavelength of Cy5. This coincides with a reduced fluorescence lifetime of Cy5 in the hairpin structure, indicating surface plasmon resonance enhanced energy transfer to the small gold nanorods. This study may provide insight on the synthesis and functionalization of small gold nanorods in biomedical sensing and therapy.

    @Article{strathprints69280,
    author = {Zendesha S. Mbalaha and Paul R. Edwards and David J.S. Birch and Yu Chen},
    title = {Synthesis of small gold nanorods and their subsequent functionalization with hairpin single stranded DNA},
    journal = {ACS Omega},
    year = {2019},
    volume = {4},
    number = {9},
    pages = {13740--13746},
    month = {July},
    abstract = {Small gold nanorods have a significantly large absorption/scattering ratio and are especially beneficial in exploiting photothermal effects, for example in photothermal therapy and remote drug release. This work systematically investigates the influence of growth conditions on the size, growth yield and stability of small gold nanorods. The silver-assisted seed-mediated growth method was optimised to synthesize stable small gold nanorods with a high growth yield ({\ensuremath{>}}85\%). Further study on the influence of silver ions on the growth facilitates the growth of small gold nanorods with tuneable longitudinal surface plasmon resonance from 613 nm to 912 nm, with average dimensions of 13-25 nm in length and 5-6 nm in diameter. Moreover, the small gold nanorods were successfully functionalized with thiol- modified hairpin oligonucleotides (hpDNA) labelled with Cy5. Fluorescence intensity measurements show an increase in the presence of target DNA and an enhanced signal/background ratio when the longitudinal surface plasmon resonance of small gold nanorods overlaps with the excitation and emission wavelength of Cy5. This coincides with a reduced fluorescence lifetime of Cy5 in the hairpin structure, indicating surface plasmon resonance enhanced energy transfer to the small gold nanorods. This study may provide insight on the synthesis and functionalization of small gold nanorods in biomedical sensing and therapy.},
    keywords = {small gold nanorods, synthesis, functionalization, surface plasmon, oligonucleotide, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/69280/},
    }

  • M. V. Yakushev, A. V. Mudryi, C. Faugeras, and R. W. Martin, "A magneto-reflectivity study of CuGaSe₂ single crystals," Physica Status Solidi (RRL) - Rapid Research Letters, vol. 13, iss. 2, p. 1800374, 2019.
    [BibTeX] [Abstract] [Download PDF]

    CuGaSe₂ single crystals are studied using magneto-reflectivity at 4.2 K in magnetic fields B up to 20 T. The A and B free excitons, observed in the optical reflectivity spectra, blue shift with increasing B. A low-field perturbation approach within the anisotropic hydrogenic model is used to fit the dependence of the spectral position of these excitons on B. The A and B exciton reduced masses of 0.115m₀ and 0.108m0 (m₀ is the free electron mass), Rydbergs of 12.9 and 12.2meV, Bohr radii 5.08 and 5.4 nm, and effective hole masses of 0.64m₀ and 0.48m₀, respectively, are determined.

    @Article{strathprints68316,
    author = {Michael V. Yakushev and Alexander V. Mudryi and Clement Faugeras and Robert W. Martin},
    journal = {Physica Status Solidi (RRL) - Rapid Research Letters},
    title = {A magneto-reflectivity study of {CuGaSe₂} single crystals},
    year = {2019},
    month = {February},
    number = {2},
    pages = {1800374},
    volume = {13},
    abstract = {CuGaSe₂ single crystals are studied using magneto-reflectivity at 4.2 K in magnetic fields B up to 20 T. The A and B free excitons, observed in the optical reflectivity spectra, blue shift with increasing B. A low-field perturbation approach within the anisotropic hydrogenic model is used to fit the dependence of the spectral position of these excitons on B. The A and B exciton reduced masses of 0.115m₀ and 0.108m0 (m₀ is the free electron mass), Rydbergs of 12.9 and 12.2meV, Bohr radii 5.08 and 5.4 nm, and effective hole masses of 0.64m₀ and 0.48m₀, respectively, are determined.},
    keywords = {CuGaSe2, excitons, magnetic field, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/68316/},
    }

  • N. K. Gunasekar, J. Bruckbauer, A. Winkelmann, X. Yu, B. Hourahine, P. R. Edwards, T. Wang, C. Trager-Cowan, and R. W. Martin, "Determining GaN nanowire polarity and its influence on light emission in the scanning electron microscope," Nano Letters, vol. 19, p. 3863–3870, 2019.
    [BibTeX] [Abstract] [Download PDF]

    The crystal polarity of non-centrosymmetric wurtzite GaN nanowires is determined non-destructively in the scanning electron microscope using electron backscatter diffraction (EBSD). The impact of the nanowire polarity on light emission is then investigated using cathodoluminescence (CL) spectroscopy. EBSD can determine polarity of non-centrosymmetric crystals by interrogating differences in the intensity distribution of bands of the EBSD pattern associated with semi-polar planes. Experimental EBSD patterns from an array of GaN nanowires are compared with theoretical patterns produced using dynamical electron simulations to reveal whether they are Ga or N-polar or, as in several cases, of mixed polarity. CL spectroscopy demonstrates the effect of the polarity on light emission, with spectra obtained from nanowires of known polarity revealing a small but measureable shift (~28 meV) in the band edge emission energy between those with Ga and N polarity. We attributed this energy shift to a difference in impurity incorporation in nanowires of different crystal polarity. This approach can be employed to non-destructively identify polarity in a wide range of non-centrosymmetric nanoscale material systems and provide direct comparison with their luminescence.

    @Article{strathprints67670,
    author = {Naresh Kumar Gunasekar and Jochen Bruckbauer and Aimo Winkelmann and Xiang Yu and Ben Hourahine and Paul R. Edwards and Tao Wang and Carol Trager-Cowan and Rober W. Martin},
    title = {Determining GaN nanowire polarity and its influence on light emission in the scanning electron microscope},
    journal = {Nano Letters},
    year = {2019},
    volume = {19},
    pages = {3863--3870},
    month = {April},
    abstract = {The crystal polarity of non-centrosymmetric wurtzite GaN nanowires is determined non-destructively in the scanning electron microscope using electron backscatter diffraction (EBSD). The impact of the nanowire polarity on light emission is then investigated using cathodoluminescence (CL) spectroscopy. EBSD can determine polarity of non-centrosymmetric crystals by interrogating differences in the intensity distribution of bands of the EBSD pattern associated with semi-polar planes. Experimental EBSD patterns from an array of GaN nanowires are compared with theoretical patterns produced using dynamical electron simulations to reveal whether they are Ga or N-polar or, as in several cases, of mixed polarity. CL spectroscopy demonstrates the effect of the polarity on light emission, with spectra obtained from nanowires of known polarity revealing a small but measureable shift (~28 meV) in the band edge emission energy between those with Ga and N polarity. We attributed this energy shift to a difference in impurity incorporation in nanowires of different crystal polarity. This approach can be employed to non-destructively identify polarity in a wide range of non-centrosymmetric nanoscale material systems and provide direct comparison with their luminescence.},
    keywords = {nanowires, polarity, electron diffraction, SEM, GaN, cathodoluminescence, Physics, Bioengineering, Chemistry(all), Condensed Matter Physics},
    url = {https://strathprints.strath.ac.uk/67670/}
    }

  • F. C-P. Massabuau, M. K. Horton, E. Pearce, S. Hammersley, P. Chen, M. S. Zielinski, T. Weatherley, G. Divitini, P. R. Edwards, M. J. Kappers, C. McAleese, M. A. Moram, C. J. Humphreys, P. Dawson, and R. A. Oliver, "Optical and structural properties of dislocations in InGaN," Journal of Applied Physics, vol. 125, p. 165701, 2019.
    [BibTeX] [Abstract] [Download PDF]

    Threading dislocations in thick layers of InₓGa₁₋ₓN (5% < x < 15%) have been investigated by means of cathodoluminescence, time-resolved cathodoluminescence and molecular dynamics. We show that indium atoms segregate near dislocations in all the samples. This promotes the formation of In-N-In chains and atomic condensates which localize carriers and hinder non-radiative recombination at dislocations. We note however that the dark halo surrounding the dislocations in the cathodoluminescence image becomes increasingly pronounced as the indium fraction of the sample increases. Using transmission electron microscopy, we attribute the dark halo to a region of lower indium content formed below the facet of the V-shaped pit that terminates the dislocation in low composition samples (x < 12%). For x > 12%, the facets of the V-defect featured dislocation bundles instead of the low indium fraction region. In this sample the origin of the dark halo may relate to a compound effect of the dislocation bundles, of a variation of surface potential and perhaps of an increase in carrier diffusion length.

    @Article{strathprints67565,
    author = {F.C-P. Massabuau and M.K. Horton and E. Pearce and S. Hammersley and P. Chen and M.S. Zielinski and T. Weatherley and G. Divitini and P.R. Edwards and M.J. Kappers and C. McAleese and M.A. Moram and C.J. Humphreys and P. Dawson and R.A. Oliver},
    title = {Optical and structural properties of dislocations in InGaN},
    journal = {Journal of Applied Physics},
    year = {2019},
    volume = {125},
    pages = {165701},
    month = {April},
    abstract = {Threading dislocations in thick layers of InₓGa₁₋ₓN (5% < x < 15%) have been investigated by means of cathodoluminescence, time-resolved cathodoluminescence and molecular dynamics. We show that indium atoms segregate near dislocations in all the samples. This promotes the formation of In-N-In chains and atomic condensates which localize carriers and hinder non-radiative recombination at dislocations. We note however that the dark halo surrounding the dislocations in the cathodoluminescence image becomes increasingly pronounced as the indium fraction of the sample increases. Using transmission electron microscopy, we attribute the dark halo to a region of lower indium content formed below the facet of the V-shaped pit that terminates the dislocation in low composition samples (x < 12%). For x > 12%, the facets of the V-defect featured dislocation bundles instead of the low indium fraction region. In this sample the origin of the dark halo may relate to a compound effect of the dislocation bundles, of a variation of surface potential and perhaps of an increase in carrier diffusion length.},
    keywords = {indium gallium nitride, cathodoluminescence, transmission electron microscopy, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/67565/}
    }

  • G. Kusch, J. Enslin, L. Spasevski, T. Teke, T. Wernicke, P. R. Edwards, M. Kneissl, and R. W. Martin, "Influence of InN and AlN concentration on the compositional inhomogeneity and formation of InN-rich regions in InxAlyGa1-x-yN," Japanese Journal of Applied Physics, vol. 58, p. SCCB18, 2019.
    [BibTeX] [Abstract] [Download PDF]

    The application of quaternary InxAlyGa1-x-yN active regions is a promising path towards high efficiency UVB-LEDs. For the utilisation of InxAlyGa1-x-yN, detailed knowledge of the interplay between growth parameters, adatom incorporation, optical and structural properties is crucial. We investigated the influence of the TMAl and TMIn flux on the composition and luminescence properties of InxAlyGa1-x-yN layers by multi-mode scanning electron microscopy. We found that varying the molar TMIn flow from 0 to 17.3 µmol/min led to an InN concentration between 0% and 3.2% and an emission energy between 4.17 eV and 3.75 eV. The variation of the molar TMAl flow from 3.5 to 35.4 µmol/min resulted in a AlN composition between 7.8% and 30.7% with an emission energy variation between 3.6 eV and 4.1 eV. Cathodoluminescence hyperspectral imaging provided evidence for the formation of nanoscale InN-rich regions. Analysing the emission properties of these InN-rich regions showed that their emission energy is inhomogeneous and varies by ~150 meV. We provide evidence that the formation of these InN-rich regions is highly dependent on the AlN and InN composition of the layer and that their formation will strongly affect the performance of InxAlyGa1-x-yN LEDs.

    @Article{strathprints67321,
    author = {Gunnar Kusch and Johannes Enslin and Lucia Spasevski and Tolga Teke and Tim Wernicke and Paul R. Edwards and Michael Kneissl and Robert W. Martin},
    title = {Influence of InN and AlN concentration on the compositional inhomogeneity and formation of InN-rich regions in InxAlyGa1-x-yN},
    journal = {Japanese Journal of Applied Physics},
    year = {2019},
    volume = {58},
    pages = {SCCB18},
    month = {March},
    abstract = {The application of quaternary InxAlyGa1-x-yN active regions is a promising path towards high efficiency UVB-LEDs. For the utilisation of InxAlyGa1-x-yN, detailed knowledge of the interplay between growth parameters, adatom incorporation, optical and structural properties is crucial. We investigated the influence of the TMAl and TMIn flux on the composition and luminescence properties of InxAlyGa1-x-yN layers by multi-mode scanning electron microscopy. We found that varying the molar TMIn flow from 0 to 17.3 µmol/min led to an InN concentration between 0% and 3.2% and an emission energy between 4.17 eV and 3.75 eV. The variation of the molar TMAl flow from 3.5 to 35.4 µmol/min resulted in a AlN composition between 7.8% and 30.7% with an emission energy variation between 3.6 eV and 4.1 eV. Cathodoluminescence hyperspectral imaging provided evidence for the formation of nanoscale InN-rich regions. Analysing the emission properties of these InN-rich regions showed that their emission energy is inhomogeneous and varies by ~150 meV. We provide evidence that the formation of these InN-rich regions is highly dependent on the AlN and InN composition of the layer and that their formation will strongly affect the performance of InxAlyGa1-x-yN LEDs.},
    keywords = {high efficiency UVB-LEDs, composition, luminescence, cathodoluminescence hyperspectral imaging, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/67321/}
    }

  • M. V. Yakushev, A. V. Mudryi, E. Kärber, P. R. Edwards, and R. W. Martin, "The band structure of CuInTe₂ studied by optical reflectivity," Applied Physics Letters, vol. 114, iss. 6, p. 62103, 2019.
    [BibTeX] [Abstract] [Download PDF]

    CuInTe₂ is a semiconductor with high potential for use as a thermoelectric material and as the absorber in thin film solar cells. Studying the optical reflectivity spectra of CuInTe₂ single crystals resolves resonances at 1.054 eV and 1.072 eV, which are assigned to the A and B free excitons. Photoluminescence spectra exhibited a peak due to the A free exciton at 1.046 eV. Varshni coefficients were found for both excitons. Zero temperature bandgaps EgA = 1.060 eV and EgB = 1.078 eV were determined for the A and B valence sub-bands, respectively. The splitting due to crystal-field ΔCF and spin-orbit effects ΔSO were calculated as -26.3 meV and 610 meV, respectively, using the determined EgA and EgB and a literature value of EgC.

    @Article{strathprints66980,
    author = {M. V. Yakushev and A. V. Mudryi and E. Kärber and P. R. Edwards and R. W. Martin},
    title = {The band structure of CuInTe₂ studied by optical reflectivity},
    journal = {Applied Physics Letters},
    year = {2019},
    volume = {114},
    number = {6},
    pages = {062103},
    month = {February},
    abstract = {CuInTe₂ is a semiconductor with high potential for use as a thermoelectric material and as the absorber in thin film solar cells. Studying the optical reflectivity spectra of CuInTe₂ single crystals resolves resonances at 1.054 eV and 1.072 eV, which are assigned to the A and B free excitons. Photoluminescence spectra exhibited a peak due to the A free exciton at 1.046 eV. Varshni coefficients were found for both excitons. Zero temperature bandgaps EgA = 1.060 eV and EgB = 1.078 eV were determined for the A and B valence sub-bands, respectively. The splitting due to crystal-field ΔCF and spin-orbit effects ΔSO were calculated as -26.3 meV and 610 meV, respectively, using the determined EgA and EgB and a literature value of EgC.},
    keywords = {optical reflectivity, CuInTe2, semiconductors, electronic band structures, chalcopyrites, Optics. Light, Atomic and Molecular Physics, and Optics},
    url = {https://strathprints.strath.ac.uk/66980/}
    }

  • E. Angioni, R. J. Marshall, N. J. Findlay, J. Bruckbauer, B. Breig, D. J. Wallis, R. W. Martin, R. S. Forgan, and P. J. Skabara, "Implementing fluorescent MOFs as down-converting layers in hybrid light-emitting diodes," Journal of Materials Chemistry. C, vol. 7, pp. 2394-2400, 2019.
    [BibTeX] [Abstract] [Download PDF]

    One of the most important non-radiative relaxation processes that limits the quantum yield of a fluorophore is related to aggregation of the molecules in the solid-state causing excimer quenching. To limit this quenching mechanism, the fluorophore can be contained within a well-ordered 3D system that minimises aggregation through rigid bonds and spatial separation in a defined topological construct. Herein, the synthesis, characterisation and application as a down-converter of a new luminescent 3D material (MOF-BTBMBA) that incorporates a building block based on a benzothiadiazole (BT) derivative (BTBMBA) in a metal-organic framework (MOF) is presented. Notably, the photoluminescence quantum yield and hybrid LED performance are significantly improved for the MOF-based device compared to that prepared with the free ligand, highlighting the effectiveness of the rigid scaffold arrangement.

    @Article{strathprints66811,
    author = {Enrico Angioni and Ross J. Marshall and Neil J. Findlay and Jochen Bruckbauer and Ben Breig and David J. Wallis and Robert W. Martin and Ross S. Forgan and Peter J. Skabara},
    title = {Implementing fluorescent MOFs as down-converting layers in hybrid light-emitting diodes},
    journal = {Journal of Materials Chemistry. C},
    year = {2019},
    volume = {7},
    pages = {2394-2400},
    month = {January},
    abstract = {One of the most important non-radiative relaxation processes that limits the quantum yield of a fluorophore is related to aggregation of the molecules in the solid-state causing excimer quenching. To limit this quenching mechanism, the fluorophore can be contained within a well-ordered 3D system that minimises aggregation through rigid bonds and spatial separation in a defined topological construct. Herein, the synthesis, characterisation and application as a down-converter of a new luminescent 3D material (MOF-BTBMBA) that incorporates a building block based on a benzothiadiazole (BT) derivative (BTBMBA) in a metal-organic framework (MOF) is presented. Notably, the photoluminescence quantum yield and hybrid LED performance are significantly improved for the MOF-based device compared to that prepared with the free ligand, highlighting the effectiveness of the rigid scaffold arrangement.},
    keywords = {fluorophore, 3D system, down-converter, Physics, Atomic and Molecular Physics, and Optics, Materials Chemistry},
    url = {https://strathprints.strath.ac.uk/66811/}
    }

  • H. Xu, G. Drozdov, B. Hourahine, P. J. Gyu, R. Sweat, T. Frauenheim, and T. Dumitrică, "Collapsed carbon nanotubes: from nano to mesoscale via density functional-based tight-binding objective molecular modeling," Carbon, vol. 143, p. 786–792, 2019.
    [BibTeX] [Abstract] [Download PDF]

    Due to the inherent spatial and temporal limitations of atomistic modeling and the lack of mesoscale models, mesoscopic simulation methods for guiding the development of super strong lightweight material systems comprising collapsed carbon nanotubes (CNTs) are missing. Here we establish a path for deriving ultra-coarse-grained mesoscopic distinct element method (mDEM) models directly from the quantum mechanical representation of a collapsed CNT. Atomistic calculations based on density functional theory-based tight-binding (DFTB) extended with Lennard-Jones interactions allow for the identification of the cross-section and elastic constants of an elastic beam idealization of a collapsed CNT. Application of the quantum treatment is possible due to the simplification in the number of atoms introduced by accounting for the helical and angular symmetries exhibited by twisted and bent CNTs. The modeling chain established here is suitable for deriving mesoscopic models for a variety of microscopic filaments with bending anisotropy.

    @Article{strathprints66672,
    author = {Hao Xu and Grigorii Drozdov and Benjamin Hourahine and Park Jin Gyu and Rebekah Sweat and Thomas Frauenheim and Traian Dumitrică},
    title = {Collapsed carbon nanotubes: from nano to mesoscale via density functional-based tight-binding objective molecular modeling},
    journal = {Carbon},
    year = {2019},
    volume = {143},
    pages = {786--792},
    month = {March},
    abstract = {Due to the inherent spatial and temporal limitations of atomistic modeling and the lack of mesoscale models, mesoscopic simulation methods for guiding the development of super strong lightweight material systems comprising collapsed carbon nanotubes (CNTs) are missing. Here we establish a path for deriving ultra-coarse-grained mesoscopic distinct element method (mDEM) models directly from the quantum mechanical representation of a collapsed CNT. Atomistic calculations based on density functional theory-based tight-binding (DFTB) extended with Lennard-Jones interactions allow for the identification of the cross-section and elastic constants of an elastic beam idealization of a collapsed CNT. Application of the quantum treatment is possible due to the simplification in the number of atoms introduced by accounting for the helical and angular symmetries exhibited by twisted and bent CNTs. The modeling chain established here is suitable for deriving mesoscopic models for a variety of microscopic filaments with bending anisotropy.},
    keywords = {coarse-grained model, carbon nanotube, density functional based tight binding, Physics, Chemistry(all), Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/66672/}
    }

  • M. A. Sulimov, M. V. Yakushev, J. Márquez-Prieto, I. Forbes, P. R. Edwards, V. D. Zhivulko, O. M. Borodavchenko, A. V. Mudryi, J. Krustok, and R. W. Martin, "Effects of selenisation temperature on photoluminescence and photoluminescence excitation spectra of ZnO/CdS/Cu₂ZnSnSe₄/Mo/glass," Thin Solid Films, vol. 672, pp. 146-151, 2019.
    [BibTeX] [Abstract] [Download PDF]

    The effect of solar cell processing (including etching in KCN along with deposition of CdS and ZnO) on photoluminescence (PL) spectra and bandgap Eg (measured at 4.2 K by photoluminescence excitation) of Cu2ZnSnSe4 films, produced by selenising metallic precursors at 450 °C, 500 °C and 550 °C, was studied. Temperature and excitation intensity analysis of the P1 dominant band in the PL spectra of solar cells suggests that after processing this band still can be assigned to the free-to-bound recombination of free electrons with holes bound at deep acceptor levels influenced by valence band-tails. However processing increased the intensity of P1 and blue shifted it. The strongest effect was observed for the film selenised at 500 °C. For the film selenised at 450 °C the blue shift and increase in the intensity were smaller and only a slight intensity rise was found for the film selenised at 550 °C. The intensity increase we assign to a reduction in the concentration of non-radiative recombination centers on the surface because of the etching and changes in doping due to inter-diffusion of Cd, S, Se and Zn after the deposition of CdS. Such an inter-diffusion depends on the elemental composition of the films defining the chemistry of defects and influencing Eg which increased in the film selenised at 500 °C but decreased in the other films. Processing increased the P1 shift rate (j-shift) with excitation power change in all the films demonstrating a higher compensation degree in the solar cells which is consistent with the formation of an interface layer containing new donors CdCu.

    @Article{strathprints66586,
    author = {M.A. Sulimov and M.V. Yakushev and J. M{\'a}rquez-Prieto and I. Forbes and P.R. Edwards and V.D. Zhivulko and O.M. Borodavchenko and A. V. Mudryi and J. Krustok and R.W. Martin},
    title = {Effects of selenisation temperature on photoluminescence and photoluminescence excitation spectra of ZnO/CdS/Cu₂ZnSnSe₄/Mo/glass},
    journal = {Thin Solid Films},
    year = {2019},
    volume = {672},
    pages = {146-151},
    month = {January},
    abstract = {The effect of solar cell processing (including etching in KCN along with deposition of CdS and ZnO) on photoluminescence (PL) spectra and bandgap Eg (measured at 4.2 K by photoluminescence excitation) of Cu2ZnSnSe4 films, produced by selenising metallic precursors at 450 °C, 500 °C and 550 °C, was studied. Temperature and excitation intensity analysis of the P1 dominant band in the PL spectra of solar cells suggests that after processing this band still can be assigned to the free-to-bound recombination of free electrons with holes bound at deep acceptor levels influenced by valence band-tails. However processing increased the intensity of P1 and blue shifted it. The strongest effect was observed for the film selenised at 500 °C. For the film selenised at 450 °C the blue shift and increase in the intensity were smaller and only a slight intensity rise was found for the film selenised at 550 °C. The intensity increase we assign to a reduction in the concentration of non-radiative recombination centers on the surface because of the etching and changes in doping due to inter-diffusion of Cd, S, Se and Zn after the deposition of CdS. Such an inter-diffusion depends on the elemental composition of the films defining the chemistry of defects and influencing Eg which increased in the film selenised at 500 °C but decreased in the other films. Processing increased the P1 shift rate (j-shift) with excitation power change in all the films demonstrating a higher compensation degree in the solar cells which is consistent with the formation of an interface layer containing new donors CdCu.},
    keywords = {copper zinc tin selenide, solar cells, photoluminescence, selenisation, optical spectroscopy, Physics, Materials Chemistry, Surfaces and Interfaces, Electronic, Optical and Magnetic Materials, Surfaces, Coatings and Films, Metals and Alloys},
    url = {https://strathprints.strath.ac.uk/66586/}
    }

  • Y. Gong, L. Jiu, J. Bruckbauer, J. Bai, R. W. Martin, and T. Wang, "Monolithic multiple colour emission from InGaN grown on patterned non-polar GaN," Scientific Reports, vol. 9, p. 986, 2019.
    [BibTeX] [Abstract] [Download PDF]

    A novel overgrowth approach has been developed in order to create a multiple-facet structure consisting of only non-polar and semi-polar GaN facets without involving any c-plane facets, allowing the major drawbacks of utilising c-plane GaN for the growth of III-nitride optoelectronics to be eliminated. Such a multiple-facet structure can be achieved by means of overgrowth on nonpolar GaN micro-rod arrays on r-plane sapphire. InGaN multiple quantum wells (MQWs) are then grown on the multiple-facet templates. Due to the different efficiencies of indium incorporation on non-polar and semi-polar GaN facets, multiple-colour InGaN/GaN MQWs have been obtained. Photoluminescence (PL) measurements have demonstrated that the multiple-colour emissions with a tunable intensity ratio of different wavelength emissions can be achieved simply through controlling the overgrowth conditions. Detailed cathodoluminescence measurements and excitation power dependent PL measurements have been performed, further validating the approach of employing the multiple facet templates for the growth of multiple colour InGaN/GaN MQWs. It is worth highlighting that the approach potentially paves the way for the growth of monolithic phosphor-free white emitters in the future.

    @Article{strathprints66339,
    author = {Y. Gong and L. Jiu and J. Bruckbauer and J. Bai and R.W. Martin and T. Wang},
    title = {Monolithic multiple colour emission from InGaN grown on patterned non-polar GaN},
    journal = {Scientific Reports},
    year = {2019},
    volume = {9},
    pages = {986},
    month = {December},
    abstract = {A novel overgrowth approach has been developed in order to create a multiple-facet structure consisting of only non-polar and semi-polar GaN facets without involving any c-plane facets, allowing the major drawbacks of utilising c-plane GaN for the growth of III-nitride optoelectronics to be eliminated. Such a multiple-facet structure can be achieved by means of overgrowth on nonpolar GaN micro-rod arrays on r-plane sapphire. InGaN multiple quantum wells (MQWs) are then grown on the multiple-facet templates. Due to the different efficiencies of indium incorporation on non-polar and semi-polar GaN facets, multiple-colour InGaN/GaN MQWs have been obtained. Photoluminescence (PL) measurements have demonstrated that the multiple-colour emissions with a tunable intensity ratio of different wavelength emissions can be achieved simply through controlling the overgrowth conditions. Detailed cathodoluminescence measurements and excitation power dependent PL measurements have been performed, further validating the approach of employing the multiple facet templates for the growth of multiple colour InGaN/GaN MQWs. It is worth highlighting that the approach potentially paves the way for the growth of monolithic phosphor-free white emitters in the future.},
    keywords = {non-polar GaN, semi-polar GaN, multiple facets, InGaN/GaN, photoluminescence, cathodoluminescence, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/66339/}
    }

  • M. V. Yakushev, M. A. Sulimov, J. Márquez-Prieto, I. Forbes, P. R. Edwards, V. D. Zhivulko, O. M. Borodavchenko, A. V. Mudryi, J. Krustok, and R. W. Martin, "A luminescence study of Cu₂ZnSnSe₄/Mo/glass films and solar cells with near stoichiometric copper content," Journal of Physics D: Applied Physics, vol. 36, p. 61208, 2019.
    [BibTeX] [Abstract] [Download PDF]

    Cu2ZnSnSe4 (CZTSe) is one of the leading candidates for the absorber layer in sustainable solar cells. Thin films of CZTSe with a near stoichiometric [Cu]/[Zn+Sn] were used to produce solar cells with conversion efficiency {\ensuremath{\eta}} = 6.4\% by a standard solar cell processing including KCN etching and the deposition of CdS and ZnO. Both CZTSe films and solar cells were examined using photoluminescence (PL) to analyse the nature of radiative recombination and photoluminescence excitation (PLE) at 4.2 K to determine the bandgap (Eg). Low temperature PL spectra of the films reveal an intense band P1 at 0.81 eV and a low intensity band P2 at 0.93 eV. Their temperature and excitation intensity dependencies suggest that they both involve recombinations of free electrons with holes localised at acceptors with the energy level influenced by potential fluctuations in the valence band . We associate P1 and P2 with different fractions of CZTSe: with a lower and higher degree of order of Cu and Zn on the cati on sub-lattice, respectively. Device processing reduced the intensity of P1 by 2.5 whereas the intensity of P2 increased by a 1.5. We assign this to a low temperature annealing due to CdS and ZnO deposition which increased the fraction of CZTSe with high d egree of Cu/Zn order and decreased the fraction with low degree of Cu/Zn order. Device processing increased Eg, blue shifted P1, decreased its width, j-shift and the mean depth of potential fluctuations. These can also be related to the annealing and/or KCN etching and the chemical effect of Cd, due to CdS replacing copper at the CdS - CZTSe interface layer. Processing induced a new broad band P3 at 1.3 eV ( quenching with Ea = 200 meV ) which we attributed to defects in the CdS layer.

    @Article{strathprints66083,
    author = {M. V. Yakushev and M.A. Sulimov and J. M{\'a}rquez-Prieto and I. Forbes and P.R. Edwards and V.D. Zhivulko and O.M. Borodavchenko and A. V. Mudryi and J. Krustok and R. W. Martin},
    title = {A luminescence study of Cu₂ZnSnSe₄/Mo/glass films and solar cells with near stoichiometric copper content},
    journal = {Journal of Physics D: Applied Physics},
    year = {2019},
    volume = {36},
    pages = {061208},
    month = {November},
    note = {This is an author-created, un-copyedited version of an article accepted for publication in Journal of Physics D: Applied Physics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at http://iopscience.iop.org/journal/0022-3727.},
    abstract = {Cu2ZnSnSe4 (CZTSe) is one of the leading candidates for the absorber layer in sustainable solar cells. Thin films of CZTSe with a near stoichiometric [Cu]/[Zn+Sn] were used to produce solar cells with conversion efficiency {\ensuremath{\eta}} = 6.4\% by a standard solar cell processing including KCN etching and the deposition of CdS and ZnO. Both CZTSe films and solar cells were examined using photoluminescence (PL) to analyse the nature of radiative recombination and photoluminescence excitation (PLE) at 4.2 K to determine the bandgap (Eg). Low temperature PL spectra of the films reveal an intense band P1 at 0.81 eV and a low intensity band P2 at 0.93 eV. Their temperature and excitation intensity dependencies suggest that they both involve recombinations of free electrons with holes localised at acceptors with the energy level influenced by potential fluctuations in the valence band . We associate P1 and P2 with different fractions of CZTSe: with a lower and higher degree of order of Cu and Zn on the cati on sub-lattice, respectively. Device processing reduced the intensity of P1 by 2.5 whereas the intensity of P2 increased by a 1.5. We assign this to a low temperature annealing due to CdS and ZnO deposition which increased the fraction of CZTSe with high d egree of Cu/Zn order and decreased the fraction with low degree of Cu/Zn order. Device processing increased Eg, blue shifted P1, decreased its width, j-shift and the mean depth of potential fluctuations. These can also be related to the annealing and/or KCN etching and the chemical effect of Cd, due to CdS replacing copper at the CdS - CZTSe interface layer. Processing induced a new broad band P3 at 1.3 eV ( quenching with Ea = 200 meV ) which we attributed to defects in the CdS layer.},
    keywords = {solar cells, photoluminescence, CZTSe, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/66083/}
    }

  • V. Brien, P. R. Edwards, P. Boulet, and K. P. O'Donnell, "Room temperature cathodoluminescence quenching of Er³⁺ in AlNOEr," Journal of Luminescence, vol. 205, p. 97–101, 2019.
    [BibTeX] [Abstract] [Download PDF]

    This paper reports a cathodoluminescence (CL) spectroscopic study of nanogranular AlNOErₓ samples with erbium content, x, in the range 0.5-3.6 atomic %. A wide range of erbium concentration was studied with the aim of understanding the concentration quenching of CL. The composition of thin films, deposited by radiofrequency reactive magnetron sputtering, was accurately determined by Energy Dispersive X-ray Spectroscopy (EDS). CL emission was investigated in the extended visible spectral range from 350 nm to 850 nm. The critical concentration of luminescent activator Er³⁺ above which CL quenching occurs is 1%; the corresponding critical distance between Er³⁺ ions in AlNOErx is about 1.0 nm. The quenching mechanism is discussed. We discount an exchange-mediated interaction in favour of a multipole-multipole phonon-assisted interaction.

    @Article{strathprints65386,
    author = {V. Brien and P.R. Edwards and P. Boulet and K.P. O'Donnell},
    title = {Room temperature cathodoluminescence quenching of {Er³⁺} in {AlNOEr}},
    journal = {Journal of Luminescence},
    year = {2019},
    volume = {205},
    pages = {97--101},
    month = {September},
    abstract = {This paper reports a cathodoluminescence (CL) spectroscopic study of nanogranular AlNOErₓ samples with erbium content, x, in the range 0.5-3.6 atomic %. A wide range of erbium concentration was studied with the aim of understanding the concentration quenching of CL. The composition of thin films, deposited by radiofrequency reactive magnetron sputtering, was accurately determined by Energy Dispersive X-ray Spectroscopy (EDS). CL emission was investigated in the extended visible spectral range from 350 nm to 850 nm. The critical concentration of luminescent activator Er³⁺ above which CL quenching occurs is 1%; the corresponding critical distance between Er³⁺ ions in AlNOErx is about 1.0 nm. The quenching mechanism is discussed. We discount an exchange-mediated interaction in favour of a multipole-multipole phonon-assisted interaction.},
    keywords = {aluminium nitrade, cathodoluminescence, rare earth, quenching, R. F. sputtering, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/65386/}
    }

  • E. Skidchenko, M. V. Yakushev, L. Spasevski, P. R. Edwards, M. A. Sulimov, and R. W. Martin, "Excitons in PL spectra of Cu(In,Ga)Se₂ single crystals," Physics of the Solid State, vol. 61, p. 918–924, 2019.
    [BibTeX] [Abstract] [Download PDF]

    A photoluminescence (PL) study of Cu(In,Ga)Se₂ (CIGSe) single crystals, (grown by the vertical Bridgman technique) with the [Ga]/[Ga + In] ratio of 7 and 12% and the [Cu]/[In + Ga] ratio greater than unity, as measured by energy dispersive spectroscopy, is presented. Analysis of the excitation intensity and temperature dependence of the PL spectra suggested the excitonic nature of the observed near-band-edge emissions peaks. Free and bound excitons in CIGSe single crystals with both 7 and 12% Ga content are clearly observed, analyzed and identified. An activation energy of 19 meV is determined for the free exciton in the PL spectra of the sample with 12% Ga. The presence of the excitons demonstrated a high structural quality of the material.

    @Article{Skidchenko2019PSS61,
    author = {Skidchenko, E. and Yakushev, M. V. and Spasevski, L. and Edwards, P. R. and Sulimov, M. A. and Martin, R. W.},
    title = {Excitons in {PL} spectra of {Cu(In,Ga)Se₂} single crystals},
    journal = {Physics of the Solid State},
    year = {2019},
    volume = {61},
    pages = {918--924},
    abstract = {A photoluminescence (PL) study of Cu(In,Ga)Se₂ (CIGSe) single crystals, (grown by the vertical Bridgman technique) with the [Ga]/[Ga + In] ratio of 7 and 12% and the [Cu]/[In + Ga] ratio greater than unity, as measured by energy dispersive spectroscopy, is presented. Analysis of the excitation intensity and temperature dependence of the PL spectra suggested the excitonic nature of the observed near-band-edge emissions peaks. Free and bound excitons in CIGSe single crystals with both 7 and 12% Ga content are clearly observed, analyzed and identified. An activation energy of 19 meV is determined for the free exciton in the PL spectra of the sample with 12% Ga. The presence of the excitons demonstrated a high structural quality of the material.},
    url = {https://dx.doi.org/10.1134/S1063783419050330}
    }

  • M. A. Sulimov, M. V. Yakushev, I. Forbes, J. M. Prieto, A. V. Mudryi, J. Krustok, P. R. Edwards, and R. W. Martin, "A PL and PLE study of high Cu content Cu₂ZnSnSe₄ films on Mo/glass and solar cells," Physics of the Solid State, vol. 61, p. 908–917, 2019.
    [BibTeX] [Abstract] [Download PDF]

    Cu₂ZnSnSe₄(CZTSe) is amongst leading candidates for the absorber layer in sustainable solar cells. We examine CZTSe thin films with [Cu]/[Zn + Sn] of 0.99 and [Zn]/[Sn] of 1.07, deposited on Mo/glass substrates, and solar cells fabricated from these films. The bandgap (Eg) of the as deposited films and solar cells was examined by photoluminescence excitation (PLE) whereas the temperature and excitation intensity dependence of photoluminescence (PL) spectra was used to examine the nature of radiative recombination. The 6 K PL spectra of CZTSe/Mo exhibit an intense broad and asymmetrical band P1 at 0.822 eV and a lower intensity band P2 at 0.93 eV. The shape of this band, high rates of blue shift with excitation intensity rise ( j-shift) j(P1) = 14 meV and j(P2) = 8 meV per decade, and red shifts of both bands with increasing temperature suggest that both bands are associated with valence band tails due to potential fluctuations caused by high populations of charged defects. The mean depth of such fluctuation γ of 24 meV was estimated from the low energy side of P1. Device processing increased Eg, blue shifted P1, decreased its width, j-shift and the mean depth of potential fluctuations. These can be due to the annealing and/or can partly be related to KCN etching and the chemical effect of Cd, from CdS replacing copper at the CdS–CZTSe interface layer. Processing induced a new broad band P3 at 1.3 eV (quenching with Ea = 200 meV). We attributed P3 to defects in the CdS layer.

    @Article{Sulimov2019PSS61,
    author = {M. A. Sulimov and M. V. Yakushev and I. Forbes and J. M. Prieto and A. V. Mudryi and Ju. Krustok and P. R. Edwards and R. W. Martin},
    title = {A {PL} and {PLE} study of high {Cu} content {Cu₂ZnSnSe₄} films on {Mo}/glass and solar cells},
    journal = {Physics of the Solid State},
    year = {2019},
    volume = {61},
    pages = {908--917},
    abstract = {Cu₂ZnSnSe₄(CZTSe) is amongst leading candidates for the absorber layer in sustainable solar cells. We examine CZTSe thin films with [Cu]/[Zn + Sn] of 0.99 and [Zn]/[Sn] of 1.07, deposited on Mo/glass substrates, and solar cells fabricated from these films. The bandgap (Eg) of the as deposited films and solar cells was examined by photoluminescence excitation (PLE) whereas the temperature and excitation intensity dependence of photoluminescence (PL) spectra was used to examine the nature of radiative recombination. The 6 K PL spectra of CZTSe/Mo exhibit an intense broad and asymmetrical band P1 at 0.822 eV and a lower intensity band P2 at 0.93 eV. The shape of this band, high rates of blue shift with excitation intensity rise ( j-shift) j(P1) = 14 meV and j(P2) = 8 meV per decade, and red shifts of both bands with increasing temperature suggest that both bands are associated with valence band tails due to potential fluctuations caused by high populations of charged defects. The mean depth of such fluctuation γ of 24 meV was estimated from the low energy side of P1. Device processing increased Eg, blue shifted P1, decreased its width, j-shift and the mean depth of potential fluctuations. These can be due to the annealing and/or can partly be related to KCN etching and the chemical effect of Cd, from CdS replacing copper at the CdS–CZTSe interface layer. Processing induced a new broad band P3 at 1.3 eV (quenching with Ea = 200 meV). We attributed P3 to defects in the CdS layer.},
    url = {https://dx.doi.org/10.1134/S1063783419050214}
    }

  • Máté. Jenei, E. Potanina, R. Zhao, K. Y. Tan, A. Rossi, T. Tanttu, K. W. Chan, V. Sevriuk, M. Möttönen, and A. Dzurak, "Waiting time distributions in a two-level fluctuator coupled to a superconducting charge detector," Physical Review Research, vol. 1, iss. 3, 2019. doi:10.1103/PhysRevResearch.1.033163
    [BibTeX] [Abstract] [Download PDF]

    We analyze charge fluctuations in a parasitic state strongly coupled to a superconducting Josephson-junction-based charge detector. The charge dynamics of the state resembles that of electron transport in a quantum dot with two charge states, and hence we refer to it as a two-level fluctuator. By constructing the distribution of waiting times from the measured detector signal and comparing it with a waiting time theory, we extract the electron in- and out-tunneling rates for the two-level fluctuator, which are severely asymmetric.

    @article{strathprints73049,
    volume = {1},
    number = {3},
    month = {December},
    title = {Waiting time distributions in a two-level fluctuator coupled to a superconducting charge detector},
    year = {2019},
    doi = {10.1103/PhysRevResearch.1.033163},
    journal = {Physical Review Research},
    keywords = {charge fluctuations, superconducting Josephson-junction-based charge detector, quantum physics, Physics, Condensed Matter Physics},
    url = {https://doi.org/10.1103/PhysRevResearch.1.033163},
    issn = {2643-1564},
    abstract = {We analyze charge fluctuations in a parasitic state strongly coupled to a superconducting Josephson-junction-based charge detector. The charge dynamics of the state resembles that of electron transport in a quantum dot with two charge states, and hence we refer to it as a two-level fluctuator. By constructing the distribution of waiting times from the measured detector signal and comparing it with a waiting time theory, we extract the electron in- and out-tunneling rates for the two-level fluctuator, which are severely asymmetric.},
    author = {Jenei, M{\'a}t{\'e} and Potanina, Elina and Zhao, Ruichen and Tan, Kuan Y. and Rossi, Alessandro and Tanttu, Tuomo and Chan, Kok W. and Sevriuk, Vasilii and M{\"o}tt{\"o}nen, Mikko and Dzurak, Andrew}
    }

  • J. Denholm and S. Redner, "Topology-controlled Potts coarsening," Physical Review E, vol. 99, p. 62142, 2019.
    [BibTeX] [Abstract] [Download PDF]

    We uncover unusual topological features in the long-time relaxation of the q-state kinetic Potts ferromagnet on the triangular lattice that is instantaneously quenched to zero temperature from a zero-magnetization initial state. For q=3, the final state is either the ground state (frequency ≈0.75), a frozen three-hexagon state (frequency ≈0.16), a two-stripe state (frequency ≈0.09), or a three-stripe state (frequency <2×10⁻⁴). Other final state topologies, such as states with more than three hexagons, occur with probability 10⁻⁵ or smaller, for q=3. The relaxation to the frozen three-hexagon state is governed by a time that scales as L² ln L. We provide a heuristic argument for this anomalous scaling and present additional new features of Potts coarsening on the triangular lattice for q=3 and for q>3.

    @Article{Denholm2019PRE99,
    author = {J. Denholm and S. Redner},
    title = {Topology-controlled Potts coarsening},
    journal = {Physical Review E},
    year = {2019},
    volume = {99},
    pages = {062142},
    abstract = {We uncover unusual topological features in the long-time relaxation of the q-state kinetic Potts ferromagnet on the triangular lattice that is instantaneously quenched to zero temperature from a zero-magnetization initial state. For q=3, the final state is either the ground state (frequency ≈0.75), a frozen three-hexagon state (frequency ≈0.16), a two-stripe state (frequency ≈0.09), or a three-stripe state (frequency <2×10⁻⁴). Other final state topologies, such as states with more than three hexagons, occur with probability 10⁻⁵ or smaller, for q=3. The relaxation to the frozen three-hexagon state is governed by a time that scales as L² ln L. We provide a heuristic argument for this anomalous scaling and present additional new features of Potts coarsening on the triangular lattice for q=3 and for q>3.},
    url = {https://doi.org/10.1103/PhysRevE.99.062142}
    }

  • S. P. Giblin, A. Fujiwara, G. Yamahata, M. Bae, N. Kim, A. Rossi, M. Möttönen, and M. Kataoka, "Evidence for universality of tunable-barrier electron pumps," Metrologia, vol. 56, iss. 4, 2019. doi:10.1088/1681-7575/ab29a5
    [BibTeX] [Abstract] [Download PDF]

    We review recent precision measurements on semiconductor tunable-barrier electron pumps operating in a ratchet mode. Seven studies on five different designs of pumps have reported measurements of the pump current with relative total uncertainties around 10-6 or less. Combined with theoretical models of electron capture by the pumps, these experimental data exhibits encouraging evidence that the pumps operate according to a universal mechanism, independent of the details of device design. Evidence for robustness of the pump current against changes in the control parameters is at a more preliminary stage, but also encouraging, with two studies reporting robustness of the pump current against three or more parameters in the range of {$\sim$}5 {$\times$} 10-7 to {$\sim$}2 {$\times$} 10-6. This review highlights the need for an agreed protocol for tuning the electron pump for optimal operation, as well as more rigorous evaluations of the robustness in a wide range of pump designs.

    @article{strathprints69532,
    volume = {56},
    number = {4},
    month = {July},
    title = {Evidence for universality of tunable-barrier electron pumps},
    year = {2019},
    doi = {10.1088/1681-7575/ab29a5},
    journal = {Metrologia},
    keywords = {current measurement, current standards, electron pumps, primary electrical metrology, Physics, Engineering(all)},
    url = {https://doi.org/10.1088/1681-7575/ab29a5},
    issn = {1681-7575},
    abstract = {We review recent precision measurements on semiconductor tunable-barrier electron pumps operating in a ratchet mode. Seven studies on five different designs of pumps have reported measurements of the pump current with relative total uncertainties around 10-6 or less. Combined with theoretical models of electron capture by the pumps, these experimental data exhibits encouraging evidence that the pumps operate according to a universal mechanism, independent of the details of device design. Evidence for robustness of the pump current against changes in the control parameters is at a more preliminary stage, but also encouraging, with two studies reporting robustness of the pump current against three or more parameters in the range of {$\sim$}5 {$\times$} 10-7 to {$\sim$}2 {$\times$} 10-6. This review highlights the need for an agreed protocol for tuning the electron pump for optimal operation, as well as more rigorous evaluations of the robustness in a wide range of pump designs.},
    author = {Giblin, Stephen P. and Fujiwara, Akira and Yamahata, Gento and Bae, Myung-Ho and Kim, Nam and Rossi, Alessandro and M{\"o}tt{\"o}nen, Mikko and Kataoka, Masaya}
    }

  • J. Moloney, O. Tesh, M. Singh, J. W. Roberts, J. C. Jarman, L. C. Lee, T. N. Huq, J. Brister, S. Karboyan, M. Kuball, P. R. Chalker, R. A. Oliver, and F. C-P. Massabuau, "Atomic layer deposited \ensuremath\alpha-Ga2O3 solar-blind photodetectors," Journal of Physics D: Applied Physics, vol. 52, iss. 47, 2019. doi:10.1088/1361-6463/ab3b76
    [BibTeX] [Abstract] [Download PDF]

    Low temperature atomic layer deposition was used to deposit {\ensuremath{\alpha}}-Ga2O3 films, which were subsequently annealed at various temperatures and atmospheres. The {\ensuremath{\alpha}}-Ga2O3 phase is stable up to 400 oC, which is also the temperature that yields the most intense and sharpest reflection by x-ray diffraction. Upon annealing at 450 oC and above, the material gradually turns into the more thermodynamically stable {\ensuremath{\epsilon}} or {\ensuremath{\beta}} phase. The suitability of the materials for solar-blind photodetector applications has been demonstrated with the best responsivity achieved being 1.2 A W?1 under 240 nm illumination and 10 V bias, for the sample annealed at 400 oC in argon. It is worth noting however that the device performance strongly depends on the annealing conditions, with the device annealed in forming gas behaving poorly. Given that the tested devices have similar microstructure, the discrepancies in device performance are attributed to hydrogen impurities.

    @article{strathprints69890,
    volume = {52},
    number = {47},
    month = {September},
    title = {Atomic layer deposited {\ensuremath{\alpha}}-Ga2O3 solar-blind photodetectors},
    year = {2019},
    doi = {10.1088/1361-6463/ab3b76},
    journal = {Journal of Physics D: Applied Physics},
    keywords = {gallium oxide, ultraviolet, photodetector, atomic layer deposition, anneal, Physics, Physics and Astronomy(all)},
    url = {https://doi.org/10.1088/1361-6463/ab3b76},
    issn = {0022-3727},
    abstract = {Low temperature atomic layer deposition was used to deposit {\ensuremath{\alpha}}-Ga2O3 films, which were subsequently annealed at various temperatures and atmospheres. The {\ensuremath{\alpha}}-Ga2O3 phase is stable up to 400 oC, which is also the temperature that yields the most intense and sharpest reflection by x-ray diffraction. Upon annealing at 450 oC and above, the material gradually turns into the more thermodynamically stable {\ensuremath{\epsilon}} or {\ensuremath{\beta}} phase. The suitability of the materials for solar-blind photodetector applications has been demonstrated with the best responsivity achieved being 1.2 A W?1 under 240 nm illumination and 10 V bias, for the sample annealed at 400 oC in argon. It is worth noting however that the device performance strongly depends on the annealing conditions, with the device annealed in forming gas behaving poorly. Given that the tested devices have similar microstructure, the discrepancies in device performance are attributed to hydrogen impurities.},
    author = {Moloney, J and Tesh, O and Singh, M and Roberts, J W and Jarman, J C and Lee, L C and Huq, T N and Brister, J and Karboyan, S and Kuball, M and Chalker, P R and Oliver, R A and Massabuau, F C-P}
    }

  • S. Schaal, A. Rossi, V. N. Ciriano-Tejel, T. Yang, S. Barraud, J. J. L. Morton, and F. M. Gonzalez-Zalba, "A CMOS dynamic random access architecture for radio-frequency readout of quantum devices," Nature Electronics, vol. 2, iss. 6, p. 236–242, 2019. doi:10.1038/s41928-019-0259-5
    [BibTeX] [Abstract] [Download PDF]

    As quantum processors become more complex, they will require efficient interfaces to deliver signals for control and readout while keeping the number of inputs manageable. Complementary metal-oxide-semiconductor (CMOS) electronics offers established solutions to signal routing and dynamic access, and the use of a CMOS platform for the qubits themselves offers the attractive proposition of integrating classical and quantum devices on-chip. Here, we report a CMOS dynamic random access architecture for readout of multiple quantum devices operating at millikelvin temperatures. Our circuit is divided into cells, each containing a control field-effect transistor and a quantum dot device, formed in the channel of a nanowire transistor. This set-up allows selective readout of the quantum dot and charge storage on the quantum dot gate, similar to one-transistor-one-capacitor (1T-1C) dynamic random access technology. We demonstrate dynamic readout of two cells by interfacing them with a single radio-frequency resonator. Our approach provides a path to reduce the number of input lines per qubit and allow large-scale device arrays to be addressed.

    @article{strathprints68781,
    volume = {2},
    number = {6},
    month = {June},
    title = {A CMOS dynamic random access architecture for radio-frequency readout of quantum devices},
    journal = {Nature Electronics},
    doi = {10.1038/s41928-019-0259-5},
    pages = {236--242},
    year = {2019},
    keywords = {quantum processors, CMOS, complementary metal oxide semiconductor (CMOS), CMOS platform, Electrical engineering. Electronics Nuclear engineering, Physics, Electronic, Optical and Magnetic Materials, Instrumentation, Electrical and Electronic Engineering},
    url = {https://doi.org/10.1038/s41928-019-0259-5},
    issn = {2520-1131},
    abstract = {As quantum processors become more complex, they will require efficient interfaces to deliver signals for control and readout while keeping the number of inputs manageable. Complementary metal-oxide-semiconductor (CMOS) electronics offers established solutions to signal routing and dynamic access, and the use of a CMOS platform for the qubits themselves offers the attractive proposition of integrating classical and quantum devices on-chip. Here, we report a CMOS dynamic random access architecture for readout of multiple quantum devices operating at millikelvin temperatures. Our circuit is divided into cells, each containing a control field-effect transistor and a quantum dot device, formed in the channel of a nanowire transistor. This set-up allows selective readout of the quantum dot and charge storage on the quantum dot gate, similar to one-transistor-one-capacitor (1T-1C) dynamic random access technology. We demonstrate dynamic readout of two cells by interfacing them with a single radio-frequency resonator. Our approach provides a path to reduce the number of input lines per qubit and allow large-scale device arrays to be addressed.},
    author = {Schaal, Simon and Rossi, Alessandro and Ciriano-Tejel, Virginia N. and Yang, Tsung-Yeh and Barraud, Sylvain and Morton, John J. L. and Gonzalez-Zalba, M. Fernando}
    }

  • A. West, B. Hensen, A. Jouan, T. Tanttu, C. Yang, A. Rossi, F. M. Gonzalez-Zalba, F. Hudson, A. Morello, D. J. Reilly, and A. S. Dzurak, "Gate-based single-shot readout of spins in silicon," Nature Nanotechnology, vol. 14, iss. 5, p. 437–443, 2019. doi:10.1038/s41565-019-0400-7
    [BibTeX] [Abstract] [Download PDF]

    Electron spins in silicon quantum dots provide a promising route towards realizing the large number of coupled qubits required for a useful quantum processor 1-7 . For the implementation of quantum algorithms and error detection 8-10 , qubit measurements are ideally performed in a single shot, which is presently achieved using on-chip charge sensors, capacitively coupled to the quantum dots 11 . However, as the number of qubits is increased, this approach becomes impractical due to the footprint and complexity of the charge sensors, combined with the required proximity to the quantum dots 12 . Alternatively, the spin state can be measured directly by detecting the complex impedance of spin-dependent electron tunnelling between quantum dots 13-15 . This can be achieved using radiofrequency reflectometry on a single gate electrode defining the quantum dot itself 15-19 , significantly reducing the gate count and architectural complexity, but thus far it has not been possible to achieve single-shot spin readout using this technique. Here, we detect single electron tunnelling in a double quantum dot and demonstrate that gate-based sensing can be used to read out the electron spin state in a single shot, with an average readout fidelity of 73\%. The result demonstrates a key step towards the readout of many spin qubits in parallel, using a compact gate design that will be needed for a large-scale semiconductor quantum processor.

    @article{strathprints68703,
    volume = {14},
    number = {5},
    month = {May},
    title = {Gate-based single-shot readout of spins in silicon},
    journal = {Nature Nanotechnology},
    doi = {10.1038/s41565-019-0400-7},
    pages = {437--443},
    year = {2019},
    keywords = {nanoscale devices, quantum processing, quantum dots, Physics, Atomic and Molecular Physics, and Optics, Condensed Matter Physics, Electrical and Electronic Engineering},
    url = {https://doi.org/10.1038/s41565-019-0400-7},
    issn = {1748-3387},
    abstract = {Electron spins in silicon quantum dots provide a promising route towards realizing the large number of coupled qubits required for a useful quantum processor 1-7 . For the implementation of quantum algorithms and error detection 8-10 , qubit measurements are ideally performed in a single shot, which is presently achieved using on-chip charge sensors, capacitively coupled to the quantum dots 11 . However, as the number of qubits is increased, this approach becomes impractical due to the footprint and complexity of the charge sensors, combined with the required proximity to the quantum dots 12 . Alternatively, the spin state can be measured directly by detecting the complex impedance of spin-dependent electron tunnelling between quantum dots 13-15 . This can be achieved using radiofrequency reflectometry on a single gate electrode defining the quantum dot itself 15-19 , significantly reducing the gate count and architectural complexity, but thus far it has not been possible to achieve single-shot spin readout using this technique. Here, we detect single electron tunnelling in a double quantum dot and demonstrate that gate-based sensing can be used to read out the electron spin state in a single shot, with an average readout fidelity of 73\%. The result demonstrates a key step towards the readout of many spin qubits in parallel, using a compact gate design that will be needed for a large-scale semiconductor quantum processor.},
    author = {West, Anderson and Hensen, Bas and Jouan, Alexis and Tanttu, Tuomo and Yang, Chih-Hwan and Rossi, Alessandro and Gonzalez-Zalba, M. Fernando and Hudson, Fay and Morello, Andrea and Reilly, David J. and Dzurak, Andrew S.}
    }

  • R. Wang, D. G. Purdie, Y. Fan, F. C. -P. Massabuau, P. Braeuninger-Weimer, O. J. Burton, R. Blume, R. Schloegl, A. Lombardo, R. S. Weatherup, and S. Hofmann, "A peeling approach for integrated manufacturing of large monolayer h-BN crystals," ACS Nano, vol. 13, iss. 2, p. 2114–2126, 2019. doi:10.1021/acsnano.8b08712
    [BibTeX] [Abstract] [Download PDF]

    Hexagonal boron nitride (h-BN) is the only known material aside from graphite with a structure composed of simple, stable, noncorrugated atomically thin layers. While historically used as a lubricant in powder form, h-BN layers have become particularly attractive as an ultimately thin insulator, barrier, or encapsulant. Practically all emerging electronic and photonic device concepts currently rely on h-BN exfoliated from small bulk crystallites, which limits device dimensions and process scalability. We here focus on a systematic understanding of Pt-catalyzed h-BN crystal formation, in order to address this integration challenge for monolayer h-BN via an integrated chemical vapor deposition (CVD) process that enables h-BN crystal domain sizes exceeding 0.5 mm and a merged, continuous layer in a growth time of less than 45 min. The process makes use of commercial, reusable Pt foils and allows a delamination process for easy and clean h-BN layer transfer. We demonstrate sequential pick-up for the assembly of graphene/h-BN heterostructures with atomic layer precision, while minimizing interfacial contamination. The approach can be readily combined with other layered materials and enables the integration of CVD h-BN into high-quality, reliable 2D material device layer stacks.

    @article{strathprints69908,
    volume = {13},
    number = {2},
    month = {February},
    title = {A peeling approach for integrated manufacturing of large monolayer h-BN crystals},
    journal = {ACS Nano},
    doi = {10.1021/acsnano.8b08712},
    pages = {2114--2126},
    year = {2019},
    keywords = {h-BN, 2D materials, CVD, transfer, catalyst, heterostructures, graphene, platinum, Chemistry, Physics, Chemistry(all), Physics and Astronomy(all)},
    url = {https://doi.org/10.1021/acsnano.8b08712},
    issn = {1936-0851},
    abstract = {Hexagonal boron nitride (h-BN) is the only known material aside from graphite with a structure composed of simple, stable, noncorrugated atomically thin layers. While historically used as a lubricant in powder form, h-BN layers have become particularly attractive as an ultimately thin insulator, barrier, or encapsulant. Practically all emerging electronic and photonic device concepts currently rely on h-BN exfoliated from small bulk crystallites, which limits device dimensions and process scalability. We here focus on a systematic understanding of Pt-catalyzed h-BN crystal formation, in order to address this integration challenge for monolayer h-BN via an integrated chemical vapor deposition (CVD) process that enables h-BN crystal domain sizes exceeding 0.5 mm and a merged, continuous layer in a growth time of less than 45 min. The process makes use of commercial, reusable Pt foils and allows a delamination process for easy and clean h-BN layer transfer. We demonstrate sequential pick-up for the assembly of graphene/h-BN heterostructures with atomic layer precision, while minimizing interfacial contamination. The approach can be readily combined with other layered materials and enables the integration of CVD h-BN into high-quality, reliable 2D material device layer stacks.},
    author = {Wang, Ruizhi and Purdie, David G. and Fan, Ye and Massabuau, Fabien C.-P. and Braeuninger-Weimer, Philipp and Burton, Oliver J. and Blume, Raoul and Schloegl, Robert and Lombardo, Antonio and Weatherup, Robert S. and Hofmann, Stephan}
    }

  • S. Mandal, C. Yuan, F. Massabuau, J. W. Pomeroy, J. Cuenca, H. Bland, E. Thomas, D. Wallis, T. Batten, D. Morgan, R. Oliver, M. Kuball, and O. A. Williams, "Thick adherent diamond films on AlN with low thermal barrier resistance," ACS Applied Materials and Interfaces, vol. 11, iss. 43, p. 40826–40834, 2019. doi:10.1021/acsami.9b13869
    [BibTeX] [Abstract] [Download PDF]

    The growth of {\ensuremath{>}}100-{\ensuremath{\mu}}m-thick diamond layers adherent on aluminum nitride with low thermal boundary resistance between diamond and AlN is presented in this work. The thermal barrier resistance was found to be in the range of 16 m 2.K/GW, which is a large improvement on the current state-of-the-art. While thick films failed to adhere on untreated AlN films, AlN films treated with hydrogen/nitrogen plasma retained the thick diamond layers. Clear differences in {\ensuremath{\zeta}}-potential measurement confirm surface modification due to hydrogen/nitrogen plasma treatment. An increase in non-diamond carbon in the initial layers of diamond grown on pretreated AlN is seen by Raman spectroscopy. The presence of non-diamond carbon has minimal effect on the thermal barrier resistance. The surfaces studied with X-ray photoelectron spectroscopy revealed a clear distinction between pretreated and untreated samples. The surface aluminum goes from a nitrogen-rich environment to an oxygen-rich environment after pretreatment. A clean interface between diamond and AlN is seen by cross-sectional transmission electron microscopy.

    @article{strathprints70192,
    volume = {11},
    number = {43},
    month = {October},
    title = {Thick adherent diamond films on AlN with low thermal barrier resistance},
    journal = {ACS Applied Materials and Interfaces},
    doi = {10.1021/acsami.9b13869},
    pages = {40826--40834},
    year = {2019},
    keywords = {diamond, aluminium nitride, thermal barrier resistance, diamond seeding, diamond growth, Physics, Materials Science(all), Surfaces and Interfaces},
    url = {https://doi.org/10.1021/acsami.9b13869},
    issn = {1944-8244},
    abstract = {The growth of {\ensuremath{>}}100-{\ensuremath{\mu}}m-thick diamond layers adherent on aluminum nitride with low thermal boundary resistance between diamond and AlN is presented in this work. The thermal barrier resistance was found to be in the range of 16 m 2.K/GW, which is a large improvement on the current state-of-the-art. While thick films failed to adhere on untreated AlN films, AlN films treated with hydrogen/nitrogen plasma retained the thick diamond layers. Clear differences in {\ensuremath{\zeta}}-potential measurement confirm surface modification due to hydrogen/nitrogen plasma treatment. An increase in non-diamond carbon in the initial layers of diamond grown on pretreated AlN is seen by Raman spectroscopy. The presence of non-diamond carbon has minimal effect on the thermal barrier resistance. The surfaces studied with X-ray photoelectron spectroscopy revealed a clear distinction between pretreated and untreated samples. The surface aluminum goes from a nitrogen-rich environment to an oxygen-rich environment after pretreatment. A clean interface between diamond and AlN is seen by cross-sectional transmission electron microscopy.},
    author = {Mandal, Soumen and Yuan, Chao and Massabuau, Fabien and Pomeroy, James W. and Cuenca, Jerome and Bland, Henry and Thomas, Evan and Wallis, David and Batten, Tim and Morgan, David and Oliver, Rachel and Kuball, Martin and Williams, Oliver A.}
    }

  • J. W. Roberts, P. R. Chalker, B. Ding, R. A. Oliver, J. T. Gibbon, L. A. H. Jones, V. R. Dhanak, L. J. Phillips, J. D. Major, and F. C. -P. Massabuau, "Low temperature growth and optical properties of \ensuremath\alpha-Ga2O3 deposited on sapphire by plasma enhanced atomic layer deposition," Journal of Crystal Growth, vol. 528, 2019. doi:10.1016/j.jcrysgro.2019.125254
    [BibTeX] [Abstract] [Download PDF]

    Plasma enhanced atomic layer deposition was used to deposit thin films of Ga2O3 on to c-plane sapphire substrates using triethylgallium and O2 plasma. The influence of substrate temperature and plasma processing parameters on the resultant crystallinity and optical properties of the Ga2O3 films were investigated. The deposition temperature was found to have a significant effect on the film crystallinity. At temperatures below 200oC amorphous Ga2O3 films were deposited. Between 250oC and 350oC the films became predominantly {\ensuremath{\alpha}}-Ga2O3. Above 350oC the deposited films showed a mixture of {\ensuremath{\alpha}}-Ga2O3 and {\ensuremath{\epsilon}}-Ga2O3 phases. Plasma power and O2 flow rate were observed to have less influence over the resultant phases present in the films. However, both parameters could be tuned to alter the strain of the film. Ultraviolet transmittance measurements on the Ga2O3 films showed that the bandgaps ranges from 5.0 eV to 5.2 eV with the largest bandgap of 5.2 eV occurring for the {\ensuremath{\alpha}}-Ga2O3 phase deposited at 250oC.

    @article{strathprints70036,
    volume = {528},
    month = {December},
    title = {Low temperature growth and optical properties of {\ensuremath{\alpha}}-Ga2O3 deposited on sapphire by plasma enhanced atomic layer deposition},
    year = {2019},
    doi = {10.1016/j.jcrysgro.2019.125254},
    journal = {Journal of Crystal Growth},
    keywords = {characterization, crystal structure, crystal morphology, x-ray diffraction, atomic layer epitaxy, gallium compounds, Physics, Materials Science(all)},
    url = {https://doi.org/10.1016/j.jcrysgro.2019.125254},
    issn = {0022-0248},
    abstract = {Plasma enhanced atomic layer deposition was used to deposit thin films of Ga2O3 on to c-plane sapphire substrates using triethylgallium and O2 plasma. The influence of substrate temperature and plasma processing parameters on the resultant crystallinity and optical properties of the Ga2O3 films were investigated. The deposition temperature was found to have a significant effect on the film crystallinity. At temperatures below 200oC amorphous Ga2O3 films were deposited. Between 250oC and 350oC the films became predominantly {\ensuremath{\alpha}}-Ga2O3. Above 350oC the deposited films showed a mixture of {\ensuremath{\alpha}}-Ga2O3 and {\ensuremath{\epsilon}}-Ga2O3 phases. Plasma power and O2 flow rate were observed to have less influence over the resultant phases present in the films. However, both parameters could be tuned to alter the strain of the film. Ultraviolet transmittance measurements on the Ga2O3 films showed that the bandgaps ranges from 5.0 eV to 5.2 eV with the largest bandgap of 5.2 eV occurring for the {\ensuremath{\alpha}}-Ga2O3 phase deposited at 250oC.},
    author = {Roberts, J. W. and Chalker, P. R. and Ding, B. and Oliver, R. A. and Gibbon, J. T. and Jones, L. A. H. and Dhanak, V. R. and Phillips, L. J. and Major, J. D. and Massabuau, F. C.-P.}
    }

2018

  • P. M. Coulon, G. Kusch, R. W. Martin, and P. A. Shields, "Deep UV emission from highly ordered AlGaN/AlN core-shell nanorods," ACS Applied Materials and Interfaces, vol. 10, iss. 39, p. 33441–33449, 2018.
    [BibTeX] [Abstract] [Download PDF]

    Three-dimensional core-shell nanostructures could resolve key problems existing in conventional planar deep UV light-emitting diode (LED) technology due to their high structural quality, high-quality nonpolar growth leading to a reduced quantum-confined Stark effect and their ability to improve light extraction. Currently, a major hurdle to their implementation in UV LEDs is the difficulty of growing such nanostructures from AlxGa1-xN materials with a bottom-up approach. In this paper, we report the successful fabrication of an AlN/AlxGa1-xN/AlN core-shell structure using an original hybrid top-down/bottom-up approach, thus representing a breakthrough in applying core-shell architecture to deep UV emission. Various AlN/AlxGa1-xN/AlN core-shell structures were grown on optimized AlN nanorod arrays. These were created using displacement Talbot lithography (DTL), a two-step dry-wet etching process, and optimized AlN metal organic vapor phase epitaxy regrowth conditions to achieve the facet recovery of straight and smooth AlN nonpolar facets, a necessary requirement for subsequent growth. Cathodoluminescence hyperspectral imaging of the emission characteristics revealed that 229 nm deep UV emission was achieved from the highly uniform array of core-shell AlN/AlxGa1-xN/AlN structures, which represents the shortest wavelength achieved so far with a core-shell architecture. This hybrid top-down/bottom-up approach represents a major advance for the fabrication of deep UV LEDs based on core-shell nanostructures.

    @article{strathprints67890,
    volume = {10},
    number = {39},
    month = {September},
    author = {Pierre Marie Coulon and Gunnar Kusch and Robert W. Martin and Philip A. Shields},
    title = {Deep UV emission from highly ordered AlGaN/AlN core-shell nanorods},
    journal = {ACS Applied Materials and Interfaces},
    pages = {33441--33449},
    year = {2018},
    keywords = {AlGaN, AlN, cathodoluminescence, core-shell, EDX, nanorod, TEM, Manufactures, Physics, Materials Science(all), Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/67890/},
    abstract = {Three-dimensional core-shell nanostructures could resolve key problems existing in conventional planar deep UV light-emitting diode (LED) technology due to their high structural quality, high-quality nonpolar growth leading to a reduced quantum-confined Stark effect and their ability to improve light extraction. Currently, a major hurdle to their implementation in UV LEDs is the difficulty of growing such nanostructures from AlxGa1-xN materials with a bottom-up approach. In this paper, we report the successful fabrication of an AlN/AlxGa1-xN/AlN core-shell structure using an original hybrid top-down/bottom-up approach, thus representing a breakthrough in applying core-shell architecture to deep UV emission. Various AlN/AlxGa1-xN/AlN core-shell structures were grown on optimized AlN nanorod arrays. These were created using displacement Talbot lithography (DTL), a two-step dry-wet etching process, and optimized AlN metal organic vapor phase epitaxy regrowth conditions to achieve the facet recovery of straight and smooth AlN nonpolar facets, a necessary requirement for subsequent growth. Cathodoluminescence hyperspectral imaging of the emission characteristics revealed that 229 nm deep UV emission was achieved from the highly uniform array of core-shell AlN/AlxGa1-xN/AlN structures, which represents the shortest wavelength achieved so far with a core-shell architecture. This hybrid top-down/bottom-up approach represents a major advance for the fabrication of deep UV LEDs based on core-shell nanostructures.}
    }

  • M. V. Yakushev, M. A. Sulimov, E. Skidchenko, J. Márquez-Prieto, I. Forbes, P. R. Edwards, M. V. Kuznetsov, V. D. Zhivulko, O. M. Borodavchenko, A. V. Mudryi, J. Krustok, and R. W. Martin, "Effects of Ar⁺ etching of Cu₂ZnSnSe₄ thin films : an x-ray photoelectron spectroscopy and photoluminescence study," Journal of Vacuum Science and Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena, vol. 36, p. 61208, 2018.
    [BibTeX] [Abstract] [Download PDF]

    Cu2ZnSnSe4 (CZTSe) is a semiconductor used as the absorber layer in highly promising sustainable thin film solar cells. The authors study the effect of Ar+ etching of copper deficient and zinc excess CZTSe thin films deposited on Mo/glass substrates on the surface elemental composition, measured by x-ray photoelectron spectroscopy, and photoluminescence (PL) spectra. Low temperature PL spectra reveal a broad asymmetrical band at 0.95 eV. The temperature and excitation intensity dependencies of this band suggest that it is a free-to-bound (FB) recombination of electrons from the conduction band with holes localized at an acceptor affected by potential fluctuations. The surface composition of the as grown films demonstrates a strong copper deficiency: [Cu]/[Zn + Sn] = 0.33. The etching of the film surface using Ar+ beam increases [Cu]/[Zn + Sn] to 0.51, which is significantly smaller than that of 0.78 in the bulk, measured by wavelength dispersive x-ray analysis, demonstrating the presence on the surface of a copper-depleted layer. The Ar+ etching drastically reduces the FB band intensity by a factor of 4.5, broadens it and develops a low energy tail. Ar ions displace atoms in CZTSe lattice creating primary radiation defects, vacancies, and interstitials, which recombine at room temperature forming antisite defects with deep energy levels. Some of them generate the observed low energy tail and increase the mean depth of potential fluctuation {\ensuremath{\gamma}}, determined from the shape of the low energy side of FB band, from 24 meV before Ar+ etching to 35 meV after. Other deep defects work as nonradiative recombination centers reducing the intensity of the FB band.

    @Article{strathprints66330,
    author = {Michael V. Yakushev and Mikhail A. Sulimov and Ekaterina Skidchenko and Jose M{\'a}rquez-Prieto and Ian Forbes and Paul R. Edwards and Mikhail V. Kuznetsov and Vadim D. Zhivulko and Olga M. Borodavchenko and Alexander V. Mudryi and Juri Krustok and Robert W. Martin},
    title = {Effects of Ar⁺ etching of Cu₂ZnSnSe₄ thin films : an x-ray photoelectron spectroscopy and photoluminescence study},
    journal = {Journal of Vacuum Science and Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena},
    year = {2018},
    volume = {36},
    pages = {061208},
    month = {November},
    abstract = {Cu2ZnSnSe4 (CZTSe) is a semiconductor used as the absorber layer in highly promising sustainable thin film solar cells. The authors study the effect of Ar+ etching of copper deficient and zinc excess CZTSe thin films deposited on Mo/glass substrates on the surface elemental composition, measured by x-ray photoelectron spectroscopy, and photoluminescence (PL) spectra. Low temperature PL spectra reveal a broad asymmetrical band at 0.95 eV. The temperature and excitation intensity dependencies of this band suggest that it is a free-to-bound (FB) recombination of electrons from the conduction band with holes localized at an acceptor affected by potential fluctuations. The surface composition of the as grown films demonstrates a strong copper deficiency: [Cu]/[Zn + Sn] = 0.33. The etching of the film surface using Ar+ beam increases [Cu]/[Zn + Sn] to 0.51, which is significantly smaller than that of 0.78 in the bulk, measured by wavelength dispersive x-ray analysis, demonstrating the presence on the surface of a copper-depleted layer. The Ar+ etching drastically reduces the FB band intensity by a factor of 4.5, broadens it and develops a low energy tail. Ar ions displace atoms in CZTSe lattice creating primary radiation defects, vacancies, and interstitials, which recombine at room temperature forming antisite defects with deep energy levels. Some of them generate the observed low energy tail and increase the mean depth of potential fluctuation {\ensuremath{\gamma}}, determined from the shape of the low energy side of FB band, from 24 meV before Ar+ etching to 35 meV after. Other deep defects work as nonradiative recombination centers reducing the intensity of the FB band.},
    keywords = {Cu2ZnSnSe4, argon ion etching, photoluminescence, Physics, Condensed Matter Physics, Renewable Energy, Sustainability and the Environment, Electronic, Optical and Magnetic Materials},
    url = {https://strathprints.strath.ac.uk/66330/}
    }

  • P. R. Edwards, K. P. O'Donnell, A. K. Singh, D. Cameron, K. Lorenz, M. Yamaga, J. H. Leach, M. J. Kappers, and M. Boćkowski, "Hysteretic photochromic switching (HPS) in doubly doped GaN(Mg):Eu–a summary of recent results," Materials, vol. 11, iss. 10, p. 1800, 2018.
    [BibTeX] [Abstract] [Download PDF]

    Europium is the most-studied and least-well-understood rare earth ion (REI) dopant in GaN. While attempting to increase the efficiency of red GaN light-emitting diodes (LEDs) by implanting Eu+ into p-type GaN templates, the Strathclyde University group, in collaboration with IST Lisbon and Unipress Warsaw, discovered hysteretic photochromic switching (HPS) in the photoluminescence spectrum of doubly doped GaN(Mg):Eu. Our recent work, summarised in this contribution, has used time-, temperature- and light-induced changes in the Eu intra-4f shell emission spectrum to deduce the microscopic nature of the Mg-Eu defects that form in this material. As well as shedding light on the Mg acceptor in GaN, we propose a possible role for these emission centres in quantum information and computing.

    @Article{strathprints65532,
    author = {Paul R. Edwards and Kevin P. O'Donnell and Akhilesh K. Singh and Douglas Cameron and Katharina Lorenz and Mitsuo Yamaga and Jacob H. Leach and Menno J. Kappers and Michal Bo{\'c}kowski},
    title = {Hysteretic photochromic switching (HPS) in doubly doped GaN(Mg):Eu{--}a summary of recent results},
    journal = {Materials},
    year = {2018},
    volume = {11},
    number = {10},
    pages = {1800},
    month = {September},
    abstract = {Europium is the most-studied and least-well-understood rare earth ion (REI) dopant in GaN. While attempting to increase the efficiency of red GaN light-emitting diodes (LEDs) by implanting Eu+ into p-type GaN templates, the Strathclyde University group, in collaboration with IST Lisbon and Unipress Warsaw, discovered hysteretic photochromic switching (HPS) in the photoluminescence spectrum of doubly doped GaN(Mg):Eu. Our recent work, summarised in this contribution, has used time-, temperature- and light-induced changes in the Eu intra-4f shell emission spectrum to deduce the microscopic nature of the Mg-Eu defects that form in this material. As well as shedding light on the Mg acceptor in GaN, we propose a possible role for these emission centres in quantum information and computing.},
    keywords = {gallium nitride, rare earth ions, europium, photoluminescence, photochromism, qubit, Physics, Materials Science(all), Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/65532/}
    }

  • I. A. Ajia, Y. Yamashita, K. Lorenz, M. M. Muhammed, L. Spasevski, D. Almalawi, J. Xu, K. Iizuka, Y. Morishima, D. H. Anjum, N. Wei, R. W. Martin, A. Kuramata, and I. S. Roqan, "GaN/AlGaN multiple quantum wells grown on transparent and conductive (-201)-oriented β-Ga₂O₃ substrate for UV vertical light emitting devices," Applied Physics Letters, vol. 113, iss. 8, p. 82102, 2018.
    [BibTeX] [Abstract] [Download PDF]

    GaN/AlGaN multiple quantum wells (MQWs) are grown on a (-201)-oriented β-Ga₂O₃ substrate. The optical and structural characteristics of the MQW structure are compared with those of a similar structure grown on sapphire. Scanning transmission electron microscopy and atomic force microscopy images show that the MQW structure exhibits higher crystalline quality of well-defined quantum wells when compared to a similar structure grown on sapphire. X-ray diffraction rocking curve and photoluminescence excitation analyses confirm the lower density of dislocation defects in the sample grown on a β-Ga₂O₃ substrate. A detailed analysis of time-integrated and time-resolved photoluminescence measurements shows that the MQWs grown on a β-Ga₂O₃ substrate are of higher optical quality. Our work indicates that the (-201)-oriented β-Ga₂O₃ substrate can be a potential candidate for UV vertical emitting devices.

    @Article{strathprints65395,
    author = {I. A. Ajia and Y. Yamashita and K. Lorenz and M. M. Muhammed and L. Spasevski and D. Almalawi and J. Xu and K. Iizuka and Y. Morishima and D. H. Anjum and N. Wei and R. W. Martin and A. Kuramata and I. S. Roqan},
    title = {GaN/AlGaN multiple quantum wells grown on transparent and conductive (-201)-oriented {β-Ga₂O₃} substrate for UV vertical light emitting devices},
    journal = {Applied Physics Letters},
    year = {2018},
    volume = {113},
    number = {8},
    pages = {082102},
    month = {August},
    abstract = {GaN/AlGaN multiple quantum wells (MQWs) are grown on a (-201)-oriented β-Ga₂O₃ substrate. The optical and structural characteristics of the MQW structure are compared with those of a similar structure grown on sapphire. Scanning transmission electron microscopy and atomic force microscopy images show that the MQW structure exhibits higher crystalline quality of well-defined quantum wells when compared to a similar structure grown on sapphire. X-ray diffraction rocking curve and photoluminescence excitation analyses confirm the lower density of dislocation defects in the sample grown on a β-Ga₂O₃ substrate. A detailed analysis of time-integrated and time-resolved photoluminescence measurements shows that the MQWs grown on a β-Ga₂O₃ substrate are of higher optical quality. Our work indicates that the (-201)-oriented β-Ga₂O₃ substrate can be a potential candidate for UV vertical emitting devices.},
    keywords = {GaN/AlGaN multiple quantum wells, MQWs, UV, ultraviolet, photoluminescence excitation, Optics. Light, Physics and Astronomy (miscellaneous)},
    url = {https://strathprints.strath.ac.uk/65395/}
    }

  • S. Sun, J. L. Zhang, K. A. Fischer, M. J. Burek, C. Dory, K. G. Lagoudakis, Y. Tzeng, M. Radulaski, Y. Kelaita, A. Safavi-Naeini, Z. Shen, N. A. Melosh, S. Chu, M. Lončar, and J. Vučković, "Cavity-enhanced Raman emission from a single color center in a solid," Physical Review Letters, vol. 121, iss. 8, p. 83601, 2018.
    [BibTeX] [Abstract] [Download PDF]

    We demonstrate cavity-enhanced Raman emission from a single atomic defect in a solid. Our platform is a single silicon-vacancy center in diamond coupled with a monolithic diamond photonic crystal cavity. The cavity enables an unprecedented frequency tuning range of the Raman emission (100 GHz) that significantly exceeds the spectral inhomogeneity of silicon-vacancy centers in diamond nanostructures. We also show that the cavity selectively suppresses the phonon-induced spontaneous emission that degrades the efficiency of Raman photon generation. Our results pave the way towards photon-mediated many-body interactions between solid-state quantum emitters in a nanophotonic platform.

    @Article{strathprints65244,
    author = {Shuo Sun and Jingyuan Linda Zhang and Kevin A. Fischer and Michael J. Burek and Constantin Dory and Konstantinos G. Lagoudakis and Yan-Kai Tzeng and Marina Radulaski and Yousif Kelaita and Amir Safavi-Naeini and Zhi-Xun Shen and Nicholas A. Melosh and Steven Chu and Marko Lon{\v c}ar and Jelena Vu{\v c}kovi{\'c}},
    title = {Cavity-enhanced Raman emission from a single color center in a solid},
    journal = {Physical Review Letters},
    year = {2018},
    volume = {121},
    number = {8},
    pages = {083601},
    month = {August},
    abstract = {We demonstrate cavity-enhanced Raman emission from a single atomic defect in a solid. Our platform is a single silicon-vacancy center in diamond coupled with a monolithic diamond photonic crystal cavity. The cavity enables an unprecedented frequency tuning range of the Raman emission (100 GHz) that significantly exceeds the spectral inhomogeneity of silicon-vacancy centers in diamond nanostructures. We also show that the cavity selectively suppresses the phonon-induced spontaneous emission that degrades the efficiency of Raman photon generation. Our results pave the way towards photon-mediated many-body interactions between solid-state quantum emitters in a nanophotonic platform.},
    keywords = {Raman emission, crystal cavity, diamond nanostructures, Solid state physics. Nanoscience, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/65244/}
    }

  • P. R. Edwards, G. Naresh-Kumar, G. Kusch, J. Bruckbauer, L. Spasevski, C. G. Brasser, M. J. Wallace, C. Trager-Cowan, and R. W. Martin, "You do what in your microprobe?! The EPMA as a multimode platform for nitride semiconductor characterization," Microscopy and Microanalysis, vol. 24, iss. S1, p. 2026–2027, 2018.
    [BibTeX] [Abstract] [Download PDF]

    While the use of electron probe microanalysis (EPMA) is widespread in the geological and metallurgical sciences, it remains less prevalent in the field of semiconductor research. For these materials, trace element (i.e. dopant) levels typically lie near or beneath the detection limit of wavelength-dispersive Xray (WDX) spectrometers, while alloy compositions of ternary mixtures and multilayer structures can more readily be determined using X-ray diffraction techniques. The electron beam measurements more commonly applied to semiconductors remain transmission electron microscopy (for structural characterization), and scanning electron microscopy (topographic, optical and electrical information). Despite this, there are many aspects of the EPMA that make it an attractive platform for all of thesetypes of semiconductor characterization, particularly when combining compositional information fromWDX with complementary and simultaneously-acquired signals. These advantages include: built-inlight optics; a stable, quantified and high-current beam; and a combined large-area and high-resolutionmapping capability. This allows the measurement of cathodoluminescence (CL), electron beam-inducedcurrent (EBIC) and electron channelling contrast imaging (ECCI) signals alongside WDX, which weapply to the investigation of visible and UV AlxInyGa1-x-yN materials, devices and nanostructures.

    @article{strathprints65206,
    volume = {24},
    number = {S1},
    month = {August},
    author = {Paul R. Edwards and G. Naresh-Kumar and Gunnar Kusch and Jochen Bruckbauer and Lucia Spasevski and Catherine G. Brasser and Michael J. Wallace and Carol Trager-Cowan and Robert W. Martin},
    title = {You do what in your microprobe?! The EPMA as a multimode platform for nitride semiconductor characterization},
    journal = {Microscopy and Microanalysis},
    pages = {2026--2027},
    year = {2018},
    keywords = {electron probe microanalysis, EPMA, electron beam, electron microscopy , Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/65206/},
    abstract = {While the use of electron probe microanalysis (EPMA) is widespread in the geological and metallurgical sciences, it remains less prevalent in the field of semiconductor research. For these materials, trace element (i.e. dopant) levels typically lie near or beneath the detection limit of wavelength-dispersive Xray (WDX) spectrometers, while alloy compositions of ternary mixtures and multilayer structures can more readily be determined using X-ray diffraction techniques. The electron beam measurements more commonly applied to semiconductors remain transmission electron microscopy (for structural characterization), and scanning electron microscopy (topographic, optical and electrical information). Despite this, there are many aspects of the EPMA that make it an attractive platform for all of thesetypes of semiconductor characterization, particularly when combining compositional information fromWDX with complementary and simultaneously-acquired signals. These advantages include: built-inlight optics; a stable, quantified and high-current beam; and a combined large-area and high-resolutionmapping capability. This allows the measurement of cathodoluminescence (CL), electron beam-inducedcurrent (EBIC) and electron channelling contrast imaging (ECCI) signals alongside WDX, which weapply to the investigation of visible and UV AlxInyGa1-x-yN materials, devices and nanostructures.}
    }

  • G. Naresh-Kumar, D. Thomson, Y. Zhang, J. Bai, L. Jiu, X. Yu, Y. P. Gong, R. S. Martin, T. Wang, and C. Trager-Cowan, "Imaging basal plane stacking faults and dislocations in (11-22) GaN using electron channelling contrast imaging," Journal of Applied Physics, vol. 124, p. 65301, 2018.
    [BibTeX] [Abstract] [Download PDF]

    Taking advantage of electron diffraction based measurements, in a scanning electron microscope, can deliver non-destructive and quantitative information on extended defects in semiconductor thin films. In the present work, we have studied a (11-22) semi-polar GaN thin film overgrown on regularly arrayed GaN micro-rod array templates grown by metal organic vapour phase epitaxy. We were able to optimise the diffraction conditions to image and quantify basal plane stacking faults (BSFs) and threading dislocations (TDs) using electron channelling contrast imaging (ECCI). Clusters of BSFs and TDs were observed with the same periodicity as the underlying micro-rod array template. The average BSF and TD density was estimated to be ≈4×10⁴ cm⁻¹ and ≈5×10⁸ cm⁻² respectively. The contrast seen for BSFs in ECCI is similar to that observed for plan-view transmission electron microscopy images, with the only difference being the former acquires the backscattered electrons and latter collects the transmitted electrons. Our present work shows the capability of ECCI for quantifying extended defects in semi-polar nitrides and represents a real step forward for optimising the growth conditions in these materials.

    @Article{strathprints64858,
    author = {G. Naresh-Kumar and David Thomson and Y. Zhang and J. Bai and L. Jiu and X. Yu and Y. P. Gong and Richard Smith Martin and Tao Wang and Carol Trager-Cowan},
    title = {Imaging basal plane stacking faults and dislocations in (11-22) GaN using electron channelling contrast imaging},
    journal = {Journal of Applied Physics},
    year = {2018},
    volume = {124},
    pages = {065301},
    month = {July},
    abstract = {Taking advantage of electron diffraction based measurements, in a scanning electron microscope, can deliver non-destructive and quantitative information on extended defects in semiconductor thin films. In the present work, we have studied a (11-22) semi-polar GaN thin film overgrown on regularly arrayed GaN micro-rod array templates grown by metal organic vapour phase epitaxy. We were able to optimise the diffraction conditions to image and quantify basal plane stacking faults (BSFs) and threading dislocations (TDs) using electron channelling contrast imaging (ECCI). Clusters of BSFs and TDs were observed with the same periodicity as the underlying micro-rod array template. The average BSF and TD density was estimated to be ≈4×10⁴ cm⁻¹ and ≈5×10⁸ cm⁻² respectively. The contrast seen for BSFs in ECCI is similar to that observed for plan-view transmission electron microscopy images, with the only difference being the former acquires the backscattered electrons and latter collects the transmitted electrons. Our present work shows the capability of ECCI for quantifying extended defects in semi-polar nitrides and represents a real step forward for optimising the growth conditions in these materials.},
    keywords = {ECCI, SEM, semiconductors, III - nitrides, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/64858/}
    }

  • M. Katsikini, V. Katchkanov, P. Boulet, P. R. Edwards, K. P. O'Donnell, and V. Brien, "Extended X-ray absorption fine structure study of Er bonding in AlNO:Erₓ films with x≤ 3.6%," Journal of Applied Physics, vol. 124, p. 85705, 2018.
    [BibTeX] [Abstract] [Download PDF]

    The structural properties of Er-doped AlNO epilayers grown by radio frequency magnetron sputtering were studied by Extended X-ray Absorption Fine Structure (EXAFS) spectra recorded at the Er L3 edge. The analysis revealed that Er substitutes for Al in all the studied samples and the increase in Er concentration from 0.5 to 3.6 at.% is not accompanied by formation of ErN, Er₂O₃ or Er clusters. Simultaneously recorded X-ray Absorption Near Edge Structure (XANES) spectra verify that the bonding configuration of Er is similar in all studied samples. The Er-N distance is constant at 2.18-2.19 Å i.e. approximately 15% larger than the Al-N bondlength, revealing that the introduction of Er in the cation sublattice causes considerable local distortion. The Debye-Waller factor, which measures the static disorder, of the second nearest shell of Al neighbors, has a local minimum for the sample containing 1% Er that coincides with the highest photoluminescence efficiency of the sample set.

    @Article{strathprints64845,
    author = {M. Katsikini and V. Katchkanov and P. Boulet and P. R. Edwards and K. P. O'Donnell and V. Brien},
    title = {Extended X-ray absorption fine structure study of {Er} bonding in {AlNO:Erₓ} films with x≤ 3.6%},
    journal = {Journal of Applied Physics},
    year = {2018},
    volume = {124},
    pages = {085705},
    month = {July},
    abstract = {The structural properties of Er-doped AlNO epilayers grown by radio frequency magnetron sputtering were studied by Extended X-ray Absorption Fine Structure (EXAFS) spectra recorded at the Er L3 edge. The analysis revealed that Er substitutes for Al in all the studied samples and the increase in Er concentration from 0.5 to 3.6 at.% is not accompanied by formation of ErN, Er₂O₃ or Er clusters. Simultaneously recorded X-ray Absorption Near Edge Structure (XANES) spectra verify that the bonding configuration of Er is similar in all studied samples. The Er-N distance is constant at 2.18-2.19 Å i.e. approximately 15% larger than the Al-N bondlength, revealing that the introduction of Er in the cation sublattice causes considerable local distortion. The Debye-Waller factor, which measures the static disorder, of the second nearest shell of Al neighbors, has a local minimum for the sample containing 1% Er that coincides with the highest photoluminescence efficiency of the sample set.},
    keywords = {EXAFS, AlN, Er, rare earth, semiconductor doping , Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/64845/}
    }

  • P. M. Coulon, G. Kusch, P. Fletcher, P. Chausse, R. W. Martin, and P. A. Shields, "Hybrid top-down/bottom-up fabrication of a highly uniform and organized faceted AlN nanorod scaffold," Materials, vol. 11, iss. 7, p. 1140, 2018.
    [BibTeX] [Abstract] [Download PDF]

    As a route to the formation of regular arrays of AlN nanorods, in contrast to other III-V materials, the use of selective area growth via metal organic vapor phase epitaxy (MOVPE) has so far not been successful. Therefore, in this work we report the fabrication of a highly uniform and ordered AlN nanorod scaffold using an alternative hybrid top-down etching and bottom-up regrowth approach. The nanorods are created across a full 2-inch AlN template by combining Displacement Talbot Lithography and lift-offto create a Ni nanodot mask, followed by chlorine-based dry etching. Additional KOH-based wet etching is used to tune the morphology and the diameter of the nanorods. The resulting smooth and straight morphology of the nanorods after the two-step dry-wet etching process is used as a template to recover the AlN facets of the nanorods via MOVPE regrowth. The facet recovery is performed for various growth times to investigate the growth mechanism and the change in morphology of the AlN nanorods. Structural characterization highlights, first, an efficient dislocation filtering resulting from the {\texttt{\char126}}130 nm diameter nanorods achieved after the two-step dry-wet etching process, and second, a dislocation bending induced by the AlN facet regrowth. A strong AlN near band edge emission is observed from the nanorods both before and after regrowth. The achievement of a highly uniform and organized faceted AlN nanorod scaffold having smooth and straight non-polar facets and improved structural and optical quality is a major stepping stone toward the fabrication of deep UV core-shell-based AlN or AlxGa1-xN templates.

    @Article{strathprints64810,
    author = {Pierre Marie Coulon and Gunnar Kusch and Philip Fletcher and Pierre Chausse and Robert W. Martin and Philip A. Shields},
    title = {Hybrid top-down/bottom-up fabrication of a highly uniform and organized faceted AlN nanorod scaffold},
    journal = {Materials},
    year = {2018},
    volume = {11},
    number = {7},
    pages = {1140},
    month = {July},
    abstract = {As a route to the formation of regular arrays of AlN nanorods, in contrast to other III-V materials, the use of selective area growth via metal organic vapor phase epitaxy (MOVPE) has so far not been successful. Therefore, in this work we report the fabrication of a highly uniform and ordered AlN nanorod scaffold using an alternative hybrid top-down etching and bottom-up regrowth approach. The nanorods are created across a full 2-inch AlN template by combining Displacement Talbot Lithography and lift-offto create a Ni nanodot mask, followed by chlorine-based dry etching. Additional KOH-based wet etching is used to tune the morphology and the diameter of the nanorods. The resulting smooth and straight morphology of the nanorods after the two-step dry-wet etching process is used as a template to recover the AlN facets of the nanorods via MOVPE regrowth. The facet recovery is performed for various growth times to investigate the growth mechanism and the change in morphology of the AlN nanorods. Structural characterization highlights, first, an efficient dislocation filtering resulting from the {\texttt{\char126}}130 nm diameter nanorods achieved after the two-step dry-wet etching process, and second, a dislocation bending induced by the AlN facet regrowth. A strong AlN near band edge emission is observed from the nanorods both before and after regrowth. The achievement of a highly uniform and organized faceted AlN nanorod scaffold having smooth and straight non-polar facets and improved structural and optical quality is a major stepping stone toward the fabrication of deep UV core-shell-based AlN or AlxGa1-xN templates.},
    keywords = {AlN, cathodoluminescence, displacement Talbot lithography (DTL), etching, nanorod, TEM, MOVPE, Electrical engineering. Electronics Nuclear engineering, Materials Science(all), Electrical and Electronic Engineering},
    url = {https://strathprints.strath.ac.uk/64810/}
    }

  • J. L. Zhang, S. Sun, M. J. Burek, C. Dory, Y. Tzeng, K. A. Fischer, Y. Kelaita, K. G. Lagoudakis, M. Radulaski, Z. Shen, N. A. Melosh, S. Chu, M. Lončar, and J. Vučković, "Strongly cavity-enhanced spontaneous emission from silicon-vacancy centers in diamond," Nano letters, vol. 18, iss. 2, p. 1360–1365, 2018.
    [BibTeX] [Abstract] [Download PDF]

    Quantum emitters are an integral component for a broad range of quantum technologies, including quantum communication, quantum repeaters, and linear optical quantum computation. Solid-state color centers are promising candidates for scalable quantum optics due to their long coherence time and small inhomogeneous broadening. However, once excited, color centers often decay through phonon-assisted processes, limiting the efficiency of single-photon generation and photon-mediated entanglement generation. Herein, we demonstrate strong enhancement of spontaneous emission rate of a single silicon-vacancy center in diamond embedded within a monolithic optical cavity, reaching a regime in which the excited-state lifetime is dominated by spontaneous emission into the cavity mode. We observe 10-fold lifetime reduction and 42-fold enhancement in emission intensity when the cavity is tuned into resonance with the optical transition of a single silicon-vacancy center, corresponding to 90\% of the excited-state energy decay occurring through spontaneous emission into the cavity mode. We also demonstrate the largest coupling strength (g/2{\ensuremath{\pi}} = 4.9 {$\pm$} 0.3 GHz) and cooperativity (C = 1.4) to date for color-center-based cavity quantum electrodynamics systems, bringing the system closer to the strong coupling regime.

    @article{strathprints64223,
    volume = {18},
    number = {2},
    month = {February},
    author = {Jingyuan Linda Zhang and Shuo Sun and Michael J. Burek and Constantin Dory and Yan-Kai Tzeng and Kevin A. Fischer and Yousif Kelaita and Konstantinos G. Lagoudakis and Marina Radulaski and Zhi-Xun Shen and Nicholas A. Melosh and Steven Chu and Marko Lon{\v c}ar and Jelena Vu{\v c}kovi{\'c}},
    title = {Strongly cavity-enhanced spontaneous emission from silicon-vacancy centers in diamond},
    journal = {Nano letters},
    pages = {1360--1365},
    year = {2018},
    keywords = {purcell enhancement, single photon generation, defect center materials, diamond, silicon vacancy center, nanophotonics, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/64223/},
    abstract = {Quantum emitters are an integral component for a broad range of quantum technologies, including quantum communication, quantum repeaters, and linear optical quantum computation. Solid-state color centers are promising candidates for scalable quantum optics due to their long coherence time and small inhomogeneous broadening. However, once excited, color centers often decay through phonon-assisted processes, limiting the efficiency of single-photon generation and photon-mediated entanglement generation. Herein, we demonstrate strong enhancement of spontaneous emission rate of a single silicon-vacancy center in diamond embedded within a monolithic optical cavity, reaching a regime in which the excited-state lifetime is dominated by spontaneous emission into the cavity mode. We observe 10-fold lifetime reduction and 42-fold enhancement in emission intensity when the cavity is tuned into resonance with the optical transition of a single silicon-vacancy center, corresponding to 90\% of the excited-state energy decay occurring through spontaneous emission into the cavity mode. We also demonstrate the largest coupling strength (g/2{\ensuremath{\pi}} = 4.9 {$\pm$} 0.3 GHz) and cooperativity (C = 1.4) to date for color-center-based cavity quantum electrodynamics systems, bringing the system closer to the strong coupling regime.}
    }

  • I. E. Svitsiankou, V. N. Pavlovskii, E. V. Lutsenko, G. P. Yablonskii, A. V. Mudryi, O. M. Borodavchenko, V. D. Zhivulko, M. V. Yakushev, and R. Martin, "Stimulated emission and optical properties of solid solutions of Cu(In,Ga)Se₂ direct band gap semiconductors," Journal of Applied Spectroscopy, vol. 85, iss. 2, p. 267–273, 2018.
    [BibTeX] [Abstract] [Download PDF]

    Stimulated emission, optical properties, and structural characteristics of non-irradiated and proton-irradiated Cu(In,Ga)Se₂ thin films deposited on soda lime glass substrates using co-evaporation of elements in a multistage process were investigated. X-ray diffraction analysis, scanning electron microscopy, X-ray spectral analysis with energy dispersion, low-temperature photoluminescence, optical transmittance and reflectance were used to study the films. Stimulated emission at low temperatures of ~20 K was found in non-irradiated and proton-irradiated Cu(In,Ga)Se₂ thin films upon excitation by laser pulses of nanosecond duration with a threshold power density of ~20 kW/cm². It was shown that the appearance and parameters of the stimulated emission depend strongly on the concentration of ion-induced defects in Cu(In,Ga)Se₂ thin films.

    @Article{strathprints64210,
    author = {I. E. Svitsiankou and V. N. Pavlovskii and E. V. Lutsenko and G. P. Yablonskii and A. V. Mudryi and O. M. Borodavchenko and V. D. Zhivulko and M. V. Yakushev and R. Martin},
    title = {Stimulated emission and optical properties of solid solutions of {Cu(In,Ga)Se₂} direct band gap semiconductors},
    journal = {Journal of Applied Spectroscopy},
    year = {2018},
    volume = {85},
    number = {2},
    pages = {267--273},
    month = {May},
    note = {This output is an English translation of an article published in published in Zhurnal Prikladnoi Spektroskopii, Vol. 85, No. 2, pp. 248-255, March-April, 2018.},
    abstract = {Stimulated emission, optical properties, and structural characteristics of non-irradiated and proton-irradiated Cu(In,Ga)Se₂ thin films deposited on soda lime glass substrates using co-evaporation of elements in a multistage process were investigated. X-ray diffraction analysis, scanning electron microscopy, X-ray spectral analysis with energy dispersion, low-temperature photoluminescence, optical transmittance and reflectance were used to study the films. Stimulated emission at low temperatures of ~20 K was found in non-irradiated and proton-irradiated Cu(In,Ga)Se₂ thin films upon excitation by laser pulses of nanosecond duration with a threshold power density of ~20 kW/cm². It was shown that the appearance and parameters of the stimulated emission depend strongly on the concentration of ion-induced defects in Cu(In,Ga)Se₂ thin films.},
    keywords = {Cu(In,Ga)Se, defect, proton, stimulated emission, thin film, Physics, Condensed Matter Physics, Spectroscopy},
    url = {https://strathprints.strath.ac.uk/64210/}
    }

  • I. E. Svitsiankou, V. N. Pavlovskii, E. V. Lutsenko, G. P. Yablonskii, A. V. Mudryi, O. M. Borodavchenko, V. D. Zhivulko, M. V. Yakushev, and R. Martin, "СТИМУЛИРОВАННОЕ ИЗЛУЧЕНИЕ И ОПТИЧЕСКИЕ СВОЙСТВА ТВЕРДЫХ РАСТВОРОВ ПРЯМОЗОННЫХ ПОЛУПРОВОДНИКОВ Cu(In,Ga)Se₂," Zhurnal Prikladnoi Spektroskopii, vol. 85, iss. 2, pp. 248-255, 2018.
    [BibTeX] [Abstract] [Download PDF]

    Stimulated emission, optical properties, and structural characteristics of non-irradiated and proton-irradiated Cu(In,Ga)Se₂ thin films deposited on soda lime glass substrates using co-evaporation of elements in a multistage process were investigated. X-ray diffraction analysis, scanning electron microscopy, X-ray spectral analysis with energy dispersion, low-temperature photoluminescence, optical transmittance and reflectance were used to study the films. Stimulated emission at low temperatures of ~20 K was found in non-irradiated and proton-irradiated Cu(In,Ga)Se₂ thin films upon excitation by laser pulses of nanosecond duration with a threshold power density of ~20 kW/cm². It was shown that the appearance and parameters of the stimulated emission depend strongly on the concentration of ion-induced defects in Cu(In,Ga)Se₂ thin films.

    @Article{strathprints64209,
    author = {I. E. Svitsiankou and V. N. Pavlovskii and E. V. Lutsenko and G. P. Yablonskii and A. V. Mudryi and O. M. Borodavchenko and V. D. Zhivulko and M. V. Yakushev and R. Martin},
    title = {СТИМУЛИРОВАННОЕ ИЗЛУЧЕНИЕ И ОПТИЧЕСКИЕ СВОЙСТВА ТВЕРДЫХ РАСТВОРОВ ПРЯМОЗОННЫХ ПОЛУПРОВОДНИКОВ {Cu(In,Ga)Se₂}},
    journal = {Zhurnal Prikladnoi Spektroskopii},
    year = {2018},
    volume = {85},
    number = {2},
    pages = {248-255},
    month = {April},
    abstract = {Stimulated emission, optical properties, and structural characteristics of non-irradiated and proton-irradiated Cu(In,Ga)Se₂ thin films deposited on soda lime glass substrates using co-evaporation of elements in a multistage process were investigated. X-ray diffraction analysis, scanning electron microscopy, X-ray spectral analysis with energy dispersion, low-temperature photoluminescence, optical transmittance and reflectance were used to study the films. Stimulated emission at low temperatures of ~20 K was found in non-irradiated and proton-irradiated Cu(In,Ga)Se₂ thin films upon excitation by laser pulses of nanosecond duration with a threshold power density of ~20 kW/cm². It was shown that the appearance and parameters of the stimulated emission depend strongly on the concentration of ion-induced defects in Cu(In,Ga)Se₂ thin films.},
    keywords = {Cu(In,Ga)Se, defect, proton, stimulated emission, thin film, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/64209/}
    }

  • P. Coulon, G. Kusch, E. L. D. Boulbar, P. Chausse, C. Bryce, R. W. Martin, and P. A. Shields, "Hybrid top-down/bottom-up fabrication of regular arrays of AlN nanorods for deep-UV core-shell LEDs," Physica Status Solidi (B) Basic Research, vol. 255, iss. 5, p. 1700445, 2018.
    [BibTeX] [Abstract] [Download PDF]

    Core-shell nanostructures are predicted to highly improve the efficiency of deep-UV light emitting diodes (LEDs), owing to their low defect density, reduced quantum-confined Stark effect, high-quality non-polar growth and improved extraction efficiency. In this paper, we report on the nanofabrication of high-quality AlN nanorod arrays using a hybrid top-down/bottom-up approach for use as a scaffold for UV LED structures. We describe the use of Displacement Talbot Lithography to fabricate a metallic hard etch mask to allow AlN nanorod arrays to be dry etched from a planar AlN template. In particular, we investigate the impact of etching parameters on the nanorod etch rate, tapering profile and mask selectivity in order to achieve vertical-sided nanorod arrays with high aspect ratios. AlN facet recovery is subsequently explored by means of regrowth using Metal Organic Vapor Phase Epitaxy. Low pressure and high V/III ratio promote straight and smooth sidewall faceting, which results in an improvement of the optical quality compared to the initial AlN template. The promising results open new perspectives for the fabrication of high-efficiency deep-UV-emitting core-shell LEDs.

    @Article{strathprints64204,
    author = {Pierre-Marie Coulon and Gunnar Kusch and Emmanuel D. Le Boulbar and Pierre Chausse and Christopher Bryce and Robert W. Martin and Philip A. Shields},
    title = {Hybrid top-down/bottom-up fabrication of regular arrays of AlN nanorods for deep-UV core-shell LEDs},
    journal = {Physica Status Solidi (B) Basic Research},
    year = {2018},
    volume = {255},
    number = {5},
    pages = {1700445},
    month = {May},
    abstract = {Core-shell nanostructures are predicted to highly improve the efficiency of deep-UV light emitting diodes (LEDs), owing to their low defect density, reduced quantum-confined Stark effect, high-quality non-polar growth and improved extraction efficiency. In this paper, we report on the nanofabrication of high-quality AlN nanorod arrays using a hybrid top-down/bottom-up approach for use as a scaffold for UV LED structures. We describe the use of Displacement Talbot Lithography to fabricate a metallic hard etch mask to allow AlN nanorod arrays to be dry etched from a planar AlN template. In particular, we investigate the impact of etching parameters on the nanorod etch rate, tapering profile and mask selectivity in order to achieve vertical-sided nanorod arrays with high aspect ratios. AlN facet recovery is subsequently explored by means of regrowth using Metal Organic Vapor Phase Epitaxy. Low pressure and high V/III ratio promote straight and smooth sidewall faceting, which results in an improvement of the optical quality compared to the initial AlN template. The promising results open new perspectives for the fabrication of high-efficiency deep-UV-emitting core-shell LEDs.},
    keywords = {AlN, MOVPE, nanorod, top-down etching, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {https://strathprints.strath.ac.uk/64204/}
    }

  • P. R. Edwards and R. W. Martin, "Corrigendum: "Cathodoluminescence nano-characterization of semiconductors" (2011 Semicond. Sci. Technol. 26 064005)," Semiconductor Science and Technology, vol. 33, p. 79501, 2018.
    [BibTeX] [Abstract] [Download PDF]

    In our original paper, we estimated the maximum field of view (FOV) that would result when collecting luminescence over a cone half-angle u and coupling this into a spectrograph with a given f /number and a slit width d. Due to the use of the low-angle approximation outwith the paraxial regime, the expression given in Equation 2 used the tangent of the angle rather than the correct sine function.

    @Article{strathprints64148,
    author = {P. R. Edwards and R. W. Martin},
    title = {Corrigendum: "Cathodoluminescence nano-characterization of semiconductors" (2011 Semicond. Sci. Technol. 26 064005)},
    journal = {Semiconductor Science and Technology},
    year = {2018},
    volume = {33},
    pages = {079501},
    month = {May},
    note = {This corrigendum to the 2011 article http://dx.doi.org/10.1088/0268-1242/26/6/064005 (Pure item ID 2838272).},
    abstract = {In our original paper, we estimated the maximum field of view (FOV) that would result when collecting luminescence over a cone half-angle u and coupling this into a spectrograph with a given f /number and a slit width d. Due to the use of the low-angle approximation outwith the paraxial regime, the expression given in Equation 2 used the tangent of the angle rather than the correct sine function.},
    keywords = {cathodoluminescence, semiconductors, Physics, Materials Chemistry, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
    url = {https://strathprints.strath.ac.uk/64148/}
    }

  • B. Hourahine, D. McArthur, and F. Papoff, "Principal modes of Maxwell's equations," in The Generalized Multipole Technique for Light Scattering, T. Wriedt and Y. Eremin, Eds., Berlin: Springer International Publishing AG, 2018.
    [BibTeX] [Abstract] [Download PDF]

    This chapter reviews the use of principal modes–states which are maximally correlated between two subspaces and hence form pairs unique up to phase factors–in solving Maxwell's equations and analysing these solutions for nanoparticles and structures. The mathematical structure of this method allows a computationally efficient generalisation of Mie's analytical approach for the sphere to obtain semi-analytical solutions for general geometries with smooth interfaces. We apply this method to investigate a range of single and multiple particle metallic structures in the linear, non-linear and non-local response regimes outside of the quasi-static limit.

    @InCollection{strathprints63767,
    author = {Benjamin Hourahine and Duncan McArthur and Francesco Papoff},
    title = {Principal modes of Maxwell's equations},
    booktitle = {The Generalized Multipole Technique for Light Scattering},
    publisher = {Springer International Publishing AG},
    year = {2018},
    editor = {Thomas Wriedt and Yuri Eremin},
    series = {Springer Series on Atomic, Optical, and Plasma Physics},
    address = {Berlin},
    month = {April},
    abstract = {This chapter reviews the use of principal modes--states which are maximally correlated between two subspaces and hence form pairs unique up to phase factors--in solving Maxwell's equations and analysing these solutions for nanoparticles and structures. The mathematical structure of this method allows a computationally efficient generalisation of Mie's analytical approach for the sphere to obtain semi-analytical solutions for general geometries with smooth interfaces. We apply this method to investigate a range of single and multiple particle metallic structures in the linear, non-linear and non-local response regimes outside of the quasi-static limit.},
    keywords = {principal modes, phase factors, Maxwell's equations, Physics, Mathematics, Mathematics(all), Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/63767/}
    }

  • C. Brasser, J. Bruckbauer, Y. P. Gong, L. Jiu, J. Bai, M. Warzecha, P. R. Edwards, T. Wang, and R. W. Martin, "Cathodoluminescence studies of chevron features in semi-polar (11-22) InGaN/GaN multiple quantum well structures," Journal of Applied Physics, vol. 123, p. 174502, 2018.
    [BibTeX] [Abstract] [Download PDF]

    Epitaxial overgrowth of semi-polar III-nitride layers and devices often leads to arrowhead-shaped surface features, referred to as chevrons. We report on a study into the optical, structural and electrical properties of these features occurring in two very different semi-polar structures, a blue-emitting multiple quantum well (MQW) structure and an amber-emitting light-emitting diode (LED). Cathodoluminescence (CL) hyperspectral imaging has highlighted shifts in their emission energy, occurring in the region of the chevron. These variations are due to different semi-polar planes introduced in the chevron arms resulting in a lack of uniformity in the InN incorporation across samples, and the disruption of the structure which could cause a narrowing of the QWs in this region. Atomic force microscopy has revealed that chevrons can penetrate over 150 nm into the sample, and quench light emission from the active layers. The dominance of non-radiative recombination in the chevron region was exposed by simultaneous measurement of CL and the electron beam-induced current (EBIC). Overall these results provide an overview of the nature and impact of chevrons on the luminescence of semi-polar devices.

    @Article{strathprints63662,
    author = {C. Brasser and J. Bruckbauer and Y.P. Gong and L. Jiu and J. Bai and M. Warzecha and P. R. Edwards and T. Wang and R. W. Martin},
    title = {Cathodoluminescence studies of chevron features in semi-polar (11-22) {InGaN/GaN} multiple quantum well structures},
    journal = {Journal of Applied Physics},
    year = {2018},
    volume = {123},
    pages = {174502},
    month = {April},
    note = {The following article has been accepted by Journal of Applied Physics. After it is published, it will be found at https://aip.scitation.org/journal/jap/.},
    abstract = {Epitaxial overgrowth of semi-polar III-nitride layers and devices often leads to arrowhead-shaped surface features, referred to as chevrons. We report on a study into the optical, structural and electrical properties of these features occurring in two very different semi-polar structures, a blue-emitting multiple quantum well (MQW) structure and an amber-emitting light-emitting diode (LED). Cathodoluminescence (CL) hyperspectral imaging has highlighted shifts in their emission energy, occurring in the region of the chevron. These variations are due to different semi-polar planes introduced in the chevron arms resulting in a lack of uniformity in the InN incorporation across samples, and the disruption of the structure which could cause a narrowing of the QWs in this region. Atomic force microscopy has revealed that chevrons can penetrate over 150 nm into the sample, and quench light emission from the active layers. The dominance of non-radiative recombination in the chevron region was exposed by simultaneous measurement of CL and the electron beam-induced current (EBIC). Overall these results provide an overview of the nature and impact of chevrons on the luminescence of semi-polar devices.},
    keywords = {semi-polar structures, cathodoluminescence, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/63662/}
    }

  • E. Pascal, S. Singh, P. G. Callahan, B. Hourahine, C. Trager-Cowan, and M. D. Graef, "Energy-weighted dynamical scattering simulations of electron diffraction modalites in the scanning electron microscope," Ultramicroscopy, vol. 187, pp. 98-106, 2018.
    [BibTeX] [Abstract] [Download PDF]

    Transmission Kikuchi diffraction (TKD) has been gaining momentum as a high resolution alternative to electron back-scattered diffraction (EBSD), adding to the existing electron diffraction modalities in the scanning electron microscope (SEM). The image simulation of any of these measurement techniques requires an energy dependent diffraction model for which, in turn, knowledge of electron energies and diffraction distances distributions is required. We identify the sample-detector geometry and the effect of inelastic events on the diffracting electron beam as the important factors to be considered when predicting these distributions. However, tractable models taking into account inelastic scattering explicitly are lacking. In this study, we expand the Monte Carlo (MC) energy-weighting dynamical simulations models used for EBSD and ECP to the TKD case. We show that the foil thickness in TKD can be used as a means of energy filtering and compare band sharpness in the different modalities. The current model is shown to correctly predict TKD patterns and, through the dictionary indexing approach, to produce higher quality indexed TKD maps than conventional Hough transform approach, especially close to grain boundaries.

    @Article{strathprints62987,
    author = {Elena Pascal and Saranch Singh and Patrick G. Callahan and Ben Hourahine and Carol Trager-Cowan and Marc De Graef},
    title = {Energy-weighted dynamical scattering simulations of electron diffraction modalites in the scanning electron microscope},
    journal = {Ultramicroscopy},
    year = {2018},
    volume = {187},
    pages = {98-106},
    month = {January},
    abstract = {Transmission Kikuchi diffraction (TKD) has been gaining momentum as a high resolution alternative to electron back-scattered diffraction (EBSD), adding to the existing electron diffraction modalities in the scanning electron microscope (SEM). The image simulation of any of these measurement techniques requires an energy dependent diffraction model for which, in turn, knowledge of electron energies and diffraction distances distributions is required. We identify the sample-detector geometry and the effect of inelastic events on the diffracting electron beam as the important factors to be considered when predicting these distributions. However, tractable models taking into account inelastic scattering explicitly are lacking. In this study, we expand the Monte Carlo (MC) energy-weighting dynamical simulations models used for EBSD and ECP to the TKD case. We show that the foil thickness in TKD can be used as a means of energy filtering and compare band sharpness in the different modalities. The current model is shown to correctly predict TKD patterns and, through the dictionary indexing approach, to produce higher quality indexed TKD maps than conventional Hough transform approach, especially close to grain boundaries.},
    keywords = {Transmission Kikuchi diffraction, TKD, electron back-scattered diffraction, EBSD, scanning electron microscopes, SEM, electrons, foil thickness, energy filtering, dynamical simulations, Monte Carlo, Plasma physics. Ionized gases, Instrumentation, Atomic and Molecular Physics, and Optics, Electronic, Optical and Magnetic Materials},
    url = {https://strathprints.strath.ac.uk/62987/}
    }

  • G. Kusch, M. Conroy, H. Li, P. R. Edwards, C. Zhao, B. S. Ooi, J. Pugh, M. J. Cryan, P. J. Parbrook, and R. W. Martin, "Multi-wavelength emission from a single InGaN/GaN nanorod analyzed by cathodoluminescence hyperspectral imaging," Scientific Reports, vol. 8, p. 1742, 2018.
    [BibTeX] [Abstract] [Download PDF]

    Multiple luminescence peaks emitted by a single InGaN/GaN quantum-well (QW) nanorod, extending from the blue to the red, were analysed by a combination of electron microscope based imaging techniques. Utilizing the capability of cathodoluminescence hyperspectral imaging it was possible to investigate spatial variations in the luminescence properties on a nanoscale. The high optical quality of a single GaN nanorod was demonstrated, evidenced by a narrow band-edge peak and the absence of any luminescence associated with the yellow defect band. Additionally two spatially confined broad luminescence bands were observed, consisting of multiple peaks ranging from 395nm to 480nm and 490nm to 650 nm. The lower energy band originates from broad c-plane QWs located at the apex of the nanorod and the higher energy band from the semipolar QWs on the pyramidal nanorod tip. Comparing the experimentally observed peak positions with peak positions obtained from plane wave modelling and 3D finite difference time domain (FDTD) modelling shows modulation of the nanorod luminescence by cavity modes. By studying the influence of these modes we demonstrate that this can be exploited as an additional parameter in engineering the emission profile of LEDs.

    @Article{strathprints62810,
    author = {Gunnar Kusch and Michele Conroy and Haoning Li and Paul R. Edwards and Chao Zhao and Boon S. Ooi and Jon Pugh and Martin J. Cryan and Peter J. Parbrook and Robert W. Martin},
    title = {Multi-wavelength emission from a single InGaN/GaN nanorod analyzed by cathodoluminescence hyperspectral imaging},
    journal = {Scientific Reports},
    year = {2018},
    volume = {8},
    pages = {1742},
    month = {January},
    abstract = {Multiple luminescence peaks emitted by a single InGaN/GaN quantum-well (QW) nanorod, extending from the blue to the red, were analysed by a combination of electron microscope based imaging techniques. Utilizing the capability of cathodoluminescence hyperspectral imaging it was possible to investigate spatial variations in the luminescence properties on a nanoscale. The high optical quality of a single GaN nanorod was demonstrated, evidenced by a narrow band-edge peak and the absence of any luminescence associated with the yellow defect band. Additionally two spatially confined broad luminescence bands were observed, consisting of multiple peaks ranging from 395nm to 480nm and 490nm to 650 nm. The lower energy band originates from broad c-plane QWs located at the apex of the nanorod and the higher energy band from the semipolar QWs on the pyramidal nanorod tip. Comparing the experimentally observed peak positions with peak positions obtained from plane wave modelling and 3D finite difference time domain (FDTD) modelling shows modulation of the nanorod luminescence by cavity modes. By studying the influence of these modes we demonstrate that this can be exploited as an additional parameter in engineering the emission profile of LEDs.},
    keywords = {luminescence, InGaN/GaN quantum-well, nanorods, hyperspectral imaging, LEDs, Optics. Light, Atomic and Molecular Physics, and Optics},
    url = {https://strathprints.strath.ac.uk/62810/}
    }

  • A. K. Singh, K. P. O'Donnell, P. R. Edwards, K. Lorenz, J. H. Leach, and M. Boćkowski, "Eu-Mg defects and donor-acceptor pairs in GaN : photodissociation and the excitation transfer problem," Journal of Physics D: Applied Physics, vol. 51, p. 65106, 2018.
    [BibTeX] [Abstract] [Download PDF]

    We have investigated temperature-dependent photoluminescence (TDPL) profiles of Eu³⁺ ions implanted in an HVPE-grown bulk GaN sample doped with Mg and of donor-acceptor pairs (DAP) involving the shallow Mg acceptor in GaN(Mg) (unimplanted) and GaN(Mg):Eu samples. Below 125 K, the TDPL of Eu³⁺ in GaN(Mg):Eu correlates with that of the DAP. Below 75 K, the intensity of Eu³⁺ emission saturates, indicating a limitation of the numbers of Eu-Mg defects available to receive excitation transferred from the host, while the DAP continues to increase, albeit more slowly in the implanted than the unimplanted sample. Prolonged exposure to UV light at low temperature results in the photodissociation of Eu-Mg defects, in their Eu1(Mg) configuration, with a corresponding increase in shallow DAP emission and the emergence of emission from unassociated EuGa (Eu2) defects.

    @Article{strathprints62526,
    author = {A.K. Singh and K.P. O'Donnell and P.R. Edwards and K. Lorenz and J.H. Leach and M. Bo{\'c}kowski},
    title = {Eu-Mg defects and donor-acceptor pairs in GaN : photodissociation and the excitation transfer problem},
    journal = {Journal of Physics D: Applied Physics},
    year = {2018},
    volume = {51},
    pages = {065106},
    month = {December},
    abstract = {We have investigated temperature-dependent photoluminescence (TDPL) profiles of Eu³⁺ ions implanted in an HVPE-grown bulk GaN sample doped with Mg and of donor-acceptor pairs (DAP) involving the shallow Mg acceptor in GaN(Mg) (unimplanted) and GaN(Mg):Eu samples. Below 125 K, the TDPL of Eu³⁺ in GaN(Mg):Eu correlates with that of the DAP. Below 75 K, the intensity of Eu³⁺ emission saturates, indicating a limitation of the numbers of Eu-Mg defects available to receive excitation transferred from the host, while the DAP continues to increase, albeit more slowly in the implanted than the unimplanted sample. Prolonged exposure to UV light at low temperature results in the photodissociation of Eu-Mg defects, in their Eu1(Mg) configuration, with a corresponding increase in shallow DAP emission and the emergence of emission from unassociated EuGa (Eu2) defects.},
    keywords = {temperature-dependent photoluminescence, photodissociation, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/62526/}
    }

  • K. P. Mingard, M. Stewart, M. G. Gee, S. Vespucci, and C. Trager-Cowan, "Practical application of direct electron detectors to EBSD mapping in 2D and 3D," Ultramicroscopy, vol. 184, iss. Part A, p. 242–251, 2018.
    [BibTeX] [Abstract] [Download PDF]

    The use of a direct electron detector for the simple acquisition of 2D electron backscatter diffraction (EBSD) maps and 3D EBSD datasets with a static sample geometry has been demonstrated in a focused ion beam scanning electron microscope. The small size and flexible connection of the Medipix direct electron detector enabled the mounting of sample and detector on the same stage at the short working distance required for the FIB. Comparison of 3D EBSD datasets acquired by this means and with conventional phosphor based EBSD detectors requiring sample movement showed that the former method with a static sample gave improved slice registration. However, for this sample detector configuration, significant heating by the detector caused sample drift. This drift and ion beam reheating both necessitated the use of fiducial marks to maintain stability during data acquisition.

    @article{strathprints62078,
    volume = {184},
    number = {Part A},
    month = {January},
    author = {K.P. Mingard and M. Stewart and M.G. Gee and S. Vespucci and C. Trager-Cowan},
    title = {Practical application of direct electron detectors to EBSD mapping in 2D and 3D},
    journal = {Ultramicroscopy},
    pages = {242--251},
    year = {2018},
    keywords = {EBSD, direct electron detector, medipix, 3D EBSD, SEM image drift, focused ion beam, Optics. Light, Instrumentation, Atomic and Molecular Physics, and Optics, Electronic, Optical and Magnetic Materials},
    url = {https://strathprints.strath.ac.uk/62078/},
    abstract = {The use of a direct electron detector for the simple acquisition of 2D electron backscatter diffraction (EBSD) maps and 3D EBSD datasets with a static sample geometry has been demonstrated in a focused ion beam scanning electron microscope. The small size and flexible connection of the Medipix direct electron detector enabled the mounting of sample and detector on the same stage at the short working distance required for the FIB. Comparison of 3D EBSD datasets acquired by this means and with conventional phosphor based EBSD detectors requiring sample movement showed that the former method with a static sample gave improved slice registration. However, for this sample detector configuration, significant heating by the detector caused sample drift. This drift and ion beam reheating both necessitated the use of fiducial marks to maintain stability during data acquisition.}
    }

  • I. Ahmed, J. A. Haigh, S. Schaal, S. Barraud, Y. Zhu, C. Lee, M. Amado, J. W. A. Robinson, A. Rossi, J. J. L. Morton, and F. M. Gonzalez-Zalba, "Radio-frequency capacitive gate-based sensing," Physical Review Applied, vol. 10, iss. 1, 2018. doi:10.1103/PhysRevApplied.10.014018
    [BibTeX] [Abstract] [Download PDF]

    Developing fast, accurate, and scalable techniques for quantum-state readout is an active area in semiconductor-based quantum computing. Here, we present results on dispersive sensing of silicon corner state quantum dots coupled to lumped-element electrical resonators via the gate. The gate capacitance of the quantum device is placed in parallel with a superconducting spiral inductor resulting in resonators with loaded Q factors in the 400-800 range. We utilize resonators operating at 330 and 616 MHz, and achieve charge sensitivities of 7.7 and 1.3{\ensuremath{\mu}}e/Hz, respectively. We perform a parametric study of the resonator to reveal its optimal operation points and perform a circuit analysis to determine the best resonator design. The results place gate-based sensing on a par with the best reported radio-frequency single-electron transistor sensitivities while providing a fast and compact method for quantum-state readout.

    @article{strathprints68587,
    volume = {10},
    number = {1},
    month = {July},
    title = {Radio-frequency capacitive gate-based sensing},
    year = {2018},
    doi = {10.1103/PhysRevApplied.10.014018},
    journal = {Physical Review Applied},
    keywords = {quantum computation, quantum information architectures \& platforms, quantum information with solid state qubits, radio frequency techniques, Physics, Physics and Astronomy(all)},
    url = {https://doi.org/10.1103/PhysRevApplied.10.014018},
    issn = {2331-7019},
    abstract = {Developing fast, accurate, and scalable techniques for quantum-state readout is an active area in semiconductor-based quantum computing. Here, we present results on dispersive sensing of silicon corner state quantum dots coupled to lumped-element electrical resonators via the gate. The gate capacitance of the quantum device is placed in parallel with a superconducting spiral inductor resulting in resonators with loaded Q factors in the 400-800 range. We utilize resonators operating at 330 and 616 MHz, and achieve charge sensitivities of 7.7 and 1.3{\ensuremath{\mu}}e/Hz, respectively. We perform a parametric study of the resonator to reveal its optimal operation points and perform a circuit analysis to determine the best resonator design. The results place gate-based sensing on a par with the best reported radio-frequency single-electron transistor sensitivities while providing a fast and compact method for quantum-state readout.},
    author = {Ahmed, Imtiaz and Haigh, James A. and Schaal, Simon and Barraud, Sylvain and Zhu, Yi and Lee, Chang-min and Amado, Mario and Robinson, Jason W. A. and Rossi, Alessandro and Morton, John J. L. and Gonzalez-Zalba, M. Fernando}
    }

  • A. Rossi, J. Klochan, J. Timoshenko, F. E. Hudson, M. Möttönen, S. Rogge, A. S. Dzurak, V. Kashcheyevs, and G. C. Tettamanzi, "Gigahertz single-electron pumping mediated by parasitic states," Nano Letters, vol. 18, iss. 7, p. 4141–4147, 2018. doi:10.1021/acs.nanolett.8b00874
    [BibTeX] [Abstract] [Download PDF]

    In quantum metrology, semiconductor single-electron pumps are used to generate accurate electric currents with the ultimate goal of implementing the emerging quantum standard of the ampere. Pumps based on electrostatically defined tunable quantum dots (QDs) have thus far shown the most promising performance in combining fast and accurate charge transfer. However, at frequencies exceeding approximately 1 GHz the accuracy typically decreases. Recently, hybrid pumps based on QDs coupled to trap states have led to increased transfer rates due to tighter electrostatic confinement. Here, we operate a hybrid electron pump in silicon obtained by coupling a QD to multiple parasitic states and achieve robust current quantization up to a few gigahertz. We show that the fidelity of the electron capture depends on the sequence in which the parasitic states become available for loading, resulting in distinctive frequency-dependent features in the pumped current.

    @article{strathprints68704,
    volume = {18},
    number = {7},
    month = {June},
    title = {Gigahertz single-electron pumping mediated by parasitic states},
    journal = {Nano Letters},
    doi = {10.1021/acs.nanolett.8b00874},
    pages = {4141--4147},
    year = {2018},
    keywords = {quantum dot, quantum electrical metrology, silicon, single-electron pump, Physics, Bioengineering, Condensed Matter Physics, Mechanical Engineering},
    url = {https://doi.org/10.1021/acs.nanolett.8b00874},
    issn = {1530-6992},
    abstract = {In quantum metrology, semiconductor single-electron pumps are used to generate accurate electric currents with the ultimate goal of implementing the emerging quantum standard of the ampere. Pumps based on electrostatically defined tunable quantum dots (QDs) have thus far shown the most promising performance in combining fast and accurate charge transfer. However, at frequencies exceeding approximately 1 GHz the accuracy typically decreases. Recently, hybrid pumps based on QDs coupled to trap states have led to increased transfer rates due to tighter electrostatic confinement. Here, we operate a hybrid electron pump in silicon obtained by coupling a QD to multiple parasitic states and achieve robust current quantization up to a few gigahertz. We show that the fidelity of the electron capture depends on the sequence in which the parasitic states become available for loading, resulting in distinctive frequency-dependent features in the pumped current.},
    author = {Rossi, Alessandro and Klochan, Jevgeny and Timoshenko, Janis and Hudson, Fay E. and M{\"o}tt{\"o}nen, Mikko and Rogge, Sven and Dzurak, Andrew S. and Kashcheyevs, Vyacheslavs and Tettamanzi, Giuseppe C.}
    }

  • J. W. Roberts, J. C. Jarman, D. N. Johnstone, P. A. Midgley, P. R. Chalker, R. A. Oliver, and F. C-P. Massabuau, "\ensuremath\alpha-Ga2O3 grown by low temperature atomic layer deposition on sapphire," Journal of Crystal Growth, vol. 487, p. 23–27, 2018. doi:10.1016/j.jcrysgro.2018.02.014
    [BibTeX] [Abstract] [Download PDF]

    {\ensuremath{\alpha}}-Ga2O3 is a metastable phase of Ga2O3 of interest for wide bandgap engineering since it is isostructural with {\ensuremath{\alpha}}-In2O3 and {\ensuremath{\alpha}}-Al2O3. {\ensuremath{\alpha}}-Ga2O3 is generally synthesised under high pressure (several GPa) or relatively high temperature ({$\sim$}500 oC). In this study, we report the growth of {\ensuremath{\alpha}}-Ga2O3 by low temperature atomic layer deposition (ALD) on sapphire substrate. The film was grown at a rate of 0.48 {\AA}/cycle, and predominantly consists of {\ensuremath{\alpha}}-Ga2O3 in the form of -oriented columns originating from the interface with the substrate. Some inclusions were also present, typically at the tips of the {\ensuremath{\alpha}} phase columns and most likely comprising {\ensuremath{\epsilon}}-Ga2O3. The remainder of the Ga2O3 film - i.e. nearer the surface and between the {\ensuremath{\alpha}}-Ga2O3 columns, was amorphous. The film was found to be highly resistive, as is expected for undoped material. This study demonstrates that {\ensuremath{\alpha}}-Ga2O3 films can be grown by low temperature ALD and suggests the possibility of a new range of ultraviolet optoelectronic and power devices grown by ALD. The study also shows that scanning electron diffraction is a powerful technique to identify the different polymorphs of Ga2O3 present in multiphase samples.

    @article{strathprints69863,
    volume = {487},
    month = {April},
    title = {{\ensuremath{\alpha}}-Ga2O3 grown by low temperature atomic layer deposition on sapphire},
    year = {2018},
    pages = {23--27},
    doi = {10.1016/j.jcrysgro.2018.02.014},
    journal = {Journal of Crystal Growth},
    keywords = {semiconducting gallium compounds, oxides, atomic layer epitaxy, x-ray diffraction, scanning electron diffraction, Physics, Physics and Astronomy(all)},
    url = {https://doi.org/10.1016/j.jcrysgro.2018.02.014},
    issn = {0022-0248},
    abstract = {{\ensuremath{\alpha}}-Ga2O3 is a metastable phase of Ga2O3 of interest for wide bandgap engineering since it is isostructural with {\ensuremath{\alpha}}-In2O3 and {\ensuremath{\alpha}}-Al2O3. {\ensuremath{\alpha}}-Ga2O3 is generally synthesised under high pressure (several GPa) or relatively high temperature ({$\sim$}500 oC). In this study, we report the growth of {\ensuremath{\alpha}}-Ga2O3 by low temperature atomic layer deposition (ALD) on sapphire substrate. The film was grown at a rate of 0.48 {\AA}/cycle, and predominantly consists of {\ensuremath{\alpha}}-Ga2O3 in the form of -oriented columns originating from the interface with the substrate. Some inclusions were also present, typically at the tips of the {\ensuremath{\alpha}} phase columns and most likely comprising {\ensuremath{\epsilon}}-Ga2O3. The remainder of the Ga2O3 film - i.e. nearer the surface and between the {\ensuremath{\alpha}}-Ga2O3 columns, was amorphous. The film was found to be highly resistive, as is expected for undoped material. This study demonstrates that {\ensuremath{\alpha}}-Ga2O3 films can be grown by low temperature ALD and suggests the possibility of a new range of ultraviolet optoelectronic and power devices grown by ALD. The study also shows that scanning electron diffraction is a powerful technique to identify the different polymorphs of Ga2O3 present in multiphase samples.},
    author = {Roberts, J. W. and Jarman, J. C. and Johnstone, D. N. and Midgley, P. A. and Chalker, P. R. and Oliver, R. A. and Massabuau, F. C-P.}
    }

2017

  • J. L. Zhang, K. G. Lagoudakis, Y. Tzeng, C. Dory, M. Radulaski, Y. Kelaita, K. A. Fischer, S. Sun, Z. Shen, N. A. Melosh, S. Chu, and J. Vučković, "Complete coherent control of silicon vacancies in diamond nanopillars containing single defect centers," Optica Applicata, vol. 4, iss. 11, p. 1317–1321, 2017.
    [BibTeX] [Abstract] [Download PDF]

    Arrays of identical and individually addressable qubits lay the foundation for the creation of scalable quantum hardware such as quantum processors and repeaters. Silicon-vacancy (SiV) centers in diamond offer excellent physical properties such as low inhomogeneous broadening, fast photon emission, and a large Debye?Waller factor. The possibility for all-optical ultrafast manipulation and techniques to extend the spin coherence times makes them promising candidates for qubits. Here, we have developed arrays of nanopillars containing single (SiV) centers with high yield, and we demonstrate ultrafast all-optical complete coherent control of the excited state population of a single SiV center at the optical transition frequency. The high quality of the chemical vapor deposition (CVD) grown SiV centers provides excellent spectral stability, which allows us to coherently manipulate and quasi-resonantly read out the excited state population of individual SiV centers on picosecond timescales using ultrafast optical pulses. This work opens new opportunities to create a scalable on-chip diamond platform for quantum information processing and scalable nanophotonics applications.

    @article{strathprints64419,
    volume = {4},
    number = {11},
    month = {November},
    author = {Jingyuan Linda Zhang and Konstantinos G. Lagoudakis and Yan-Kai Tzeng and Constantin Dory and Marina Radulaski and Yousif Kelaita and Kevin A. Fischer and Shuo Sun and Zhi-Xun Shen and Nicholas A. Melosh and Steven Chu and Jelena Vu{\v c}kovi{\'c}},
    title = {Complete coherent control of silicon vacancies in diamond nanopillars containing single defect centers},
    journal = {Optica Applicata},
    pages = {1317--1321},
    year = {2017},
    keywords = {coherent optical effects, defect-center materials , quantum optics, Physics, Electronic, Optical and Magnetic Materials, Atomic and Molecular Physics, and Optics},
    url = {https://strathprints.strath.ac.uk/64419/},
    abstract = {Arrays of identical and individually addressable qubits lay the foundation for the creation of scalable quantum hardware such as quantum processors and repeaters. Silicon-vacancy (SiV) centers in diamond offer excellent physical properties such as low inhomogeneous broadening, fast photon emission, and a large Debye?Waller factor. The possibility for all-optical ultrafast manipulation and techniques to extend the spin coherence times makes them promising candidates for qubits. Here, we have developed arrays of nanopillars containing single (SiV) centers with high yield, and we demonstrate ultrafast all-optical complete coherent control of the excited state population of a single SiV center at the optical transition frequency. The high quality of the chemical vapor deposition (CVD) grown SiV centers provides excellent spectral stability, which allows us to coherently manipulate and quasi-resonantly read out the excited state population of individual SiV centers on picosecond timescales using ultrafast optical pulses. This work opens new opportunities to create a scalable on-chip diamond platform for quantum information processing and scalable nanophotonics applications.}
    }

  • M. Radulaski, K. A. Fischer, K. G. Lagoudakis, J. L. Zhang, and J. Vučković, "Photon blockade in two-emitter-cavity systems," Physical Review Letters, vol. 96, iss. 1, p. 011801(R), 2017.
    [BibTeX] [Abstract] [Download PDF]

    The photon blockade (PB) effect in emitter-cavity systems depends on the anharmonicity of the ladder of dressed energy eigenstates. The recent developments in color center photonics are leading toward experimental demonstrations of multiemitter-cavity solid-state systems with an expanded set of energy levels compared to the traditionally studied single-emitter systems. We focus on the case of N=2 nonidentical quasiatoms strongly coupled to a nanocavity in the bad cavity regime (with parameters within reach of the color center systems), and discover three PB mechanisms: polaritonic, subradiant, and unconventional. The polaritonic PB, which is the conventional mechanism studied in single-emitter-cavity systems, also occurs at the polariton frequencies in multiemitter systems. The subradiant PB is a new interference effect owing to the inhomogeneous broadening of the emitters which results in a purer and a more robust single-photon emission than the polaritonic PB. The unconventional PB in the modeled system corresponds to the suppression of the single- and two-photon correlation statistics and the enhancement of the three-photon correlation statistic. Using the effective Hamiltonian approach, we unravel the origin and the time-domain evolution of these phenomena.

    @Article{strathprints64418,
    author = {Marina Radulaski and Kevin A. Fischer and Konstantinos G. Lagoudakis and Jingyuan Linda Zhang and Jelena Vu{\v c}kovi{\'c}},
    title = {Photon blockade in two-emitter-cavity systems},
    journal = {Physical Review Letters},
    year = {2017},
    volume = {96},
    number = {1},
    pages = {011801(R)},
    month = {July},
    abstract = {The photon blockade (PB) effect in emitter-cavity systems depends on the anharmonicity of the ladder of dressed energy eigenstates. The recent developments in color center photonics are leading toward experimental demonstrations of multiemitter-cavity solid-state systems with an expanded set of energy levels compared to the traditionally studied single-emitter systems. We focus on the case of N=2 nonidentical quasiatoms strongly coupled to a nanocavity in the bad cavity regime (with parameters within reach of the color center systems), and discover three PB mechanisms: polaritonic, subradiant, and unconventional. The polaritonic PB, which is the conventional mechanism studied in single-emitter-cavity systems, also occurs at the polariton frequencies in multiemitter systems. The subradiant PB is a new interference effect owing to the inhomogeneous broadening of the emitters which results in a purer and a more robust single-photon emission than the polaritonic PB. The unconventional PB in the modeled system corresponds to the suppression of the single- and two-photon correlation statistics and the enhancement of the three-photon correlation statistic. Using the effective Hamiltonian approach, we unravel the origin and the time-domain evolution of these phenomena.},
    keywords = {cavity quantum electrodynamics, photon statistics, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/64418/}
    }

  • K. A. Fischer, Y. A. Kelaita, N. V. Sapra, C. Dory, K. G. Lagoudakis, K. Müller, and J. Vučković, "On-chip architecture for self-homodyned nonclassical light," Phys. Rev. Applied, vol. 7, iss. 4, p. 44002, 2017.
    [BibTeX] [Abstract] [Download PDF]

    In the last decade, there has been remarkable progress on the practical integration of on-chip quantum photonic devices, yet quantum-state generators remain an outstanding challenge. Simultaneously, the quantum-dot photonic-crystal-resonator platform has demonstrated a versatility for creating nonclassical light with tunable quantum statistics thanks to a newly discovered self-homodyning interferometric effect that preferentially selects the quantum light over the classical light when using an optimally tuned Fano resonance. In this work, we propose a general structure for the cavity quantum electrodynamical generation of quantum states from a waveguide-integrated version of the quantum-dot photonic-crystal-resonator platform, which is specifically tailored for preferential quantum-state transmission. We support our results with rigorous finite-difference time-domain and quantum-optical simulations and show how our proposed device can serve as a robust generator of highly pure single- and even multiphoton states.

    @Article{strathprints64417,
    author = {Kevin A. Fischer and Yousif A. Kelaita and Neil V. Sapra and Constantin Dory and Konstantinos G. Lagoudakis and Kai M{\"u}ller and Jelena Vu{\v c}kovi{\'c}},
    title = {On-chip architecture for self-homodyned nonclassical light},
    journal = {Phys. Rev. Applied},
    year = {2017},
    volume = {7},
    number = {4},
    pages = {044002},
    month = {April},
    abstract = {In the last decade, there has been remarkable progress on the practical integration of on-chip quantum photonic devices, yet quantum-state generators remain an outstanding challenge. Simultaneously, the quantum-dot photonic-crystal-resonator platform has demonstrated a versatility for creating nonclassical light with tunable quantum statistics thanks to a newly discovered self-homodyning interferometric effect that preferentially selects the quantum light over the classical light when using an optimally tuned Fano resonance. In this work, we propose a general structure for the cavity quantum electrodynamical generation of quantum states from a waveguide-integrated version of the quantum-dot photonic-crystal-resonator platform, which is specifically tailored for preferential quantum-state transmission. We support our results with rigorous finite-difference time-domain and quantum-optical simulations and show how our proposed device can serve as a robust generator of highly pure single- and even multiphoton states.},
    keywords = {optics, photonics, quantum physics, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/64417/}
    }

  • K.  G. Lagoudakis, K.  A. Fischer, T. Sarmiento, P.  L. McMahon, M. Radulaski, J.  L. Zhang, Y. Kelaita, C. Dory, K. Müller, and J. Vučković, "Observation of mollow triplets with tunable interactions in double lambda systems of individual hole spins," Phys. Rev. Lett., vol. 118, iss. 1, p. 13602, 2017.
    [BibTeX] [Abstract] [Download PDF]

    Although individual spins in quantum dots have been studied extensively as qubits, their investigation under strong resonant driving in the scope of accessing Mollow physics is still an open question. Here, we have grown high quality positively charged quantum dots embedded in a planar microcavity that enable enhanced light-matter interactions. Under a strong magnetic field in the Voigt configuration, individual positively charged quantum dots provide a double lambda level structure. Using a combination of above-band and resonant excitation, we observe the formation of Mollow triplets on all optical transitions. We find that when the strong resonant drive power is used to tune the Mollow-triplet lines through each other, we observe anticrossings. We also demonstrate that the interaction that gives rise to the anticrossings can be controlled in strength by tuning the polarization of the resonant laser drive. Quantum-optical modeling of our system fully captures the experimentally observed spectra and provides insight on the complicated level structure that results from the strong driving of the double lambda system.

    @Article{strathprints64416,
    author = {K. G. Lagoudakis and K. A. Fischer and T. Sarmiento and P. L. McMahon and M. Radulaski and J. L. Zhang and Y. Kelaita and C. Dory and K. M{\"u}ller and J. Vu{\v c}kovi{\'c}},
    title = {Observation of mollow triplets with tunable interactions in double lambda systems of individual hole spins},
    journal = {Phys. Rev. Lett.},
    year = {2017},
    volume = {118},
    number = {1},
    pages = {013602},
    month = {January},
    abstract = {Although individual spins in quantum dots have been studied extensively as qubits, their investigation under strong resonant driving in the scope of accessing Mollow physics is still an open question. Here, we have grown high quality positively charged quantum dots embedded in a planar microcavity that enable enhanced light-matter interactions. Under a strong magnetic field in the Voigt configuration, individual positively charged quantum dots provide a double lambda level structure. Using a combination of above-band and resonant excitation, we observe the formation of Mollow triplets on all optical transitions. We find that when the strong resonant drive power is used to tune the Mollow-triplet lines through each other, we observe anticrossings. We also demonstrate that the interaction that gives rise to the anticrossings can be controlled in strength by tuning the polarization of the resonant laser drive. Quantum-optical modeling of our system fully captures the experimentally observed spectra and provides insight on the complicated level structure that results from the strong driving of the double lambda system.},
    keywords = {magneto-optical spectra, semiconductor quantum optics, trions, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/64416/}
    }

  • C. Dory, K. A. Fischer, K. Müller, K. G. Lagoudakis, T. Sarmiento, A. Rundquist, J. L. Zhang, Y. Kelaita, N. V. Sapra, and J. Vučković, "Tuning the photon statistics of a strongly coupled nanophotonic system," Physical Review Letters, vol. 95, iss. 2, p. 23804, 2017.
    [BibTeX] [Abstract] [Download PDF]

    We investigate the dynamics of single- and multiphoton emission from detuned strongly coupled systems based on the quantum-dot?photonic-crystal resonator platform. Transmitting light through such systems can generate a range of nonclassical states of light with tunable photon counting statistics due to the nonlinear ladder of hybridized light-matter states. By controlling the detuning between emitter and resonator, the transmission can be tuned to strongly enhance either single- or two-photon emission processes. Despite the strongly dissipative nature of these systems, we find that by utilizing a self-homodyne interference technique combined with frequency filtering we are able to find a strong two-photon component of the emission in the multiphoton regime. In order to explain our correlation measurements, we propose rate equation models that capture the dominant processes of emission in both the single- and multiphoton regimes. These models are then supported by quantum-optical simulations that fully capture the frequency filtering of emission from our solid-state system.

    @Article{strathprints64415,
    author = {Constantin Dory and Kevin A. Fischer and Kai M{\"u}ller and Konstantinos G. Lagoudakis and Tomas Sarmiento and Armand Rundquist and Jingyuan L. Zhang and Yousif Kelaita and Neil V. Sapra and Jelena Vu{\v c}kovi{\'c}},
    title = {Tuning the photon statistics of a strongly coupled nanophotonic system},
    journal = {Physical Review Letters},
    year = {2017},
    volume = {95},
    number = {2},
    pages = {023804},
    month = {February},
    abstract = {We investigate the dynamics of single- and multiphoton emission from detuned strongly coupled systems based on the quantum-dot?photonic-crystal resonator platform. Transmitting light through such systems can generate a range of nonclassical states of light with tunable photon counting statistics due to the nonlinear ladder of hybridized light-matter states. By controlling the detuning between emitter and resonator, the transmission can be tuned to strongly enhance either single- or two-photon emission processes. Despite the strongly dissipative nature of these systems, we find that by utilizing a self-homodyne interference technique combined with frequency filtering we are able to find a strong two-photon component of the emission in the multiphoton regime. In order to explain our correlation measurements, we propose rate equation models that capture the dominant processes of emission in both the single- and multiphoton regimes. These models are then supported by quantum-optical simulations that fully capture the frequency filtering of emission from our solid-state system.},
    keywords = {photon statistics, photonic crystals, photonics, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/64415/}
    }

  • M. Radulaski, M. Widmann, M. Niethammer, J. L. Zhang, S. Lee, T. Rendler, K. G. Lagoudakis, N. T. Son, E. Janzén, T. Ohshima, J. Wrachtrup, and J. Vučković, "Scalable quantum photonics with single color centers in silicon carbide," Nano Letters, vol. 17, iss. 3, p. 1782–1786, 2017.
    [BibTeX] [Abstract] [Download PDF]

    Silicon carbide is a promising platform for single photon sources, quantum bits (qubits), and nanoscale sensors based on individual color centers. Toward this goal, we develop a scalable array of nanopillars incorporating single silicon vacancy centers in 4H-SiC, readily available for efficient interfacing with free-space objective and lensed-fibers. A commercially obtained substrate is irradiated with 2 MeV electron beams to create vacancies. Subsequent lithographic process forms 800 nm tall nanopillars with 400?1400 nm diameters. We obtain high collection efficiency of up to 22 kcounts/s optical saturation rates from a single silicon vacancy center while preserving the single photon emission and the optically induced electron-spin polarization properties. Our study demonstrates silicon carbide as a readily available platform for scalable quantum photonics architecture relying on single photon sources and qubits.

    @article{strathprints64412,
    volume = {17},
    number = {3},
    month = {February},
    author = {Marina Radulaski and Matthias Widmann and Matthias Niethammer and Jingyuan Linda Zhang and Sang-Yun Lee and Torsten Rendler and Konstantinos G. Lagoudakis and Nguyen Tien Son and Erik Janz{\'e}n and Takeshi Ohshima and J{\"o}rg Wrachtrup and Jelena Vu{\v c}kovi{\'c}},
    note = {This document is the Accepted Manuscript version of a Published Work that appeared in final form in Nano Letters, copyright {\copyright} American Chemical Society after peer review and technical editing by t he publisher. To access the final edited and published work see https://doi.org/10.1021/acs.nanolett.6b05102.},
    title = {Scalable quantum photonics with single color centers in silicon carbide},
    year = {2017},
    journal = {Nano Letters},
    pages = {1782--1786},
    keywords = {color centers, nanopillars, photonics, silicon carbide, spin-qubits, spintronics, Physics, Bioengineering, Materials Science(all), Chemistry(all), Mechanical Engineering, Condensed Matter Physics},
    url = {https://strathprints.strath.ac.uk/64412/},
    abstract = {Silicon carbide is a promising platform for single photon sources, quantum bits (qubits), and nanoscale sensors based on individual color centers. Toward this goal, we develop a scalable array of nanopillars incorporating single silicon vacancy centers in 4H-SiC, readily available for efficient interfacing with free-space objective and lensed-fibers. A commercially obtained substrate is irradiated with 2 MeV electron beams to create vacancies. Subsequent lithographic process forms 800 nm tall nanopillars with 400?1400 nm diameters. We obtain high collection efficiency of up to 22 kcounts/s optical saturation rates from a single silicon vacancy center while preserving the single photon emission and the optically induced electron-spin polarization properties. Our study demonstrates silicon carbide as a readily available platform for scalable quantum photonics architecture relying on single photon sources and qubits.}
    }

  • E. Pascal, S. Singh, B. Hourahine, C. Trager-Cowan, and M. D. Graef, "Dynamical simulations of transmission Kikuchi diffraction (TKD) patterns," Microscopy and Microanalysis, vol. 23, iss. S1, p. 540–541, 2017.
    [BibTeX] [Abstract] [Download PDF]

    Truly nanostructured materials pose a significant spatial resolution challenge to the conventional Electron Backscatter Diffraction (EBSD) characterization technique. Nevertheless, the interaction volume can be reduced by the use of electron transparent samples and the acquisition of electron backscatterlike patterns (EBSP) in transmission mode instead. These transmission Kikuchi diffraction (TKD) patterns are typically acquired by mounting a thin foil, similar to transmission electron microscopy (TEM), and tilting it at a slight angle (20°-30°) from horizontal towards a standard EBSD camera.

    @Article{strathprints63158,
    author = {Elena Pascal and Saransh Singh and Ben Hourahine and Carol Trager-Cowan and Marc De Graef},
    title = {Dynamical simulations of transmission Kikuchi diffraction (TKD) patterns},
    journal = {Microscopy and Microanalysis},
    year = {2017},
    volume = {23},
    number = {S1},
    pages = {540--541},
    month = {August},
    abstract = {Truly nanostructured materials pose a significant spatial resolution challenge to the conventional Electron Backscatter Diffraction (EBSD) characterization technique. Nevertheless, the interaction volume can be reduced by the use of electron transparent samples and the acquisition of electron backscatterlike patterns (EBSP) in transmission mode instead. These transmission Kikuchi diffraction (TKD) patterns are typically acquired by mounting a thin foil, similar to transmission electron microscopy (TEM), and tilting it at a slight angle (20°-30°) from horizontal towards a standard EBSD camera.},
    keywords = {electron backscatter diffraction (EBSD), electron backscatterlike patterns (EBSP), transmission Kikuchi diffraction (TKD) patterns, Physics, Instrumentation},
    url = {https://strathprints.strath.ac.uk/63158/}
    }

  • E. Pascal, B. Hourahine, G. Naresh-Kumar, K. Mingard, and C. Trager-Cowan, "Dislocation contrast in electron channelling contrast images as projections of strain-like components," Materials Today: Proceedings, vol. 5, iss. 6, pp. 14652-14661, 2017.
    [BibTeX] [Abstract] [Download PDF]

    The forward scattering geometry in the scanning electron microscope enables the acquisition of electron channelling contrast imaging (ECCI) micrographs. These images contain diffraction information from the beam of electrons ``channelling in'' into the sample. Since small, localised strains strongly affect the electron diffraction, defects which introduce lattice displacement in the region of the surface the electron beam is interacting with will be revealed as district variation in backscattered electron intensity. By acquiring multiple images from the same area in different diffraction conditions and comparing them against modelled predictions of defect strain sampled by diffraction, it is possible to characterise these defects. Here we discuss the relation between the elastic strain introduced by a threading dislocation intersecting the surface and the contrast features observed in the electron channelling contrast image of that region. Preliminary simulated channelling contrast images are shown for dislocations with known line direction and Burgers vectors using a two-beam dynamical diffraction model. These are demonstrated to be in qualitative agreement with measured images of dislocated polar wurtzite GaN acquired with two different diffraction condition.

    @Article{strathprints63048,
    author = {E. Pascal and B. Hourahine and G. Naresh-Kumar and K. Mingard and C. Trager-Cowan},
    title = {Dislocation contrast in electron channelling contrast images as projections of strain-like components},
    journal = {Materials Today: Proceedings},
    year = {2017},
    volume = {5},
    number = {6},
    pages = {14652-14661},
    month = {July},
    abstract = {The forward scattering geometry in the scanning electron microscope enables the acquisition of electron channelling contrast imaging (ECCI) micrographs. These images contain diffraction information from the beam of electrons ``channelling in'' into the sample. Since small, localised strains strongly affect the electron diffraction, defects which introduce lattice displacement in the region of the surface the electron beam is interacting with will be revealed as district variation in backscattered electron intensity. By acquiring multiple images from the same area in different diffraction conditions and comparing them against modelled predictions of defect strain sampled by diffraction, it is possible to characterise these defects. Here we discuss the relation between the elastic strain introduced by a threading dislocation intersecting the surface and the contrast features observed in the electron channelling contrast image of that region. Preliminary simulated channelling contrast images are shown for dislocations with known line direction and Burgers vectors using a two-beam dynamical diffraction model. These are demonstrated to be in qualitative agreement with measured images of dislocated polar wurtzite GaN acquired with two different diffraction condition.},
    keywords = {electron channelling contrast imaging (ECCI), dislocations, scanning electron microscope (SEM), image simulation, defect imaging, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/63048/}
    }

  • D. Graham, R. Goodacre, H. Arnolds, J. Masson, G. Schatz, J. Baumberg, D. Kim, J. Aizpurua, W. Lum, A. Silvestri, B. de Nijs, Y. Xu, G. D. Martino, M. Natan, S. Schlücker, P. Wuytens, I. Bruzas, C. Kuttner, M. Hardy, R. Chikkaraddy, N. M. Sabanés, I. Delfino, P. Dawson, S. Gawinkowski, N. Bontempi, S. Mahajan, S. Reich, B. Hourahine, S. Bell, A. Królikowska, M. Porter, A. Keeler, M. Kamp, A. Fountain, C. Fasolato, F. Giorgis, J. C. Otero, C. Matricardi, R. V. Duyne, J. Lombardi, V. Deckert, and L. Velleman, "Theory of SERS enhancement : general discussion," Faraday Discussions, vol. 205, p. 173–211, 2017.
    [BibTeX] [Abstract] [Download PDF]

    Rohit Chikkaraddy opened the discussion of the Introductory Lecture: Regarding quantifying the chemical enhancement, you showed a systematic change in the SERS enhancement for halide substituted molecules due to charge transfer from the metal. Is the extra enhancement due to an inherent increase in the Raman cross-section of the molecule? How do you go about referencing, as the charge transfer changes the vibrational frequency?

    @article{strathprints62740,
    volume = {205},
    month = {November},
    title = {Theory of SERS enhancement : general discussion},
    author = {Duncan Graham and Roy Goodacre and Heike Arnolds and Jean-Francois Masson and George Schatz and Jeremy Baumberg and Dong-Ho Kim and Javier Aizpurua and William Lum and Alessandro Silvestri and Bart de Nijs and Yikai Xu and Giuliana Di Martino and Michael Natan and Sebastian Schl{\"u}cker and Pieter Wuytens and Ian Bruzas and Christian Kuttner and Mike Hardy and Rohit Chikkaraddy and Natalia Mart{\'i}n Saban{\'e}s and Ines Delfino and Paul Dawson and Sylwester Gawinkowski and Nicol{\`o} Bontempi and Sumeet Mahajan and Stephanie Reich and Ben Hourahine and Steven Bell and Agata Kr{\'o}likowska and Marc Porter and Alex Keeler and Marlous Kamp and Augustus Fountain and Claudia Fasolato and Fabrizio Giorgis and Juan C Otero and Cristiano Matricardi and Richard Van Duyne and John Lombardi and Volker Deckert and Leonora Velleman},
    year = {2017},
    pages = {173--211},
    journal = {Faraday Discussions},
    keywords = {surface enhanced Raman scattering, spectral range, Physics, Physical and Theoretical Chemistry},
    url = {https://strathprints.strath.ac.uk/62740/},
    abstract = {Rohit Chikkaraddy opened the discussion of the Introductory Lecture: Regarding quantifying the chemical enhancement, you showed a systematic change in the SERS enhancement for halide substituted molecules due to charge transfer from the metal. Is the extra enhancement due to an inherent increase in the Raman cross-section of the molecule? How do you go about referencing, as the charge transfer changes the vibrational frequency?}
    }

  • P. -M. Coulon, J. R. Pugh, M. Athanasiou, G. Kusch, L. E. D. Boulbar, A. Sarua, R. Smith, R. W. Martin, T. Wang, M. Cryan, D. W. E. Allsopp, and P. A. Shields, "Optical properties and resonant cavity modes in axial InGaN/GaN nanotube microcavities," Optics Express, vol. 25, iss. 23, p. 28246–28257, 2017.
    [BibTeX] [Abstract] [Download PDF]

    Microcavities based on group-III nitride material offer a notable platform for the investigation of light-matter interactions as well as the development of devices such as high efficiency light emitting diodes (LEDs) and low-threshold nanolasers. Disk or tube geometries in particular are attractive for low-threshold lasing applications due to their ability to support high finesse whispering gallery modes (WGMs) and small modal volumes. In this article we present the fabrication of homogenous and dense arrays of axial InGaN/GaN nanotubes via a combination of displacement Talbot lithography (DTL) for patterning and inductively coupled plasma top-down dry-etching. Optical characterization highlights the homogeneous emission from nanotube structures. Power-dependent continuous excitation reveals a non-uniform light distribution within a single nanotube, with vertical confinement between the bottom and top facets, and radial confinement within the active region. Finite-difference time-domain simulations, taking into account the particular shape of the outer diameter, indicate that the cavity mode of a single nanotube has a mixed WGM-vertical Fabry-Perot mode (FPM) nature. Additional simulations demonstrate that the improvement of the shape symmetry and dimensions primarily influence the Q-factor of the WGMs whereas the position of the active region impacts the coupling efficiency with one or a family of vertical FPMs. These results show that regular arrays of axial InGaN/GaN nanotubes can be achieved via a low-cost, fast and large-scale process based on DTL and top-down etching. These techniques open a new perspective for cost effective fabrication of nano-LED and nano-laser structures along with bio-chemical sensing applications.

    @article{strathprints62687,
    volume = {25},
    number = {23},
    month = {November},
    author = {P. -M. Coulon and J. R. Pugh and M. Athanasiou and G. Kusch and E. D. Le Boulbar and A. Sarua and R. Smith and R. W. Martin and T. Wang and M. Cryan and D. W.E. Allsopp and P. A. Shields},
    title = {Optical properties and resonant cavity modes in axial InGaN/GaN nanotube microcavities},
    journal = {Optics Express},
    pages = {28246--28257},
    year = {2017},
    keywords = {microcavities, light-matter interactions, light emitting diodes, axial InGaN/GaN nanotubes, displacement Talbot lithography (DTL), Optics. Light, Atomic and Molecular Physics, and Optics},
    url = {https://strathprints.strath.ac.uk/62687/},
    abstract = {Microcavities based on group-III nitride material offer a notable platform for the investigation of light-matter interactions as well as the development of devices such as high efficiency light emitting diodes (LEDs) and low-threshold nanolasers. Disk or tube geometries in particular are attractive for low-threshold lasing applications due to their ability to support high finesse whispering gallery modes (WGMs) and small modal volumes. In this article we present the fabrication of homogenous and dense arrays of axial InGaN/GaN nanotubes via a combination of displacement Talbot lithography (DTL) for patterning and inductively coupled plasma top-down dry-etching. Optical characterization highlights the homogeneous emission from nanotube structures. Power-dependent continuous excitation reveals a non-uniform light distribution within a single nanotube, with vertical confinement between the bottom and top facets, and radial confinement within the active region. Finite-difference time-domain simulations, taking into account the particular shape of the outer diameter, indicate that the cavity mode of a single nanotube has a mixed WGM-vertical Fabry-Perot mode (FPM) nature. Additional simulations demonstrate that the improvement of the shape symmetry and dimensions primarily influence the Q-factor of the WGMs whereas the position of the active region impacts the coupling efficiency with one or a family of vertical FPMs. These results show that regular arrays of axial InGaN/GaN nanotubes can be achieved via a low-cost, fast and large-scale process based on DTL and top-down etching. These techniques open a new perspective for cost effective fabrication of nano-LED and nano-laser structures along with bio-chemical sensing applications.}
    }

  • I. A. Ajia, P. R. Edwards, Y. Pak, E. Belekov, M. A. Roldan, N. Wei, Z. Liu, R. W. Martin, and I. S. Roqan, "Generated carrier dynamics in V-pit enhanced InGaN/GaN light emitting diode," ACS Photonics, vol. 5, pp. 820-826, 2017.
    [BibTeX] [Abstract] [Download PDF]

    We investigate the effects of V-pits on the optical properties of a state-of-the art highly efficient, blue InGaN/GaN multi-quantum-well (MQW) light emitting diode (LED) with high internal quantum efficiency (IQE) of > 80%. The LED is structurally enhanced by incorporating pre-MQW InGaN strain relief layer with low InN content and patterned sapphire substrate. For comparison, a conventional (unenhanced) InGaN/GaN MQW LED (with IQE of 46%) grown under similar conditions was subjected to the same measurements. Scanning transmission electron microscopy (STEM) reveals the absence of V-pits in the unenhanced LED, whereas in the enhanced LED, V-pits with {10-11} facets, emerging from threading dislocations (TDs) were prominent. Cathodoluminescence mapping reveals the luminescence properties near the V-pits, showing that the formation of V-pit defects can encourage the growth of defect-neutralizing barriers around TD defect states. The diminished contribution of TDs in the MQWs allows indium-rich localization sites to act as efficient recombination centers. Photoluminescence and time-resolved spectroscopy measurements suggest that the V-pits play a significant role in the generated carrier rate and droop mechanism, showing that the quantum confined Stark effect is suppressed at low generated carrier density, after which the carrier dynamics and droop are governed by the carrier overflow effect.

    @Article{strathprints62657,
    author = {Idris. A. Ajia and Paul R. Edwards and Yusin Pak and Ermek Belekov and Manuel A. Roldan and Nini Wei and Zhiqiang Liu and Robert W. Martin and Iman S. Roqan},
    title = {Generated carrier dynamics in V-pit enhanced InGaN/GaN light emitting diode},
    journal = {ACS Photonics},
    year = {2017},
    volume = {5},
    pages = {820-826},
    month = {December},
    abstract = {We investigate the effects of V-pits on the optical properties of a state-of-the art highly efficient, blue InGaN/GaN multi-quantum-well (MQW) light emitting diode (LED) with high internal quantum efficiency (IQE) of > 80%. The LED is structurally enhanced by incorporating pre-MQW InGaN strain relief layer with low InN content and patterned sapphire substrate. For comparison, a conventional (unenhanced) InGaN/GaN MQW LED (with IQE of 46%) grown under similar conditions was subjected to the same measurements. Scanning transmission electron microscopy (STEM) reveals the absence of V-pits in the unenhanced LED, whereas in the enhanced LED, V-pits with {10-11} facets, emerging from threading dislocations (TDs) were prominent. Cathodoluminescence mapping reveals the luminescence properties near the V-pits, showing that the formation of V-pit defects can encourage the growth of defect-neutralizing barriers around TD defect states. The diminished contribution of TDs in the MQWs allows indium-rich localization sites to act as efficient recombination centers. Photoluminescence and time-resolved spectroscopy measurements suggest that the V-pits play a significant role in the generated carrier rate and droop mechanism, showing that the quantum confined Stark effect is suppressed at low generated carrier density, after which the carrier dynamics and droop are governed by the carrier overflow effect.},
    keywords = {InGaN, efficiency droop, light emitting diode, carrier dynamics, time-resolved spectroscopy, Optics. Light, Atomic and Molecular Physics, and Optics},
    url = {https://strathprints.strath.ac.uk/62657/}
    }

  • A. K. Singh, K. P. O'Donnell, P. R. Edwards, D. Cameron, K. Lorenz, M. J. Kappers, M. Boćkowski, M. Yamaga, and R. Prakash, "Luminescence of Eu³⁺ in GaN(Mg, Eu) : transitions from the ⁵D₁ level," Applied Physics Letters, vol. 111, p. 241105, 2017.
    [BibTeX] [Abstract] [Download PDF]

    Eu-doped GaN(Mg) exemplifies hysteretic photochromic switching between two configurations, Eu0 and Eu1(Mg), of the same photoluminescent defect. Using above bandgap excitation, we studied the temperature dependence of photoluminescence (TDPL) of transitions from the excited ⁵D₁ level of Eu³⁺ for both configurations of this defect. During sample cooling, ⁵D₁→⁷F₀,₁,₂ transitions of Eu0 manifest themselves at temperatures below ~200 K, while those of Eu1(Mg) appear only during switching. The observed line positions verify crystal field energies of the ⁷F₀,₁,₂ levels. TDPL profiles of ⁵D₁→⁷F₁ and ⁵D₀→7FJ transitions of Eu0 show an onset of observable emission from the ⁵D₁ level coincident with the previously observed, but hitherto unexplained, decrease in the intensity of its ⁵D₀→⁷FJ emission on cooling below 200 K. Hence the ⁵D₀→⁷FJ TDPL anomaly signals a back-up of ⁵D₁ population due to a reduction in phonon-assisted relaxation between ⁵D₁ and ⁵D₀ levels at lower temperatures. We discuss this surprising result in the light of temperature-dependent transient luminescence measurements of Eu0.

    @Article{strathprints62516,
    author = {A.K. Singh and K.P. O'Donnell and P.R. Edwards and D. Cameron and K. Lorenz and M.J. Kappers and M. Bo{\'c}kowski and M. Yamaga and R. Prakash},
    title = {Luminescence of Eu³⁺ in GaN(Mg, Eu) : transitions from the ⁵D₁ level},
    journal = {Applied Physics Letters},
    year = {2017},
    volume = {111},
    pages = {241105},
    month = {November},
    abstract = {Eu-doped GaN(Mg) exemplifies hysteretic photochromic switching between two configurations, Eu0 and Eu1(Mg), of the same photoluminescent defect. Using above bandgap excitation, we studied the temperature dependence of photoluminescence (TDPL) of transitions from the excited ⁵D₁ level of Eu³⁺ for both configurations of this defect. During sample cooling, ⁵D₁→⁷F₀,₁,₂ transitions of Eu0 manifest themselves at temperatures below ~200 K, while those of Eu1(Mg) appear only during switching. The observed line positions verify crystal field energies of the ⁷F₀,₁,₂ levels. TDPL profiles of ⁵D₁→⁷F₁ and ⁵D₀→7FJ transitions of Eu0 show an onset of observable emission from the ⁵D₁ level coincident with the previously observed, but hitherto unexplained, decrease in the intensity of its ⁵D₀→⁷FJ emission on cooling below 200 K. Hence the ⁵D₀→⁷FJ TDPL anomaly signals a back-up of ⁵D₁ population due to a reduction in phonon-assisted relaxation between ⁵D₁ and ⁵D₀ levels at lower temperatures. We discuss this surprising result in the light of temperature-dependent transient luminescence measurements of Eu0.},
    keywords = {photoluminescence, bandgap, temperature, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/62516/}
    }

  • D. Tiwari, E. Skidchenko, J. Bowers, M. V. Yakushev, R. Martin, and D. J. Fermin, "Spectroscopic and electrical signatures of acceptor states in solution processed Cu₂ZnSn(S,Se)₄ solar cells," Journal of Materials Chemistry. C, vol. 5, p. 12647–12858, 2017.
    [BibTeX] [Abstract] [Download PDF]

    The nature and dynamics of acceptor states in solution-processed Cu₂ZnSn(S,Se)₄ (CZTSSe) thin films are investigated by variable temperature photoluminescence (PL) and electrical impedance spectroscopy. Highly pure I-4 phase CZTSSe with the composition Cu₁.₆ZnSn₀.₉(S₀.₂₃Se₀.₇₇)₄ is synthesized by sequentially spin coating of dimethyl-formamide/isopropanol solutions containing metal salts and thiourea onto Mo coated glass, followed by annealing in an Se atmosphere at 540 C. As annealed films are highly compact with a thickness of 1.3 micron and grain sizes above 800 nm, with a band gap of 1.18 eV. Photovoltaic devices of 0.25 cm² with the architecture glass/Mo/CZTSSe/CdS/i-ZnO/Al:ZnO demonstrate a power conversion efficiency reaching up to 5.7% in the absence of an antireflective coating. Under AM 1.5G illumination at 296 K, the best device shows a 396 mV open-circuit voltage (VOC), 27.8 mA cm⁻² short-circuit current (Jsc) and 52% fill factor (FF). The overall dispersion of these parameters is under 15% for a total of 20 devices. In the near IR region, PL spectra are dominated by two broad and asymmetrical bands at 1.14 eV (PL1) and 0.95 eV (PL2) with characteristic power and temperature dependences. Analysis of the device electrical impedance spectra also reveals two electron acceptor states with the same activation energy as those observed by PL. This allows assigning PL1 as a radiative recombination at localized copper vacancies (VCu), while PL2 is associated with CuZn antisites, broadened by potential fluctuations (band tails). The impact of these states on device performance as well as other parameters, such as barrier collection heights introduced by partial selenization of the back contact, are discussed.

    @Article{strathprints62349,
    author = {Devendra Tiwari and Ekaterina Skidchenko and Jake Bowers and M. V. Yakushev and Robert Martin and David J. Fermin},
    title = {Spectroscopic and electrical signatures of acceptor states in solution processed Cu₂ZnSn(S,Se)₄ solar cells},
    journal = {Journal of Materials Chemistry. C},
    year = {2017},
    volume = {5},
    pages = {12647--12858},
    month = {October},
    abstract = {The nature and dynamics of acceptor states in solution-processed Cu₂ZnSn(S,Se)₄ (CZTSSe) thin films are investigated by variable temperature photoluminescence (PL) and electrical impedance spectroscopy. Highly pure I-4 phase CZTSSe with the composition Cu₁.₆ZnSn₀.₉(S₀.₂₃Se₀.₇₇)₄ is synthesized by sequentially spin coating of dimethyl-formamide/isopropanol solutions containing metal salts and thiourea onto Mo coated glass, followed by annealing in an Se atmosphere at 540 C. As annealed films are highly compact with a thickness of 1.3 micron and grain sizes above 800 nm, with a band gap of 1.18 eV. Photovoltaic devices of 0.25 cm² with the architecture glass/Mo/CZTSSe/CdS/i-ZnO/Al:ZnO demonstrate a power conversion efficiency reaching up to 5.7% in the absence of an antireflective coating. Under AM 1.5G illumination at 296 K, the best device shows a 396 mV open-circuit voltage (VOC), 27.8 mA cm⁻² short-circuit current (Jsc) and 52% fill factor (FF). The overall dispersion of these parameters is under 15% for a total of 20 devices. In the near IR region, PL spectra are dominated by two broad and asymmetrical bands at 1.14 eV (PL1) and 0.95 eV (PL2) with characteristic power and temperature dependences. Analysis of the device electrical impedance spectra also reveals two electron acceptor states with the same activation energy as those observed by PL. This allows assigning PL1 as a radiative recombination at localized copper vacancies (VCu), while PL2 is associated with CuZn antisites, broadened by potential fluctuations (band tails). The impact of these states on device performance as well as other parameters, such as barrier collection heights introduced by partial selenization of the back contact, are discussed.},
    keywords = {thin films, acceptor states, spectroscopy, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/62349/}
    }

  • R. Zhao, A. Rossi, S. P. Giblin, J. D. Fletcher, F. E. Hudson, M. Möttönen, M. Kataoka, and A. S. Dzurak, "Thermal-error regime in high-accuracy gigahertz single-electron pumping," Physical Review Applied, vol. 8, iss. 4, 2017. doi:10.1103/PhysRevApplied.8.044021
    [BibTeX] [Abstract] [Download PDF]

    Single-electron pumps based on semiconductor quantum dots are promising candidates for the emerging quantum standard of electrical current. They can transfer discrete charges with part-per-million (ppm) precision in nanosecond time scales. Here, we employ a metal-oxide-semiconductor silicon quantum dot to experimentally demonstrate high-accuracy gigahertz single-electron pumping in the regime where the number of electrons trapped in the dot is determined by the thermal distribution in the reservoir leads. In a measurement with traceability to primary voltage and resistance standards, the averaged pump current over the quantized plateau, driven by a 1-GHz sinusoidal wave in the absence of a magnetic field, is equal to the ideal value of ef within a measurement uncertainty as low as 0.27 ppm.

    @article{strathprints68705,
    volume = {8},
    number = {4},
    month = {October},
    title = {Thermal-error regime in high-accuracy gigahertz single-electron pumping},
    year = {2017},
    doi = {10.1103/PhysRevApplied.8.044021},
    journal = {Physical Review Applied},
    keywords = {single electron pump, quantum dots, acoustic waves, Physics, Physics and Astronomy(all)},
    url = {https://doi.org/10.1103/PhysRevApplied.8.044021},
    issn = {2331-7019},
    abstract = {Single-electron pumps based on semiconductor quantum dots are promising candidates for the emerging quantum standard of electrical current. They can transfer discrete charges with part-per-million (ppm) precision in nanosecond time scales. Here, we employ a metal-oxide-semiconductor silicon quantum dot to experimentally demonstrate high-accuracy gigahertz single-electron pumping in the regime where the number of electrons trapped in the dot is determined by the thermal distribution in the reservoir leads. In a measurement with traceability to primary voltage and resistance standards, the averaged pump current over the quantized plateau, driven by a 1-GHz sinusoidal wave in the absence of a magnetic field, is equal to the ideal value of ef within a measurement uncertainty as low as 0.27 ppm.},
    author = {Zhao, R. and Rossi, A. and Giblin, S. P. and Fletcher, J. D. and Hudson, F. E. and M{\"o}tt{\"o}nen, M. and Kataoka, M. and Dzurak, A. S.}
    }

  • A. Rossi, R. Zhao, A. S. Dzurak, and M. F. Gonzalez-Zalba, "Dispersive readout of a silicon quantum dot with an accumulation-mode gate sensor," Applied Physics Letters, vol. 110, iss. 21, 2017. doi:10.1063/1.4984224
    [BibTeX] [Abstract] [Download PDF]

    Sensitive charge detection has enabled qubit readout in solid-state systems. Recently, an alternative to the well-established charge detection via on-chip electrometers has emerged, based on in situ gate detectors and radio-frequency dispersive readout techniques. This approach promises to facilitate scalability by removing the need for additional device components devoted to sensing. Here, we perform gate-based dispersive readout of an accumulation-mode silicon quantum dot. We observe that the response of an accumulation-mode gate detector is significantly affected by its bias voltage, particularly if this exceeds the threshold for electron accumulation. We discuss and explain these results in light of the competing capacitive contributions to the dispersive response.

    @article{strathprints68706,
    volume = {110},
    number = {21},
    month = {May},
    title = {Dispersive readout of a silicon quantum dot with an accumulation-mode gate sensor},
    year = {2017},
    doi = {10.1063/1.4984224},
    journal = {Applied Physics Letters},
    keywords = {charge detection, silicon quantum dots, quantum dots, Physics, Physics and Astronomy (miscellaneous)},
    url = {https://doi.org/10.1063/1.4984224},
    issn = {0003-6951},
    abstract = {Sensitive charge detection has enabled qubit readout in solid-state systems. Recently, an alternative to the well-established charge detection via on-chip electrometers has emerged, based on in situ gate detectors and radio-frequency dispersive readout techniques. This approach promises to facilitate scalability by removing the need for additional device components devoted to sensing. Here, we perform gate-based dispersive readout of an accumulation-mode silicon quantum dot. We observe that the response of an accumulation-mode gate detector is significantly affected by its bias voltage, particularly if this exceeds the threshold for electron accumulation. We discuss and explain these results in light of the competing capacitive contributions to the dispersive response.},
    author = {Rossi, A. and Zhao, R. and Dzurak, A. S. and Gonzalez-Zalba, M. F.}
    }

  • F. C-P. Massabuau, P. Chen, M. K. Horton, S. L. Rhode, C. X. Ren, T. J. O'Hanlon, A. Kovács, M. J. Kappers, C. J. Humphreys, R. E. Dunin-Borkowski, and R. A. Oliver, "Carrier localization in the vicinity of dislocations in InGaN," Journal of Applied Physics, vol. 121, iss. 1, 2017. doi:10.1063/1.4973278
    [BibTeX] [Abstract] [Download PDF]

    We present a multi-microscopy study of dislocations in InGaN, whereby the same threading dislocation was observed under several microscopes (atomic force microscopy, scanning electron microscopy, cathodoluminescence imaging and spectroscopy, transmission electron microscopy), and its morphological optical and structural properties directly correlated. We achieved this across an ensemble of defects large enough to be statistically significant. Our results provide evidence that carrier localization occurs in the direct vicinity of the dislocation through the enhanced formation of In-N chains and atomic condensates, thus limiting non-radiative recombination of carriers at the dislocation core. We highlight that the localization properties in the vicinity of threading dislocations arise as a consequence of the strain field of the individual dislocation and the additional strain field building between interacting neighboring dislocations. Our study therefore suggests that careful strain and dislocation distribution engineering may further improve the resilience of InGaN-based devices to threading dislocations. Besides providing a new understanding of dislocations in InGaN, this paper presents a proof-of-concept for a methodology which is relevant to many problems in materials science.

    @article{strathprints69871,
    volume = {121},
    number = {1},
    month = {January},
    note = {This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Massabuau, F, Chen, P, Horton, MK, Rhode, SL, Ren, CX, O'Hanlon, TJ, Kovacs, A, Kappers, MJ, Humphreys, CJ, Dunin-Borkowski, RE \& Oliver, RA 2017, 'Carrier localization in the vicinity of dislocations in InGaN' Journal of Applied Physics, vol. 121, no. 013104 and may be found at https://doi.org/10.1063/1.4973278.},
    title = {Carrier localization in the vicinity of dislocations in InGaN},
    journal = {Journal of Applied Physics},
    doi = {10.1063/1.4973278},
    year = {2017},
    keywords = {materials science, atomic force microscopy, chemical elements, luminescence, Monte Carlo methods, semiconductors, Physics, Physics and Astronomy(all)},
    url = {https://doi.org/10.1063/1.4973278},
    issn = {0021-8979},
    abstract = {We present a multi-microscopy study of dislocations in InGaN, whereby the same threading dislocation was observed under several microscopes (atomic force microscopy, scanning electron microscopy, cathodoluminescence imaging and spectroscopy, transmission electron microscopy), and its morphological optical and structural properties directly correlated. We achieved this across an ensemble of defects large enough to be statistically significant. Our results provide evidence that carrier localization occurs in the direct vicinity of the dislocation through the enhanced formation of In-N chains and atomic condensates, thus limiting non-radiative recombination of carriers at the dislocation core. We highlight that the localization properties in the vicinity of threading dislocations arise as a consequence of the strain field of the individual dislocation and the additional strain field building between interacting neighboring dislocations. Our study therefore suggests that careful strain and dislocation distribution engineering may further improve the resilience of InGaN-based devices to threading dislocations. Besides providing a new understanding of dislocations in InGaN, this paper presents a proof-of-concept for a methodology which is relevant to many problems in materials science.},
    author = {Massabuau, F. C-P. and Chen, P. and Horton, M. K. and Rhode, S. L. and Ren, C. X. and O'Hanlon, T. J. and Kov{\'a}cs, A. and Kappers, M. J. and Humphreys, C. J. and Dunin-Borkowski, R. E. and Oliver, R. A.}
    }

  • F. C-P. Massabuau, S. L. Rhode, M. K. Horton, T. J. O'Hanlon, A. Kovacs, M. S. Zielinski, M. J. Kappers, R. E. Dunin-Borkowski, C. J. Humphreys, and R. A. Oliver, "Dislocations in AlGaN: core structure, atom segregation, and optical properties," Nano Letters, vol. 17, iss. 8, p. 4846–4852, 2017. doi:10.1021/acs.nanolett.7b01697
    [BibTeX] [Abstract] [Download PDF]

    We conducted a comprehensive investigation of dislocations in Al0.46Ga0.54N. Using aberration-corrected scanning transmission electron microscopy and energy dispersive X-ray spectroscopy, the atomic structure and atom distribution at the dislocation core have been examined. We report that the core configuration of dislocations in AlGaN is consistent with that of other materials in the III-Nitride system. However, we observed that the dissociation of mixed-type dislocations is impeded by alloying GaN with AlN, which is confirmed by our experimental observation of Ga and Al atom segregation in the tensile and compressive parts of the dislocations, respectively. Investigation of the optical properties of the dislocations shows that the atom segregation at dislocations has no significant effect on the intensity recorded by cathodoluminescence in the vicinity of the dislocations. These results are in contrast with the case of dislocations in In0.09Ga0.91N where segregation of In and Ga atoms also occurs but results in carrier localization limiting non-radiative recombination at the dislocation. This study therefore sheds light on why InGaN-based devices are generally more resilient to dislocations than their AlGaN-based counterparts.

    @article{strathprints69879,
    volume = {17},
    number = {8},
    month = {August},
    note = {his document is the Accepted Manuscript version of a Published Work that appeared in final form in Nano Letters, copyright {\copyright} American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.nanolett.7b01697.},
    title = {Dislocations in AlGaN: core structure, atom segregation, and optical properties},
    year = {2017},
    journal = {Nano Letters},
    doi = {10.1021/acs.nanolett.7b01697},
    pages = {4846--4852},
    keywords = {AlGaN, InGaN, dislocation, aberration-corrected TEM, cathodoluminescence, Physics, Physics and Astronomy(all)},
    url = {https://doi.org/10.1021/acs.nanolett.7b01697},
    issn = {1530-6992},
    abstract = {We conducted a comprehensive investigation of dislocations in Al0.46Ga0.54N. Using aberration-corrected scanning transmission electron microscopy and energy dispersive X-ray spectroscopy, the atomic structure and atom distribution at the dislocation core have been examined. We report that the core configuration of dislocations in AlGaN is consistent with that of other materials in the III-Nitride system. However, we observed that the dissociation of mixed-type dislocations is impeded by alloying GaN with AlN, which is confirmed by our experimental observation of Ga and Al atom segregation in the tensile and compressive parts of the dislocations, respectively. Investigation of the optical properties of the dislocations shows that the atom segregation at dislocations has no significant effect on the intensity recorded by cathodoluminescence in the vicinity of the dislocations. These results are in contrast with the case of dislocations in In0.09Ga0.91N where segregation of In and Ga atoms also occurs but results in carrier localization limiting non-radiative recombination at the dislocation. This study therefore sheds light on why InGaN-based devices are generally more resilient to dislocations than their AlGaN-based counterparts.},
    author = {Massabuau, Fabien C-P. and Rhode, Sneha L. and Horton, Matthew K. and O'Hanlon, Thomas J. and Kovacs, Andras and Zielinski, Marcin S. and Kappers, Menno J. and Dunin-Borkowski, Rafal E. and Humphreys, Colin J. and Oliver, Rachel A.}
    }

  • C. G. Bryce, E. D. Le Boulbar, P. -M. Coulon, P. R. Edwards, I. Gîrgel, D. W. E. Allsopp, P. A. Shields, and R. W. Martin, "Quantum well engineering in InGaN/GaN core-shell nanorod structures," Journal of Physics D: Applied Physics, vol. 50, p. 42LT01, 2017.
    [BibTeX] [Abstract] [Download PDF]

    We report the ability to control relative InN incorporation in InGaN/GaN quantum wells (QWs) grown on the semi-polar and non-polar facets of a core-shell nanorod LED structure by varying the growth conditions. A study of the cathodoluminescence emitted from series of structures with different growth temperatures and pressures for the InGaN QW layer revealed that increasing the growth pressure had the effect of increasing InN incorporation on the semi-polar facets, while increasing the growth temperature improves the uniformity of light emission from the QWs on the non-polar facets.

    @Article{strathprints61754,
    author = {C. G. Bryce and Le Boulbar, E. D. and P.-M. Coulon and P. R. Edwards and I. G{\^i}rgel and D. W. E. Allsopp and P. A. Shields and R. W. Martin},
    title = {Quantum well engineering in InGaN/GaN core-shell nanorod structures},
    journal = {Journal of Physics D: Applied Physics},
    year = {2017},
    volume = {50},
    pages = {42LT01},
    month = {September},
    abstract = {We report the ability to control relative InN incorporation in InGaN/GaN quantum wells (QWs) grown on the semi-polar and non-polar facets of a core-shell nanorod LED structure by varying the growth conditions. A study of the cathodoluminescence emitted from series of structures with different growth temperatures and pressures for the InGaN QW layer revealed that increasing the growth pressure had the effect of increasing InN incorporation on the semi-polar facets, while increasing the growth temperature improves the uniformity of light emission from the QWs on the non-polar facets.},
    keywords = {cathodoluminescence, quantum wells, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/61754/}
    }

  • G. Naresh-Kumar, A. Vilalta-Clemente, H. Jussila, A. Winkelmann, G. Nolze, S. Vespucci, S. Nagarajan, A. J. Wilkinson, and C. Trager-Cowan, "Quantitative imaging of anti-phase domains by polarity sensitive orientation mapping using electron backscatter diffraction," Scientific Reports, vol. 7, p. 10916, 2017.
    [BibTeX] [Abstract] [Download PDF]

    Advanced structural characterisation techniques which are rapid to use, non-destructive and structurally definitive on the nanoscale are in demand, especially for a detailed understanding of extended-defects and their influence on the properties of materials. We have applied the electron backscatter diffraction (EBSD) technique in a scanning electron microscope to non-destructively characterise and quantify antiphase domains (APDs) in GaP thin films grown on different (001) Si substrates with different offcuts. We were able to image and quantify APDs by relating the asymmetrical intensity distributions observed in the EBSD patterns acquired experimentally and comparing the same with the dynamical electron diffraction simulations. Additionally mean angular error maps were also plotted using automated cross-correlation based approaches to image APDs. Samples grown on substrates with a 4° offcut from the [110] do not show any APDs, whereas samples grown on the exactly oriented substrates contain APDs. The procedures described in our work can be adopted for characterising a wide range of other material systems possessing non-centrosymmetric point groups.

    @Article{strathprints61621,
    author = {G. Naresh-Kumar and A. Vilalta-Clemente and H. Jussila and A. Winkelmann and G. Nolze and S. Vespucci and S. Nagarajan and A.J. Wilkinson and C. Trager-Cowan},
    title = {Quantitative imaging of anti-phase domains by polarity sensitive orientation mapping using electron backscatter diffraction},
    journal = {Scientific Reports},
    year = {2017},
    volume = {7},
    pages = {10916},
    month = {August},
    abstract = {Advanced structural characterisation techniques which are rapid to use, non-destructive and structurally definitive on the nanoscale are in demand, especially for a detailed understanding of extended-defects and their influence on the properties of materials. We have applied the electron backscatter diffraction (EBSD) technique in a scanning electron microscope to non-destructively characterise and quantify antiphase domains (APDs) in GaP thin films grown on different (001) Si substrates with different offcuts. We were able to image and quantify APDs by relating the asymmetrical intensity distributions observed in the EBSD patterns acquired experimentally and comparing the same with the dynamical electron diffraction simulations. Additionally mean angular error maps were also plotted using automated cross-correlation based approaches to image APDs. Samples grown on substrates with a 4° offcut from the [110] do not show any APDs, whereas samples grown on the exactly oriented substrates contain APDs. The procedures described in our work can be adopted for characterising a wide range of other material systems possessing non-centrosymmetric point groups.},
    keywords = {quantitative imaging, orientation mapping, thin films, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/61621/}
    }

  • J. Bruckbauer, Z. Li, N. Gunasekar, M. Warzecha, P. Edwards, L. Jiu, J. Bai, T. Wang, C. Trager-Cowan, and R. Martin, "Spatially-resolved optical and structural properties of semi-polar (11-22) AlₓGa₁₋ₓN with x up to 0.56," Scientific Reports, vol. 7, p. 10804, 2017.
    [BibTeX] [Abstract] [Download PDF]

    Pushing the emission wavelength of efficient ultraviolet (UV) emitters further into the deep-UV requires material with high crystal quality, while also reducing the detrimental effects of built-in electric fields. Crack-free semi-polar (11-22) AlₓGa₁₋ₓN epilayers with AlN contents up to x=0.56 and high crystal quality were achieved using an overgrowth method employing GaN microrods on m-sapphire. Two dominant emission peaks were identified using cathodoluminescence hyperspectral imaging. The longer wavelength peak originates near and around chevron-shaped features, whose density is greatly increased for higher contents. The emission from the majority of the surface is dominated by the shorter wavelength peak, influenced by the presence of basal-plane stacking faults (BSFs). Due to the overgrowth technique BSFs are bunched up in parallel stripes where the lower wavelength peak is broadened and hence appears slightly redshifted compared with the higher quality regions in-between. Additionally, the density of threading dislocations in these region is one order of magnitude lower compared with areas affected by BSFs as ascertained by electron channelling contrast imaging. Overall, the luminescence properties of semi-polar AlGaN epilayers are strongly influenced by the overgrowth method, which shows that reducing the density of extended defects improves the optical performance of high AlN content AlGaN structures.

    @Article{strathprints61607,
    author = {Jochen Bruckbauer and Zhi Li and Naresh Gunasekar and Monika Warzecha and Paul Edwards and Ling Jiu and Jie Bai and Tao Wang and Carol Trager-Cowan and Robert Martin},
    title = {Spatially-resolved optical and structural properties of semi-polar (11-22) {AlₓGa₁₋ₓN} with x up to 0.56},
    journal = {Scientific Reports},
    year = {2017},
    volume = {7},
    pages = {10804},
    month = {August},
    abstract = {Pushing the emission wavelength of efficient ultraviolet (UV) emitters further into the deep-UV requires material with high crystal quality, while also reducing the detrimental effects of built-in electric fields. Crack-free semi-polar (11-22) AlₓGa₁₋ₓN epilayers with AlN contents up to x=0.56 and high crystal quality were achieved using an overgrowth method employing GaN microrods on m-sapphire. Two dominant emission peaks were identified using cathodoluminescence hyperspectral imaging. The longer wavelength peak originates near and around chevron-shaped features, whose density is greatly increased for higher contents. The emission from the majority of the surface is dominated by the shorter wavelength peak, influenced by the presence of basal-plane stacking faults (BSFs). Due to the overgrowth technique BSFs are bunched up in parallel stripes where the lower wavelength peak is broadened and hence appears slightly redshifted compared with the higher quality regions in-between. Additionally, the density of threading dislocations in these region is one order of magnitude lower compared with areas affected by BSFs as ascertained by electron channelling contrast imaging. Overall, the luminescence properties of semi-polar AlGaN epilayers are strongly influenced by the overgrowth method, which shows that reducing the density of extended defects improves the optical performance of high AlN content AlGaN structures.},
    keywords = {emission wavelengths, III-nitride structures, cathodoluminescence, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/61607/}
    }

  • C. Ni, G. J. Hedley, J. Payne, V. Svrcek, C. McDonald, L. K. Jagadamma, P. Edwards, R. Martin, D. Mariotti, P. Maguire, I. Samuel, and J. Irvine, "Charge carrier localised in zero-dimensional (CH₃NH₃)₃Bi₂I₉ clusters," Nature Communications, vol. 8, p. 170, 2017.
    [BibTeX] [Abstract] [Download PDF]

    A metal-organic hybrid perovskite with 3-D framework of metal halide octahedra has been reported as a low-cost, solution processable absorber for a thin film solar cell with a power conversion efficiency over 20%. Low-dimensional layered perovskites with metal halide slabs separated by the insulating organic layers are reported to show higher stability, but the efficiencies of the solar cells are limited by the anisotropy of the crystals because of the confinement of excitons. In order to explore the confinement and transport of excitons in zero-dimensional metal-organic hybrid materials, a highly-orientated film of methylammonium bismuth halide, (CH₃NH₃)₃Bi₂I₉, with a nanometer sized core cluster of Bi₂I₉³⁻ surrounded by insulating CH₃NH₃⁺, was deposited on a quartz substrate via solution processing. The (CH₃NH₃)₃Bi₂I₉ film shows highly anisotropic photoluminescence emission and excitation due to the large proportion of localized excitons coupled with a small number of delocalised excitons from inter-cluster energy transfer. The abrupt increase in photoluminescence quantum yield at excitation energy above twice band gap could indicate a quantum cutting due to the low dimensionality.

    @Article{strathprints60928,
    author = {Chengsheng Ni and Gordon J. Hedley and Julia Payne and Vladimir Svrcek and Calum McDonald and Lethy Krishnan Jagadamma and Paul Edwards and Robert Martin and Davide Mariotti and Paul Maguire and Ifor Samuel and John Irvine},
    title = {Charge carrier localised in zero-dimensional {(CH₃NH₃)₃Bi₂I₉} clusters},
    journal = {Nature Communications},
    year = {2017},
    volume = {8},
    pages = {170},
    abstract = {A metal-organic hybrid perovskite with 3-D framework of metal halide octahedra has been reported as a low-cost, solution processable absorber for a thin film solar cell with a power conversion efficiency over 20%. Low-dimensional layered perovskites with metal halide slabs separated by the insulating organic layers are reported to show higher stability, but the efficiencies of the solar cells are limited by the anisotropy of the crystals because of the confinement of excitons. In order to explore the confinement and transport of excitons in zero-dimensional metal-organic hybrid materials, a highly-orientated film of methylammonium bismuth halide, (CH₃NH₃)₃Bi₂I₉, with a nanometer sized core cluster of Bi₂I₉³⁻ surrounded by insulating CH₃NH₃⁺, was deposited on a quartz substrate via solution processing. The (CH₃NH₃)₃Bi₂I₉ film shows highly anisotropic photoluminescence emission and excitation due to the large proportion of localized excitons coupled with a small number of delocalised excitons from inter-cluster energy transfer. The abrupt increase in photoluminescence quantum yield at excitation energy above twice band gap could indicate a quantum cutting due to the low dimensionality.},
    keywords = {perovskite, metal-organic, metal-halide, octahedra, absorber , thin-film solar cell, photoluminescence quantum yield, Physics, Chemistry, Biochemistry, Genetics and Molecular Biology(all), Chemistry(all), Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/60928/}
    }

  • A. Núñez-Cascajero, S. Valdueza-Felip, L. Monteagudo-Lerma, E. Monroy, E. Taylor-Shaw, R. W. Martin, M. González-Herráez, and F. B. Naranjo, "In-rich AlₓIn₁₋ₓN grown by RF-sputtering on sapphire : from closely-packed columnar to high-surface quality compact layers," Journal of Physics D: Applied Physics, vol. 50, iss. 6, p. 65101, 2017.
    [BibTeX] [Abstract] [Download PDF]

    The structural, morphological, electrical and optical properties of In-rich AlₓIn₁₋ₓN(0 < x < 0.39) layers grown by reactive radio-frequency (RF) sputtering on sapphire areinvestigated as a function of the deposition parameters. The RF power applied to the aluminumtarget (0 W - 150 W) and substrate temperature (300 °C - 550 °C) are varied. X-ray diffraction measurements reveal that all samples have a wurtzite crystallographic structure oriented withthe c-axis along the growth direction. The aluminum composition is tuned by changing thepower applied to the aluminum target while keeping the power applied to the indium targetfixed at 40 W. When increasing the Al content from 0 to 0.39, the room-temperature opticalband gap is observed to blue-shift from 1.76 eV to 2.0 eV, strongly influenced by the Burstein-Moss effect. Increasing the substrate temperature, results in an evolution of the morphologyfrom closely-packed columnar to compact. For a substrate temperature of 500 °C and RFpower for Al of 150 W, compact Al₀.₃₉In₀.₆₁N films with a smooth surface (root-mean-squaresurface roughness below 1 nm) are produced.

    @Article{strathprints60907,
    author = {A N{\'u}{\~n}ez-Cascajero and S Valdueza-Felip and L Monteagudo-Lerma and E Monroy and E Taylor-Shaw and R W Martin and M Gonz{\'a}lez-Herr{\'a}ez and F B Naranjo},
    title = {In-rich {AlₓIn₁₋ₓN} grown by RF-sputtering on sapphire : from closely-packed columnar to high-surface quality compact layers},
    journal = {Journal of Physics D: Applied Physics},
    year = {2017},
    volume = {50},
    number = {6},
    pages = {065101},
    month = {January},
    abstract = {The structural, morphological, electrical and optical properties of In-rich AlₓIn₁₋ₓN(0 < x < 0.39) layers grown by reactive radio-frequency (RF) sputtering on sapphire areinvestigated as a function of the deposition parameters. The RF power applied to the aluminumtarget (0 W - 150 W) and substrate temperature (300 °C - 550 °C) are varied. X-ray diffraction measurements reveal that all samples have a wurtzite crystallographic structure oriented withthe c-axis along the growth direction. The aluminum composition is tuned by changing thepower applied to the aluminum target while keeping the power applied to the indium targetfixed at 40 W. When increasing the Al content from 0 to 0.39, the room-temperature opticalband gap is observed to blue-shift from 1.76 eV to 2.0 eV, strongly influenced by the Burstein-Moss effect. Increasing the substrate temperature, results in an evolution of the morphologyfrom closely-packed columnar to compact. For a substrate temperature of 500 °C and RFpower for Al of 150 W, compact Al₀.₃₉In₀.₆₁N films with a smooth surface (root-mean-squaresurface roughness below 1 nm) are produced.},
    keywords = {III-nitrides, AlInN, RF-sputtering, semiconductor, Physics, Surfaces, Coatings and Films, Acoustics and Ultrasonics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/60907/}
    }

  • A. V. Mudryi, M. V. Yakushev, V. A. Volkov, V. D. Zhivulko, O. M. Borodavchenko, and R. W. Martin, "Influence of the growth method on the photoluminescence spectra and electronic properties of CuInS₂ single crystals," Journal of Luminescence, vol. 186, p. 123–126, 2017.
    [BibTeX] [Abstract] [Download PDF]

    A comparative analysis of free and bound excitons in the photoluminescence (PL) spectra of CuInS₂ single crystals grown by the traveling heater (THM) and the chemical vapor transport (CVT) methods is presented. The values of the binding energy of the A free exciton (18.5 and 19.7 meV), determined by measurements of the spectral positions of the ground and excited states, allowed the Bohr radii (3.8 and 3.7 nm), bandgaps (1.5536 and 1.5548 eV) and dielectric constants (10.2 and 9.9) to be calculated for CuInS₂ crystals grown by THM and CVT, respectively.

    @Article{strathprints60887,
    author = {A.V. Mudryi and M.V. Yakushev and V.A. Volkov and V.D. Zhivulko and O.M. Borodavchenko and R.W. Martin},
    title = {Influence of the growth method on the photoluminescence spectra and electronic properties of {CuInS₂} single crystals},
    journal = {Journal of Luminescence},
    year = {2017},
    volume = {186},
    pages = {123--126},
    month = {June},
    abstract = {A comparative analysis of free and bound excitons in the photoluminescence (PL) spectra of CuInS₂ single crystals grown by the traveling heater (THM) and the chemical vapor transport (CVT) methods is presented. The values of the binding energy of the A free exciton (18.5 and 19.7 meV), determined by measurements of the spectral positions of the ground and excited states, allowed the Bohr radii (3.8 and 3.7 nm), bandgaps (1.5536 and 1.5548 eV) and dielectric constants (10.2 and 9.9) to be calculated for CuInS₂ crystals grown by THM and CVT, respectively.},
    keywords = {photoluminescence, CuInS2, excitons, traveling heater, chemical vapor transport, chalcopyrite semiconductor, growth method, Optics. Light, Atomic and Molecular Physics, and Optics},
    url = {http://strathprints.strath.ac.uk/60887/}
    }

  • J. T. Griffiths, C. X. Ren, P. -M. Coulon, L. E. D. Boulbar, C. G. Bryce, I. Girgel, A. Howkins, I. Boyd, R. W. Martin, D. W. E. Allsopp, P. A. Shields, C. J. Humphreys, and R. A. Oliver, "Structural impact on the nanoscale optical properties of InGaN core-shell nanorods," Applied Physics Letters, vol. 110, p. 172105, 2017.
    [BibTeX] [Abstract] [Download PDF]

    III-nitride core-shell nanorods are promising for the development of high efficiency light emitting diodes and novel optical devices. We reveal the nanoscale optical and structural properties of core-shell InGaN nanorods formed by combined top-down etching and regrowth to achieve non-polar sidewalls with a low density of extended defects. While the luminescence is uniform along the non-polar {1-100} sidewalls, nano-cathodoluminescence shows a sharp reduction in the luminescent intensity at the intersection of the non-polar {1-100} facets. The reduction in the luminescent intensity is accompanied by a reduction in the emission energy localised at the apex of the corners. Correlative compositional analysis reveals an increasing indium content towards the corner except at the apex itself. We propose that the observed variations in the structure and chemistry are responsible for the changes in the optical properties at the corners of the nanorods. The insights revealed by nano-cathodoluminescence will aid in the future development of higher efficiency core-shell nanorods.

    @Article{strathprints60877,
    author = {J. T. Griffiths and C. X. Ren and P.-M. Coulon and E. D. Le Boulbar and C. G. Bryce and I. Girgel and A. Howkins and I. Boyd and R. W. Martin and D. W. E. Allsopp and P. A. Shields and C. J. Humphreys and R. A. Oliver},
    title = {Structural impact on the nanoscale optical properties of InGaN core-shell nanorods},
    journal = {Applied Physics Letters},
    year = {2017},
    volume = {110},
    pages = {172105},
    month = {April},
    abstract = {III-nitride core-shell nanorods are promising for the development of high efficiency light emitting diodes and novel optical devices. We reveal the nanoscale optical and structural properties of core-shell InGaN nanorods formed by combined top-down etching and regrowth to achieve non-polar sidewalls with a low density of extended defects. While the luminescence is uniform along the non-polar {1-100} sidewalls, nano-cathodoluminescence shows a sharp reduction in the luminescent intensity at the intersection of the non-polar {1-100} facets. The reduction in the luminescent intensity is accompanied by a reduction in the emission energy localised at the apex of the corners. Correlative compositional analysis reveals an increasing indium content towards the corner except at the apex itself. We propose that the observed variations in the structure and chemistry are responsible for the changes in the optical properties at the corners of the nanorods. The insights revealed by nano-cathodoluminescence will aid in the future development of higher efficiency core-shell nanorods.},
    keywords = {nanorods, light emitting diodes, nano-cathodoluminescence, nitride semiconductors, quantum confined Stark effect, efficiency droop, Optics. Light, Physics and Astronomy (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/60877/}
    }

  • S. Magalhães, N. Franco, I. M. Watson, R. W. Martin, K. P. O'Donnell, H. P. D. Schenk, F. Tang, T. C. Sadler, M. J. Kappers, R. A. Oliver, T. Monteiro, T. L. Martin, P. A. J. Bagot, M. P. Moody, E. Alves, and K. Lorenz, "Validity of Vegard's rule for AlₓIn₁₋ₓN (0.08 < x < 0.28) thin films grown on GaN templates," Journal of Physics D: Applied Physics, vol. 50, iss. 20, p. 205107, 2017.
    [BibTeX] [Abstract] [Download PDF]

    In this work, comparative x-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS) measurements allow a comprehensive characterization of AlₓIn₁₋ₓN thin films grown on GaN. Within the limits of experimental accuracy, and in the compositional range 0.08  <  x <  0.28, the lattice parameters of the alloys generally obey Vegard's rule, varying linearly with the InN fraction. Results are also consistent with the small deviation from linear behaviour suggested by Darakchieva et al (2008 Appl. Phys. Lett. 93 261908). However, unintentional incorporation of Ga, revealed by atom probe tomography (APT) at levels below the detection limit for RBS, may also affect the lattice parameters. Furthermore, in certain samples the compositions determined by XRD and RBS differ significantly. This fact, which was interpreted in earlier publications as an indication of a deviation from Vegard's rule, may rather be ascribed to the influence of defects or impurities on the lattice parameters of the alloy. The wide-ranging set of AlₓIn₁₋ₓN films studied allowed furthermore a detailed investigation of the composition leading to lattice-matching of AlₓIn₁₋ₓN/GaN bilayers.

    @Article{strathprints60823,
    author = {S Magalh{\~a}es and N Franco and I M Watson and R W Martin and K P O'Donnell and H P D Schenk and F Tang and T C Sadler and M J Kappers and R A Oliver and T Monteiro and T L Martin and P A J Bagot and M P Moody and E Alves and K Lorenz},
    title = {Validity of Vegard's rule for {AlₓIn₁₋ₓN (0.08 < x < 0.28)} thin films grown on GaN templates},
    journal = {Journal of Physics D: Applied Physics},
    year = {2017},
    volume = {50},
    number = {20},
    pages = {205107},
    month = {April},
    note = {This is an author-created, un-copyedited version of an article accepted for publication in Journal of Physics D: Applied Physics. The publisher is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at https://doi.org/10.1088/1361-6463/aa69dc},
    abstract = {In this work, comparative x-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS) measurements allow a comprehensive characterization of AlₓIn₁₋ₓN thin films grown on GaN. Within the limits of experimental accuracy, and in the compositional range 0.08  <  x <  0.28, the lattice parameters of the alloys generally obey Vegard's rule, varying linearly with the InN fraction. Results are also consistent with the small deviation from linear behaviour suggested by Darakchieva et al (2008 Appl. Phys. Lett. 93 261908). However, unintentional incorporation of Ga, revealed by atom probe tomography (APT) at levels below the detection limit for RBS, may also affect the lattice parameters. Furthermore, in certain samples the compositions determined by XRD and RBS differ significantly. This fact, which was interpreted in earlier publications as an indication of a deviation from Vegard's rule, may rather be ascribed to the influence of defects or impurities on the lattice parameters of the alloy. The wide-ranging set of AlₓIn₁₋ₓN films studied allowed furthermore a detailed investigation of the composition leading to lattice-matching of AlₓIn₁₋ₓN/GaN bilayers.},
    keywords = {comparative x-ray diffraction, Rutherford backscattering spectrometry , Vegard's rule, lattice parameters, gallium, thin films, Physics, Surfaces, Coatings and Films, Acoustics and Ultrasonics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/60823/}
    }

  • M. V. Yakushev, M. A. Sulimov, J. Márquez-Prieto, I. Forbes, J. Krustok, P. R. Edwards, V. D. Zhivulko, O. M. Borodavchenko, A. V. Mudryi, and R. W. Martin, "Influence of the copper content on the optical properties of CZTSe thin films," Solar Energy Materials and Solar Cells, vol. 168, pp. 69-77, 2017.
    [BibTeX] [Abstract] [Download PDF]

    We present an optical spectroscopy study of Cu₂ZnSnSe₄ (CZTSe) thin films deposited on Mo/glass substrates. The [Cu]/[Zn+Sn] ratio in these films varies from nearly stoichiometric to strongly Cu deficient and Zn rich. Increasing Cu deficiency and Zn excess widens the bandgap Eg, determined using photoluminescence excitation (PLE) at 4.2 K, from 0.99 eV to 1.03 eV and blue shifts the dominant band in the photoluminescence (PL) spectra from 0.83 eV to 0.95 eV. The PL spectra of the near stoichiometric film reveal two bands: a dominant band centred at 0.83 eV and a lower intensity one at 0.93 eV. The temperature and excitation intensity dependence of the PL spectra help to identify the recombination mechanisms of the observed emission bands as free-to-bound: recombination of free electrons with holes localised at acceptors affected by randomly distributed potential fluctuations. Both the mean depth of such fluctuations, determined by analysing the shape of the dominant bands, and the broadening energy, estimated from the PLE spectra, become smaller with increasing Cu deficiency and Zn excess which also widens Eg due to an improved ordering of the Cu/Zn atoms. These changes in the elemental composition induce a significant blue shift of the PL bands exceeding the Eg widening. This is attributed to a change of the dominant acceptor for a shallow one, and is beneficial for the solar cell performance. Film regions with a higher degree of Cu/Zn ordering are present in the near stoichiometric film generating the second PL band at 0.93 eV.

    @Article{strathprints60524,
    author = {M. V. Yakushev and M. A. Sulimov and J. M{\'a}rquez-Prieto and I. Forbes and J. Krustok and P. R. Edwards and V. D. Zhivulko and O. M. Borodavchenko and A. V. Mudryi and R. W. Martin},
    title = {Influence of the copper content on the optical properties of {CZTSe} thin films},
    journal = {Solar Energy Materials and Solar Cells},
    year = {2017},
    volume = {168},
    pages = {69-77},
    month = {April},
    abstract = {We present an optical spectroscopy study of Cu₂ZnSnSe₄ (CZTSe) thin films deposited on Mo/glass substrates. The [Cu]/[Zn+Sn] ratio in these films varies from nearly stoichiometric to strongly Cu deficient and Zn rich. Increasing Cu deficiency and Zn excess widens the bandgap Eg, determined using photoluminescence excitation (PLE) at 4.2 K, from 0.99 eV to 1.03 eV and blue shifts the dominant band in the photoluminescence (PL) spectra from 0.83 eV to 0.95 eV. The PL spectra of the near stoichiometric film reveal two bands: a dominant band centred at 0.83 eV and a lower intensity one at 0.93 eV. The temperature and excitation intensity dependence of the PL spectra help to identify the recombination mechanisms of the observed emission bands as free-to-bound: recombination of free electrons with holes localised at acceptors affected by randomly distributed potential fluctuations. Both the mean depth of such fluctuations, determined by analysing the shape of the dominant bands, and the broadening energy, estimated from the PLE spectra, become smaller with increasing Cu deficiency and Zn excess which also widens Eg due to an improved ordering of the Cu/Zn atoms. These changes in the elemental composition induce a significant blue shift of the PL bands exceeding the Eg widening. This is attributed to a change of the dominant acceptor for a shallow one, and is beneficial for the solar cell performance. Film regions with a higher degree of Cu/Zn ordering are present in the near stoichiometric film generating the second PL band at 0.93 eV.},
    keywords = {copper, thin films, optical spectroscopy, photoluminescence excitation, stoichiometric film, Cu2ZnSnSe4, defects, zinc, Physics, Surfaces, Coatings and Films, Electronic, Optical and Magnetic Materials, Renewable Energy, Sustainability and the Environment},
    url = {http://strathprints.strath.ac.uk/60524/},
    }

  • A. Winkelmann, G. Nolze, S. Vespucci, N. Gunasekar, C. Trager-Cowan, A. Vilalta-Clemente, A. J. Wilkinson, and M. Vos, "Diffraction effects and inelastic electron transport in angle-resolved microscopic imaging applications," Journal of Microscopy, vol. 267, iss. 3, p. 330–346, 2017.
    [BibTeX] [Abstract] [Download PDF]

    We analyze the signal formation process for scanning electron microscopic imaging applications on crystalline specimens. In accordance with previous investigations, we find nontrivial effects of incident beam diffraction on the backscattered electron distribution in energy and momentum. Specifically, incident beam diffraction causes angular changes of the backscattered electron distribution which we identify as the dominant mechanism underlying pseudocolor orientation imaging using multiple, angle-resolving detectors. Consequently, diffraction effects of the incident beam and their impact on the subsequent coherent and incoherent electron transport need to be taken into account for an in-depth theoretical modeling of the energy and momentum distribution of electrons backscattered from crystalline sample regions. Our findings have implications for the level of theoretical detail that can be necessary for the interpretation of complex imaging modalities such as electron channeling contrast imaging (ECCI) of defects in crystals. If the solid angle of detection is limited to specific regions of the backscattered electron momentum distribution, the image contrast that is observed in ECCI and similar applications can be strongly affected by incident beam diffraction and topographic effects from the sample surface. As an application, we demonstrate characteristic changes in the resulting images if different properties of the backscattered electron distribution are used for the analysis of a GaN thin film sample containing dislocations.

    @Article{strathprints60424,
    author = {Aimo Winkelmann and Gert Nolze and Stefano Vespucci and Naresh Gunasekar and Carol Trager-Cowan and Arantxa Vilalta-Clemente and Angus J. Wilkinson and Maarten Vos},
    title = {Diffraction effects and inelastic electron transport in angle-resolved microscopic imaging applications},
    journal = {Journal of Microscopy},
    year = {2017},
    volume = {267},
    number = {3},
    pages = {330--346},
    month = {March},
    abstract = {We analyze the signal formation process for scanning electron microscopic imaging applications on crystalline specimens. In accordance with previous investigations, we find nontrivial effects of incident beam diffraction on the backscattered electron distribution in energy and momentum. Specifically, incident beam diffraction causes angular changes of the backscattered electron distribution which we identify as the dominant mechanism underlying pseudocolor orientation imaging using multiple, angle-resolving detectors. Consequently, diffraction effects of the incident beam and their impact on the subsequent coherent and incoherent electron transport need to be taken into account for an in-depth theoretical modeling of the energy and momentum distribution of electrons backscattered from crystalline sample regions. Our findings have implications for the level of theoretical detail that can be necessary for the interpretation of complex imaging modalities such as electron channeling contrast imaging (ECCI) of defects in crystals. If the solid angle of detection is limited to specific regions of the backscattered electron momentum distribution, the image contrast that is observed in ECCI and similar applications can be strongly affected by incident beam diffraction and topographic effects from the sample surface. As an application, we demonstrate characteristic changes in the resulting images if different properties of the backscattered electron distribution are used for the analysis of a GaN thin film sample containing dislocations.},
    keywords = {electron diffraction, electron microscope, cathodoluminescence, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/60424/}
    }

  • E. D. Le Boulbar, J. Priesol, M. Nouf-Allehiani, G. Naresh-Kumar, S. Fox, C. Trager-Cowan, A. Šatka, D. W. E. Allsopp, and P. A. Shields, "Design and fabrication of enhanced lateral growth for dislocation reduction in GaN using nanodashes," Journal of Crystal Growth, vol. 466, p. 30–38, 2017.
    [BibTeX] [Abstract] [Download PDF]

    The semiconductor gallium nitride is the material at the centre of energy-efficient solid-state lighting and is becoming increasingly important in high-power and high-frequency electronics. Reducing the dislocation density of gallium nitride planar layers is important for improving the performance and reliability of devices, such as light-emitting diodes and high-electron-mobility transistors. The patterning of selective growth masks is one technique for forcing a three-dimensional growth mode in order to control the propagation of threading defects to the active device layers. The morphology of the three-dimensional growth front is determined by the relative growth rates of the different facets that are formed, and for GaN is typically limited by the slow-growing \{1 ?1 0 1\} facets. We demonstrate how the introduction of nanodash growth windows can be oriented in an array to preserve fast-growing \{1 1 ?2 2\} facets at the early stage of growth to accelerate coalescence of three-dimensional structures into a continuous GaN layer. Cathodoluminescence and Electron Channelling Contrast Imaging methods, both used to measure the threading dislocation density, reveal that the dislocations are organised and form a distinctive pattern according to the underlying mask. By optimising the arrangement of nanodashes and the nanodash density, the threading dislocation density of GaN on sapphire epilayers can be reduced significantly from 109 cm?2 to 3.0 {$\times$} 107 cm?2. Raman spectroscopy, used to monitor the strain in the overgrown GaN epilayers, shows that the position of the GaN E2H phonon mode peak was reduced as the dash density increases for a sample grown via pendeo-epitaxy whilst no obvious change was recorded for a sample grown via more conventional epitaxial lateral overgrowth. These results show how growth mask design can be used to circumvent limitations imposed by the growth dynamics. Moreover, they have revealed a greater understanding of the influence of the growth process on the dislocation density which will lead to higher performing electronic and optoelectronic devices as a result of the lower dislocation densities achieved.

    @Article{strathprints60304,
    author = {Le Boulbar, E. D. and J. Priesol and M. Nouf-Allehiani and G. Naresh-Kumar and S. Fox and C. Trager-Cowan and A. {\v S}atka and D. W. E. Allsopp and P. A. Shields},
    title = {Design and fabrication of enhanced lateral growth for dislocation reduction in {GaN} using nanodashes},
    journal = {Journal of Crystal Growth},
    year = {2017},
    volume = {466},
    pages = {30--38},
    month = {May},
    abstract = {The semiconductor gallium nitride is the material at the centre of energy-efficient solid-state lighting and is becoming increasingly important in high-power and high-frequency electronics. Reducing the dislocation density of gallium nitride planar layers is important for improving the performance and reliability of devices, such as light-emitting diodes and high-electron-mobility transistors. The patterning of selective growth masks is one technique for forcing a three-dimensional growth mode in order to control the propagation of threading defects to the active device layers. The morphology of the three-dimensional growth front is determined by the relative growth rates of the different facets that are formed, and for GaN is typically limited by the slow-growing \{1 ?1 0 1\} facets. We demonstrate how the introduction of nanodash growth windows can be oriented in an array to preserve fast-growing \{1 1 ?2 2\} facets at the early stage of growth to accelerate coalescence of three-dimensional structures into a continuous GaN layer. Cathodoluminescence and Electron Channelling Contrast Imaging methods, both used to measure the threading dislocation density, reveal that the dislocations are organised and form a distinctive pattern according to the underlying mask. By optimising the arrangement of nanodashes and the nanodash density, the threading dislocation density of GaN on sapphire epilayers can be reduced significantly from 109 cm?2 to 3.0 {$\times$} 107 cm?2. Raman spectroscopy, used to monitor the strain in the overgrown GaN epilayers, shows that the position of the GaN E2H phonon mode peak was reduced as the dash density increases for a sample grown via pendeo-epitaxy whilst no obvious change was recorded for a sample grown via more conventional epitaxial lateral overgrowth. These results show how growth mask design can be used to circumvent limitations imposed by the growth dynamics. Moreover, they have revealed a greater understanding of the influence of the growth process on the dislocation density which will lead to higher performing electronic and optoelectronic devices as a result of the lower dislocation densities achieved.},
    keywords = {defects, metalorganic chemical vapour epitaxy, pendeoepitaxy, selective epitaxy, nitrides, semiconducting III-V materials, gallium nitride, solid-state lighting, cathodoluminescence, electron channelling contrast imaging, Optics. Light, Electrical engineering. Electronics Nuclear engineering, Physics and Astronomy(all), Electrical and Electronic Engineering},
    url = {http://strathprints.strath.ac.uk/60304/}
    }

  • S. Vespucci, G. Naresh-Kumar, C. Trager-Cowan, K. P. Mingard, D. Maneuski, V. O'Shea, and A. Winkelmann, "Diffractive triangulation of radiative point sources," Applied Physics Letters, vol. 110, iss. 12, p. 124103, 2017.
    [BibTeX] [Abstract] [Download PDF]

    We describe a general method to determine the location of a point source of waves relative to a two-dimensional single-crystalline active pixel detector. Based on the inherent structural sensitivity of crystalline sensor materials, characteristic detector diffraction patterns can be used to triangulate the location of a wave emitter. The principle described here can be applied to various types of waves provided that the detector elements are suitably structured. As a prototypical practical application of the general detection principle, a digital hybrid pixel detector is used to localize a source of electrons for Kikuchi diffraction pattern measurements in the scanning electron microscope. This approach provides a promising alternative method to calibrate Kikuchi patterns for accurate measurements of microstructural crystal orientations, strains, and phase distributions.

    @Article{strathprints60196,
    author = {S. Vespucci and G. Naresh-Kumar and C. Trager-Cowan and K. P. Mingard and D. Maneuski and V. O'Shea and A. Winkelmann},
    title = {Diffractive triangulation of radiative point sources},
    journal = {Applied Physics Letters},
    year = {2017},
    volume = {110},
    number = {12},
    pages = {124103},
    month = {March},
    abstract = {We describe a general method to determine the location of a point source of waves relative to a two-dimensional single-crystalline active pixel detector. Based on the inherent structural sensitivity of crystalline sensor materials, characteristic detector diffraction patterns can be used to triangulate the location of a wave emitter. The principle described here can be applied to various types of waves provided that the detector elements are suitably structured. As a prototypical practical application of the general detection principle, a digital hybrid pixel detector is used to localize a source of electrons for Kikuchi diffraction pattern measurements in the scanning electron microscope. This approach provides a promising alternative method to calibrate Kikuchi patterns for accurate measurements of microstructural crystal orientations, strains, and phase distributions.},
    keywords = {instrumentation, pixel detector, crystalline sensor materials, Physics, Physics and Astronomy (miscellaneous), Radiation},
    url = {http://strathprints.strath.ac.uk/60196/}
    }

  • Z. Li, L. Wang, L. Jiu, J. Bruckbauer, Y. Gong, Y. Zhang, J. Bai, R. W. Martin, and T. Wang, "Optical investigation of semi-polar (11-22) AlₓGa₁₋ₓN with high Al composition," Applied Physics Letters, vol. 110, iss. 9, p. 91102, 2017.
    [BibTeX] [Abstract] [Download PDF]

    Exciton localization disturbs uniform population inversion, leading to an increase in threshold current for lasing. High Al content AlGaN is required for the fabrication of deep ultra-violet LDs, generating exciton localization. Photoluminescence and cathodoluminescence measurements have been performed on high quality semi-polar (11-22) AlxGa1-xN alloys with high Al composition in order to study the optical properties of both the near-band-edge (NBE) emission and the basal-plane stacking faults (BSFs) related emission, demonstrating different behaviours. Further comparison with the exciton localization of their c-plane counterparts exhibits that the exciton localization in semi-polar (11-22) AlGaN is much smaller than that in c-plane AlGaN.

    @Article{strathprints59864,
    author = {Z. Li and L. Wang and L. Jiu and J. Bruckbauer and Y. Gong and Y. Zhang and J. Bai and R. W. Martin and T. Wang},
    title = {Optical investigation of semi-polar (11-22) {AlₓGa₁₋ₓN} with high {Al} composition},
    journal = {Applied Physics Letters},
    year = {2017},
    volume = {110},
    number = {9},
    pages = {091102},
    month = {February},
    abstract = {Exciton localization disturbs uniform population inversion, leading to an increase in threshold current for lasing. High Al content AlGaN is required for the fabrication of deep ultra-violet LDs, generating exciton localization. Photoluminescence and cathodoluminescence measurements have been performed on high quality semi-polar (11-22) AlxGa1-xN alloys with high Al composition in order to study the optical properties of both the near-band-edge (NBE) emission and the basal-plane stacking faults (BSFs) related emission, demonstrating different behaviours. Further comparison with the exciton localization of their c-plane counterparts exhibits that the exciton localization in semi-polar (11-22) AlGaN is much smaller than that in c-plane AlGaN.},
    keywords = {exciton localization, uniform population inversion, threshold current, lasing, aluminium, ultra violet laser diodes, photoluminescence, temperature dependence, near-band-edge emission, basal-plane stacking faults, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/59864/}
    }

  • D. McArthur, B. Hourahine, and F. Papoff, "Enhancing ultraviolet spontaneous emission with a designed quantum vacuum," Optics Express, vol. 25, iss. 4, p. 4162–4179, 2017.
    [BibTeX] [Abstract] [Download PDF]

    We determine how to alter the properties of the quantum vacuum at ultraviolet wavelengths to simultaneously enhance the spontaneous transition rates and the far field detection rate of quantum emitters. We find the response of several complex nanostructures in the 200 ? 400 nm range, where many organic molecules have fluorescent responses, using an analytic decomposition of the electromagnetic response in terms of continuous spectra of plane waves and discrete sets of modes. Coupling a nanorod with an aluminum substrate gives decay rates up to 2.7 {$\times$} 103 times larger than the decay rate in vacuum and enhancements of 824 for the far field emission into the entire upper semi-space and of 2.04 {$\times$} 103 for emission within a cone with a 60? semi-angle. This effect is due to both an enhancement of the field at the emitter?s position and a reshaping of the radiation patterns near mode resonances and cannot be obtained by replacing the aluminum substrate with a second nanoparticle or with a fused silica substrate. These large decay rates and far field enhancement factors will be very useful in the detection of fluorescence signals, as these resonances can be shifted by changing the dimensions of th nanorod. Moreover, these nanostructures have potential for nano-lasing because the Q factors of these resonances can reach 107.9, higher than the Q factors observed in nano-lasers.

    @article{strathprints59693,
    volume = {25},
    number = {4},
    month = {February},
    author = {Duncan McArthur and Benjamin Hourahine and Francesco Papoff},
    title = {Enhancing ultraviolet spontaneous emission with a designed quantum vacuum},
    journal = {Optics Express},
    pages = {4162--4179},
    year = {2017},
    keywords = {subwavelength structures , ultraviolet, fluorescence, fluctuations, relaxations, and noise, Optics. Light, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/59693/},
    abstract = {We determine how to alter the properties of the quantum vacuum at ultraviolet wavelengths to simultaneously enhance the spontaneous transition rates and the far field detection rate of quantum emitters. We find the response of several complex nanostructures in the 200 ? 400 nm range, where many organic molecules have fluorescent responses, using an analytic decomposition of the electromagnetic response in terms of continuous spectra of plane waves and discrete sets of modes. Coupling a nanorod with an aluminum substrate gives decay rates up to 2.7 {$\times$} 103 times larger than the decay rate in vacuum and enhancements of 824 for the far field emission into the entire upper semi-space and of 2.04 {$\times$} 103 for emission within a cone with a 60? semi-angle. This effect is due to both an enhancement of the field at the emitter?s position and a reshaping of the radiation patterns near mode resonances and cannot be obtained by replacing the aluminum substrate with a second nanoparticle or with a fused silica substrate. These large decay rates and far field enhancement factors will be very useful in the detection of fluorescence signals, as these resonances can be shifted by changing the dimensions of th nanorod. Moreover, these nanostructures have potential for nano-lasing because the Q factors of these resonances can reach 107.9, higher than the Q factors observed in nano-lasers.}
    }

  • P. Coulon, S. H. Vajargah, A. Bao, P. R. Edwards, E. D. Le Boulbar, I. Gîrgel, R. W. Martin, C. J. Humphreys, R. A. Oliver, D. W. E. Allsopp, and P. A. Shields, "Evolution of the m-plane quantum well morphology and composition within a GaN/InGaN core-shell structure," Crystal Growth and Design, vol. 17, iss. 2, p. 474–482, 2017.
    [BibTeX] [Abstract] [Download PDF]

    GaN/InGaN core-shell nanorods are promising for optoelectronic applications due to the absence of polarization-related electric fields on the sidewalls, a lower defect density, a larger emission volume and strain relaxation at the free surfaces. The core-shell geometry allows the growth of thicker InGaN shell layers, which would benefit the efficiency of light emitting diodes. However, the growth mode of such layers by metal organic vapor phase epitaxy is poorly understood. Through a combination of nanofabrication, epitaxial growth and detailed characterization, this work reveals an evolution in the growth mode of InGaN epitaxial shells, from a two dimensional (2D) growth mode to three dimensional (3D) striated growth without additional line defect formation with increasing layer thickness. Measurements of the indium distribution show fluctuations along the {\ensuremath{<}}10-10{\ensuremath{>}} directions, with low and high indium composition associated with the 2D and 3D growth modes, respectively. Atomic steps at the GaN/InGaN core-shell interface were observed to occur with a similar frequency as quasi-periodic indium fluctuations along [0001] observed within the 2D layer, to provide evidence that the resulting local strain relief at the steps acts as the trigger for a change of growth mode by elastic relaxation. This study demonstrates that misfit dislocation generation during the growth of wider InGaN shell layers can be avoided by using pre-etched GaN nanorods. Significantly, this enables the growth of absorption-based devices and light-emitting diodes with emissive layers wide enough to mitigate efficiency droop.

    @Article{strathprints59627,
    author = {Coulon, Pierre-Marie and Shahrzad Hosseini Vajargah and An Bao and Paul R. Edwards and Le Boulbar, Emmanuel D. and Ionut G{\^i}rgel and Robert W. Martin and Colin J. Humphreys and Rachel A. Oliver and Duncan W. E. Allsopp and Philip A. Shields},
    title = {Evolution of the m-plane quantum well morphology and composition within a {GaN/InGaN} core-shell structure},
    journal = {Crystal Growth and Design},
    year = {2017},
    volume = {17},
    number = {2},
    pages = {474--482},
    month = {February},
    abstract = {GaN/InGaN core-shell nanorods are promising for optoelectronic applications due to the absence of polarization-related electric fields on the sidewalls, a lower defect density, a larger emission volume and strain relaxation at the free surfaces. The core-shell geometry allows the growth of thicker InGaN shell layers, which would benefit the efficiency of light emitting diodes. However, the growth mode of such layers by metal organic vapor phase epitaxy is poorly understood. Through a combination of nanofabrication, epitaxial growth and detailed characterization, this work reveals an evolution in the growth mode of InGaN epitaxial shells, from a two dimensional (2D) growth mode to three dimensional (3D) striated growth without additional line defect formation with increasing layer thickness. Measurements of the indium distribution show fluctuations along the {\ensuremath{<}}10-10{\ensuremath{>}} directions, with low and high indium composition associated with the 2D and 3D growth modes, respectively. Atomic steps at the GaN/InGaN core-shell interface were observed to occur with a similar frequency as quasi-periodic indium fluctuations along [0001] observed within the 2D layer, to provide evidence that the resulting local strain relief at the steps acts as the trigger for a change of growth mode by elastic relaxation. This study demonstrates that misfit dislocation generation during the growth of wider InGaN shell layers can be avoided by using pre-etched GaN nanorods. Significantly, this enables the growth of absorption-based devices and light-emitting diodes with emissive layers wide enough to mitigate efficiency droop.},
    keywords = {nanorod, core-shell, InGaN, m-plane, morphology, AFM, TEM, EDX, nanofabrication, epitaxial growth, Chemistry, Physics, Materials Science(all), Chemistry(all), Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/59627/}
    }

  • A. Vilalta-Clemente, G. Naresh-Kumar, M. Nouf-Allehiani, P. Gamarra, M. A. di Forte-Poisson, C. Trager-Cowan, and A. J. Wilkinson, "Cross-correlation based high resolution electron backscatter diffraction and electron channelling contrast imaging for strain mapping and dislocation distributions in InAlN thin films," Acta Materialia, vol. 125, p. 125–135, 2017.
    [BibTeX] [Abstract] [Download PDF]

    We describe the development of cross-correlation based high resolution electron backscatter diffraction (HR-EBSD) and electron channelling contrast imaging (ECCI), in the scanning electron microscope (SEM), to quantitatively map the strain variation and lattice rotation and determine the density and identify dislocations in nitride semiconductor thin films. These techniques can provide quantitative, rapid, non-destructive analysis of the structural properties of materials with a spatial resolution of order of tens of nanometers. HR-EBSD has a sensitivity to changes of strain and rotation of the order of 10?4 and 0.01? respectively, while ECCI can be used to image single dislocations up to a dislocation density of order 1010 cm?2. In the present work, we report the application of the cross-correlation based HR-EBSD approach to determine the tilt, twist, elastic strain and the distribution and type of threading dislocations in InAlN/AlN/GaN high electron mobility transistor (HEMT) structures grown on two different substrates, namely SiC and sapphire. We describe our procedure to estimate the distribution of geometrically necessary dislocations (GND) based on Nye-Kroner analysis and compare them with the direct imaging of threading dislocations (TDs) by ECCI. Combining data from HR-EBSD and ECCI observations allowed the densities of pure edge, mixed and pure screw threading dislocations to be fully separated.

    @Article{strathprints59588,
    author = {A. Vilalta-Clemente and G. Naresh-Kumar and M. Nouf-Allehiani and P. Gamarra and M.A. di Forte-Poisson and C. Trager-Cowan and A.J. Wilkinson},
    title = {Cross-correlation based high resolution electron backscatter diffraction and electron channelling contrast imaging for strain mapping and dislocation distributions in {InAlN} thin films},
    journal = {Acta Materialia},
    year = {2017},
    volume = {125},
    pages = {125--135},
    month = {February},
    abstract = {We describe the development of cross-correlation based high resolution electron backscatter diffraction (HR-EBSD) and electron channelling contrast imaging (ECCI), in the scanning electron microscope (SEM), to quantitatively map the strain variation and lattice rotation and determine the density and identify dislocations in nitride semiconductor thin films. These techniques can provide quantitative, rapid, non-destructive analysis of the structural properties of materials with a spatial resolution of order of tens of nanometers. HR-EBSD has a sensitivity to changes of strain and rotation of the order of 10?4 and 0.01? respectively, while ECCI can be used to image single dislocations up to a dislocation density of order 1010 cm?2. In the present work, we report the application of the cross-correlation based HR-EBSD approach to determine the tilt, twist, elastic strain and the distribution and type of threading dislocations in InAlN/AlN/GaN high electron mobility transistor (HEMT) structures grown on two different substrates, namely SiC and sapphire. We describe our procedure to estimate the distribution of geometrically necessary dislocations (GND) based on Nye-Kroner analysis and compare them with the direct imaging of threading dislocations (TDs) by ECCI. Combining data from HR-EBSD and ECCI observations allowed the densities of pure edge, mixed and pure screw threading dislocations to be fully separated.},
    keywords = {EBSD, ECCI, dislocations, InAIN, HEMTs, electron backscatter diffraction, electron channelling contrast imaging, scanning electron microscope, nitride semiconductor thin films, geometrically necessary dislocations, Physics, Polymers and Plastics, Metals and Alloys, Ceramics and Composites, Electronic, Optical and Magnetic Materials},
    url = {http://strathprints.strath.ac.uk/59588/}
    }

  • S. Vespucci, A. Winkelmann, K. Mingard, D. Maneuski, V. O'Shea, and C. Trager-Cowan, "Exploring transmission Kikuchi diffraction using a Timepix detector," Journal of Instrumentation, vol. 12, iss. 2, p. C02075, 2017.
    [BibTeX] [Abstract] [Download PDF]

    Electron backscatter diffraction (EBSD) is a well-established scanning electron microscope (SEM)-based technique [1]. It allows the non-destructive mapping of the crystal structure, texture, crystal phase and strain with a spatial resolution of tens of nanometers. Conventionally this is performed by placing an electron sensitive screen, typically consisting of a phosphor screen combined with a charge coupled device (CCD) camera, in front of a specimen, usually tilted 70? to the normal of the exciting electron beam. Recently, a number of authors have shown that a significant increase in spatial resolution is achievable when Kikuchi diffraction patterns are acquired in transmission geometry; that is when diffraction patterns are generated by electrons transmitted through an electron-transparent, usually thinned, specimen. The resolution of this technique, called transmission Kikuchi diffraction (TKD), has been demonstrated to be better than 10 nm [2, 3]. We have recently demonstrated the advantages of a direct electron detector, Timepix [4, 5], for the acquisition of standard EBSD patterns [5]. In this article we will discuss the advantages of Timepix to perform TKD and for acquiring spot diffraction patterns and more generally for acquiring scanning transmission electron microscopy micrographs in the SEM. Particularly relevant for TKD, is its very compact size, which allows much more flexibility in the positioning of the detector in the SEM chamber. We will furthermore show recent results using Timepix as a virtual forward scatter detector, and will illustrate the information derivable on producing images through processing of data acquired from different areas of the detector. We will show results from samples ranging from gold nanoparticles to nitride semiconductor nanorods.

    @Article{strathprints59555,
    author = {S. Vespucci and A. Winkelmann and K. Mingard and D. Maneuski and V. O'Shea and C. Trager-Cowan},
    title = {Exploring transmission {K}ikuchi diffraction using a {T}imepix detector},
    journal = {Journal of Instrumentation},
    year = {2017},
    volume = {12},
    number = {2},
    pages = {C02075},
    month = {February},
    abstract = {Electron backscatter diffraction (EBSD) is a well-established scanning electron microscope (SEM)-based technique [1]. It allows the non-destructive mapping of the crystal structure, texture, crystal phase and strain with a spatial resolution of tens of nanometers. Conventionally this is performed by placing an electron sensitive screen, typically consisting of a phosphor screen combined with a charge coupled device (CCD) camera, in front of a specimen, usually tilted 70? to the normal of the exciting electron beam. Recently, a number of authors have shown that a significant increase in spatial resolution is achievable when Kikuchi diffraction patterns are acquired in transmission geometry; that is when diffraction patterns are generated by electrons transmitted through an electron-transparent, usually thinned, specimen. The resolution of this technique, called transmission Kikuchi diffraction (TKD), has been demonstrated to be better than 10 nm [2, 3]. We have recently demonstrated the advantages of a direct electron detector, Timepix [4, 5], for the acquisition of standard EBSD patterns [5]. In this article we will discuss the advantages of Timepix to perform TKD and for acquiring spot diffraction patterns and more generally for acquiring scanning transmission electron microscopy micrographs in the SEM. Particularly relevant for TKD, is its very compact size, which allows much more flexibility in the positioning of the detector in the SEM chamber. We will furthermore show recent results using Timepix as a virtual forward scatter detector, and will illustrate the information derivable on producing images through processing of data acquired from different areas of the detector. We will show results from samples ranging from gold nanoparticles to nitride semiconductor nanorods.},
    keywords = {radiation, imaging detectors, electron backscatter diffraction, Kikuchi diffraction patterns, transmission Kikuchi diffraction, direct electron detector, Timepix, scanning transmission electron microscopy micrographs, Physics, Instrumentation, Mathematical Physics},
    url = {http://strathprints.strath.ac.uk/59555/}
    }

  • M. D. Smith, D. Thomson, V. Z. Zubialevich, H. Li, G. Naresh-Kumar, C. Trager-Cowan, and P. J. Parbrook, "Nanoscale fissure formation in AlₓGa₁₋ₓN/GaN heterostructures and their influence on Ohmic contact formation," Physica Status Solidi A, vol. 214, iss. 1, p. 1600353, 2017.
    [BibTeX] [Abstract] [Download PDF]

    Nanoscale surface fissures on AlₓGa₁₋ₓN/GaN (15 nm/1 ?m) heterostructures grown by metalorganic vapour phase epitaxy (MOVPE) were imaged using tapping-mode atomic force microscopy (AFM) and electron channelling contrast imaging (ECCI). Fissure formation was linked to threading dislocations, and was only observed in samples cooled under H2 and NH3, developing with increasing barrier layer Al content. No strain relaxation was detected regardless of fissure formation up to barrier layer Al composition fractions of x = 0.37. A reduction of measured channel carrier density was found in fissured samples at low temperature. This instability is attributed to shallow trap formation associated with fissure boundaries. For Ti/Al/Ni/Au Ohmic contact formation to high Al content barrier layers, fissures were found to offer conduction routes to the 2DEG that allow for low resistance contacts, with fissure-free samples requiring additional optimisation of the metal stack and anneal conditions to achieve contact resistivity of order those measured in fissured samples. In addition, the effects of fissures were found to be detrimental to thermal stability of sheet and contact resistance, suggesting that fissure formation compromises the integrity of the 2DEG.

    @Article{strathprints59478,
    author = {M. D. Smith and D. Thomson and V. Z. Zubialevich and H. Li and G. Naresh-Kumar and C. Trager-Cowan and P. J. Parbrook},
    title = {Nanoscale fissure formation in {AlₓGa₁₋ₓN/GaN} heterostructures and their influence on {O}hmic contact formation},
    journal = {Physica Status Solidi A},
    year = {2017},
    volume = {214},
    number = {1},
    pages = {1600353},
    month = {January},
    abstract = {Nanoscale surface fissures on AlₓGa₁₋ₓN/GaN (15 nm/1 ?m) heterostructures grown by metalorganic vapour phase epitaxy (MOVPE) were imaged using tapping-mode atomic force microscopy (AFM) and electron channelling contrast imaging (ECCI). Fissure formation was linked to threading dislocations, and was only observed in samples cooled under H2 and NH3, developing with increasing barrier layer Al content. No strain relaxation was detected regardless of fissure formation up to barrier layer Al composition fractions of x = 0.37. A reduction of measured channel carrier density was found in fissured samples at low temperature. This instability is attributed to shallow trap formation associated with fissure boundaries. For Ti/Al/Ni/Au Ohmic contact formation to high Al content barrier layers, fissures were found to offer conduction routes to the 2DEG that allow for low resistance contacts, with fissure-free samples requiring additional optimisation of the metal stack and anneal conditions to achieve contact resistivity of order those measured in fissured samples. In addition, the effects of fissures were found to be detrimental to thermal stability of sheet and contact resistance, suggesting that fissure formation compromises the integrity of the 2DEG.},
    keywords = {nanoscale surface fissures, electron channelling, contrast imaging, AlxGa1?xN/GaN, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/59478/}
    }

  • G. Kusch, F. Mehnke, J. Enslin, P. R. Edwards, T. Wernicke, M. Kneissl, and R. W. Martin, "Analysis of doping concentration and composition in wide bandgap AlGaN:Si by wavelength dispersive X-ray spectroscopy," Semiconductor Science and Technology, vol. 32, iss. 3, p. 35020, 2017.
    [BibTeX] [Abstract] [Download PDF]

    Detailed knowledge of the dopant concentration and composition of wide band gap AlxGa1?xN layers is of crucial importance for the fabrication of ultra violet (UV) light emitting diodes (LEDs). This paper demonstrates the capabilities of wavelength dispersive X-ray (WDX) spectroscopy in accurately determining these parameters and compares the results with those from high resolution X-ray diffraction (HR-XRD) and secondary ion mass spectrometry (SIMS). WDX spectroscopy has been carried out on different silicon-doped wide bandgap AlxGa1?xN samples (x between 0.80 and 1). This study found a linear increase in the Si concentration with the SiH4/group-III ratio, measuring Si concentrations between 3{$\times$}1018 cm?3 and 2.8{$\times$}1019 cm?3, while no direct correlation between the AlN composition and the Si incorporation ratio was found. Comparison between the composition obtained by WDX and by HR-XRD showed very good agreement in the range investigated, while comparison of the donor concentration between WDX and SIMS found only partial agreement, which we attribute to a number of effects.

    @Article{strathprints59282,
    author = {Gunnar Kusch and Frank Mehnke and Johannes Enslin and Paul R Edwards and Tim Wernicke and Michael Kneissl and Robert W Martin},
    title = {Analysis of doping concentration and composition in wide bandgap {AlGaN:Si} by wavelength dispersive {X}-ray spectroscopy},
    journal = {Semiconductor Science and Technology},
    year = {2017},
    volume = {32},
    number = {3},
    pages = {035020},
    month = {February},
    abstract = {Detailed knowledge of the dopant concentration and composition of wide band gap AlxGa1?xN layers is of crucial importance for the fabrication of ultra violet (UV) light emitting diodes (LEDs). This paper demonstrates the capabilities of wavelength dispersive X-ray (WDX) spectroscopy in accurately determining these parameters and compares the results with those from high resolution X-ray diffraction (HR-XRD) and secondary ion mass spectrometry (SIMS). WDX spectroscopy has been carried out on different silicon-doped wide bandgap AlxGa1?xN samples (x between 0.80 and 1). This study found a linear increase in the Si concentration with the SiH4/group-III ratio, measuring Si concentrations between 3{$\times$}1018 cm?3 and 2.8{$\times$}1019 cm?3, while no direct correlation between the AlN composition and the Si incorporation ratio was found. Comparison between the composition obtained by WDX and by HR-XRD showed very good agreement in the range investigated, while comparison of the donor concentration between WDX and SIMS found only partial agreement, which we attribute to a number of effects.},
    keywords = {dopant concentration, ultra violet light emitting diodes, wavelength dispersive X-ray spectroscopy, high resolution X-ray diffraction, doping concentrations, semiconductors, dopant composition, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/59282/}
    }

  • A. K. Singh, K. P. O'Donnell, P. R. Edwards, K. Lorenz, M. J. Kappers, and M. Boćkowski, "Hysteretic photochromic switching of Eu-Mg defects in GaN links the shallow transient and deep ground states of the Mg acceptor," Scientific Reports, vol. 7, p. 41982, 2017.
    [BibTeX] [Abstract] [Download PDF]

    Although p-type activation of GaN by Mg underpins a mature commercial technology, the nature of the Mg acceptor in GaN is still controversial. Here, we use implanted Eu as a `spectator ion' to probe the lattice location of Mg in doubly doped GaN(Mg):Eu. Photoluminescence spectroscopy of this material exemplifies hysteretic photochromic switching (HPS) between two configurations, Eu0 and Eu1(Mg), of the same Eu-Mg defect, with a hyperbolic time dependence on `switchdown' from Eu0 to Eu1(Mg). The sample temperature and the incident light intensity at 355 nm tune the characteristic switching time over several orders of magnitude, from less than a second at 12.5 K, ~100 mW/cm2 to (an estimated) several hours at 50 K, 1 mW/cm². Linking the distinct Eu-Mg defect configurations with the shallow transient and deep ground states of the Mg acceptor in the Lany-Zunger model, we determine the energy barrier between the states to be 27.7(4) meV, in good agreement with the predictions of theory. The experimental results further suggest that at low temperatures holes in deep ground states are localized on N atoms axially bonded to Mg acceptors.

    @Article{strathprints59279,
    author = {A. K. Singh and K. P. O'Donnell and P. R. Edwards and K. Lorenz and M. J. Kappers and M. Bo{\'c}kowski},
    title = {Hysteretic photochromic switching of {Eu-Mg} defects in {GaN} links the shallow transient and deep ground states of the {Mg} acceptor},
    journal = {Scientific Reports},
    year = {2017},
    volume = {7},
    pages = {41982},
    month = {February},
    abstract = {Although p-type activation of GaN by Mg underpins a mature commercial technology, the nature of the Mg acceptor in GaN is still controversial. Here, we use implanted Eu as a `spectator ion' to probe the lattice location of Mg in doubly doped GaN(Mg):Eu. Photoluminescence spectroscopy of this material exemplifies hysteretic photochromic switching (HPS) between two configurations, Eu0 and Eu1(Mg), of the same Eu-Mg defect, with a hyperbolic time dependence on `switchdown' from Eu0 to Eu1(Mg). The sample temperature and the incident light intensity at 355 nm tune the characteristic switching time over several orders of magnitude, from less than a second at 12.5 K, ~100 mW/cm2 to (an estimated) several hours at 50 K, 1 mW/cm². Linking the distinct Eu-Mg defect configurations with the shallow transient and deep ground states of the Mg acceptor in the Lany-Zunger model, we determine the energy barrier between the states to be 27.7(4) meV, in good agreement with the predictions of theory. The experimental results further suggest that at low temperatures holes in deep ground states are localized on N atoms axially bonded to Mg acceptors.},
    keywords = {photoluminescence spectroscopy, hysteretic photochromic switching, Lany-Zunger model, energy barrier, shallow transient state, deep ground state, acceptor state, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/59279/}
    }

  • P. Tian, P. R. Edwards, M. J. Wallace, R. W. Martin, J. J. D. McKendry, E. Gu, M. D. Dawson, Z. Qiu, C. Jia, Z. Chen, G. Zhang, L. Zheng, and R. Liu, "Characteristics of GaN-based light emitting diodes with different thicknesses of buffer layer grown by HVPE and MOCVD," Journal of Physics D: Applied Physics, vol. 50, iss. 7, p. 75101, 2017.
    [BibTeX] [Abstract] [Download PDF]

    GaN-based light emitting diodes (LEDs) have been fabricated on sapphire substrates with different thicknesses of GaN buffer layer grown by a combination of hydride vapor phase epitaxy and metalorganic chemical vapor deposition. We analyzed the LED efficiency and modulation characteristics with buffer thicknesses of 12 ?m and 30 ?m. With the buffer thickness increase, cathodoluminescence hyperspectral imaging shows that the dislocation density in the buffer layer decreases from {$\sim$}1.3X10 8 cm-2 to{$\sim$}1.0 X 10 8 cm-2, and Raman spectra suggest that the compressive stress in the quantum wells is partly relaxed, which leads to a large blue shift in the peak emission wavelength of the photoluminescence and electroluminescent spectra. The combined effects of the low dislocation density and stress relaxation lead to improvements in the efficiency of LEDs with the 30 ?m GaN buffer, but the electrical-to-optical modulation bandwidth is higher for the LEDs with the 12 ?m GaN buffer. A rate equation analysis suggests that defect-related nonradiative recombination can help increase the modulation bandwidth but reduce the LED efficiency at low currents, suggesting that a compromise should be made in the choice of defect density.

    @Article{strathprints58427,
    author = {Pengfei Tian and Paul R. Edwards and Michael J. Wallace and Robert W. Martin and Jonathan J.D. McKendry and Erdan Gu and Martin D. Dawson and Zhi-Jun Qiu and Chuanyu Jia and Zhizhong Chen and Guoyi Zhang and Lirong Zheng and Ran Liu},
    title = {Characteristics of {GaN}-based light emitting diodes with different thicknesses of buffer layer grown by {HVPE} and {MOCVD}},
    journal = {Journal of Physics D: Applied Physics},
    year = {2017},
    volume = {50},
    number = {7},
    pages = {075101},
    month = {January},
    abstract = {GaN-based light emitting diodes (LEDs) have been fabricated on sapphire substrates with different thicknesses of GaN buffer layer grown by a combination of hydride vapor phase epitaxy and metalorganic chemical vapor deposition. We analyzed the LED efficiency and modulation characteristics with buffer thicknesses of 12 ?m and 30 ?m. With the buffer thickness increase, cathodoluminescence hyperspectral imaging shows that the dislocation density in the buffer layer decreases from {$\sim$}1.3X10 8 cm-2 to{$\sim$}1.0 X 10 8 cm-2, and Raman spectra suggest that the compressive stress in the quantum wells is partly relaxed, which leads to a large blue shift in the peak emission wavelength of the photoluminescence and electroluminescent spectra. The combined effects of the low dislocation density and stress relaxation lead to improvements in the efficiency of LEDs with the 30 ?m GaN buffer, but the electrical-to-optical modulation bandwidth is higher for the LEDs with the 12 ?m GaN buffer. A rate equation analysis suggests that defect-related nonradiative recombination can help increase the modulation bandwidth but reduce the LED efficiency at low currents, suggesting that a compromise should be made in the choice of defect density.},
    keywords = {light emitting diode, GaN, buffer, modulation bandwidth, solid state lighting, Optics. Light, Surfaces, Coatings and Films, Acoustics and Ultrasonics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/58427/}
    }

  • D. McArthur, B. Hourahine, and F. Papoff, "Coherent control of plasmons in nanoparticles with nonlocal response," Optics Communications, vol. 382, p. 258–265, 2017.
    [BibTeX] [Abstract] [Download PDF]

    We discuss a scheme for the coherent control of light and plasmons in nanoparticles that have nonlocal dielectric permittivity and contain nonlinear impurities or color centers. We consider particles which have a response to light that is strongly influenced by plasmons over a broad range of frequencies. Our coherent control method enables the reduction of absorption and/or suppression of scattering.

    @article{strathprints57291,
    volume = {382},
    month = {January},
    title = {Coherent control of plasmons in nanoparticles with nonlocal response},
    author = {D. McArthur and B. Hourahine and F. Papoff},
    year = {2017},
    pages = {258--265},
    journal = {Optics Communications},
    keywords = {plasmonics, nanoparticles, nonlocality, optical routing, Optics. Light, Atomic and Molecular Physics, and Optics},
    url = {http://strathprints.strath.ac.uk/57291/},
    abstract = {We discuss a scheme for the coherent control of light and plasmons in nanoparticles that have nonlocal dielectric permittivity and contain nonlinear impurities or color centers. We consider particles which have a response to light that is strongly influenced by plasmons over a broad range of frequencies. Our coherent control method enables the reduction of absorption and/or suppression of scattering.}
    }

2016

  • G. Naresh-Kumar, D. Thomson, M. Nouf-Allehiani, J. Bruckbauer, P. R. Edwards, B. Hourahine, R. W. Martin, and C. Trager-Cowan, "Reprint of : Electron channelling contrast imaging for III-nitride thin film structures," Materials Science in Semiconductor Processing, vol. 55, p. 19–25, 2016.
    [BibTeX] [Abstract] [Download PDF]

    Electron channelling contrast imaging (ECCI) performed in a scanning electron microscope (SEM) is a rapid and non-destructive structural characterisation technique for imaging, identifying and quantifying extended defects in crystalline materials. In this review, we will demonstrate the application of ECCI to the characterisation of III-nitride semiconductor thin films grown on different substrates and with different crystal orientations. We will briefly describe the history and the theory behind electron channelling and the experimental setup and conditions required to perform ECCI. We will discuss the advantages of using ECCI, especially in combination with other SEM based techniques, such as cathodoluminescence imaging. The challenges in using ECCI are also briefly discussed.

    @article{strathprints72242,
    volume = {55},
    month = {November},
    title = {Reprint of : Electron channelling contrast imaging for III-nitride thin film structures},
    author = {G. Naresh-Kumar and D. Thomson and M. Nouf-Allehiani and J. Bruckbauer and P. R. Edwards and B. Hourahine and R. W. Martin and C. Trager-Cowan},
    year = {2016},
    pages = {19--25},
    journal = {Materials Science in Semiconductor Processing},
    keywords = {ECCI, extended defects, III-nitrides, SEM and thin films, Physics, Materials Science(all), Condensed Matter Physics, Mechanics of Materials, Mechanical Engineering},
    url = {https://strathprints.strath.ac.uk/72242/},
    abstract = {Electron channelling contrast imaging (ECCI) performed in a scanning electron microscope (SEM) is a rapid and non-destructive structural characterisation technique for imaging, identifying and quantifying extended defects in crystalline materials. In this review, we will demonstrate the application of ECCI to the characterisation of III-nitride semiconductor thin films grown on different substrates and with different crystal orientations. We will briefly describe the history and the theory behind electron channelling and the experimental setup and conditions required to perform ECCI. We will discuss the advantages of using ECCI, especially in combination with other SEM based techniques, such as cathodoluminescence imaging. The challenges in using ECCI are also briefly discussed.}
    }

  • Y. A. Kelaita, K. A. Fischer, T. M. Babinec, K. G. Lagoudakis, T. Sarmiento, A. Rundquist, A. Majumdar, and J. Vučković, "Hybrid metal-dielectric nanocavity for enhanced light-matter interactions," Optical Materials Express, vol. 7, iss. 1, p. 231–239, 2016.
    [BibTeX] [Abstract] [Download PDF]

    Despite tremendous advances in the fundamentals and applications of cavity quantum electrodynamics (CQED), investigations in this field have primarily been limited to optical cavities composed of purely dielectric materials. Here, we demonstrate a hybrid metal-dielectric nanocavity design and realize it in the InAs/GaAs quantum photonics platform utilizing angled rotational metal evaporation. Key features of our nanometallic light-matter interface include: (i) order of magnitude reduction in mode volume compared to that of leading photonic crystal CQED systems; (ii) surface-emitting nanoscale cylindrical geometry and therefore good collection efficiency; and finally (iii) strong and broadband spontaneous emission rate enhancement (Purcell factor textasciitilde 8) of single photons. This light-matter interface may play an important role in quantum technologies.

    @article{strathprints64413,
    volume = {7},
    number = {1},
    month = {December},
    author = {Yousif A. Kelaita and Kevin A. Fischer and Thomas M. Babinec and Konstantinos G. Lagoudakis and Tomas Sarmiento and Armand Rundquist and Arka Majumdar and Jelena Vu{\v c}kovi{\'c}},
    title = {Hybrid metal-dielectric nanocavity for enhanced light-matter interactions},
    journal = {Optical Materials Express},
    pages = {231--239},
    year = {2016},
    keywords = {nanostructure fabrication, metal optics, resonators, Physics, Electronic, Optical and Magnetic Materials},
    url = {https://strathprints.strath.ac.uk/64413/},
    abstract = {Despite tremendous advances in the fundamentals and applications of cavity quantum electrodynamics (CQED), investigations in this field have primarily been limited to optical cavities composed of purely dielectric materials. Here, we demonstrate a hybrid metal-dielectric nanocavity design and realize it in the InAs/GaAs quantum photonics platform utilizing angled rotational metal evaporation. Key features of our nanometallic light-matter interface include: (i) order of magnitude reduction in mode volume compared to that of leading photonic crystal CQED systems; (ii) surface-emitting nanoscale cylindrical geometry and therefore good collection efficiency; and finally (iii) strong and broadband spontaneous emission rate enhancement (Purcell factor textasciitilde 8) of single photons. This light-matter interface may play an important role in quantum technologies.}
    }

  • K. A. Fischer, K. Müller, K. G. Lagoudakis, and J. Vučković, "Dynamical modeling of pulsed two-photon interference," New Journal of Physics, vol. 18, iss. 11, p. 113053, 2016.
    [BibTeX] [Abstract] [Download PDF]

    Single-photon sources are at the heart of quantum-optical networks, with their uniquely quantum emission and phenomenon of two-photon interference allowing for the generation and transfer of nonclassical states. Although a few analytical methods have been briefly investigated for describing pulsed single-photon sources, these methods apply only to either perfectly ideal or at least extremely idealized sources. Here, we present the first complete picture of pulsed single-photon sources by elaborating how to numerically and fully characterize non-ideal single-photon sources operating in a pulsed regime. In order to achieve this result, we make the connection between quantum Monte-Carlo simulations, experimental characterizations, and an extended form of the quantum regression theorem. We elaborate on how an ideal pulsed single-photon source is connected to its photocount distribution and its measured degree of second- and first-order optical coherence. By doing so, we provide a description of the relationship between instantaneous source correlations and the typical experimental interferometers (Hanbury-Brown and Twiss, Hong?Ou?Mandel, and Mach?Zehnder) used to characterize such sources. Then, we use these techniques to explore several prototypical quantum systems and their non-ideal behaviors. As an example numerical result, we show that for the most popular single-photon source{–}a resonantly excited two-level system{–}its error probability is directly related to its excitation pulse length. We believe that the intuition gained from these representative systems and characters can be used to interpret future results with more complicated source Hamiltonians and behaviors. Finally, we have thoroughly documented our simulation methods with contributions to the Quantum Optics Toolbox in Python in order to make our work easily accessible to other scientists and engineers.

    @article{strathprints64408,
    volume = {18},
    number = {11},
    month = {November},
    author = {Kevin A Fischer and Kai M{\"u}ller and Konstantinos G Lagoudakis and Jelena Vu{\v c}kovi{\'c}},
    title = {Dynamical modeling of pulsed two-photon interference},
    journal = {New Journal of Physics},
    pages = {113053},
    year = {2016},
    keywords = {quantum-optical networks, photons, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/64408/},
    abstract = {Single-photon sources are at the heart of quantum-optical networks, with their uniquely quantum emission and phenomenon of two-photon interference allowing for the generation and transfer of nonclassical states. Although a few analytical methods have been briefly investigated for describing pulsed single-photon sources, these methods apply only to either perfectly ideal or at least extremely idealized sources. Here, we present the first complete picture of pulsed single-photon sources by elaborating how to numerically and fully characterize non-ideal single-photon sources operating in a pulsed regime. In order to achieve this result, we make the connection between quantum Monte-Carlo simulations, experimental characterizations, and an extended form of the quantum regression theorem. We elaborate on how an ideal pulsed single-photon source is connected to its photocount distribution and its measured degree of second- and first-order optical coherence. By doing so, we provide a description of the relationship between instantaneous source correlations and the typical experimental interferometers (Hanbury-Brown and Twiss, Hong?Ou?Mandel, and Mach?Zehnder) used to characterize such sources. Then, we use these techniques to explore several prototypical quantum systems and their non-ideal behaviors. As an example numerical result, we show that for the most popular single-photon source{--}a resonantly excited two-level system{--}its error probability is directly related to its excitation pulse length. We believe that the intuition gained from these representative systems and characters can be used to interpret future results with more complicated source Hamiltonians and behaviors. Finally, we have thoroughly documented our simulation methods with contributions to the Quantum Optics Toolbox in Python in order to make our work easily accessible to other scientists and engineers.}
    }

  • K. G. Lagoudakis, P. L. McMahon, C. Dory, K. A. Fischer, K. Müller, V. Borish, D. Dalacu, P. J. Poole, M. E. Reimer, V. Zwiller, Y. Yamamoto, and J. Vuckovic, "Ultrafast coherent manipulation of trions in site-controlled nanowire quantum dots," Optica, vol. 3, iss. 12, p. 1430–1435, 2016.
    [BibTeX] [Abstract] [Download PDF]

    Physical implementations of large-scale quantum processors based on solid-state platforms benefit from realizations of quantum bits positioned in regular arrays. Self-assembled quantum dots are well established as promising candidates for quantum optics and quantum information processing, but they are randomly positioned. Site-controlled quantum dots, on the other hand, are grown in pre-defined locations but have not yet been sufficiently developed to be used as a platform for quantum information processing. In this paper, we demonstrate all-optical ultrafast complete coherent control of a qubit formed by the single-spin/trion states of a charged site-controlled nanowire quantum dot. Our results show that site-controlled quantum dots in nanowires are promising hosts of charged-exciton qubits and that these qubits can be cleanly manipulated in the same fashion as has been demonstrated in randomly positioned quantum dot samples. Our findings suggest that many of the related excitonic qubit experiments that have been performed over the past 15 years may work well in the more scalable, site-controlled systems, making them very promising for the realization of quantum hardware.

    @article{strathprints64407,
    volume = {3},
    number = {12},
    month = {November},
    author = {K. G. Lagoudakis and P. L. McMahon and C. Dory and K. A. Fischer and K. M{\"u}ller and V. Borish and D. Dalacu and P. J. Poole and M. E. Reimer and V. Zwiller and Y. Yamamoto and J. Vuckovic},
    note = {{\copyright} 2016 Optical Society of America. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited.},
    title = {Ultrafast coherent manipulation of trions in site-controlled nanowire quantum dots},
    year = {2016},
    journal = {Optica},
    pages = {1430--1435},
    keywords = {optics, quantum dots, Physics, Electronic, Optical and Magnetic Materials, Atomic and Molecular Physics, and Optics},
    url = {https://strathprints.strath.ac.uk/64407/},
    abstract = {Physical implementations of large-scale quantum processors based on solid-state platforms benefit from realizations of quantum bits positioned in regular arrays. Self-assembled quantum dots are well established as promising candidates for quantum optics and quantum information processing, but they are randomly positioned. Site-controlled quantum dots, on the other hand, are grown in pre-defined locations but have not yet been sufficiently developed to be used as a platform for quantum information processing. In this paper, we demonstrate all-optical ultrafast complete coherent control of a qubit formed by the single-spin/trion states of a charged site-controlled nanowire quantum dot. Our results show that site-controlled quantum dots in nanowires are promising hosts of charged-exciton qubits and that these qubits can be cleanly manipulated in the same fashion as has been demonstrated in randomly positioned quantum dot samples. Our findings suggest that many of the related excitonic qubit experiments that have been performed over the past 15 years may work well in the more scalable, site-controlled systems, making them very promising for the realization of quantum hardware.}
    }

  • K. Müller, K. A. Fischer, C. Dory, T. Sarmiento, K. G. Lagoudakis, A. Rundquist, Y. A. Kelaita, and J. Vučković, "Self-homodyne-enabled generation of indistinguishable photons," Optica, vol. 3, iss. 9, p. 931–936, 2016.
    [BibTeX] [Abstract] [Download PDF]

    The rapid generation of non-classical light serves as the foundation for exploring quantum optics and developing applications such as secure communications or the generation of NOON states. While strongly coupled quantum dot-photonic crystal resonator systems have great potential as non-classical light sources due to their promise of tailored output statistics, the generation of indistinguishable photons has been obscured due to the strongly dissipative nature of such systems. Here, we demonstrate that the recently discovered self-homodyne suppression technique can be used to overcome this limitation and tune the quantum statistics of transmitted light, achieving indistinguishable photon emission competitive with state-of-the-art metrics. Furthermore, our nanocavity-based platform directly lends itself to scalable on-chip architectures for quantum information.

    @article{strathprints64406,
    volume = {3},
    number = {9},
    month = {September},
    author = {Kai M{\"u}ller and Kevin A. Fischer and Constantin Dory and Tomas Sarmiento and Konstantinos G. Lagoudakis and Armand Rundquist and Yousif A. Kelaita and Jelena Vu{\v c}kovi{\'c}},
    note = {{\copyright} 2016 Optical Society of America. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited.},
    title = {Self-homodyne-enabled generation of indistinguishable photons},
    year = {2016},
    journal = {Optica},
    pages = {931--936},
    keywords = {quantum electrodynamics, photon statistics, Physics, Electronic, Optical and Magnetic Materials, Atomic and Molecular Physics, and Optics},
    url = {https://strathprints.strath.ac.uk/64406/},
    abstract = {The rapid generation of non-classical light serves as the foundation for exploring quantum optics and developing applications such as secure communications or the generation of NOON states. While strongly coupled quantum dot-photonic crystal resonator systems have great potential as non-classical light sources due to their promise of tailored output statistics, the generation of indistinguishable photons has been obscured due to the strongly dissipative nature of such systems. Here, we demonstrate that the recently discovered self-homodyne suppression technique can be used to overcome this limitation and tune the quantum statistics of transmitted light, achieving indistinguishable photon emission competitive with state-of-the-art metrics. Furthermore, our nanocavity-based platform directly lends itself to scalable on-chip architectures for quantum information.}
    }

  • K. G. Lagoudakis, P. L. McMahon, K. A. Fischer, S. Puri, K. Müller, D. Dalacu, P. J. Poole, M. E. Reimer, V. Zwiller, Y. Yamamoto, and J. Vučković, "Initialization of a spin qubit in a site-controlled nanowire quantum dot," New Journal of Physics, vol. 18, iss. 5, 2016.
    [BibTeX] [Abstract] [Download PDF]

    A fault-tolerant quantum repeater or quantum computer using solid-state spin-based quantum bits will likely require a physical implementation with many spins arranged in a grid. Self-assembled quantum dots (QDs) have been established as attractive candidates for building spin-based quantum information processing devices, but such QDs are randomly positioned, which makes them unsuitable for constructing large-scale processors. Recent efforts have shown that QDs embedded in nanowires can be deterministically positioned in regular arrays, can store single charges, and have excellent optical properties, but so far there have been no demonstrations of spin qubit operations using nanowire QDs. Here we demonstrate optical pumping of individual spins trapped in site-controlled nanowire QDs, resulting in high-fidelity spin-qubit initialization. This represents the next step towards establishing spins in nanowire QDs as quantum memories suitable for use in a large-scale, fault-tolerant quantum computer or repeater based on all-optical control of the spin qubits.

    @article{strathprints64405,
    volume = {18},
    number = {5},
    month = {April},
    title = {Initialization of a spin qubit in a site-controlled nanowire quantum dot},
    author = {Konstantinos G Lagoudakis and Peter L McMahon and Kevin A Fischer and Shruti Puri and Kai M{\"u}ller and Dan Dalacu and Philip J Poole and Michael E Reimer and Val Zwiller and Yoshihisa Yamamoto and Jelena Vu{\v c}kovi{\'c}},
    year = {2016},
    journal = {New Journal of Physics},
    keywords = {quantuim dot, spin qubit, optical pumping, spectroscopy, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/64405/},
    abstract = {A fault-tolerant quantum repeater or quantum computer using solid-state spin-based quantum bits will likely require a physical implementation with many spins arranged in a grid. Self-assembled quantum dots (QDs) have been established as attractive candidates for building spin-based quantum information processing devices, but such QDs are randomly positioned, which makes them unsuitable for constructing large-scale processors. Recent efforts have shown that QDs embedded in nanowires can be deterministically positioned in regular arrays, can store single charges, and have excellent optical properties, but so far there have been no demonstrations of spin qubit operations using nanowire QDs. Here we demonstrate optical pumping of individual spins trapped in site-controlled nanowire QDs, resulting in high-fidelity spin-qubit initialization. This represents the next step towards establishing spins in nanowire QDs as quantum memories suitable for use in a large-scale, fault-tolerant quantum computer or repeater based on all-optical control of the spin qubits.}
    }

  • C. Dory, K. A. Fischer, K. Müller, K. G. Lagoudakis, T. Sarmiento, A. Rundquist, J. L. Zhang, Y. Kelaita, and J. Vučković, "Complete coherent control of a quantum dot strongly coupled to a nanocavity," Scientific Reports, vol. 6, 2016.
    [BibTeX] [Abstract] [Download PDF]

    Strongly coupled quantum dot-cavity systems provide a non-linear configuration of hybridized light-matter states with promising quantum-optical applications. Here, we investigate the coherent interaction between strong laser pulses and quantum dot-cavity polaritons. Resonant excitation of polaritonic states and their interaction with phonons allow us to observe coherent Rabi oscillations and Ramsey fringes. Furthermore, we demonstrate complete coherent control of a quantum dot-photonic crystal cavity based quantum-bit. By controlling the excitation power and phase in a two-pulse excitation scheme we achieve access to the full Bloch sphere. Quantum-optical simulations are in good agreement with our experiments and provide insight into the decoherence mechanisms.

    @article{strathprints64401,
    volume = {6},
    month = {April},
    title = {Complete coherent control of a quantum dot strongly coupled to a nanocavity},
    author = {Constantin Dory and Kevin A. Fischer and Kai M{\"u}ller and Konstantinos G. Lagoudakis and Tomas Sarmiento and Armand Rundquist and Jingyuan L. Zhang and Yousif Kelaita and Jelena Vu{\v c}kovi{\'c}},
    year = {2016},
    journal = {Scientific Reports},
    keywords = {quantum dot, quantum-optical, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/64401/},
    abstract = {Strongly coupled quantum dot-cavity systems provide a non-linear configuration of hybridized light-matter states with promising quantum-optical applications. Here, we investigate the coherent interaction between strong laser pulses and quantum dot-cavity polaritons. Resonant excitation of polaritonic states and their interaction with phonons allow us to observe coherent Rabi oscillations and Ramsey fringes. Furthermore, we demonstrate complete coherent control of a quantum dot-photonic crystal cavity based quantum-bit. By controlling the excitation power and phase in a two-pulse excitation scheme we achieve access to the full Bloch sphere. Quantum-optical simulations are in good agreement with our experiments and provide insight into the decoherence mechanisms.}
    }

  • M. V. Yakushev, V. A. Volkov, N. N. Mursakulov, C. E. Sabzaliyeva, and R. W. Martin, "RBS-channeling study of radiation damage in Ar⁺ implanted CuInSe₂ crystals," Journal of Vacuum Science and Technology A, vol. 34, iss. 5, p. 51203, 2016.
    [BibTeX] [Abstract] [Download PDF]

    Chalcopyrite solar cells are reported to have a high tolerance to irradiation by high energy electrons or ions, but the origin of this is not well understood. This work studies the evolution of damage in Ar⁺-bombarded CuInSe₂ single crystal using Rutherford backscattering/channeling analysis. Ar⁺ ions of 30 keV were implanted with doses in the range from 10¹² to 3×10¹⁶ cm⁻² at room temperature. Implantation was found to create two layers of damage: (1) on the surface, caused by preferential sputtering of Se and Cu atoms; (2) at the layer of implanted Ar, possibly consisting of stacking faults and dislocation loops. The damage in the second layer was estimated to be less than 2% of the theoretical prediction suggesting efficient healing of primary implantation defects.

    @Article{strathprints62361,
    author = {Michael V. Yakushev and Vladimir A. Volkov and Niyazi N. Mursakulov and Chimnaz E. Sabzaliyeva and Robert W. Martin},
    title = {RBS-channeling study of radiation damage in {Ar⁺} implanted {CuInSe₂} crystals},
    journal = {Journal of Vacuum Science and Technology A},
    year = {2016},
    volume = {34},
    number = {5},
    pages = {051203},
    month = {September},
    abstract = {Chalcopyrite solar cells are reported to have a high tolerance to irradiation by high energy electrons or ions, but the origin of this is not well understood. This work studies the evolution of damage in Ar⁺-bombarded CuInSe₂ single crystal using Rutherford backscattering/channeling analysis. Ar⁺ ions of 30 keV were implanted with doses in the range from 10¹² to 3×10¹⁶ cm⁻² at room temperature. Implantation was found to create two layers of damage: (1) on the surface, caused by preferential sputtering of Se and Cu atoms; (2) at the layer of implanted Ar, possibly consisting of stacking faults and dislocation loops. The damage in the second layer was estimated to be less than 2% of the theoretical prediction suggesting efficient healing of primary implantation defects.},
    keywords = {radiation damage, CuInSe2, solar cells, Rutherford backscattering, RBS, Chalcopyrite, Physics, Surfaces, Coatings and Films, Surfaces and Interfaces, Condensed Matter Physics},
    url = {https://strathprints.strath.ac.uk/62361/}
    }

  • J. K. Gamble, P. Harvey-Collard, T. N. Jacobson, A. D. Baczewski, E. Nielsen, L. Maurer, I. Montaño, M. Rudolph, M. S. Carroll, C. H. Yang, A. Rossi, A. S. Dzurak, and R. P. Muller, "Valley splitting of single-electron Si MOS quantum dots," Applied Physics Letters, vol. 109, iss. 25, 2016. doi:10.1063/1.4972514
    [BibTeX] [Abstract] [Download PDF]

    Silicon-based metal-oxide-semiconductor quantum dots are prominent candidates for high-fidelity, manufacturable qubits. Due to silicon's band structure, additional low-energy states persist in these devices, presenting both challenges and opportunities. Although the physics governing these valley states has been the subject of intense study, quantitative agreement between experiment and theory remains elusive. Here, we present data from an experiment probing the valley states of quantum dot devices and develop a theory that is in quantitative agreement with both this and a recently reported experiment. Through sampling millions of realistic cases of interface roughness, our method provides evidence that the valley physics between the two samples is essentially the same.

    @article{strathprints68707,
    volume = {109},
    number = {25},
    month = {December},
    title = {Valley splitting of single-electron Si MOS quantum dots},
    year = {2016},
    doi = {10.1063/1.4972514},
    journal = {Applied Physics Letters},
    keywords = {quantum dots, silicon quantum dots, manufacturable qubits, Physics, Physics and Astronomy (miscellaneous)},
    url = {https://doi.org/10.1063/1.4972514},
    issn = {0003-6951},
    abstract = {Silicon-based metal-oxide-semiconductor quantum dots are prominent candidates for high-fidelity, manufacturable qubits. Due to silicon's band structure, additional low-energy states persist in these devices, presenting both challenges and opportunities. Although the physics governing these valley states has been the subject of intense study, quantitative agreement between experiment and theory remains elusive. Here, we present data from an experiment probing the valley states of quantum dot devices and develop a theory that is in quantitative agreement with both this and a recently reported experiment. Through sampling millions of realistic cases of interface roughness, our method provides evidence that the valley physics between the two samples is essentially the same.},
    author = {Gamble, John King and Harvey-Collard, Patrick and Jacobson, N. Tobias and Baczewski, Andrew D. and Nielsen, Erik and Maurer, Leon and Monta{\~n}o, In{\`e}s and Rudolph, Martin and Carroll, M. S. and Yang, C. H. and Rossi, A. and Dzurak, A. S. and Muller, Richard P.}
    }

  • T. Tanttu, A. Rossi, K. Y. Tan, A. Mäkinen, K. W. Chan, A. S. Dzurak, and M. Möttönen, "Three-waveform bidirectional pumping of single electrons with a silicon quantum dot," Scientific Reports, vol. 6, 2016. doi:10.1038/srep36381
    [BibTeX] [Abstract] [Download PDF]

    Semiconductor-based quantum dot single-electron pumps are currently the most promising candidates for the direct realization of the emerging quantum standard of the ampere in the International System of Units. Here, we discuss a silicon quantum dot single-electron pump with radio frequency control over the transparencies of entrance and exit barriers as well as the dot potential. We show that our driving protocol leads to robust bidirectional pumping: one can conveniently reverse the direction of the quantized current by changing only the phase shift of one driving waveform with respect to the others. We anticipate that this pumping technique may be used in the future to perform error counting experiments by pumping the electrons into and out of a reservoir island monitored by a charge sensor.

    @article{strathprints68652,
    volume = {6},
    month = {November},
    title = {Three-waveform bidirectional pumping of single electrons with a silicon quantum dot},
    year = {2016},
    doi = {10.1038/srep36381},
    journal = {Scientific Reports},
    keywords = {international system of units, silicon quantum dots, waveform, Physics, General, Physics and Astronomy(all)},
    url = {https://doi.org/10.1038/srep36381},
    issn = {2045-2322},
    abstract = {Semiconductor-based quantum dot single-electron pumps are currently the most promising candidates for the direct realization of the emerging quantum standard of the ampere in the International System of Units. Here, we discuss a silicon quantum dot single-electron pump with radio frequency control over the transparencies of entrance and exit barriers as well as the dot potential. We show that our driving protocol leads to robust bidirectional pumping: one can conveniently reverse the direction of the quantized current by changing only the phase shift of one driving waveform with respect to the others. We anticipate that this pumping technique may be used in the future to perform error counting experiments by pumping the electrons into and out of a reservoir island monitored by a charge sensor.},
    author = {Tanttu, Tuomo and Rossi, Alessandro and Tan, Kuan Yen and M{\"a}kinen, Akseli and Chan, Kok Wai and Dzurak, Andrew S. and M{\"o}tt{\"o}nen, Mikko}
    }

  • M. Conroy, H. Li, V. Z. Zubialevich, G. Kusch, M. Schmidt, T. Collins, C. Glynn, R. W. Martin, C. O'Dwyer, J. D. Holmes, P. J. Parbrook, and M. D. Morris, "Self-healing thermal annealing : surface morphological restructuring control of GaN nanorods," Crystal Growth and Design, vol. 16, iss. 12, p. 6769–6775, 2016.
    [BibTeX] [Abstract] [Download PDF]

    With advances in nanolithography and dry etching, top-down methods of nanostructuring have become a widely used tool for improving the efficiency of optoelectronics. These nano dimensions can offer various benefits to the device performance in terms of light extraction and efficiency, but often at the expense of emission color quality. Broadening of the target emission peak and unwanted yellow luminescence are characteristic defect-related effects due to the ion beam etching damage, particularly for III?N based materials. In this article we focus on GaN based nanorods, showing that through thermal annealing the surface roughness and deformities of the crystal structure can be ?self-healed?. Correlative electron microscopy and atomic force microscopy show the change from spherical nanorods to faceted hexagonal structures, revealing the temperature-dependent surface morphology faceting evolution. The faceted nanorods were shown to be strain- and defect-free by cathodoluminescence hyperspectral imaging, micro-Raman, and transmission electron microscopy (TEM). In-situ TEM thermal annealing experiments allowed for real time observation of dislocation movements and surface restructuring observed in ex-situ annealing TEM sampling. This thermal annealing investigation gives new insight into the redistribution path of GaN material and dislocation movement post growth, allowing for improved understanding and in turn advances in optoelectronic device processing of compound semiconductors.

    @article{strathprints60967,
    volume = {16},
    number = {12},
    month = {December},
    author = {Michelle Conroy and Haoning Li and Vitaly Z. Zubialevich and Gunnar Kusch and Michael Schmidt and Timothy Collins and Colm Glynn and Robert W. Martin and Colm O'Dwyer and Justin D. Holmes and Peter J. Parbrook and Michael D. Morris},
    note = {This document is the Accepted Manuscript version of a Published Work that appeared in final form in Crystal Growth and Design, copyright {\copyright} American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.cgd.6b00756},
    title = {Self-healing thermal annealing : surface morphological restructuring control of GaN nanorods},
    year = {2016},
    journal = {Crystal Growth and Design},
    pages = {6769--6775},
    keywords = {nanolithography, nanostructuring, nanorods, thermal annealing, self-healing, gallium nitride, cathodoluminescence hyperspectral imaging, micro-Raman, Physics, Materials Science(all), Chemistry(all), Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/60967/},
    abstract = {With advances in nanolithography and dry etching, top-down methods of nanostructuring have become a widely used tool for improving the efficiency of optoelectronics. These nano dimensions can offer various benefits to the device performance in terms of light extraction and efficiency, but often at the expense of emission color quality. Broadening of the target emission peak and unwanted yellow luminescence are characteristic defect-related effects due to the ion beam etching damage, particularly for III?N based materials. In this article we focus on GaN based nanorods, showing that through thermal annealing the surface roughness and deformities of the crystal structure can be ?self-healed?. Correlative electron microscopy and atomic force microscopy show the change from spherical nanorods to faceted hexagonal structures, revealing the temperature-dependent surface morphology faceting evolution. The faceted nanorods were shown to be strain- and defect-free by cathodoluminescence hyperspectral imaging, micro-Raman, and transmission electron microscopy (TEM). In-situ TEM thermal annealing experiments allowed for real time observation of dislocation movements and surface restructuring observed in ex-situ annealing TEM sampling. This thermal annealing investigation gives new insight into the redistribution path of GaN material and dislocation movement post growth, allowing for improved understanding and in turn advances in optoelectronic device processing of compound semiconductors.}
    }

  • E. Taylor-Shaw, E. Angioni, N. J. Findlay, B. Breig, A. R. Inigo, J. Bruckbauer, D. J. Wallis, P. J. Skabara, and R. W. Martin, "Cool to warm white light emission from hybrid inorganic/organic light-emitting diodes," Journal of Materials Chemistry. C, vol. 4, iss. 48, p. 11499–11507, 2016.
    [BibTeX] [Abstract] [Download PDF]

    The synthesis and characterisation of two novel organic down-converting molecules is disclosed, together with their performance as functional colour-converters in combination with inorganic blue light-emitting diodes (LEDs). Each molecule contains two fluorene-triphenylamine arms, connected to either a benzothiadiazole or bisbenzothiadiazole core. These molecules have been selected on the basis that they are free from absorption bands in the green region of the visible spectrum to maximise their performance and offer improvements compared with previous BODIPY-containing analogues. The inorganic InGaN/GaN LED emits at 444 nm, overlying the absorption of each of the organic molecules. The combination of the blue (inorganic) and yellow (organic) emission is shown to produce reasonable quality, white light-emitting hybrid devices for both down-converter molecules. Cool to warm white light is achieved for both molecules by increasing the concentration. An optimum colour rendering index (CRI) value of 66 is obtained for the mono-benzothiadiazole molecule. Also a high blue-to-white efficacy (defined as white luminous flux (lm)/blue radiant flux (W)) of 368 lm/W is achieved, superseding the current phosphor converters of 200-300 lm/W. A comparison of these down-converting molecules to the older generation BODIPY-containing molecules is also provided.

    @article{strathprints58749,
    volume = {4},
    number = {48},
    month = {November},
    author = {Elaine Taylor-Shaw and Enrico Angioni and Neil J. Findlay and Benjamin Breig and Anto R. Inigo and Jochen Bruckbauer and David J. Wallis and Peter J. Skabara and Robert W. Martin},
    title = {Cool to warm white light emission from hybrid inorganic/organic light-emitting diodes},
    journal = {Journal of Materials Chemistry. C},
    pages = {11499--11507},
    year = {2016},
    keywords = {light-emitting diodes (LEDs), organic down-converting molecules, light emission, c InGaN/GaN LED, Chemistry, Chemistry (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/58749/},
    abstract = {The synthesis and characterisation of two novel organic down-converting molecules is disclosed, together with their performance as functional colour-converters in combination with inorganic blue light-emitting diodes (LEDs). Each molecule contains two fluorene-triphenylamine arms, connected to either a benzothiadiazole or bisbenzothiadiazole core. These molecules have been selected on the basis that they are free from absorption bands in the green region of the visible spectrum to maximise their performance and offer improvements compared with previous BODIPY-containing analogues. The inorganic InGaN/GaN LED emits at 444 nm, overlying the absorption of each of the organic molecules. The combination of the blue (inorganic) and yellow (organic) emission is shown to produce reasonable quality, white light-emitting hybrid devices for both down-converter molecules. Cool to warm white light is achieved for both molecules by increasing the concentration. An optimum colour rendering index (CRI) value of 66 is obtained for the mono-benzothiadiazole molecule. Also a high blue-to-white efficacy (defined as white luminous flux (lm)/blue radiant flux (W)) of 368 lm/W is achieved, superseding the current phosphor converters of 200-300 lm/W. A comparison of these down-converting molecules to the older generation BODIPY-containing molecules is also provided.}
    }

  • W. L. Sarney, S. P. Svensson, M. Ting, N. Segercrantz, W. Walukiewicz, K. M. Yu, R. W. Martin, S. V. Novikov, and T. C. T. Foxon, "Intermixing studies in GaN₁₋ₓSbₓ highly mismatched alloys," Applied Optics, vol. 56, iss. 3, p. B64–B69, 2016.
    [BibTeX] [Abstract] [Download PDF]

    GaN1?xSbx with x{\texttt{\char126}} 5-7\% is a highly mismatched alloy predicted to have favorable properties for application as an electrode in a photo-electrochemical cell for solar water splitting. In this study, we grew GaN1?xSbx under conditions intended to induce phase segregation. Prior experiments with the similar alloy GaN1?xAsx, the tendency of Sb to surfact, and the low growth temperatures needed to incorporate Sb, all suggested that GaN1?xSbx alloys would likely exhibit phase segregation. We found that, except for very high Sb compositions, this was not the case, and that instead interdiffusion dominated. Characteristics measured by optical absorption were similar to intentionally grown bulk alloys for the same composition. Furthermore, the alloys produced by this method maintained crystallinity for very high Sb compositions, and allowed higher overall Sb compositions. This method may allow higher temperature growth while still achieving needed Sb compositions for solar water splitting applications.

    @Article{strathprints58170,
    author = {Wendy L. Sarney and Stefan P. Svensson and Min Ting and Natalie Segercrantz and Wladek Walukiewicz and Kin Man Yu and Robert W. Martin and Sergei V. Novikov and C.T. Thomas Foxon},
    title = {Intermixing studies in {GaN₁₋ₓSbₓ} highly mismatched alloys},
    journal = {Applied Optics},
    year = {2016},
    volume = {56},
    number = {3},
    pages = {B64--B69},
    month = {September},
    note = {{\copyright} 2016 Optical Society of America. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited.},
    abstract = {GaN1?xSbx with x{\texttt{\char126}} 5-7\% is a highly mismatched alloy predicted to have favorable properties for application as an electrode in a photo-electrochemical cell for solar water splitting. In this study, we grew GaN1?xSbx under conditions intended to induce phase segregation. Prior experiments with the similar alloy GaN1?xAsx, the tendency of Sb to surfact, and the low growth temperatures needed to incorporate Sb, all suggested that GaN1?xSbx alloys would likely exhibit phase segregation. We found that, except for very high Sb compositions, this was not the case, and that instead interdiffusion dominated. Characteristics measured by optical absorption were similar to intentionally grown bulk alloys for the same composition. Furthermore, the alloys produced by this method maintained crystallinity for very high Sb compositions, and allowed higher overall Sb compositions. This method may allow higher temperature growth while still achieving needed Sb compositions for solar water splitting applications.},
    keywords = {semiconductor materials, optoelectronics, thin films, Optics. Light, Atomic and Molecular Physics, and Optics},
    url = {http://strathprints.strath.ac.uk/58170/}
    }

  • D. Verma, B. Hourahine, T. Frauenheim, R. D. James, and T. Dumitrică, "Directional-dependent thickness and bending rigidity of phosphorene," Physical Review B (Condensed Matter), vol. 94, iss. 12, p. 121404(R), 2016.
    [BibTeX] [Abstract] [Download PDF]

    The strong mechanical anisotropy of phosphorene combined with the atomic-scale thickness challenges the commonly employed elastic continuum idealizations. Using objective boundary conditions and a density functional-based potential, we directly uncover the flexibility of individual {\ensuremath{\alpha}}, {\ensuremath{\beta}} and {\ensuremath{\gamma}} phosphorene allotrope layers along an arbitrary bending direction. A correlation analysis with the in-plane elasticity finds that although a monolayer thickness cannot be defined in the classical continuum sense, an unusual orthotropic plate with a directional-dependent thickness can unambiguously describe the out-of-plane deformation of {\ensuremath{\alpha}} and {\ensuremath{\gamma}} allotropes. Such decoupling of the in-plane and out-of-plane nanomechanics might be generic for two-dimensional materials beyond graphene.

    @Article{strathprints57606,
    author = {Deepti Verma and Benjamin Hourahine and Thomas Frauenheim and Richard D. James and Traian Dumitrică},
    title = {Directional-dependent thickness and bending rigidity of phosphorene},
    journal = {Physical Review B (Condensed Matter)},
    year = {2016},
    volume = {94},
    number = {12},
    pages = {121404(R)},
    month = {September},
    note = {{\copyright} 2016 American Physical Society.},
    abstract = {The strong mechanical anisotropy of phosphorene combined with the atomic-scale thickness challenges the commonly employed elastic continuum idealizations. Using objective boundary conditions and a density functional-based potential, we directly uncover the flexibility of individual {\ensuremath{\alpha}}, {\ensuremath{\beta}} and {\ensuremath{\gamma}} phosphorene allotrope layers along an arbitrary bending direction. A correlation analysis with the in-plane elasticity finds that although a monolayer thickness cannot be defined in the classical continuum sense, an unusual orthotropic plate with a directional-dependent thickness can unambiguously describe the out-of-plane deformation of {\ensuremath{\alpha}} and {\ensuremath{\gamma}} allotropes. Such decoupling of the in-plane and out-of-plane nanomechanics might be generic for two-dimensional materials beyond graphene.},
    keywords = {phosphorene, atomic-scale thickness, nanomechanics, Physics, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/57606/}
    }

  • J. Bruckbauer, C. Brasser, N. J. Findlay, P. R. Edwards, D. J. Wallis, P. J. Skabara, and R. W. Martin, "Colour tuning in white hybrid inorganic/organic light-emitting diodes," Journal of Physics D: Applied Physics, vol. 49, iss. 40, p. 405103, 2016.
    [BibTeX] [Abstract] [Download PDF]

    White hybrid inorganic/organic light-emitting diodes (LEDs) were fabricated by combining a novel organic colour converter with a blue inorganic LED. An organic small molecule was specifically synthesised to act as down-converter. The characteristics of the white colour were controlled by changing the concentration of the organic molecule based on the BODIPY unit, which was embedded in a transparent matrix, and volume of the molecule and encapsulant mixture. The concentration has a critical effect on the conversion efficiency, i.e. how much of the absorbed blue light is converted into yellow light. With increasing concentration the conversion efficiency decreases. This quenching effect is due to aggregation of the organic molecule at higher concentrations. Increasing the deposited amount of the converter does not increase the yellow emission despite more blue light being absorbed. Degradation of the organic converter was also observed during a period of 15 months from LED fabrication. Angular-dependent measurements revealed slight deviation from a Lambertian profile for the blue and yellow emission peaks leading to a small change in "whiteness" with emission angle. Warm white and cool white light with correlated colour temperatures of 2770 K and 7680 K, respectively, were achieved using different concentrations of the converter molecule. Although further work is needed to improve the lifetime and poor colour rendering, these hybrid LEDs show promising results as an alternative approach for generating white LEDs compared with phosphor-based white LEDs.

    @Article{strathprints57454,
    author = {Jochen Bruckbauer and Catherine Brasser and Neil J. Findlay and Paul R. Edwards and David J. Wallis and Peter J. Skabara and Robert W. Martin},
    title = {Colour tuning in white hybrid inorganic/organic light-emitting diodes},
    journal = {Journal of Physics D: Applied Physics},
    year = {2016},
    volume = {49},
    number = {40},
    pages = {405103},
    month = {September},
    abstract = {White hybrid inorganic/organic light-emitting diodes (LEDs) were fabricated by combining a novel organic colour converter with a blue inorganic LED. An organic small molecule was specifically synthesised to act as down-converter. The characteristics of the white colour were controlled by changing the concentration of the organic molecule based on the BODIPY unit, which was embedded in a transparent matrix, and volume of the molecule and encapsulant mixture. The concentration has a critical effect on the conversion efficiency, i.e. how much of the absorbed blue light is converted into yellow light. With increasing concentration the conversion efficiency decreases. This quenching effect is due to aggregation of the organic molecule at higher concentrations. Increasing the deposited amount of the converter does not increase the yellow emission despite more blue light being absorbed. Degradation of the organic converter was also observed during a period of 15 months from LED fabrication. Angular-dependent measurements revealed slight deviation from a Lambertian profile for the blue and yellow emission peaks leading to a small change in "whiteness" with emission angle. Warm white and cool white light with correlated colour temperatures of 2770 K and 7680 K, respectively, were achieved using different concentrations of the converter molecule. Although further work is needed to improve the lifetime and poor colour rendering, these hybrid LEDs show promising results as an alternative approach for generating white LEDs compared with phosphor-based white LEDs.},
    keywords = {light-emitting diode, energy efficiencey, organic converter, colour converter, hybrid white LED, inorganic blue LED, Optics. Light, Atomic and Molecular Physics, and Optics, Surfaces, Coatings and Films, Energy Engineering and Power Technology},
    url = {http://strathprints.strath.ac.uk/57454/}
    }

  • M. Conroy, H. Li, G. Kusch, C. Zhao, B. Ooi, P. R. Edwards, R. W. Martin, J. D. Holmes, and P. J. Parbrook, "Correction: Site controlled red-yellow-green light emitting InGaN quantum discs on nano-tipped GaN rods," Nanoscale, vol. 8, iss. 27, p. 13521, 2016.
    [BibTeX] [Abstract] [Download PDF]

    Correction for 'Site controlled red-yellow-green light emitting InGaN quantum discs on nano-tipped GaN rods' by M. Conroy et al., Nanoscale, 2016, 8 , 11019-11026.

    @Article{strathprints57022,
    author = {M. Conroy and H. Li and G. Kusch and C. Zhao and B. Ooi and P. R. Edwards and R. W. Martin and J. D. Holmes and P. J. Parbrook},
    title = {Correction: {S}ite controlled red-yellow-green light emitting {InGaN} quantum discs on nano-tipped {GaN} rods},
    journal = {Nanoscale},
    year = {2016},
    volume = {8},
    number = {27},
    pages = {13521},
    month = {July},
    abstract = {Correction for 'Site controlled red-yellow-green light emitting InGaN quantum discs on nano-tipped GaN rods' by M. Conroy et al., Nanoscale, 2016, 8 , 11019-11026.},
    keywords = {InGaN quantum discs, nano-tipped GaN rods, nanorods, electron microscopy, energy-dispersive x-ray, cathodoluminescence, Solid state physics. Nanoscience, Materials Science(all)},
    url = {http://strathprints.strath.ac.uk/57022/}
    }

  • K. M. Yu, W. L. Sarney, S. V. Novikov, N. Segercrantz, M. Ting, M. Shaw, S. P. Svensson, R. W. Martin, W. Walukiewicz, and C. T. Foxon, "Highly mismatched GaN₁₋ₓSbₓ alloys : synthesis, structure and electronic properties," Semiconductor Science and Technology, vol. 31, iss. 8, p. 83001, 2016.
    [BibTeX] [Abstract] [Download PDF]

    Highly mismatched alloys (HMAs) is a class of semiconductor alloys whose constituents are distinctly different in terms of size, ionicity and/or electronegativity. Electronic properties of the alloys deviate significantly from an interpolation scheme based on small deviations from the virtual crystal approximation. Most of the HMAs were only studied in a dilute composition limit. Recent advances in understanding of the semiconductor synthesis processes allowed growth of thin films of HMAs under non-equilibrium conditions. Thus reducing the growth temperature allowed synthesis of group III-N-V HMAs over almost the entire composition range. This paper focuses on the GaNxSb1-x HMA which has been suggested as a potential material for solar water dissociation devices. Here we review our recent work on the synthesis, structural and optical characterization of GaN1-xSbx HMA. Theoretical modeling studies on its electronic structure based on the band anticrossing (BAC) model are also reviewed. In particular we discuss the effects of growth temperature, Ga flux and Sb flux on the incorporation of Sb, film microstructure and optical properties of the alloys. Results obtained from two separate MBE growths are directly compared. Our work demonstrates that a large range of direct bandgap energies from 3.4 eV to below 1.0 eV can be achieved for this alloy grown at low temperature. We show that the electronic band structure of GaN1-xSbx HMA over the entire composition range is well described by a modified the BAC model which includes the dependence of the host matrix band edges as well as the BAC model coupling parameters on composition. We emphasize that the modified BAC model of the electronic band structure developed for the full composition of GaNxSb1-x is general and is applicable to any HMA.

    @Article{strathprints56601,
    author = {K M Yu and W L Sarney and S V Novikov and N Segercrantz and M Ting and M Shaw and S P Svensson and R W Martin and W Walukiewicz and C T Foxon},
    title = {Highly mismatched {GaN₁₋ₓSbₓ} alloys : synthesis, structure and electronic properties},
    journal = {Semiconductor Science and Technology},
    year = {2016},
    volume = {31},
    number = {8},
    pages = {083001},
    month = {June},
    abstract = {Highly mismatched alloys (HMAs) is a class of semiconductor alloys whose constituents are distinctly different in terms of size, ionicity and/or electronegativity. Electronic properties of the alloys deviate significantly from an interpolation scheme based on small deviations from the virtual crystal approximation. Most of the HMAs were only studied in a dilute composition limit. Recent advances in understanding of the semiconductor synthesis processes allowed growth of thin films of HMAs under non-equilibrium conditions. Thus reducing the growth temperature allowed synthesis of group III-N-V HMAs over almost the entire composition range. This paper focuses on the GaNxSb1-x HMA which has been suggested as a potential material for solar water dissociation devices. Here we review our recent work on the synthesis, structural and optical characterization of GaN1-xSbx HMA. Theoretical modeling studies on its electronic structure based on the band anticrossing (BAC) model are also reviewed. In particular we discuss the effects of growth temperature, Ga flux and Sb flux on the incorporation of Sb, film microstructure and optical properties of the alloys. Results obtained from two separate MBE growths are directly compared. Our work demonstrates that a large range of direct bandgap energies from 3.4 eV to below 1.0 eV can be achieved for this alloy grown at low temperature. We show that the electronic band structure of GaN1-xSbx HMA over the entire composition range is well described by a modified the BAC model which includes the dependence of the host matrix band edges as well as the BAC model coupling parameters on composition. We emphasize that the modified BAC model of the electronic band structure developed for the full composition of GaNxSb1-x is general and is applicable to any HMA.},
    keywords = {highly mismatched alloys (HMAs), semiconductor alloys, semiconductor synthesis, band anticrossing , Solid state physics. Nanoscience, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/56601/}
    }

  • M. Conroy, H. Li, G. Kusch, C. Zhao, B. Ooi, P. R. Edwards, R. W. Martin, J. D. Holmes, and P. J. Parbrook, "Site controlled red-yellow-green light emitting InGaN quantum discs on nano-tipped GaN rods," Nanoscale, vol. 8, iss. 21, p. 11019–11026, 2016.
    [BibTeX] [Abstract] [Download PDF]

    We report a method of growing site controlled InGaN multiple quantum discs (QDs) at uniform wafer scale on coalescence free ultra-high density ({\ensuremath{>}}80\%) nanorod templates by metal organic chemical vapour deposition (MOCVD). The dislocation and coalescence free nature of the GaN space filling nanorod arrays eliminates the well-known emission problems seen in InGaN based visible light sources that these types of crystallographic defects cause. Correlative scanning transmission electron microscopy (STEM), energy-dispersive X-ray (EDX) mapping and cathodoluminescence (CL) hyperspectral imaging illustrates the controlled site selection of the red, yellow and green (RYG) emission at these nano tips. This article reveals that the nanorod tips? broad emission in the RYG visible range is in fact achieved by manipulating the InGaN QD?s confinement dimensions, rather than significantly increasing the In\%. This article details the easily controlled method of manipulating the QDs dimensions producing high crystal quality InGaN without complicated growth conditions needed for strain relaxation and alloy compositional changes seen for bulk planar GaN templates.

    @Article{strathprints56549,
    author = {M. Conroy and H. Li and G. Kusch and C. Zhao and B. Ooi and P. R. Edwards and R. W. Martin and J. D. Holmes and P. J. Parbrook},
    title = {Site controlled red-yellow-green light emitting {InGaN} quantum discs on nano-tipped {GaN} rods},
    journal = {Nanoscale},
    year = {2016},
    volume = {8},
    number = {21},
    pages = {11019--11026},
    month = {May},
    abstract = {We report a method of growing site controlled InGaN multiple quantum discs (QDs) at uniform wafer scale on coalescence free ultra-high density ({\ensuremath{>}}80\%) nanorod templates by metal organic chemical vapour deposition (MOCVD). The dislocation and coalescence free nature of the GaN space filling nanorod arrays eliminates the well-known emission problems seen in InGaN based visible light sources that these types of crystallographic defects cause. Correlative scanning transmission electron microscopy (STEM), energy-dispersive X-ray (EDX) mapping and cathodoluminescence (CL) hyperspectral imaging illustrates the controlled site selection of the red, yellow and green (RYG) emission at these nano tips. This article reveals that the nanorod tips? broad emission in the RYG visible range is in fact achieved by manipulating the InGaN QD?s confinement dimensions, rather than significantly increasing the In\%. This article details the easily controlled method of manipulating the QDs dimensions producing high crystal quality InGaN without complicated growth conditions needed for strain relaxation and alloy compositional changes seen for bulk planar GaN templates.},
    keywords = {InGaN quantum discs, nano-tipped GaN rods, nanorods, electron microscopy, energy-dispersive x-ray, cathodoluminescence, Solid state physics. Nanoscience, Materials Science(all)},
    url = {http://strathprints.strath.ac.uk/56549/}
    }

  • C. X. Ren, B. Rouet-Leduc, J. T. Griffiths, E. Bohacek, M. J. Wallace, P. R. Edwards, M. A. Hopkins, D. W. E. Allsopp, M. J. Kappers, R. W. Martin, and R. A. Oliver, "Analysis of defect-related inhomogeneous electroluminescence in InGaN/GaN QW LEDs," Superlattices and Microstructures, vol. 99, p. 118–124, 2016.
    [BibTeX] [Abstract] [Download PDF]

    The inhomogeneous electroluminescence (EL) of InGaN/GaN quantum well light emitting diode structures was investigated in this study. Electroluminescence hyperspectral images showed that inhomogeneities in the form of bright spots exhibited spectrally blue-shifted and broadened emission. Scanning electron microscopy combined with cathodoluminescence (SEM-CL) was used to identify hexagonal pits at the centre of approximately 20\% of these features. Scanning transmission electron microscopy imaging with energy dispersive X-ray spectroscopy (STEM-EDX) indicated there may be p-doped AlGaN within the active region caused by the presence of the pit. Weak beam dark-field TEM (WBDF-TEM) revealed the presence of bundles of dislocations associated with the pit, suggesting the surface features which cause the inhomogeneous EL may occur at coalescence boundaries, supported by trends in the number of features observed across the wafer.

    @Article{strathprints56455,
    author = {C.X. Ren and B. Rouet-Leduc and J.T. Griffiths and E. Bohacek and M.J. Wallace and P.R. Edwards and M.A. Hopkins and D.W.E. Allsopp and M.J. Kappers and R.W. Martin and R.A. Oliver},
    title = {Analysis of defect-related inhomogeneous electroluminescence in {InGaN/GaN QW LED}s},
    journal = {Superlattices and Microstructures},
    year = {2016},
    volume = {99},
    pages = {118--124},
    abstract = {The inhomogeneous electroluminescence (EL) of InGaN/GaN quantum well light emitting diode structures was investigated in this study. Electroluminescence hyperspectral images showed that inhomogeneities in the form of bright spots exhibited spectrally blue-shifted and broadened emission. Scanning electron microscopy combined with cathodoluminescence (SEM-CL) was used to identify hexagonal pits at the centre of approximately 20\% of these features. Scanning transmission electron microscopy imaging with energy dispersive X-ray spectroscopy (STEM-EDX) indicated there may be p-doped AlGaN within the active region caused by the presence of the pit. Weak beam dark-field TEM (WBDF-TEM) revealed the presence of bundles of dislocations associated with the pit, suggesting the surface features which cause the inhomogeneous EL may occur at coalescence boundaries, supported by trends in the number of features observed across the wafer.},
    keywords = {semiconductor, LED, defect, electroluminescence, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/56455/}
    }

  • E. D. Le Boulbar, P. R. Edwards, S. H. Vajargah, I. Griffiths, I. Gîrgel, P. Coulon, D. Cherns, R. W. Martin, C. J. Humphreys, C. R. Bowen, D. W. E. Allsopp, and P. A. Shields, "Structural and optical emission uniformity of m-plane InGaN single quantum wells in core-shell nanorods," Crystal Growth and Design, vol. 16, iss. 4, p. 1907–1916, 2016.
    [BibTeX] [Abstract] [Download PDF]

    Controlling the long-range homogeneity of core-shell InGaN/GaN layers is essential for their use in light-emitting devices. This paper demonstrates variations in optical emission energy as low as {\texttt{\char126}}7 meV.?m-1 along the m-plane facets from core-shell InGaN/GaN single quantum wells as measured through high-resolution cathodoluminescence hyperspectral imaging. The layers were grown by metal organic vapor phase epitaxy on etched GaN nanorod arrays with a pitch of 2 ?m. High-resolution transmission electron microscopy and spatially-resolved energy-dispersive X-ray spectroscopy measurements demonstrate a long-range InN-content and thickness homogeneity along the entire 1.2 {\ensuremath{\mu}}m length of the m-plane. Such homogeneous emission was found on the m-plane despite the observation of short range compositional fluctuations in the InGaN single quantum well. The ability to achieve this uniform optical emission from InGaN/GaN core-shell layers is critical to enable them to compete with and replace conventional planar light-emitting devices.

    @Article{strathprints56115,
    author = {Le Boulbar, Emmanuel D. and Paul R. Edwards and Shahrzad Hosseini Vajargah and Ian Griffiths and Ionut G{\^i}rgel and Coulon, Pierre-Marie and David Cherns and Robert W. Martin and C. J. Humphreys and Chris R. Bowen and D. W. E. Allsopp and P. A. Shields},
    title = {Structural and optical emission uniformity of m-plane {InGaN} single quantum wells in core-shell nanorods},
    journal = {Crystal Growth and Design},
    year = {2016},
    volume = {16},
    number = {4},
    pages = {1907--1916},
    month = {April},
    abstract = {Controlling the long-range homogeneity of core-shell InGaN/GaN layers is essential for their use in light-emitting devices. This paper demonstrates variations in optical emission energy as low as {\texttt{\char126}}7 meV.?m-1 along the m-plane facets from core-shell InGaN/GaN single quantum wells as measured through high-resolution cathodoluminescence hyperspectral imaging. The layers were grown by metal organic vapor phase epitaxy on etched GaN nanorod arrays with a pitch of 2 ?m. High-resolution transmission electron microscopy and spatially-resolved energy-dispersive X-ray spectroscopy measurements demonstrate a long-range InN-content and thickness homogeneity along the entire 1.2 {\ensuremath{\mu}}m length of the m-plane. Such homogeneous emission was found on the m-plane despite the observation of short range compositional fluctuations in the InGaN single quantum well. The ability to achieve this uniform optical emission from InGaN/GaN core-shell layers is critical to enable them to compete with and replace conventional planar light-emitting devices.},
    keywords = {gallium nitride, GaN, nanostructured materials, LED, cathodoluminescence, CL, SEM, TEM, EDX, MOVPE, Physics, Physics and Astronomy(all), Materials Science(all), Engineering(all)},
    url = {http://strathprints.strath.ac.uk/56115/}
    }

  • M. V. Yakushev, J. Krustok, M. Grossberg, V. A. Volkov, A. V. Mudryi, and R. W. Martin, "A photoluminescence study of CuInSe₂ single crystals ion implanted with 5 keV hydrogen," Journal of Physics D: Applied Physics, vol. 49, iss. 10, p. 105108, 2016.
    [BibTeX] [Abstract] [Download PDF]

    CuInSe2 single crystals ion implanted with 5 keV hydrogen at doses from 3 {$\times$} 1014 to 1016 cm-2 are studied by photoluminescence (PL). The PL spectra before and after implantation reveal two bands, a main dominant band centred at 0.96 eV and a lower intensity band centred at 0.93 eV. Detailed analysis of the shape of these bands, their temperature and excitation intensity dependencies allow the recombination mechanisms to be identified as band-to-tail (BT) and band-to-impurity (BI), respectively. The implantation causes gradual red shifts of the bands increasing linearly with the dose. The average depth of potential fluctuations is also estimated to increase with the dose and saturates for doses above 1015 cm-2. A model is proposed which associates the potential fluctuations with the antisite defects copper on indium site and indium on copper site. The saturation is explained by full randomization of copper and indium atoms on the cation sub-lattice.

    @Article{strathprints56024,
    author = {M. V. Yakushev and J. Krustok and M. Grossberg and V. A. Volkov and A. V. Mudryi and R. W. Martin},
    title = {A photoluminescence study of {CuInSe₂} single crystals ion implanted with 5 {keV} hydrogen},
    journal = {Journal of Physics D: Applied Physics},
    year = {2016},
    volume = {49},
    number = {10},
    pages = {105108},
    month = {February},
    abstract = {CuInSe2 single crystals ion implanted with 5 keV hydrogen at doses from 3 {$\times$} 1014 to 1016 cm-2 are studied by photoluminescence (PL). The PL spectra before and after implantation reveal two bands, a main dominant band centred at 0.96 eV and a lower intensity band centred at 0.93 eV. Detailed analysis of the shape of these bands, their temperature and excitation intensity dependencies allow the recombination mechanisms to be identified as band-to-tail (BT) and band-to-impurity (BI), respectively. The implantation causes gradual red shifts of the bands increasing linearly with the dose. The average depth of potential fluctuations is also estimated to increase with the dose and saturates for doses above 1015 cm-2. A model is proposed which associates the potential fluctuations with the antisite defects copper on indium site and indium on copper site. The saturation is explained by full randomization of copper and indium atoms on the cation sub-lattice.},
    keywords = {CuInSe, ion-implantation, photoluminescence, Physics, Condensed Matter Physics, Electronic, Optical and Magnetic Materials, Acoustics and Ultrasonics, Surfaces, Coatings and Films},
    url = {http://strathprints.strath.ac.uk/56024/}
    }

  • I. E. Svitsiankou, V. N. Pavlovskii, E. Lutsenko, G. P. Yablonskii, A. V. Mudryi, V. Zhivulko, M. V. Yakushev, and R. W. Martin, "Stimulated emission and lasing in Cu(In,Ga)Se₂ thin films," Journal of Physics D: Applied Physics, vol. 49, iss. 9, p. 95106, 2016.
    [BibTeX] [Abstract] [Download PDF]

    Stimulated emission and lasing in Cu(In,Ga)Se 2 thin films have been demonstrated at a temperature of 20 K using excitation by a nanosecond pulsed N 2 laser with power densities in the range from 2 to 100 kW cm ? 2 . Sharp narrowing of the photoluminescence band, superlinear dependence of its intensity on excitation laser power, as well as stabilization of the spectral position and of the full-width at half-maximum of the band were observed in the films at increasing excitation intensity. The stimulated emission threshold was determined to be 20 kW cm ? 2 . A gain value of 94 cm ? 1 has been estimated using the variable stripe length method. Several sharp laser modes near 1.13 eV were observed above the laser threshold of I thr {\texttt{\char126}} 50 kW cm ? 2

    @Article{strathprints55973,
    author = {I E Svitsiankou and V N Pavlovskii and EV Lutsenko and G P Yablonskii and A V Mudryi and VD Zhivulko and M V Yakushev and R W Martin},
    title = {Stimulated emission and lasing in {Cu(In,Ga)Se₂} thin films},
    journal = {Journal of Physics D: Applied Physics},
    year = {2016},
    volume = {49},
    number = {9},
    pages = {095106},
    month = {February},
    abstract = {Stimulated emission and lasing in Cu(In,Ga)Se 2 thin films have been demonstrated at a temperature of 20 K using excitation by a nanosecond pulsed N 2 laser with power densities in the range from 2 to 100 kW cm ? 2 . Sharp narrowing of the photoluminescence band, superlinear dependence of its intensity on excitation laser power, as well as stabilization of the spectral position and of the full-width at half-maximum of the band were observed in the films at increasing excitation intensity. The stimulated emission threshold was determined to be 20 kW cm ? 2 . A gain value of 94 cm ? 1 has been estimated using the variable stripe length method. Several sharp laser modes near 1.13 eV were observed above the laser threshold of I thr {\texttt{\char126}} 50 kW cm ? 2},
    keywords = {stimulated emission, lasing, gain, threshold, photoluminescence, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/55973/}
    }

  • J. Márquez-Prieto, M. V. Yakushev, I. Forbes, J. Krustok, P. R. Edwards, V. D. Zhivulko, O. M. Borodavchenko, A. V. Mudryi, M. Dimitrievska, V. Izquierdo-Roca, N. Pearsall, and R. W. Martin, "Impact of the selenisation temperature on the structural and optical properties of CZTSe absorbers," Solar Energy Materials and Solar Cells, vol. 152, p. 42–50, 2016.
    [BibTeX] [Abstract] [Download PDF]

    We present structural and optical spectroscopy studies of thin films of Cu2ZnSnSe4 (CZTSe) with strong copper deficiency deposited on Mo/Glass substrates and selenised at 450, 500 or 550 ?C. Solar cells fabricated from these films demonstrated efficiencies up to 7.4\% for selenisation at 500 ?C. Structural analysis based on X-ray diffraction and Raman spectroscopy revealed the presence of SnSe2 in the film selenised at 450 ?C but not detected in the films selenised at higher temperatures. A progressive decrease of the Sn and Se content was observed as the selenisation temperature increased. Photoluminescence excitation was used to determine the bandgaps at 4.2 K. Detailed measurements of the temperature and excitation intensity dependencies of the photoluminescence spectra allow the recombination mechanisms of the observed emission bands to be identified as band-to-impurity and band-to-band transitions, and their evolution with selenisation temperature changes to be analysed. The strongest band-to-band transition is recorded in the PL spectra of the film selenised at 500 ?C and can be observed from 6 K to room temperature. The compositional and structural changes in the films and their influence on the optoelectronic properties of CZTSe and solar cells are discussed.

    @Article{strathprints55956,
    author = {J. M{\'a}rquez-Prieto and M. V. Yakushev and I. Forbes and J. Krustok and P. R. Edwards and V. D. Zhivulko and O. M. Borodavchenko and A. V. Mudryi and M. Dimitrievska and V. Izquierdo-Roca and N. Pearsall and R. W. Martin},
    title = {Impact of the selenisation temperature on the structural and optical properties of {CZTSe} absorbers},
    journal = {Solar Energy Materials and Solar Cells},
    year = {2016},
    volume = {152},
    pages = {42--50},
    month = {April},
    abstract = {We present structural and optical spectroscopy studies of thin films of Cu2ZnSnSe4 (CZTSe) with strong copper deficiency deposited on Mo/Glass substrates and selenised at 450, 500 or 550 ?C. Solar cells fabricated from these films demonstrated efficiencies up to 7.4\% for selenisation at 500 ?C. Structural analysis based on X-ray diffraction and Raman spectroscopy revealed the presence of SnSe2 in the film selenised at 450 ?C but not detected in the films selenised at higher temperatures. A progressive decrease of the Sn and Se content was observed as the selenisation temperature increased. Photoluminescence excitation was used to determine the bandgaps at 4.2 K. Detailed measurements of the temperature and excitation intensity dependencies of the photoluminescence spectra allow the recombination mechanisms of the observed emission bands to be identified as band-to-impurity and band-to-band transitions, and their evolution with selenisation temperature changes to be analysed. The strongest band-to-band transition is recorded in the PL spectra of the film selenised at 500 ?C and can be observed from 6 K to room temperature. The compositional and structural changes in the films and their influence on the optoelectronic properties of CZTSe and solar cells are discussed.},
    keywords = {Cu2ZnSnSe4, selenisation, optical spectroscopy, structure, Physics, Surfaces, Coatings and Films, Electronic, Optical and Magnetic Materials, Renewable Energy, Sustainability and the Environment},
    url = {http://strathprints.strath.ac.uk/55956/}
    }

  • I. Gîrgel, P. R. Edwards, E. Le Boulbar, P. Coulon, S. Sahonta, D. W. E. Allsopp, R. W. Martin, C. J. Humphreys, and P. A. Shields, "Investigation of indium gallium nitride facet-dependent nonpolar growth rates and composition for core-shell light-emitting diodes," Journal of Nanophotonics, vol. 10, iss. 1, p. 16010, 2016.
    [BibTeX] [Abstract] [Download PDF]

    Core?shell indium gallium nitride (InGaN)/gallium nitride (GaN) structures are attractive as light emitters due to the large nonpolar surface of rod-like cores with their longitudinal axis aligned along the c-direction. These facets do not suffer from the quantum-confined Stark effect that limits the thickness of quantum wells and efficiency in conventional light-emitting devices. Understanding InGaN growth on these submicron three-dimensional structures is important to optimize optoelectronic device performance. In this work, the influence of reactor parameters was determined and compared. GaN nanorods (NRs) with both \{11-20\} a-plane and \{10-10\} m-plane nonpolar facets were prepared to investigate the impact of metalorganic vapor phase epitaxy reactor parameters on the characteristics of a thick (38 to 85 nm) overgrown InGaN shell. The morphology and optical emission properties of the InGaN layers were investigated by scanning electron microscopy, transmission electron microscopy, and cathodoluminescence hyperspectral imaging. The study reveals that reactor pressure has an important impact on the InN mole fraction on the \{10-10\} m-plane facets, even at a reduced growth rate. The sample grown at 750?C and 100 mbar had an InN mole fraction of 25\% on the \{10-10\} facets of the NRs.

    @Article{strathprints55885,
    author = {Ionut G{\^i}rgel and Paul R. Edwards and Le Boulbar, Emmanuel and Pierre-Marie Coulon and Suman-Lata Sahonta and Duncan W. E. Allsopp and Robert W. Martin and Colin J. Humphreys and Philip A. Shields},
    title = {Investigation of indium gallium nitride facet-dependent nonpolar growth rates and composition for core-shell light-emitting diodes},
    journal = {Journal of Nanophotonics},
    year = {2016},
    volume = {10},
    number = {1},
    pages = {016010},
    month = {March},
    abstract = {Core?shell indium gallium nitride (InGaN)/gallium nitride (GaN) structures are attractive as light emitters due to the large nonpolar surface of rod-like cores with their longitudinal axis aligned along the c-direction. These facets do not suffer from the quantum-confined Stark effect that limits the thickness of quantum wells and efficiency in conventional light-emitting devices. Understanding InGaN growth on these submicron three-dimensional structures is important to optimize optoelectronic device performance. In this work, the influence of reactor parameters was determined and compared. GaN nanorods (NRs) with both \{11-20\} a-plane and \{10-10\} m-plane nonpolar facets were prepared to investigate the impact of metalorganic vapor phase epitaxy reactor parameters on the characteristics of a thick (38 to 85 nm) overgrown InGaN shell. The morphology and optical emission properties of the InGaN layers were investigated by scanning electron microscopy, transmission electron microscopy, and cathodoluminescence hyperspectral imaging. The study reveals that reactor pressure has an important impact on the InN mole fraction on the \{10-10\} m-plane facets, even at a reduced growth rate. The sample grown at 750?C and 100 mbar had an InN mole fraction of 25\% on the \{10-10\} facets of the NRs.},
    keywords = {core-shell, indium gallium nitride, m-plane, a-plane, nonpolar, cathodoluminescence, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/55885/}
    }

  • G. Naresh-Kumar, D. Thomson, M. Nouf-Allehiani, J. Bruckbauer, P. R. Edwards, B. Hourahine, R. W. Martin, and C. Trager-Cowan, "Electron channelling contrast imaging for III-nitride thin film structures," Materials Science in Semiconductor Processing, vol. 47, p. 44–50, 2016.
    [BibTeX] [Abstract] [Download PDF]

    Electron channelling contrast imaging (ECCI) performed in a scanning electron microscope (SEM) is a rapid and non-destructive structural characterisation technique for imaging, identifying and quantifying extended defects in crystalline materials. In this review, we will demonstrate the application of ECCI to the characterisation of III-nitride semiconductor thin films grown on different substrates and with different crystal orientations. We will briefly describe the history and the theory behind electron channelling and the experimental setup and conditions required to perform ECCI. We will discuss the advantages of using ECCI; especially in combination with other SEM based techniques, such as cathodoluminescence imaging. The challenges in using ECCI are also briefly discussed.

    @Article{strathprints55555,
    author = {G. Naresh-Kumar and D. Thomson and M. Nouf-Allehiani and J. Bruckbauer and P. R. Edwards and B. Hourahine and R.W. Martin and C. Trager-Cowan},
    title = {Electron channelling contrast imaging for {III}-nitride thin film structures},
    journal = {Materials Science in Semiconductor Processing},
    year = {2016},
    volume = {47},
    pages = {44--50},
    month = {June},
    abstract = {Electron channelling contrast imaging (ECCI) performed in a scanning electron microscope (SEM) is a rapid and non-destructive structural characterisation technique for imaging, identifying and quantifying extended defects in crystalline materials. In this review, we will demonstrate the application of ECCI to the characterisation of III-nitride semiconductor thin films grown on different substrates and with different crystal orientations. We will briefly describe the history and the theory behind electron channelling and the experimental setup and conditions required to perform ECCI. We will discuss the advantages of using ECCI; especially in combination with other SEM based techniques, such as cathodoluminescence imaging. The challenges in using ECCI are also briefly discussed.},
    keywords = {III - nitrides, extended defects, thin films, Physics, Condensed Matter Physics, Electronic, Optical and Magnetic Materials, Instrumentation},
    url = {http://strathprints.strath.ac.uk/55555/}
    }

  • K. P. O'Donnell, P. R. Edwards, M. Yamaga, K. Lorenz, M. J. Kappers, and M. Boćkowski, "Crystalfield symmetries of luminescent Eu³⁺ centers in GaN : the importance of the ⁵D₀ to ⁷F₁ transition," Applied Physics Letters, vol. 108, iss. 2, p. 22102, 2016.
    [BibTeX] [Abstract] [Download PDF]

    Eu-doped GaN is a promising material with potential application not only in optoelectronics but also in magneto-optical and quantum optical devices ?beyond the light emitting diode?. Its interesting spectroscopy is unfortunately complicated by spectral overlaps due to ?site multiplicity?, the existence in a given sample of multiple composite centers in which Eu ions associate with intrinsic or extrinsic defects. We show here that elementary crystalfield analysis of the 5D0 to 7F1 transition can critically distinguish such sites. Hence, we find that the center involved in the hysteretic photochromic switching (HPS) observed in GaN(Mg):Eu, proposed as the basis of a new solid state qubit material, is not in fact Eu1, as previously reported, but a related defect, Eu1(Mg). Furthermore, the decomposition of the crystalfield distortions of Eu0, Eu1(Mg) and Eu1 into axial and non-axial components strongly suggests reasonable microscopic models for the defects themselves.

    @Article{strathprints55347,
    author = {K. P. O'Donnell and P. R. Edwards and M. Yamaga and K. Lorenz and M. J. Kappers and M. Bo{\'c}kowski},
    title = {Crystalfield symmetries of luminescent {Eu³⁺} centers in {GaN} : the importance of the {⁵D₀} to {⁷F₁} transition},
    journal = {Applied Physics Letters},
    year = {2016},
    volume = {108},
    number = {2},
    pages = {022102},
    month = {January},
    abstract = {Eu-doped GaN is a promising material with potential application not only in optoelectronics but also in magneto-optical and quantum optical devices ?beyond the light emitting diode?. Its interesting spectroscopy is unfortunately complicated by spectral overlaps due to ?site multiplicity?, the existence in a given sample of multiple composite centers in which Eu ions associate with intrinsic or extrinsic defects. We show here that elementary crystalfield analysis of the 5D0 to 7F1 transition can critically distinguish such sites. Hence, we find that the center involved in the hysteretic photochromic switching (HPS) observed in GaN(Mg):Eu, proposed as the basis of a new solid state qubit material, is not in fact Eu1, as previously reported, but a related defect, Eu1(Mg). Furthermore, the decomposition of the crystalfield distortions of Eu0, Eu1(Mg) and Eu1 into axial and non-axial components strongly suggests reasonable microscopic models for the defects themselves.},
    keywords = {crystal defects, III-V semiconductors, visible spectra, rare earth ions, emission spectra, Physics, Condensed Matter Physics, Electronic, Optical and Magnetic Materials},
    url = {http://strathprints.strath.ac.uk/55347/}
    }

  • L. Ohnoutek, M. Hakl, M. Veis, B. A. Piot, C. Faugeras, G. Martinez, M. V. Yakushev, R. W. Martin, C. Drasar, A. Materna, G. Strzelecka, A. Hruban, M. Potemski, and M. Orlita, "Strong interband Faraday rotation in 3D topological insulator Bi₂Se₃," Scientific Reports, vol. 6, p. 19087, 2016.
    [BibTeX] [Abstract] [Download PDF]

    The Faraday effect is a representative magneto-optical phenomenon, resulting from the transfer of angular momentum between interacting light and matter in which time-reversal symmetry has been broken by an externally applied magnetic field. Here we report on the Faraday rotation induced in the prominent 3D topological insulator Bi2Se3 due to bulk interband excitations. The origin of this non-resonant effect, extraordinarily strong among other non-magnetic materials, is traced back to the specific Dirac-type Hamiltonian for Bi2Se3, which implies that electrons and holes in this material closely resemble relativistic particles with a non-zero rest mass.

    @Article{strathprints55246,
    author = {L. Ohnoutek and M. Hakl and M. Veis and B. A. Piot and C. Faugeras and G. Martinez and M. V. Yakushev and R. W. Martin and C. Drasar and A. Materna and G. Strzelecka and A. Hruban and M. Potemski and M. Orlita},
    title = {Strong interband Faraday rotation in 3{D} topological insulator {Bi₂Se₃}},
    journal = {Scientific Reports},
    year = {2016},
    volume = {6},
    pages = {19087},
    month = {January},
    abstract = {The Faraday effect is a representative magneto-optical phenomenon, resulting from the transfer of angular momentum between interacting light and matter in which time-reversal symmetry has been broken by an externally applied magnetic field. Here we report on the Faraday rotation induced in the prominent 3D topological insulator Bi2Se3 due to bulk interband excitations. The origin of this non-resonant effect, extraordinarily strong among other non-magnetic materials, is traced back to the specific Dirac-type Hamiltonian for Bi2Se3, which implies that electrons and holes in this material closely resemble relativistic particles with a non-zero rest mass.},
    keywords = {Faraday rotation, 3D topological insulator, Bi3Se3, time reversal symmetry, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/55246/}
    }

  • N. A. K. Kaufmann, L. Lahourcade, B. Hourahine, D. Martin, and N. Grandjean, "Critical impact of Ehrlich-Schwöbel barrier on GaN surface morphology during homoepitaxial growth," Journal of Crystal Growth, vol. 433, p. 36–42, 2016.
    [BibTeX] [Abstract] [Download PDF]

    We discuss the impact of kinetics, and in particular the effect of the Ehrlich-Schwöbel barrier (ESB), on the growth and surface morphology of homoepitaxial GaN layers. The presence of an ESB can lead to various self-assembled surface features, which strongly affect the surface roughness. We present an in-depth study of this phenomenon on GaN homoepitaxial layers grown by metal organic vapor phase epitaxy and molecular beam epitaxy. We show how a proper tuning of the growth parameters allows for the control of the surface morphology, independent of the growth technique.

    @Article{strathprints53732,
    author = {Nils A.K. Kaufmann and L. Lahourcade and B. Hourahine and D. Martin and N. Grandjean},
    title = {Critical impact of {E}hrlich-{S}chw{\"o}bel barrier on {GaN} surface morphology during homoepitaxial growth},
    journal = {Journal of Crystal Growth},
    year = {2016},
    volume = {433},
    pages = {36--42},
    month = {January},
    abstract = {We discuss the impact of kinetics, and in particular the effect of the Ehrlich-Schw{\"o}bel barrier (ESB), on the growth and surface morphology of homoepitaxial GaN layers. The presence of an ESB can lead to various self-assembled surface features, which strongly affect the surface roughness. We present an in-depth study of this phenomenon on GaN homoepitaxial layers grown by metal organic vapor phase epitaxy and molecular beam epitaxy. We show how a proper tuning of the growth parameters allows for the control of the surface morphology, independent of the growth technique.},
    keywords = {growth models, metalorganic chemical vapor deposition, molecular beam epitaxy, nitrides, semiconducting gallium compounds, surface structure, Physics, Electronic, Optical and Magnetic Materials, Surfaces and Interfaces},
    url = {http://strathprints.strath.ac.uk/53732/}
    }

2015

  • K. Müller, K. A. Fischer, A. Rundquist, C. Dory, K. G. Lagoudakis, T. Sarmiento, Y. A. Kelaita, V. Borish, and J. Vučković, "Ultrafast polariton-phonon dynamics of strongly coupled quantum dot-nanocavity systems," Physical Review X, vol. 5, iss. 3, 2015.
    [BibTeX] [Abstract] [Download PDF]

    We investigate the influence of exciton-phonon coupling on the dynamics of a strongly coupled quantum dot-photonic crystal cavity system and explore the effects of this interaction on different schemes for nonclassical light generation. By performing time-resolved measurements, we map out the detuning-dependent polariton lifetime and extract the spectrum of the polariton-to-phonon coupling with unprecedented precision. Photon-blockade experiments for different pulse-length and detuning conditions (supported by quantum optical simulations) reveal that achieving high-fidelity photon blockade requires an intricate understanding of the phonons' influence on the system dynamics. Finally, we achieve direct coherent control of the polariton states of a strongly coupled system and demonstrate that their efficient coupling to phonons can be exploited for novel concepts in high-fidelity single-photon generation.

    @article{strathprints71794,
    volume = {5},
    number = {3},
    month = {July},
    title = {Ultrafast polariton-phonon dynamics of strongly coupled quantum dot-nanocavity systems},
    author = {Kai M{\"u}ller and Kevin A. Fischer and Armand Rundquist and Constantin Dory and Konstantinos G. Lagoudakis and Tomas Sarmiento and Yousif A. Kelaita and Victoria Borish and Jelena Vu{\v c}kovi{\'c}},
    year = {2015},
    journal = {Physical Review X},
    keywords = {condensed matter, photonics, quantum physics, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/71794/},
    abstract = {We investigate the influence of exciton-phonon coupling on the dynamics of a strongly coupled quantum dot-photonic crystal cavity system and explore the effects of this interaction on different schemes for nonclassical light generation. By performing time-resolved measurements, we map out the detuning-dependent polariton lifetime and extract the spectrum of the polariton-to-phonon coupling with unprecedented precision. Photon-blockade experiments for different pulse-length and detuning conditions (supported by quantum optical simulations) reveal that achieving high-fidelity photon blockade requires an intricate understanding of the phonons' influence on the system dynamics. Finally, we achieve direct coherent control of the polariton states of a strongly coupled system and demonstrate that their efficient coupling to phonons can be exploited for novel concepts in high-fidelity single-photon generation.}
    }

  • T. Tanttu, A. Rossi, K. Y. Tan, K. E. Huhtinen, K. W. Chan, M. Möttönen, and A. S. Dzurak, "Electron counting in a silicon single-electron pump," New Journal of Physics, vol. 17, iss. 10, 2015. doi:10.1088/1367-2630/17/10/103030
    [BibTeX] [Abstract] [Download PDF]

    We report electron counting experiments in a silicon metal-oxide-semiconductor quantum dot architecture which has been previously demonstrated to generate a quantized current in excess of 80 pA with uncertainty below 30 parts per million. Single-shot detection of electrons pumped into a reservoir dot is performed using a capacitively coupled single-electron transistor. We extract the full probability distribution of the transfer of n electrons per pumping cycle for We find that the probabilities extracted from the counting experiment are in agreement with direct current measurements in a broad range of dc electrochemical potentials of the pump. The electron counting technique is also used to confirm the improving robustness of the pumping mechanism with increasing electrostatic confinement of the quantum dot.

    @article{strathprints68708,
    volume = {17},
    number = {10},
    month = {October},
    title = {Electron counting in a silicon single-electron pump},
    year = {2015},
    doi = {10.1088/1367-2630/17/10/103030},
    journal = {New Journal of Physics},
    keywords = {charge pumping, electron counting, quantum dots, single-electron pump, Physics, Physics and Astronomy(all)},
    url = {https://doi.org/10.1088/1367-2630/17/10/103030},
    issn = {1367-2630},
    abstract = {We report electron counting experiments in a silicon metal-oxide-semiconductor quantum dot architecture which has been previously demonstrated to generate a quantized current in excess of 80 pA with uncertainty below 30 parts per million. Single-shot detection of electrons pumped into a reservoir dot is performed using a capacitively coupled single-electron transistor. We extract the full probability distribution of the transfer of n electrons per pumping cycle for We find that the probabilities extracted from the counting experiment are in agreement with direct current measurements in a broad range of dc electrochemical potentials of the pump. The electron counting technique is also used to confirm the improving robustness of the pumping mechanism with increasing electrostatic confinement of the quantum dot.},
    author = {Tanttu, Tuomo and Rossi, Alessandro and Tan, Kuan Yen and Huhtinen, Kukka Emilia and Chan, Kok Wai and M{\"o}tt{\"o}nen, Mikko and Dzurak, Andrew S.}
    }

  • S. V. Novikov, C. R. Staddon, R. Martin, A. J. Kent, and T. C. Foxon, "Molecular beam epitaxy of free-standing wurtzite AlₓGa₁₋ₓN layers," Journal of Crystal Growth, vol. 425, p. 125–128, 2015.
    [BibTeX] [Abstract] [Download PDF]

    Recent developments with group III nitrides present AlxGa1xN based LEDs as realistic devices for new alternative deep ultra-violet light sources. Because there is a significant difference in the lattice parameters of GaN and AlN, AlxGa1xN substrates would be preferable to either GaN or AlN for ultraviolet device applications. We have studied the growth of free-standing wurtzite AlxGa1xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE). Thick wurtzite AlxGa1xN films were grown by PA-MBE on 2-in. GaAs (111)B substrates and were removed from the GaAs substrate after growth to provide free standing AlxGa1xN samples. X-ray microanalysis measurements confirm that the AlN fraction is uniform across the wafer and mass spectroscopy measurements show that the composition is also uniform in depth. We have demonstrated that free-standing wurtzite AlxGa1xN wafers can be achieved by PA-MBE for a wide range of AlN fractions. In order to develop a commercially viable process for the growth of wurtzite AlxGa1xN substrates, we have used a novel Riber plasma source and have demonstrated growth rates of GaN up to 1.8 mm/h on 2-in. diameter GaAs and sapphire wafers

    @Article{strathprints56186,
    author = {Sergei V. Novikov and C.R. Staddon and Robert Martin and A.J. Kent and C. Thomas Foxon},
    title = {Molecular beam epitaxy of free-standing wurtzite {AlₓGa₁₋ₓN} layers},
    journal = {Journal of Crystal Growth},
    year = {2015},
    volume = {425},
    pages = {125--128},
    month = {February},
    abstract = {Recent developments with group III nitrides present AlxGa1xN based LEDs as realistic devices for new alternative deep ultra-violet light sources. Because there is a significant difference in the lattice parameters of GaN and AlN, AlxGa1xN substrates would be preferable to either GaN or AlN for ultraviolet device applications. We have studied the growth of free-standing wurtzite AlxGa1xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE). Thick wurtzite AlxGa1xN films were grown by PA-MBE on 2-in. GaAs (111)B substrates and were removed from the GaAs substrate after growth to provide free standing AlxGa1xN samples. X-ray microanalysis measurements confirm that the AlN fraction is uniform across the wafer and mass spectroscopy measurements show that the composition is also uniform in depth. We have demonstrated that free-standing wurtzite AlxGa1xN wafers can be achieved by PA-MBE for a wide range of AlN fractions. In order to develop a commercially viable process for the growth of wurtzite AlxGa1xN substrates, we have used a novel Riber plasma source and have demonstrated growth rates of GaN up to 1.8 mm/h on 2-in. diameter GaAs and sapphire wafers},
    keywords = {substrates, molecular beam epitaxy, nitrides, semiconducting III-V materials, Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/56186/}
    }

  • M. V. Yakushev, I. Forbes, A. V. Mudryi, M. Grossberg, J. Krustok, N. S. Beattie, M. Moynihan, A. Rockett, and R. W. Martin, "Optical spectroscopy studies of Cu₂ZnSnSe₄ thin films," Thin Solid Films, vol. 582, p. 154–157, 2015.
    [BibTeX] [Abstract] [Download PDF]

    Cu2ZnSnSe4 thin films were synthesised by selenisation of magnetron sputtered metal precursors. The band gap determined from the absorption spectra increases from 1.01 eV at 300 K to 1.05 eV at 4.2 K. In lower quality films photoluminescence spectra show a broad, low intensity asymmetric band associated with a recombination of free electrons and holes localised on acceptors in the presence of spatial potential fluctuations. In high quality material the luminescence band becomes intense and narrow resolving two phonon replicas. Its shifts at changing excitation power suggest donor?acceptor pair recombination mechanisms. The proposed model involving two pairs of donors and acceptors is supported by the evolution of the band intensity and spectral position with temperature. Energy levels of the donors and acceptors are estimated using Arrhenius quenching analysis.

    @Article{strathprints55983,
    author = {M.V. Yakushev and I. Forbes and A.V. Mudryi and M. Grossberg and J. Krustok and N.S. Beattie and M. Moynihan and A. Rockett and R.W. Martin},
    title = {Optical spectroscopy studies of {Cu₂ZnSnSe₄} thin films},
    journal = {Thin Solid Films},
    year = {2015},
    volume = {582},
    pages = {154--157},
    month = {May},
    abstract = {Cu2ZnSnSe4 thin films were synthesised by selenisation of magnetron sputtered metal precursors. The band gap determined from the absorption spectra increases from 1.01 eV at 300 K to 1.05 eV at 4.2 K. In lower quality films photoluminescence spectra show a broad, low intensity asymmetric band associated with a recombination of free electrons and holes localised on acceptors in the presence of spatial potential fluctuations. In high quality material the luminescence band becomes intense and narrow resolving two phonon replicas. Its shifts at changing excitation power suggest donor?acceptor pair recombination mechanisms. The proposed model involving two pairs of donors and acceptors is supported by the evolution of the band intensity and spectral position with temperature. Energy levels of the donors and acceptors are estimated using Arrhenius quenching analysis.},
    keywords = {Cu2ZnSnSe4, thin films, photoluminescence, defects, absorption, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/55983/}
    }

  • A. Redondo-Cubero, K. Lorenz, E. Wendler, S. Magalhães, E. Alves, D. Carvalho, T. Ben, F. M. Morales, R. García, K. P. O'Donnell, and C. Wetzel, "Analysis of the stability of InGaN/GaN multiquantum wells against ion beam intermixing," Nanotechnology, vol. 26, iss. 42, p. 425703, 2015.
    [BibTeX] [Abstract] [Download PDF]

    Ion-induced damage and intermixing was evaluated in InGaN/GaN multi-quantum wells (MQWs) using 35 keV N+ implantation at room temperature. In situ ion channeling measurements show that damage builds up with a similar trend for In and Ga atoms, with a high threshold for amorphization. The extended defects induced during the implantation, basal and prismatic stacking faults, are uniformly distributed across the quantum well structure. Despite the extremely high fluences used (up to 4 {$\times$}1016 cm-2), the InGaN MQWs exhibit a high stability against ion beam mixing.

    @Article{strathprints55103,
    author = {A Redondo-Cubero and K Lorenz and E Wendler and S Magalh{\~a}es and E Alves and D Carvalho and T Ben and F M Morales and R Garc{\'i}a and K P O'Donnell and C Wetzel},
    title = {Analysis of the stability of {InGaN/GaN} multiquantum wells against ion beam intermixing},
    journal = {Nanotechnology},
    year = {2015},
    volume = {26},
    number = {42},
    pages = {425703},
    month = {October},
    abstract = {Ion-induced damage and intermixing was evaluated in InGaN/GaN multi-quantum wells (MQWs) using 35 keV N+ implantation at room temperature. In situ ion channeling measurements show that damage builds up with a similar trend for In and Ga atoms, with a high threshold for amorphization. The extended defects induced during the implantation, basal and prismatic stacking faults, are uniformly distributed across the quantum well structure. Despite the extremely high fluences used (up to 4 {$\times$}1016 cm-2), the InGaN MQWs exhibit a high stability against ion beam mixing.},
    keywords = {implantation, InGaN, ion beam mixing, quantum wells, Solid state physics. Nanoscience, Bioengineering, Chemistry(all), Electrical and Electronic Engineering, Mechanical Engineering, Mechanics of Materials, Materials Science(all)},
    url = {http://strathprints.strath.ac.uk/55103/}
    }

  • M. V. Yakushev, A. V. Mudryi, O. M. Borodavchenko, V. A. Volkov, and R. W. Martin, "A photoluminescence study of excitonic grade CuInSe₂ single crystals irradiated with 6 MeV electrons," Journal of Applied Physics, vol. 118, iss. 15, p. 155703, 2015.
    [BibTeX] [Abstract] [Download PDF]

    High-quality single crystals of CuInSe2 with near-stoichiometric elemental compositions were irradiated with 6 MeV electrons, at doses from 1015 to 3 {$\times$} 1018 cm?2, and studied using photoluminescence (PL) at temperatures from 4.2 to 300 K. Before irradiation, the photoluminescence spectra reveal a number of sharp and well resolved lines associated with free- and bound-excitons. The spectra also show broader bands relating to free-to-bound transitions and their phonon replicas in the lower energy region below 1.0 eV. The irradiation with 6 MeV electrons reduces the intensity of the free- and the majority of the bound-exciton peaks. Such a reduction can be seen for doses above 1016 cm?2. The irradiation induces new PL lines at 1.0215 eV and 0.9909 eV and also enhances the intensity of the lines at 1.0325 and 1.0102 eV present in the photoluminescence spectra before the irradiation. Two broad bands at 0.902 and 0.972 eV, respectively, are tentatively associated with two acceptor-type defects: namely, interstitial selenium (Sei) and copper on indium site (Cu In). After irradiation, these become more intense suggesting an increase in the concentration of these defects due to irradiation.

    @Article{strathprints54688,
    author = {M. V. Yakushev and A. V. Mudryi and O. M. Borodavchenko and V. A. Volkov and R. W. Martin},
    title = {A photoluminescence study of excitonic grade {CuInSe₂} single crystals irradiated with 6 {MeV} electrons},
    journal = {Journal of Applied Physics},
    year = {2015},
    volume = {118},
    number = {15},
    pages = {155703},
    month = {October},
    abstract = {High-quality single crystals of CuInSe2 with near-stoichiometric elemental compositions were irradiated with 6 MeV electrons, at doses from 1015 to 3 {$\times$} 1018 cm?2, and studied using photoluminescence (PL) at temperatures from 4.2 to 300 K. Before irradiation, the photoluminescence spectra reveal a number of sharp and well resolved lines associated with free- and bound-excitons. The spectra also show broader bands relating to free-to-bound transitions and their phonon replicas in the lower energy region below 1.0 eV. The irradiation with 6 MeV electrons reduces the intensity of the free- and the majority of the bound-exciton peaks. Such a reduction can be seen for doses above 1016 cm?2. The irradiation induces new PL lines at 1.0215 eV and 0.9909 eV and also enhances the intensity of the lines at 1.0325 and 1.0102 eV present in the photoluminescence spectra before the irradiation. Two broad bands at 0.902 and 0.972 eV, respectively, are tentatively associated with two acceptor-type defects: namely, interstitial selenium (Sei) and copper on indium site (Cu In). After irradiation, these become more intense suggesting an increase in the concentration of these defects due to irradiation.},
    keywords = {CuInSe2 , radiation damage, photoluminescence, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/54688/}
    }

  • M. V. Yakushev, J. Márquez-Prieto, I. Forbes, P. R. Edwards, V. D. Zhivulko, A. V. Mudryi, J. Krustok, and R. W. Martin, "Radiative recombination in Cu₂ZnSnSe₄ thin films with Cu deficiency and Zn excess," Journal of Physics D: Applied Physics, vol. 48, iss. 47, p. 475109, 2015.
    [BibTeX] [Abstract] [Download PDF]

    Thin films of Cu2ZnSnSe4 (CZTSe) with copper de?ficiency and zinc excess were fabricated at Northumbria University by the selenisation of metallic precursors deposited on Mo/glass and bare glass substrates. Absorption and photoluminescence (PL) measurements were used to examine the ?film on glass whereas fi?lms on Mo/glass were used to produce a solar cell with ef?ficiency of 8.1\%. Detailed temperature and excitation intensity analysis of PL spectra allows identifi?cation of the main recombination mechanisms as band-to-tail and band-to-band transitions. The latter transition was observed in the spectra from 6 to 300 K.

    @Article{strathprints54657,
    author = {M V Yakushev and J M{\'a}rquez-Prieto and I Forbes and P R Edwards and V D Zhivulko and A V Mudryi and J Krustok and R W Martin},
    title = {Radiative recombination in {Cu₂ZnSnSe₄} thin films with {Cu} deficiency and {Zn} excess},
    journal = {Journal of Physics D: Applied Physics},
    year = {2015},
    volume = {48},
    number = {47},
    pages = {475109},
    month = {November},
    abstract = {Thin films of Cu2ZnSnSe4 (CZTSe) with copper de?ficiency and zinc excess were fabricated at Northumbria University by the selenisation of metallic precursors deposited on Mo/glass and bare glass substrates. Absorption and photoluminescence (PL) measurements were used to examine the ?film on glass whereas fi?lms on Mo/glass were used to produce a solar cell with ef?ficiency of 8.1\%. Detailed temperature and excitation intensity analysis of PL spectra allows identifi?cation of the main recombination mechanisms as band-to-tail and band-to-band transitions. The latter transition was observed in the spectra from 6 to 300 K.},
    keywords = {Cu2ZnSnSe4, solar cells, photoluminescence, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/54657/}
    }

  • D. McArthur, B. Hourahine, and F. Papoff, "Dataset on coherent control of fields and induced currents in nonlinear multiphoton processes in a nanosphere," Scientific Data, vol. 2, p. 150064, 2015.
    [BibTeX] [Abstract] [Download PDF]

    We model a scheme for the coherent control of light waves and currents in metallic nanospheres which applies independently of the nonlinear multiphoton processes at the origin of waves and currents. Using exact mathematical formulae, we calculate numerically with a custom fortran code the effect of an external control field which enable us to change the radiation pattern and suppress radiative losses or to reduce absorption, enabling the particle to behave as a perfect scatterer or as a perfect absorber. Data are provided in tabular, comma delimited value format and illustrate narrow features in the response of the particles that result in high sensitivity to small variations in the local environment, including subwavelength spatial shifts.

    @Article{strathprints54610,
    author = {Duncan McArthur and Ben Hourahine and Francesco Papoff},
    title = {Dataset on coherent control of fields and induced currents in nonlinear multiphoton processes in a nanosphere},
    journal = {Scientific Data},
    year = {2015},
    volume = {2},
    pages = {150064},
    month = {November},
    abstract = {We model a scheme for the coherent control of light waves and currents in metallic nanospheres which applies independently of the nonlinear multiphoton processes at the origin of waves and currents. Using exact mathematical formulae, we calculate numerically with a custom fortran code the effect of an external control field which enable us to change the radiation pattern and suppress radiative losses or to reduce absorption, enabling the particle to behave as a perfect scatterer or as a perfect absorber. Data are provided in tabular, comma delimited value format and illustrate narrow features in the response of the particles that result in high sensitivity to small variations in the local environment, including subwavelength spatial shifts.},
    keywords = {physical sciences , nonlinear optics, optical physics, nanophotonics and plasmonics physical sciences, Optics. Light, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/54610/}
    }

  • B. N. Sedrine, T. C. Esteves, J. Rodrigues, L. Rino, M. R. Correia, M. C. Sequeira, A. J. Neves, E. Alves, M. Boćkowski, P. R. Edwards, K. P. O'Donnell, K. Lorenz, and T. Monteiro, "Photoluminescence studies of a perceived white light emission from a monolithic InGaN/GaN quantum well structure," Scientific Reports, vol. 5, p. 13739, 2015.
    [BibTeX] [Abstract] [Download PDF]

    In this work we demonstrate by photoluminescence studies white light emission from a monolithic InGaN/GaN single quantum well structure grown by metal organic chemical vapour deposition. As-grown and thermally annealed samples at high temperature (1000 ?C, 1100 ?C and 1200 ?C) and high pressure (1.1 GPa) were analysed by spectroscopic techniques, and the annealing effect on the photoluminescence is deeply explored. Under laser excitation of 3.8 eV at room temperature, the as-grown structure exhibits two main emission bands: a yellow band peaked at 2.14 eV and a blue band peaked at 2.8 eV resulting in white light perception. Interestingly, the stability of the white light is preserved after annealing at the lowest temperature (1000 ?C), but suppressed for higher temperatures due to a deterioration of the blue quantum well emission. Moreover, the control of the yellow/blue bands intensity ratio, responsible for the white colour coordinate temperatures, could be achieved after annealing at 1000 ?C. The room temperature white emission is studied as a function of incident power density, and the correlated colour temperature values are found to be in the warm white range: 3260?4000 K.

    @Article{strathprints54228,
    author = {N. Ben Sedrine and T. C. Esteves and J. Rodrigues and L. Rino and M. R. Correia and M. C. Sequeira and A. J. Neves and E. Alves and M. Bo{\'c}kowski and P. R. Edwards and K.P. O'Donnell and K. Lorenz and T. Monteiro},
    title = {Photoluminescence studies of a perceived white light emission from a monolithic {InGaN/GaN} quantum well structure},
    journal = {Scientific Reports},
    year = {2015},
    volume = {5},
    pages = {13739},
    abstract = {In this work we demonstrate by photoluminescence studies white light emission from a monolithic InGaN/GaN single quantum well structure grown by metal organic chemical vapour deposition. As-grown and thermally annealed samples at high temperature (1000 ?C, 1100 ?C and 1200 ?C) and high pressure (1.1 GPa) were analysed by spectroscopic techniques, and the annealing effect on the photoluminescence is deeply explored. Under laser excitation of 3.8 eV at room temperature, the as-grown structure exhibits two main emission bands: a yellow band peaked at 2.14 eV and a blue band peaked at 2.8 eV resulting in white light perception. Interestingly, the stability of the white light is preserved after annealing at the lowest temperature (1000 ?C), but suppressed for higher temperatures due to a deterioration of the blue quantum well emission. Moreover, the control of the yellow/blue bands intensity ratio, responsible for the white colour coordinate temperatures, could be achieved after annealing at 1000 ?C. The room temperature white emission is studied as a function of incident power density, and the correlated colour temperature values are found to be in the warm white range: 3260?4000 K.},
    keywords = {light emitting diodes, LEDs, photoluminescence, Physics, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/54228/}
    }

  • S. Vespucci, A. Winkelmann, G. Naresh-Kumar, K. P. Mingard, D. Maneuski, P. R. Edwards, A. P. Day, V. O'Shea, and C. Trager-Cowan, "Digital direct electron imaging of energy-filtered electron backscatter diffraction patterns," Physical Review B (Condensed Matter), vol. 92, iss. 20, p. 205301, 2015.
    [BibTeX] [Abstract] [Download PDF]

    Electron backscatter diffraction is a scanning electron microscopy technique used to obtain crystallographic information on materials. It allows the nondestructive mapping of crystal structure, texture, and strain with a lateral and depth resolution on the order of tens of nanometers. Electron backscatter diffraction patterns (EBSPs) are presently acquired using a detector comprising a scintillator coupled to a digital camera, and the crystallographic information obtainable is limited by the conversion of electrons to photons and then back to electrons again. In this article we will report the direct acquisition of energy-filtered EBSPs using a digital complementary metal-oxide-semiconductor hybrid pixel detector, Timepix. We show results from a range of samples with different mass and density, namely diamond, silicon, and GaN. Direct electron detection allows the acquisition of EBSPs at lower ({$\leq$}5 keV) electron beam energies. This results in a reduction in the depth and lateral extension of the volume of the specimen contributing to the pattern and will lead to a significant improvement in lateral and depth resolution. Direct electron detection together with energy filtering (electrons having energy below a specific value are excluded) also leads to an improvement in spatial resolution but in addition provides an unprecedented increase in the detail in the acquired EBSPs. An increase in contrast and higher-order diffraction features are observed. In addition, excess-deficiency effects appear to be suppressed on energy filtering. This allows the fundamental physics of pattern formation to be interrogated and will enable a change in the use of electron backscatter diffraction (EBSD) for crystal phase identification and the mapping of strain. The enhancement in the contrast in high-pass energy-filtered EBSD patterns is found to be stronger for lighter, less dense materials. The improved contrast for such materials will enable the application of the EBSD technique to be expanded to materials for which conventional EBSD analysis is not presently practicable.

    @Article{strathprints54220,
    author = {S. Vespucci and A. Winkelmann and G. Naresh-Kumar and K. P. Mingard and D. Maneuski and P. R. Edwards and A. P. Day and V. O'Shea and C. Trager-Cowan},
    title = {Digital direct electron imaging of energy-filtered electron backscatter diffraction patterns},
    journal = {Physical Review B (Condensed Matter)},
    year = {2015},
    volume = {92},
    number = {20},
    pages = {205301},
    month = {November},
    abstract = {Electron backscatter diffraction is a scanning electron microscopy technique used to obtain crystallographic information on materials. It allows the nondestructive mapping of crystal structure, texture, and strain with a lateral and depth resolution on the order of tens of nanometers. Electron backscatter diffraction patterns (EBSPs) are presently acquired using a detector comprising a scintillator coupled to a digital camera, and the crystallographic information obtainable is limited by the conversion of electrons to photons and then back to electrons again. In this article we will report the direct acquisition of energy-filtered EBSPs using a digital complementary metal-oxide-semiconductor hybrid pixel detector, Timepix. We show results from a range of samples with different mass and density, namely diamond, silicon, and GaN. Direct electron detection allows the acquisition of EBSPs at lower ({$\leq$}5 keV) electron beam energies. This results in a reduction in the depth and lateral extension of the volume of the specimen contributing to the pattern and will lead to a significant improvement in lateral and depth resolution. Direct electron detection together with energy filtering (electrons having energy below a specific value are excluded) also leads to an improvement in spatial resolution but in addition provides an unprecedented increase in the detail in the acquired EBSPs. An increase in contrast and higher-order diffraction features are observed. In addition, excess-deficiency effects appear to be suppressed on energy filtering. This allows the fundamental physics of pattern formation to be interrogated and will enable a change in the use of electron backscatter diffraction (EBSD) for crystal phase identification and the mapping of strain. The enhancement in the contrast in high-pass energy-filtered EBSD patterns is found to be stronger for lighter, less dense materials. The improved contrast for such materials will enable the application of the EBSD technique to be expanded to materials for which conventional EBSD analysis is not presently practicable.},
    keywords = {electron backscatter diffraction patterns, EBSPs, electron detection, energy filtering, Physics, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/54220/}
    }

  • M. Yamaga, H. Watanabe, M. Kurahashi, K. P. O'Donnell, K. Lorenz, and M. Boćkowski, "Indirect excitation of Eu³⁺ in GaN codoped with Mg and Eu," Journal of Physics: Conference Series, vol. 619, iss. 1, p. 12025, 2015.
    [BibTeX] [Abstract] [Download PDF]

    Temperature-dependent Eu3+ luminescence spectra in GaN(Mg):Eu can be assigned to, at least, two distinct Eu3+ centres, denoted by Eu0 and Eu1. The splitting energy levels of the 7FJ (J=1,2) multiplets for the Eu0 and Eu1 centres have been calculated using the equivalent operator Hamiltonian for C3v crystal field with the addition of an odd parity distortion.

    @Article{strathprints54146,
    author = {M Yamaga and H Watanabe and M Kurahashi and K P O'Donnell and K Lorenz and M Bo{\'c}kowski},
    title = {Indirect excitation of {Eu³⁺} in {GaN} codoped with {Mg} and {Eu}},
    journal = {Journal of Physics: Conference Series},
    year = {2015},
    volume = {619},
    number = {1},
    pages = {012025},
    month = {June},
    abstract = {Temperature-dependent Eu3+ luminescence spectra in GaN(Mg):Eu can be assigned to, at least, two distinct Eu3+ centres, denoted by Eu0 and Eu1. The splitting energy levels of the 7FJ (J=1,2) multiplets for the Eu0 and Eu1 centres have been calculated using the equivalent operator Hamiltonian for C3v crystal field with the addition of an odd parity distortion.},
    keywords = {GaN films , crystal structure, optical spectroscopy, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/54146/}
    }

  • G. Kusch, M. Nouf-Allehiani, F. Mehnke, C. Kuhn, P. R. Edwards, T. Wernicke, A. Knauer, V. Kueller, G. Naresh-Kumar, M. Weyers, M. Kneissl, C. Trager-Cowan, and R. W. Martin, "Spatial clustering of defect luminescence centers in Si-doped low resistivity Al₀.₈₂Ga₀.₁₈N," Applied Physics Letters, vol. 107, iss. 7, p. 72103, 2015.
    [BibTeX] [Abstract] [Download PDF]

    A series of Si-doped AlN-rich AlGaN layers with low resistivities was characterized by a combination of nanoscale imaging techniques. Utilizing the capability of scanning electron microscopy to reliably investigate the same sample area with different techniques, it was possible to determine the effect of doping concentration, defect distribution, and morphology on the luminescence properties of these layers. Cathodoluminescence shows that the dominant defect luminescence depends on the Si-doping concentration. For lower doped samples, the most intense peak was centered between 3.36 eV and 3.39 eV, while an additional, stronger peak appears at 3 eV for the highest doped sample. These peaks were attributed to the (VIII-ON)2? complex and the VIII3? vacancy, respectively. Multimode imaging using cathodoluminescence, secondary electrons, electron channeling contrast, and atomic force microscopy demonstrates that the luminescence intensity of these peaks is not homogeneously distributed but shows a strong dependence on the topography and on the distribution of screw dislocations.

    @Article{strathprints54134,
    author = {Gunnar Kusch and M. Nouf-Allehiani and Frank Mehnke and Christian Kuhn and Paul R. Edwards and Tim Wernicke and Arne Knauer and Viola Kueller and G. Naresh-Kumar and Markus Weyers and Michael Kneissl and Carol Trager-Cowan and Robert W. Martin},
    title = {Spatial clustering of defect luminescence centers in {Si}-doped low resistivity {Al₀.₈₂Ga₀.₁₈N}},
    journal = {Applied Physics Letters},
    year = {2015},
    volume = {107},
    number = {7},
    pages = {072103},
    month = {August},
    abstract = {A series of Si-doped AlN-rich AlGaN layers with low resistivities was characterized by a combination of nanoscale imaging techniques. Utilizing the capability of scanning electron microscopy to reliably investigate the same sample area with different techniques, it was possible to determine the effect of doping concentration, defect distribution, and morphology on the luminescence properties of these layers. Cathodoluminescence shows that the dominant defect luminescence depends on the Si-doping concentration. For lower doped samples, the most intense peak was centered between 3.36 eV and 3.39 eV, while an additional, stronger peak appears at 3 eV for the highest doped sample. These peaks were attributed to the (VIII-ON)2? complex and the VIII3? vacancy, respectively. Multimode imaging using cathodoluminescence, secondary electrons, electron channeling contrast, and atomic force microscopy demonstrates that the luminescence intensity of these peaks is not homogeneously distributed but shows a strong dependence on the topography and on the distribution of screw dislocations.},
    keywords = {nanoscale imaging, cathodoluminescence, multimode imaging, Physics, Physics and Astronomy (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/54134/}
    }

  • I. Gîrgel, P. R. Edwards, E. L. Boulbar, D. W. E. Allsopp, R. W. Martin, and P. A. Shields, "Investigation of facet-dependent InGaN growth for core-shell LEDs," Proceedings of SPIE, vol. 9363, p. 93631V, 2015.
    [BibTeX] [Abstract] [Download PDF]

    In this work we used vertically aligned GaN nanowires with well-defined crystal facets, i.e. the \{11-20\} a-plane, \{10-10\} m-plane, (0001) c-plane and \{1-101\} semi-polar planes, to investigate the impact of MOVPE reactor parameters on the characteristics of an InGaN layer. The morphology and optical characteristics of the InGaN layers grown of each facet were investigated by cathodoluminescence (CL) hyperspectral imaging and scanning electron microscopy (SEM). The influence of reactor parameters on growth rate and alloy fraction were determined and compared. The study revealed that pressure can have an important impact on the incorporation of InN on the \{10-10\} m-plane facets. The growth performed at 750?C and 100mbar led to a homogeneous high InN fraction of 25\% on the \{10-10\} facets of the nanowires. This work suggests homogeneous good quality GaN/InGaN core-shell structure could be grown in the near future.

    @Article{strathprints54092,
    author = {Ionut G{\^i}rgel and Paul R. Edwards and Emmanuel Le Boulbar and Duncan W. E. Allsopp and Robert W. Martin and Philip A. Shields},
    title = {Investigation of facet-dependent {InGaN} growth for core-shell {LED}s},
    journal = {Proceedings of SPIE},
    year = {2015},
    volume = {9363},
    pages = {93631V},
    month = {March},
    note = {Copyright 2015 Society of Photo-Optical Instrumentation Engineers. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.},
    abstract = {In this work we used vertically aligned GaN nanowires with well-defined crystal facets, i.e. the \{11-20\} a-plane, \{10-10\} m-plane, (0001) c-plane and \{1-101\} semi-polar planes, to investigate the impact of MOVPE reactor parameters on the characteristics of an InGaN layer. The morphology and optical characteristics of the InGaN layers grown of each facet were investigated by cathodoluminescence (CL) hyperspectral imaging and scanning electron microscopy (SEM). The influence of reactor parameters on growth rate and alloy fraction were determined and compared. The study revealed that pressure can have an important impact on the incorporation of InN on the \{10-10\} m-plane facets. The growth performed at 750?C and 100mbar led to a homogeneous high InN fraction of 25\% on the \{10-10\} facets of the nanowires. This work suggests homogeneous good quality GaN/InGaN core-shell structure could be grown in the near future.},
    keywords = {indium gallium nitride, light emitting diodes, nanofibers, indium nitride, gallium nitride, hyperspectral Imaging, scanning electron microscopy, metalorganic chemical vapor deposition, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/54092/}
    }

  • D. Carvalho, F. M. Morales, T. Ben, R. García, A. Redondo-Cubero, E. Alves, K. Lorenz, P. R. Edwards, K. P. O'Donnell, and C. Wetzel, "Quantitative chemical mapping of InGaN quantum wells from calibrated high-angle annular dark field micrographs," Microscopy and Microanalysis, vol. 21, iss. 4, p. 994–1005, 2015.
    [BibTeX] [Abstract] [Download PDF]

    We present a simple and robust method to acquire quantitative maps of compositional fluctuations in nanostructures from low magnification high-angle annular dark field (HAADF) micrographs calibrated by energy-dispersive X-ray (EDX) spectroscopy in scanning transmission electron microscopy (STEM) mode. We show that a nonuniform background in HAADF-STEM micrographs can be eliminated, to a first approximation, by use of a suitable analytic function. The uncertainty in probe position when collecting an EDX spectrum renders the calibration of HAADF-STEM micrographs indirect, and a statistical approach has been developed to determine the position with confidence. Our analysis procedure, presented in a flowchart to facilitate the successful implementation of the method by users, was applied to discontinuous InGaN/GaN quantum wells in order to obtain quantitative determinations of compositional fluctuations on the nanoscale.

    @Article{strathprints53715,
    author = {Daniel Carvalho and Francisco M. Morales and Teresa Ben and Rafael Garc{\'i}a and Andr{\'e}s Redondo-Cubero and Eduardo Alves and Katharina Lorenz and Paul R. Edwards and Kevin P. O'Donnell and Christian Wetzel},
    title = {Quantitative chemical mapping of {InGaN} quantum wells from calibrated high-angle annular dark field micrographs},
    journal = {Microscopy and Microanalysis},
    year = {2015},
    volume = {21},
    number = {4},
    pages = {994--1005},
    month = {August},
    abstract = {We present a simple and robust method to acquire quantitative maps of compositional fluctuations in nanostructures from low magnification high-angle annular dark field (HAADF) micrographs calibrated by energy-dispersive X-ray (EDX) spectroscopy in scanning transmission electron microscopy (STEM) mode. We show that a nonuniform background in HAADF-STEM micrographs can be eliminated, to a first approximation, by use of a suitable analytic function. The uncertainty in probe position when collecting an EDX spectrum renders the calibration of HAADF-STEM micrographs indirect, and a statistical approach has been developed to determine the position with confidence. Our analysis procedure, presented in a flowchart to facilitate the successful implementation of the method by users, was applied to discontinuous InGaN/GaN quantum wells in order to obtain quantitative determinations of compositional fluctuations on the nanoscale.},
    keywords = {chemical mapping, energy dispersive X-ray spectroscopy, InGaN/GaN quantum well , HAADF-STEM, RBS, EDX, Physics, Instrumentation},
    url = {http://strathprints.strath.ac.uk/53715/}
    }

  • F. Papoff, D. McArthur, and B. Hourahine, "Coherent control of radiation patterns of nonlinear multiphoton processes in nanoparticles," Scientific Reports, vol. 5, p. 12040, 2015.
    [BibTeX] [Abstract] [Download PDF]

    We propose a scheme for the coherent control of light waves and currents in metallic nanospheres which applies independently of the nonlinear multiphoton processes at the origin of waves and currents. We derive conditions on the external control field which enable us to change the radiation pattern and suppress radiative losses or to reduce absorption, enabling the particle to behave as a perfect scatterer or as a perfect absorber. The control introduces narrow features in the response of the particles that result in high sensitivity to small variations in the local environment, including subwavelength spatial shifts.

    @Article{strathprints53589,
    author = {Francesco Papoff and Duncan McArthur and Ben Hourahine},
    title = {Coherent control of radiation patterns of nonlinear multiphoton processes in nanoparticles},
    journal = {Scientific Reports},
    year = {2015},
    volume = {5},
    pages = {12040},
    month = {July},
    abstract = {We propose a scheme for the coherent control of light waves and currents in metallic nanospheres which applies independently of the nonlinear multiphoton processes at the origin of waves and currents. We derive conditions on the external control field which enable us to change the radiation pattern and suppress radiative losses or to reduce absorption, enabling the particle to behave as a perfect scatterer or as a perfect absorber. The control introduces narrow features in the response of the particles that result in high sensitivity to small variations in the local environment, including subwavelength spatial shifts.},
    keywords = {nanoparticles, coherent control, metallic nanospheres, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/53589/}
    }

  • S. Magalhães, I. M. Watson, S. Pereira, N. Franco, L. T. Tan, R. W. Martin, K. P. O'Donnell, E. Alves, J. P. Araújo, T. Monteiro, and K. Lorenz, "Composition, structure and morphology of Al1-xInxN thin films grown on Al1-yGayN templates with different GaN contents," Journal of Physics D: Applied Physics, vol. 48, iss. 1, p. 15103, 2015.
    [BibTeX] [Abstract] [Download PDF]

    Four nominally Al0.85In0.15N thin films with different strains were grown simultaneously on Al1-yGayN (y = 1, 0.93, 0.87 and 0.69) templates by metal organic chemical vapour deposition. The nominal InN content of {$\sim$}15\% was chosen to achieve close lattice matching of Al1-xInxN with the templates of intermediate GaN molar fractions, a small tensile strain for growth on GaN, and compressive strain for the template with the lowest GaN fraction. The film deposited on GaN reveals the highest structural quality, the lowest surface roughness and a homogeneous composition with depth. For growth on the Al1-yGayN ternary templates, the film roughness and the surface pit density both increase with decreasing GaN content, in line with the roughening of the growth templates themselves. Detailed study indicates that the structural and morphological qualities of the templates influence not only the quality of the Al1-xInxN films but also their composition profile. Results suggest that surface roughness of the template and growth on the inclined facets lead to compositional gradients due to a decreased InN incorporation on these facets and to the formation of V-pits.

    @Article{strathprints53524,
    author = {S. Magalh{\~a}es and I. M. Watson and S. Pereira and N. Franco and L. T. Tan and R. W. Martin and K. P. O'Donnell and E. Alves and J. P. Ara{\'u}jo and T. Monteiro and K. Lorenz},
    title = {Composition, structure and morphology of {Al1-xInxN} thin films grown on {Al1-yGayN} templates with different {GaN} contents},
    journal = {Journal of Physics D: Applied Physics},
    year = {2015},
    volume = {48},
    number = {1},
    pages = {015103},
    month = {January},
    abstract = {Four nominally Al0.85In0.15N thin films with different strains were grown simultaneously on Al1-yGayN (y = 1, 0.93, 0.87 and 0.69) templates by metal organic chemical vapour deposition. The nominal InN content of {$\sim$}15\% was chosen to achieve close lattice matching of Al1-xInxN with the templates of intermediate GaN molar fractions, a small tensile strain for growth on GaN, and compressive strain for the template with the lowest GaN fraction. The film deposited on GaN reveals the highest structural quality, the lowest surface roughness and a homogeneous composition with depth. For growth on the Al1-yGayN ternary templates, the film roughness and the surface pit density both increase with decreasing GaN content, in line with the roughening of the growth templates themselves. Detailed study indicates that the structural and morphological qualities of the templates influence not only the quality of the Al1-xInxN films but also their composition profile. Results suggest that surface roughness of the template and growth on the inclined facets lead to compositional gradients due to a decreased InN incorporation on these facets and to the formation of V-pits.},
    keywords = {AlGaN, AlInN, RBS, XRD, Physics, Condensed Matter Physics, Electronic, Optical and Magnetic Materials, Acoustics and Ultrasonics, Surfaces, Coatings and Films},
    url = {http://strathprints.strath.ac.uk/53524/}
    }

  • K. P. O'Donnell, "The temperature dependence of the luminescence of rare-earth-doped semiconductors : 25 years after Favennec," Physica Status Solidi C, vol. 12, iss. 4-5, p. 466–468, 2015.
    [BibTeX] [Abstract] [Download PDF]

    Twentyfive years after the publication of P. N. Favennec's seminal paper on luminescence from rare-earth-doped semiconductors (Electron. Lett. 25, 718-719 (1989), with 390+ citations to date) we examine the long shadow it has cast on recent studies of europium-doped GaN, aimed at substituting for InN-rich InGaN in red-light-emitting devices (LEDs). According to Favennec's principle, wider band gap semiconductors should show weaker thermal quenching, thus favouring the III-nitrides hugely. The conventional approach to fitting temperature dependences of light emission, based on competition between radiative and non-radiative transitions, is presented here in simplified form and an alternative fitting equation proposed. The original data of Favennec (op. cit.) is re-examined in the light of these fitting models.

    @Article{strathprints53482,
    author = {K. P. O'Donnell},
    title = {The temperature dependence of the luminescence of rare-earth-doped semiconductors : 25 years after {F}avennec},
    journal = {Physica Status Solidi C},
    year = {2015},
    volume = {12},
    number = {4-5},
    pages = {466--468},
    month = {April},
    abstract = {Twentyfive years after the publication of P. N. Favennec's seminal paper on luminescence from rare-earth-doped semiconductors (Electron. Lett. 25, 718-719 (1989), with 390+ citations to date) we examine the long shadow it has cast on recent studies of europium-doped GaN, aimed at substituting for InN-rich InGaN in red-light-emitting devices (LEDs). According to Favennec's principle, wider band gap semiconductors should show weaker thermal quenching, thus favouring the III-nitrides hugely. The conventional approach to fitting temperature dependences of light emission, based on competition between radiative and non-radiative transitions, is presented here in simplified form and an alternative fitting equation proposed. The original data of Favennec (op. cit.) is re-examined in the light of these fitting models.},
    keywords = {luminescence, rare earth doping, semiconductors, temperature dependence, Physics, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/53482/}
    }

  • M. A. Sousa, T. C. Esteves, N. B. Sedrine, J. Rodrigues, M. B. Lourenço, A. Redondo-Cubero, E. Alves, K. P. O'Donnell, M. Bockowski, C. Wetzel, M. R. Correia, K. Lorenz, and T. Monteiro, "Luminescence studies on green emitting InGaN/GaN MQWs implanted with nitrogen," Scientific Reports, vol. 5, p. 9703, 2015.
    [BibTeX] [Abstract] [Download PDF]

    We studied the optical properties of metalorganic chemical vapour deposited (MOCVD) InGaN/GaN multiple quantum wells (MQW) subjected to nitrogen (N) implantation and post-growth annealing treatments. The optical characterization was carried out by means of temperature and excitation density-dependent steady state photoluminescence (PL) spectroscopy, supplemented by room temperature PL excitation (PLE) and PL lifetime (PLL) measurements. The as-grown and as-implanted samples were found to exhibit a single green emission band attributed to localized excitons in the QW, although the N implantation leads to a strong reduction of the PL intensity. The green band was found to be surprisingly stable on annealing up to 1400?C. A broad blue band dominates the low temperature PL after thermal annealing in both samples. This band is more intense for the implanted sample, suggesting that defects generated by N implantation, likely related to the diffusion/segregation of indium (In), have been optically activated by the thermal treatment.

    @Article{strathprints53469,
    author = {Marco A. Sousa and Teresa C. Esteves and Nabiha Ben Sedrine and Joana Rodrigues and M{\'a}rcio B. Louren{\c c}o and Andr{\'e}s Redondo-Cubero and Eduardo Alves and Kevin P. O'Donnell and Michal Bockowski and Christian Wetzel and Maria R. Correia and Katharina Lorenz and Teresa Monteiro},
    title = {Luminescence studies on green emitting {InGaN/GaN MQW}s implanted with nitrogen},
    journal = {Scientific Reports},
    year = {2015},
    volume = {5},
    pages = {9703},
    month = {April},
    abstract = {We studied the optical properties of metalorganic chemical vapour deposited (MOCVD) InGaN/GaN multiple quantum wells (MQW) subjected to nitrogen (N) implantation and post-growth annealing treatments. The optical characterization was carried out by means of temperature and excitation density-dependent steady state photoluminescence (PL) spectroscopy, supplemented by room temperature PL excitation (PLE) and PL lifetime (PLL) measurements. The as-grown and as-implanted samples were found to exhibit a single green emission band attributed to localized excitons in the QW, although the N implantation leads to a strong reduction of the PL intensity. The green band was found to be surprisingly stable on annealing up to 1400?C. A broad blue band dominates the low temperature PL after thermal annealing in both samples. This band is more intense for the implanted sample, suggesting that defects generated by N implantation, likely related to the diffusion/segregation of indium (In), have been optically activated by the thermal treatment.},
    keywords = {multi quantum wells, luminescence , light emitting diodes, Physics, Optics. Light, General, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/53469/}
    }

  • M. J. Wallace, P. R. Edwards, M. J. Kappers, M. A. Hopkins, F. Oehler, S. Sivaraya, R. A. Oliver, C. J. Humphreys, D. W. E. Allsopp, and R. W. Martin, "Effect of the barrier growth mode on the luminescence and conductivity micron scale uniformity of InGaN light emitting diodes," Journal of Applied Physics, vol. 117, iss. 11, p. 115705, 2015.
    [BibTeX] [Abstract] [Download PDF]

    In this paper we present a combined cathodoluminescence and electron beam induced current study of the optical and electrical properties of InGaN LEDs grown using different active region growth methods. In one device, both the quantum wells and quantum barriers were deposited at their optimum temperatures (2T) whereas in the other device, each barrier was grown in a two step process, with the first few nanometers at a lower temperature (Q2T). It was found that, in the Q2T sample, small micron scale domains of lower emission intensity correlate strongly to a lower EBIC signal, whereas in the 2T sample which has a more uniform emission pattern and an anti-correlation exists between CL emission intensity and EBIC signal.

    @Article{strathprints52285,
    author = {M. J. Wallace and P. R. Edwards and M. J. Kappers and M. A. Hopkins and F. Oehler and S. Sivaraya and R. A. Oliver and C. J. Humphreys and D. W. E. Allsopp and R. W. Martin},
    title = {Effect of the barrier growth mode on the luminescence and conductivity micron scale uniformity of {InGaN} light emitting diodes},
    journal = {Journal of Applied Physics},
    year = {2015},
    volume = {117},
    number = {11},
    pages = {115705},
    abstract = {In this paper we present a combined cathodoluminescence and electron beam induced current study of the optical and electrical properties of InGaN LEDs grown using different active region growth methods. In one device, both the quantum wells and quantum barriers were deposited at their optimum temperatures (2T) whereas in the other device, each barrier was grown in a two step process, with the first few nanometers at a lower temperature (Q2T). It was found that, in the Q2T sample, small micron scale domains of lower emission intensity correlate strongly to a lower EBIC signal, whereas in the 2T sample which has a more uniform emission pattern and an anti-correlation exists between CL emission intensity and EBIC signal.},
    keywords = {cathodoluminescence, light emitting diode, electron beams, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/52285/}
    }

2014

  • A. Y. Piggott, J. Lu, T. M. Babinec, K. G. Lagoudakis, J. Petykiewicz, and J. Vučković, "Inverse design and implementation of a wavelength demultiplexing grating coupler," Scientific Reports, vol. 4, p. 1–5, 2014.
    [BibTeX] [Abstract] [Download PDF]

    Nanophotonics has emerged as a powerful tool for manipulating light on chips. Almost all of today's devices, however, have been designed using slow and ineffective brute-force search methods, leading in many cases to limited device performance. In this article, we provide a complete demonstration of our recently proposed inverse design technique, wherein the user specifies design constraints in the form of target fields rather than a dielectric constant profile, and in particular we use this method to demonstrate a new demultiplexing grating. The novel grating, which has not been developed using conventional techniques, accepts a vertical-incident Gaussian beam from a free-space and separates O-band (1300 nm) and C-band (1550 nm) light into separate waveguides. This inverse design concept is simple and extendable to a broad class of highly compact devices including frequency filters, mode converters, and spatial mode multiplexers.

    @article{strathprints64398,
    volume = {4},
    month = {November},
    title = {Inverse design and implementation of a wavelength demultiplexing grating coupler},
    author = {Alexander Y. Piggott and Jesse Lu and Thomas M. Babinec and Konstantinos G. Lagoudakis and Jan Petykiewicz and Jelena Vu{\v c}kovi{\'c}},
    year = {2014},
    pages = {1--5},
    journal = {Scientific Reports},
    keywords = {applied mathematics, optical materials and structures, Mathematics, Applied Mathematics},
    url = {https://strathprints.strath.ac.uk/64398/},
    abstract = {Nanophotonics has emerged as a powerful tool for manipulating light on chips. Almost all of today's devices, however, have been designed using slow and ineffective brute-force search methods, leading in many cases to limited device performance. In this article, we provide a complete demonstration of our recently proposed inverse design technique, wherein the user specifies design constraints in the form of target fields rather than a dielectric constant profile, and in particular we use this method to demonstrate a new demultiplexing grating. The novel grating, which has not been developed using conventional techniques, accepts a vertical-incident Gaussian beam from a free-space and separates O-band (1300 nm) and C-band (1550 nm) light into separate waveguides. This inverse design concept is simple and extendable to a broad class of highly compact devices including frequency filters, mode converters, and spatial mode multiplexers.}
    }

  • A. Y. Piggott, K. G. Lagoudakis, T. Sarmiento, M. Bajcsy, G. Shambat, and J. Vučković, "Photo-oxidative tuning of individual and coupled GaAs photonic crystal cavities," Optics Express, vol. 22, iss. 12, p. 15017–15023, 2014.
    [BibTeX] [Abstract] [Download PDF]

    We demonstrate a photo-induced oxidation technique for tuning GaAs photonic crystal cavities using a low-power 390 nm pulsed laser. The laser oxidizes a small (textless 1 {\ensuremath{\mu}}m) diameter spot, reducing the local index of refraction and blueshifting the cavity. The tuning progress can be actively monitored in real time. We also demonstrate tuning an individual cavity within a pair of proximity-coupled cavities, showing that this method can be used to tune individual cavities in a cavity network, with applications in quantum simulations and quantum computing.

    @article{strathprints64358,
    volume = {22},
    number = {12},
    month = {June},
    author = {Alexander Y. Piggott and Konstantinos G. Lagoudakis and Tomas Sarmiento and Michal Bajcsy and Gary Shambat and Jelena Vu{\v c}kovi{\'c}},
    title = {Photo-oxidative tuning of individual and coupled GaAs photonic crystal cavities},
    journal = {Optics Express},
    pages = {15017--15023},
    year = {2014},
    keywords = {microcavities, photonic crystals, microcavity devices, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/64358/},
    abstract = {We demonstrate a photo-induced oxidation technique for tuning GaAs photonic crystal cavities using a low-power 390 nm pulsed laser. The laser oxidizes a small (textless 1 {\ensuremath{\mu}}m) diameter spot, reducing the local index of refraction and blueshifting the cavity. The tuning progress can be actively monitored in real time. We also demonstrate tuning an individual cavity within a pair of proximity-coupled cavities, showing that this method can be used to tune individual cavities in a cavity network, with applications in quantum simulations and quantum computing.}
    }

  • T. Ferrus, A. Rossi, A. Andreev, T. Kodera, T. Kambara, W. Lin, S. Oda, and D. A. Williams, "GHz photon-activated hopping between localized states in a silicon quantum dot," New Journal of Physics, vol. 16, 2014. doi:10.1088/1367-2630/16/1/013016
    [BibTeX] [Abstract] [Download PDF]

    We discuss the effects of gigahertz photon irradiation on a degenerately phosphorus-doped silicon quantum dot, in particular, the creation of voltage offsets on gate leads and the tunneling of one or two electrons via Coulomb blockade lifting at 4.2 K. A semi-analytical model is derived that explains the main features observed experimentally. Ultimately both effects may provide an efficient way to optically control and operate electrically isolated structures by microwave pulses. In quantum computing architectures, these results may lead to the use of microwave multiplexing to manipulate quantum states in a multi-qubit configuration.

    @article{strathprints68709,
    volume = {16},
    month = {January},
    title = {GHz photon-activated hopping between localized states in a silicon quantum dot},
    year = {2014},
    doi = {10.1088/1367-2630/16/1/013016},
    journal = {New Journal of Physics},
    keywords = {quantum states, gigahertz photon irradiation, silicon quantum dots, Physics, Physics and Astronomy(all)},
    url = {https://doi.org/10.1088/1367-2630/16/1/013016},
    issn = {1367-2630},
    abstract = {We discuss the effects of gigahertz photon irradiation on a degenerately phosphorus-doped silicon quantum dot, in particular, the creation of voltage offsets on gate leads and the tunneling of one or two electrons via Coulomb blockade lifting at 4.2 K. A semi-analytical model is derived that explains the main features observed experimentally. Ultimately both effects may provide an efficient way to optically control and operate electrically isolated structures by microwave pulses. In quantum computing architectures, these results may lead to the use of microwave multiplexing to manipulate quantum states in a multi-qubit configuration.},
    author = {Ferrus, T. and Rossi, A. and Andreev, A. and Kodera, T. and Kambara, T. and Lin, W. and Oda, S. and Williams, D. A.}
    }

  • M. V. Yakushev, A. V. Rodina, G. M. Shuchalin, R. P. Seisian, M. A. Abdullaev, A. Rockett, V. D. Zhivulko, A. V. Mudryi, C. Faugeras, and R. W. Martin, "Landau levels of the C-exciton in CuInSe₂ studied by magneto-transmission," Applied Physics Letters, vol. 105, iss. 14, p. 142103, 2014.
    [BibTeX] [Abstract] [Download PDF]

    The electronic structure of the solar cell absorber CuInSe2 is studied using magneto-transmission in thin polycrystalline films at magnetic fields up to 29 T. A, B, and C free excitons are resolved in absorption spectra at zero field and a Landau level fan generated by diamagnetic exciton recombination is observed for fields above 7 T. The dependence of the C band exciton binding energy on magnetic fields, calculated using a hydrogenic approximation, is used to determine the C exciton Rydberg at 0 T (8.5 meV), band gap (1.2828 eV), and hole effective mass mso = (0.31 {$\pm$} 0.12)m0 for the C valence sub-band.

    @Article{strathprints53685,
    author = {M. V. Yakushev and A. V. Rodina and G. M. Shuchalin and R. P. Seisian and M. A. Abdullaev and A. Rockett and V. D. Zhivulko and A. V. Mudryi and C. Faugeras and R. W. Martin},
    title = {Landau levels of the {C}-exciton in {CuInSe₂} studied by magneto-transmission},
    journal = {Applied Physics Letters},
    year = {2014},
    volume = {105},
    number = {14},
    pages = {142103},
    month = {October},
    note = {Copyright 2014 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Yakushev, M. V., Rodina, A. V., Shuchalin, G. M., Seisian, R. P., Abdullaev, M. A., Rockett, A., ... Martin, R. W. (2014). Landau levels of the C-exciton in CuInSe2 studied by magneto-transmission. Applied Physics Letters, 105(14), [142103]. and may be found at http://dx.doi.org/10.1063/1.4897995},
    abstract = {The electronic structure of the solar cell absorber CuInSe2 is studied using magneto-transmission in thin polycrystalline films at magnetic fields up to 29 T. A, B, and C free excitons are resolved in absorption spectra at zero field and a Landau level fan generated by diamagnetic exciton recombination is observed for fields above 7 T. The dependence of the C band exciton binding energy on magnetic fields, calculated using a hydrogenic approximation, is used to determine the C exciton Rydberg at 0 T (8.5 meV), band gap (1.2828 eV), and hole effective mass mso = (0.31 {$\pm$} 0.12)m0 for the C valence sub-band.},
    keywords = {excitons, magnetic fields, absorption spectra, Physics, Physics and Astronomy (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/53685/}
    }

  • M. D. Smith, E. Taylor, T. C. Sadler, V. Z. Zubialevich, K. Lorenz, H. N. Li, J. O'Connell, E. Alves, J. D. Holmes, R. W. Martin, and P. J. Parbrook, "Determination of Ga auto-incorporation in nominal InAlN epilayers grown by MOCVD," Journal of Materials Chemistry. C, vol. 2, iss. 29, p. 5787–5792, 2014.
    [BibTeX] [Abstract] [Download PDF]

    We report on the consistent measurement of gallium incorporation in nominal InAlN layers using various complimentary techniques, underpinned by X-ray diffraction. Nominal InAlN layers with similar growth conditions were prepared, and the change in unintended Ga content in the group III sublattice ranged from similar to 24\% to similar to 12\% when the total reactor flow rate was increased from 8000 to 24 000 standard cubic centimetres per minute. Ultra-thin InAlN/GaN HEMT layers were grown in a clean reactor to minimize Ga auto-incorporation, and measured using X-ray photoelectron spectroscopy and secondary ion mass spectrometry. The implications of Ga incorporation in InAlN layers within optoelectronic and power devices is discussed.

    @article{strathprints51372,
    volume = {2},
    number = {29},
    title = {Determination of Ga auto-incorporation in nominal InAlN epilayers grown by MOCVD},
    author = {M. D. Smith and E. Taylor and T. C. Sadler and V. Z. Zubialevich and K. Lorenz and H. N. Li and J. O'Connell and E. Alves and J. D. Holmes and R. W. Martin and P. J. Parbrook},
    year = {2014},
    pages = {5787--5792},
    journal = {Journal of Materials Chemistry. C},
    keywords = {auto-incorporation, epilayers, gallium, InAlN, GaN, Physical and theoretical chemistry, Solid state physics. Nanoscience, Physical and Theoretical Chemistry},
    url = {http://strathprints.strath.ac.uk/51372/},
    abstract = {We report on the consistent measurement of gallium incorporation in nominal InAlN layers using various complimentary techniques, underpinned by X-ray diffraction. Nominal InAlN layers with similar growth conditions were prepared, and the change in unintended Ga content in the group III sublattice ranged from similar to 24\% to similar to 12\% when the total reactor flow rate was increased from 8000 to 24 000 standard cubic centimetres per minute. Ultra-thin InAlN/GaN HEMT layers were grown in a clean reactor to minimize Ga auto-incorporation, and measured using X-ray photoelectron spectroscopy and secondary ion mass spectrometry. The implications of Ga incorporation in InAlN layers within optoelectronic and power devices is discussed.}
    }

  • K. M. Yu, S. V. Novikov, M. Ting, W. L. Sarney, S. P. Svensson, M. Shaw, R. W. Martin, W. Walukiewicz, and C. T. Foxon, "Growth and characterization of highly mismatched GaN₁₋ₓSbₓ alloys," Journal of Applied Physics, vol. 116, iss. 12, p. 123704, 2014.
    [BibTeX] [Abstract] [Download PDF]

    A systematic investigation on the effects of growth temperature, Ga flux, and Sb flux on the incorporation of Sb, film structure, and optical properties of the GaN1-xSbx highly mismatched alloys (HMAs) was carried out. We found that the direct bandgap ranging from 3.4 eV to below 1.0 eV for the alloys grown at low temperature. At the growth temperature of 80 degrees C, GaN1-xSbx with x{\ensuremath{>}}6\% losses crystallinity and becomes primarily amorphous with small crystallites of 2-5 nm. Despite the range of microstructures found for GaN1-xSbx alloys with different composition, a well-developed absorption edge shifts from 3.4 eV (GaN) to close to 2 eV for samples with a small amount, less than 10\% of Sb. Luminescence from dilute GaN1-xSbx alloys grown at high temperature and the bandgap energy for alloys with higher Sb content are consistent with a localized substitutional Sb level E-Sb at similar to 1.1 eV above the valence band of GaN. The decrease in the bandgap of GaN1-xSbx HMAs is consistent with the formation of a Sb-derived band due to the anticrossing interaction of the Sb states with the valence band of GaN.

    @Article{strathprints51371,
    author = {K.M. Yu and S. V. Novikov and Min Ting and W.L. Sarney and S.P. Svensson and M. Shaw and R.W. Martin and W. Walukiewicz and C.T. Foxon},
    title = {Growth and characterization of highly mismatched {GaN₁₋ₓSbₓ} alloys},
    journal = {Journal of Applied Physics},
    year = {2014},
    volume = {116},
    number = {12},
    pages = {123704},
    month = {September},
    note = {{\copyright} 2015 AIP Publishing},
    abstract = {A systematic investigation on the effects of growth temperature, Ga flux, and Sb flux on the incorporation of Sb, film structure, and optical properties of the GaN1-xSbx highly mismatched alloys (HMAs) was carried out. We found that the direct bandgap ranging from 3.4 eV to below 1.0 eV for the alloys grown at low temperature. At the growth temperature of 80 degrees C, GaN1-xSbx with x{\ensuremath{>}}6\% losses crystallinity and becomes primarily amorphous with small crystallites of 2-5 nm. Despite the range of microstructures found for GaN1-xSbx alloys with different composition, a well-developed absorption edge shifts from 3.4 eV (GaN) to close to 2 eV for samples with a small amount, less than 10\% of Sb. Luminescence from dilute GaN1-xSbx alloys grown at high temperature and the bandgap energy for alloys with higher Sb content are consistent with a localized substitutional Sb level E-Sb at similar to 1.1 eV above the valence band of GaN. The decrease in the bandgap of GaN1-xSbx HMAs is consistent with the formation of a Sb-derived band due to the anticrossing interaction of the Sb states with the valence band of GaN.},
    keywords = {GaN, GaSb, HMAs, film structures, growth temperature, Ga flux, Sb flux, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/51371/}
    }

  • S. V. Novikov, M. Ting, K. M. Yu, W. L. Sarney, R. W. Martin, S. P. Svensson, W. Walukiewicz, and C. T. Foxon, "Tellurium n-type doping of highly mismatched amorphous GaN₁₋ₓAsₓ alloys in plasma-assisted molecular beam epitaxy," Journal of Crystal Growth, vol. 404, p. 9–13, 2014.
    [BibTeX] [Abstract] [Download PDF]

    In this paper we report our study on n-type Te doping of amorphous GaNi1-xAsx layers grown by plasma assisted molecular beam epitaxy. We have used a low temperature PbTe source as a source of tellurium. Reproducible and uniform tellurium incorporation in amorphous GaNi1-xAsx layers has been successfully achieved with a maximum Te concentration of 9 x 10(20) cm(-3). Tellurium incorporation resulted in n-doping of GaN1-xAsx layers with Hall carrier concentrations up to 3 x 10(19) cm(-3) and mobilities of similar to 1 cm(2)/V s. The optimal growth temperature window for efficient Te doping of the amorphous GaNi1-xAsx layers has been determined.

    @Article{strathprints51167,
    author = {S. V. Novikov and M. Ting and K.M. Yu and W.L. Sarney and R.W. Martin and S.P. Svensson and W. Walukiewicz and C.T. Foxon},
    title = {Tellurium n-type doping of highly mismatched amorphous {GaN₁₋ₓAsₓ} alloys in plasma-assisted molecular beam epitaxy},
    journal = {Journal of Crystal Growth},
    year = {2014},
    volume = {404},
    pages = {9--13},
    month = {October},
    abstract = {In this paper we report our study on n-type Te doping of amorphous GaNi1-xAsx layers grown by plasma assisted molecular beam epitaxy. We have used a low temperature PbTe source as a source of tellurium. Reproducible and uniform tellurium incorporation in amorphous GaNi1-xAsx layers has been successfully achieved with a maximum Te concentration of 9 x 10(20) cm(-3). Tellurium incorporation resulted in n-doping of GaN1-xAsx layers with Hall carrier concentrations up to 3 x 10(19) cm(-3) and mobilities of similar to 1 cm(2)/V s. The optimal growth temperature window for efficient Te doping of the amorphous GaNi1-xAsx layers has been determined.},
    keywords = {GaNAs, GaN, tellurium, Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/51167/}
    }

  • M. V. Yakushev, P. Maiello, T. Raadik, M. J. Shaw, P. R. Edwards, J. Krustok, A. V. Mudryi, I. Forbes, and R. W. Martin, "Investigation of the structural, optical and electrical properties of Cu3BiS3 semiconducting thin films," Energy Procedia, vol. 60, p. 166–172, 2014.
    [BibTeX] [Abstract] [Download PDF]

    The elemental composition, structural, optical and electronic properties of p-type Cu3BiS3 thin films are investigated. The films are shown to be single phase orthorhombic, with a measured composition of Cu3.00Bi0.92S3.02. A surface oxidation layer is also clarified using energy dependent X-ray microanalysis. Photoreflectance spectra demonstrate two band gaps (EgX =1.24 eV and EgY =1.53 eV at 4 K) associated with the X and Y valence sub-bands. The photocurrent excitation measurements suggest a direct allowed nature of EgX. Photoluminescence spectra at 5 K reveal two broad emission bands at 0.84 and 0.99 eV quenching with an activation energy of 40 meV.

    @article{strathprints51127,
    volume = {60},
    title = {Investigation of the structural, optical and electrical properties of Cu3BiS3 semiconducting thin films},
    author = {M.V. Yakushev and P. Maiello and T. Raadik and M.J. Shaw and P.R. Edwards and J. Krustok and A.V. Mudryi and I. Forbes and R.W. Martin},
    year = {2014},
    pages = {166--172},
    journal = {Energy Procedia},
    keywords = {Cu3BiS3, thin films, solar cells, Raman spectroscopy, photoluminescence, photoreflectance, Physics, Energy(all)},
    url = {http://strathprints.strath.ac.uk/51127/},
    abstract = {The elemental composition, structural, optical and electronic properties of p-type Cu3BiS3 thin films are investigated. The films are shown to be single phase orthorhombic, with a measured composition of Cu3.00Bi0.92S3.02. A surface oxidation layer is also clarified using energy dependent X-ray microanalysis. Photoreflectance spectra demonstrate two band gaps (EgX =1.24 eV and EgY =1.53 eV at 4 K) associated with the X and Y valence sub-bands. The photocurrent excitation measurements suggest a direct allowed nature of EgX. Photoluminescence spectra at 5 K reveal two broad emission bands at 0.84 and 0.99 eV quenching with an activation energy of 40 meV.}
    }

  • G. Naresh-Kumar, A. Vilalta-Clemente, S. Pandey, D. Skuridina, H. Behmenburg, P. Gamarra, G. Patriarche, I. Vickridge, M. A. di Forte-Poisson, P. Vogt, M. Kneissl, M. Morales, P. Ruterana, A. Cavallini, D. Cavalcoli, C. Giesen, M. Heuken, and C. Trager-Cowan, "Multicharacterization approach for studying InAl(Ga)N/Al(Ga)N/GaN heterostructures for high electron mobility transistors," AIP Advances, vol. 4, iss. 12, p. 127101, 2014.
    [BibTeX] [Abstract] [Download PDF]

    We report on our multi?pronged approach to understand the structural and electrical properties of an InAl(Ga)N(33nm barrier)/Al(Ga)N(1nm interlayer)/GaN(3{\ensuremath{\mu}}m)/AlN(100nm)/Al2O3 high electron mobility transistor (HEMT) heterostructure grown by metal organic vapor phase epitaxy (MOVPE). In particular we reveal and discuss the role of unintentional Ga incorporation in the barrier and also in the interlayer. The observation of unintentional Ga incorporation by using energy dispersive X?ray spectroscopy analysis in a scanning transmission electron microscope is supported with results obtained for samples with a range of AlN interlayer thicknesses grown under both the showerhead as well as the horizontal type MOVPE reactors. Poisson?Schrödinger simulations show that for high Ga incorporation in the Al(Ga)N interlayer, an additional triangular well with very small depth may be exhibited in parallel to the main 2?DEG channel. The presence of this additional channel may cause parasitic conduction and severe issues in device characteristics and processing. Producing a HEMT structure with InAlGaN as the barrier and AlGaN as the interlayer with appropriate alloy composition may be a possible route to optimization, as it might be difficult to avoid Ga incorporation while continuously depositing the layers using the MOVPE growth method. Our present work shows the necessity of a multicharacterization approach to correlate structural and electrical properties to understand device structures and their performance.

    @Article{strathprints50638,
    author = {G. Naresh-Kumar and A. Vilalta-Clemente and S. Pandey and D. Skuridina and H. Behmenburg and P. Gamarra and G. Patriarche and I. Vickridge and M. A. di Forte-Poisson and P. Vogt and M. Kneissl and M. Morales and P. Ruterana and A. Cavallini and D. Cavalcoli and C. Giesen and M. Heuken and C. Trager-Cowan},
    title = {Multicharacterization approach for studying InAl(Ga)N/Al(Ga)N/GaN heterostructures for high electron mobility transistors},
    journal = {AIP Advances},
    year = {2014},
    volume = {4},
    number = {12},
    pages = {127101},
    month = {December},
    abstract = {We report on our multi?pronged approach to understand the structural and electrical properties of an InAl(Ga)N(33nm barrier)/Al(Ga)N(1nm interlayer)/GaN(3{\ensuremath{\mu}}m)/AlN(100nm)/Al2O3 high electron mobility transistor (HEMT) heterostructure grown by metal organic vapor phase epitaxy (MOVPE). In particular we reveal and discuss the role of unintentional Ga incorporation in the barrier and also in the interlayer. The observation of unintentional Ga incorporation by using energy dispersive X?ray spectroscopy analysis in a scanning transmission electron microscope is supported with results obtained for samples with a range of AlN interlayer thicknesses grown under both the showerhead as well as the horizontal type MOVPE reactors. Poisson?Schr{\"o}dinger simulations show that for high Ga incorporation in the Al(Ga)N interlayer, an additional triangular well with very small depth may be exhibited in parallel to the main 2?DEG channel. The presence of this additional channel may cause parasitic conduction and severe issues in device characteristics and processing. Producing a HEMT structure with InAlGaN as the barrier and AlGaN as the interlayer with appropriate alloy composition may be a possible route to optimization, as it might be difficult to avoid Ga incorporation while continuously depositing the layers using the MOVPE growth method. Our present work shows the necessity of a multicharacterization approach to correlate structural and electrical properties to understand device structures and their performance.},
    keywords = {Ga incorporation, III-V semiconductors, Rutherford backscattering, Physics, Electronic, Optical and Magnetic Materials, Physics and Astronomy (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/50638/}
    }

  • I. Nikiforov, B. Hourahine, T. Frauenheim, and T. D. u a, "Formation of helices in graphene nanoribbons under torsion," Journal of Physical Chemistry Letters, vol. 5, iss. 23, p. 4083–4087, 2014.
    [BibTeX] [Abstract] [Download PDF]

    We use objective boundary conditions and self-consistent charge density-functional-based tight-binding to simulate at the atomistic scale the formation of helices in narrow graphene nanoribbons with armchair edges terminated with fluorine and hydrogen. We interpret the microscopic data using an inextensible, unshearable elastic rod model, which considers both bending and torsional strains. When fitted to the atomistic data, the simple rod model uses closed-form solutions for a cubic equation to predict the strain energy and morphology at a given twist angle and the crossover point between pure torsion and a helix. Our modeling and simulation bring key insights into the origin of the helical graphene morphologies stored inside of carbon nanotubes. They can be useful for designing chiral nanoribbons with tailored properties.

    @Article{strathprints50426,
    author = {Ilia Nikiforov and Benjamin Hourahine and Thomas Frauenheim and Traian Dumitric{\u a}},
    title = {Formation of helices in graphene nanoribbons under torsion},
    journal = {Journal of Physical Chemistry Letters},
    year = {2014},
    volume = {5},
    number = {23},
    pages = {4083--4087},
    month = {December},
    abstract = {We use objective boundary conditions and self-consistent charge density-functional-based tight-binding to simulate at the atomistic scale the formation of helices in narrow graphene nanoribbons with armchair edges terminated with fluorine and hydrogen. We interpret the microscopic data using an inextensible, unshearable elastic rod model, which considers both bending and torsional strains. When fitted to the atomistic data, the simple rod model uses closed-form solutions for a cubic equation to predict the strain energy and morphology at a given twist angle and the crossover point between pure torsion and a helix. Our modeling and simulation bring key insights into the origin of the helical graphene morphologies stored inside of carbon nanotubes. They can be useful for designing chiral nanoribbons with tailored properties.},
    keywords = {objective boundary conditions, graphene nanoribbons, strain energy, morphology, Physics, Materials Science(all)},
    url = {http://strathprints.strath.ac.uk/50426/}
    }

  • N. J. Findlay, J. Bruckbauer, A. R. Inigo, B. Breig, S. Arumugam, D. J. Wallis, R. W. Martin, and P. J. Skabara, "Light-emitting diodes : an organic down-converting material for white-light emission from hybrid LEDs (Adv. Mater. 43/2014)," Advanced Materials, vol. 26, iss. 43, p. 7415, 2014.
    [BibTeX] [Abstract] [Download PDF]

    Combining a yellow-emitting organic material with a blue-emitting inorganic light-emitting device (LED) provides a hybrid white-light LED. This approach, demonstrated by R. W. Martin, P. J. Skabara, and co-workers on page 7290, couples very efficient blue emission with the flexibility, fast modulation speed, and cost effectiveness of the organic material. For the latter, BODIPY is used as the emitter and linked to a conjugated unit designed to efficiently absorb a suitable fraction of the blue light.

    @Article{strathprints50349,
    author = {Neil J. Findlay and Jochen Bruckbauer and Anto R. Inigo and Benjamin Breig and Sasikumar Arumugam and David J. Wallis and Robert W. Martin and Peter J. Skabara},
    title = {Light-emitting diodes : an organic down-converting material for white-light emission from hybrid {LED}s ({A}dv. {M}ater. 43/2014)},
    journal = {Advanced Materials},
    year = {2014},
    volume = {26},
    number = {43},
    pages = {7415},
    month = {November},
    abstract = {Combining a yellow-emitting organic material with a blue-emitting inorganic light-emitting device (LED) provides a hybrid white-light LED. This approach, demonstrated by R. W. Martin, P. J. Skabara, and co-workers on page 7290, couples very efficient blue emission with the flexibility, fast modulation speed, and cost effectiveness of the organic material. For the latter, BODIPY is used as the emitter and linked to a conjugated unit designed to efficiently absorb a suitable fraction of the blue light.},
    keywords = {hybrid light emitting diodes, colorimetry, BODIPY, energy down-converters, Physics, Mechanics of Materials, Materials Science(all), Mechanical Engineering},
    url = {http://strathprints.strath.ac.uk/50349/}
    }

  • M. Shaw, K. M. Yu, M. Ting, R. E. L. Powell, W. L. Sarney, S. P. Svensson, A. J. Kent, W. Walukiewicz, C. T. Foxon, S. V. Novikov, and R. W. Martin, "Composition and optical properties of dilute-Sb GaN₁₋ₓSbₓ highly mismatched alloys grown by MBE," Journal of Physics D: Applied Physics, vol. 47, iss. 46, p. 465102, 2014.
    [BibTeX] [Abstract] [Download PDF]

    In this work the compositional and optical characterization of three series of dilute-Sb GaN1 ? xSbx alloys grown with various Sb flux, under N and Ga-rich conditions, are presented. Using wavelength dispersive x-ray microanalysis and Rutherford backscattering spectroscopy it is found that the N-rich samples (Ga flux {\ensuremath{<}} 2.3 {$\times$} 10?7 Torr) incorporate a higher magnitude of GaSb than the Ga-rich samples (Ga flux {\ensuremath{>}} 2.3 {$\times$} 10?7 Torr) under the same growth conditions. The optical properties of the Ga-rich samples are measured using room temperature cathodoluminescence (CL), photoluminescence (PL) and absorption measurements. A broad luminescence peak is observed around 2.2 eV. The nature and properties of this peak are considered, as is the suitability of these dilute-Sb alloys for use in solar energy conversion devices.

    @Article{strathprints50168,
    author = {Martin Shaw and K.M. Yu and M. Ting and R. E. L. Powell and W. L. Sarney and S. P. Svensson and A. J. Kent and W. Walukiewicz and C. T. Foxon and S. V. Novikov and Robert W. Martin},
    title = {Composition and optical properties of dilute-{Sb} {GaN₁₋ₓSbₓ} highly mismatched alloys grown by {MBE}},
    journal = {Journal of Physics D: Applied Physics},
    year = {2014},
    volume = {47},
    number = {46},
    pages = {465102},
    month = {October},
    abstract = {In this work the compositional and optical characterization of three series of dilute-Sb GaN1 ? xSbx alloys grown with various Sb flux, under N and Ga-rich conditions, are presented. Using wavelength dispersive x-ray microanalysis and Rutherford backscattering spectroscopy it is found that the N-rich samples (Ga flux {\ensuremath{<}} 2.3 {$\times$} 10?7 Torr) incorporate a higher magnitude of GaSb than the Ga-rich samples (Ga flux {\ensuremath{>}} 2.3 {$\times$} 10?7 Torr) under the same growth conditions. The optical properties of the Ga-rich samples are measured using room temperature cathodoluminescence (CL), photoluminescence (PL) and absorption measurements. A broad luminescence peak is observed around 2.2 eV. The nature and properties of this peak are considered, as is the suitability of these dilute-Sb alloys for use in solar energy conversion devices.},
    keywords = {dilute-Sb GaN1 ? xSbx alloys, solar energy conversion devices, Highly Mismatched Alloys (HMAs), semiconductor alloys, Optics. Light, Atomic and Molecular Physics, and Optics, Electronic, Optical and Magnetic Materials},
    url = {http://strathprints.strath.ac.uk/50168/}
    }

  • Y. D. Zhuang, J. Bruckbauer, P. A. Shields, P. R. Edwards, R. W. Martin, and D. W. E. Allsopp, "Influence of stress on optical transitions in GaN nanorods containing a single InGaN/GaN quantum disk," Journal of Applied Physics, vol. 116, iss. 17, p. 174305, 2014.
    [BibTeX] [Abstract] [Download PDF]

    Cathodoluminescence (CL) hyperspectral imaging has been performed on GaN nanorods containing a single InGaN quantum disk (SQD) with controlled variations in excitation conditions. Two different nanorod diameters (200 and 280 nm) have been considered. Systematic changes in the CL spectra from the SQD were observed as the accelerating voltage of the electron beam and its position of incidence are varied. It is shown that the dominant optical transition in the SQD varies across the nanorod as a result of interplay between the contributions of the deformation potential and the quantum-confined Stark effect to the transition energy as consequence of radial variation in the pseudomorphic strain.

    @Article{strathprints50121,
    author = {Y. D. Zhuang and J. Bruckbauer and P. A. Shields and P. R. Edwards and R. W. Martin and D. W. E. Allsopp},
    title = {Influence of stress on optical transitions in {GaN} nanorods containing a single {InGaN/GaN} quantum disk},
    journal = {Journal of Applied Physics},
    year = {2014},
    volume = {116},
    number = {17},
    pages = {174305},
    month = {November},
    abstract = {Cathodoluminescence (CL) hyperspectral imaging has been performed on GaN nanorods containing a single InGaN quantum disk (SQD) with controlled variations in excitation conditions. Two different nanorod diameters (200 and 280 nm) have been considered. Systematic changes in the CL spectra from the SQD were observed as the accelerating voltage of the electron beam and its position of incidence are varied. It is shown that the dominant optical transition in the SQD varies across the nanorod as a result of interplay between the contributions of the deformation potential and the quantum-confined Stark effect to the transition energy as consequence of radial variation in the pseudomorphic strain.},
    keywords = {nanorods, electron beams, emission spectra, quantum wells, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/50121/}
    }

  • E. Taylor, M. D. Smith, T. C. Sadler, K. Lorenz, H. N. Li, E. Alves, P. J. Parbrook, and R. W. Martin, "Structural and optical properties of Ga auto-incorporated InAlN epilayers," Journal of Crystal Growth, vol. 408, p. 97–101, 2014.
    [BibTeX] [Abstract] [Download PDF]

    InAlN epilayers deposited on thick GaN buffer layers grown by metalorganic chemical vapour deposition (MOCVD) revealed an auto-incorporation of Ga when analysed by wavelength dispersive x-ray (WDX) spectroscopy and Rutherford backscattering spectrometry (RBS). Samples were grown under similar conditions with the change in reactor flow rate resulting in varying Ga contents of 12-24\%. The increase in flow rate from 8000 to 24 000 sccm suppressed the Ga auto-incorporation which suggests that the likely cause is from residual Ga left behind from previous growth runs. The luminescence properties of the resultant InAlGaN layers were investigated using cathodoluminescence (CL) measurements.

    @Article{strathprints49667,
    author = {E. Taylor and M.D. Smith and T.C. Sadler and K. Lorenz and H.N. Li and E. Alves and P.J. Parbrook and R.W. Martin},
    title = {Structural and optical properties of {Ga} auto-incorporated {InAlN} epilayers},
    journal = {Journal of Crystal Growth},
    year = {2014},
    volume = {408},
    pages = {97--101},
    month = {December},
    abstract = {InAlN epilayers deposited on thick GaN buffer layers grown by metalorganic chemical vapour deposition (MOCVD) revealed an auto-incorporation of Ga when analysed by wavelength dispersive x-ray (WDX) spectroscopy and Rutherford backscattering spectrometry (RBS). Samples were grown under similar conditions with the change in reactor flow rate resulting in varying Ga contents of 12-24\%. The increase in flow rate from 8000 to 24 000 sccm suppressed the Ga auto-incorporation which suggests that the likely cause is from residual Ga left behind from previous growth runs. The luminescence properties of the resultant InAlGaN layers were investigated using cathodoluminescence (CL) measurements.},
    keywords = {metalorganic chemical vapour deposition, wavelength dispersive x-ray, Rutherford backscattering spectrometry, InAlN, InAlGaN, Ga incorporation, MOCVD, Physics, Chemical engineering, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/49667/}
    }

  • P. R. Edwards and M. R. Lee, "Cathodoluminescence hyperspectral imaging in geoscience," in Cathodoluminescence and its Application to Geoscience, I. M. Coulson, Ed., Québec: Mineralogical Association of Canada, 2014, p. 29–45.
    [BibTeX] [Abstract] [Download PDF]

    Cathodoluminescence (CL) is the electron-stimulated emission of low-energy (IR/visible/UV) photons from a solid material. Electron irradiation raises sample electrons to an excited state, which then emit a photon as they return to a lower energy state. The resultant luminescence can be analysed both spatially and spectrally, and until recently only one of these two approaches could be used for a given measurement. This chapter outlines the conventional spatial and spectral techniques, then describes the more recent approach of hyperspectral imaging, in which a single CL dataset simultaneously contains both spatial and spectral information.

    @incollection{strathprints49411,
    author = {Paul R. Edwards and Martin R. Lee},
    series = {Short Course Series},
    booktitle = {Cathodoluminescence and its Application to Geoscience},
    editor = {Ian M. Coulson},
    address = {Qu{\'e}bec},
    title = {Cathodoluminescence hyperspectral imaging in geoscience},
    publisher = {Mineralogical Association of Canada},
    year = {2014},
    pages = {29--45},
    keywords = {cathodoluminescence, CL, hyperspectral imaging (HSI), Physics, Geology, Physics and Astronomy (miscellaneous), Geophysics},
    url = {http://strathprints.strath.ac.uk/49411/},
    abstract = {Cathodoluminescence (CL) is the electron-stimulated emission of low-energy (IR/visible/UV) photons from a solid material. Electron irradiation raises sample electrons to an excited state, which then emit a photon as they return to a lower energy state. The resultant luminescence can be analysed both spatially and spectrally, and until recently only one of these two approaches could be used for a given measurement. This chapter outlines the conventional spatial and spectral techniques, then describes the more recent approach of hyperspectral imaging, in which a single CL dataset simultaneously contains both spatial and spectral information.}
    }

  • P. R. Edwards, M. J. Wallace, G. Kusch, G. Naresh-Kumar, J. Bruckbauer, C. Trager-Cowan, K. P. O'Donnell, and R. W. Martin, "Cathodoluminescence hyperspectral imaging of nitride semiconductors : introducing new variables," Microscopy and Microanalysis, vol. 20, iss. S3, p. 906–907, 2014.
    [BibTeX] [Download PDF]
    @article{strathprints49410,
    volume = {20},
    number = {S3},
    month = {August},
    author = {Paul R. Edwards and Michael J. Wallace and Gunnar Kusch and Gunasekar Naresh-Kumar and Jochen Bruckbauer and Carol Trager-Cowan and Kevin P. O'Donnell and Robert W. Martin},
    title = {Cathodoluminescence hyperspectral imaging of nitride semiconductors : introducing new variables},
    journal = {Microscopy and Microanalysis},
    pages = {906--907},
    year = {2014},
    keywords = {Physics, Instrumentation},
    url = {http://strathprints.strath.ac.uk/49410/}
    }

  • C. Trager-Cowan, G. Naresh-Kumar, N. Allehiani, S. Kraeusel, B. Hourahine, S. Vespucci, D. Thomson, J. Bruckbauer, G. Kusch, P. R. Edwards, R. W. Martin, C. Mauder, A. P. Day, A. Winkelmann, A. Vilalta-Clemente, A. J. Wilkinson, P. J. Parbrook, M. J. Kappers, M. A. Moram, R. A. Oliver, C. J. Humphreys, P. Shields, L. E. D. Boulbar, D. Maneuski, V. O'Shea, and K. P. Mingard, "Electron channeling contrast imaging of defects in III-nitride semiconductors," Microscopy and Microanalysis, vol. 20, iss. S3, p. 1024–1025, 2014.
    [BibTeX] [Download PDF]
    @Article{strathprints49409,
    author = {C. Trager-Cowan and G. Naresh-Kumar and N. Allehiani and S. Kraeusel and B. Hourahine and S. Vespucci and D. Thomson and J. Bruckbauer and G. Kusch and P. R. Edwards and R. W. Martin and C. Mauder and A. P. Day and A. Winkelmann and A. Vilalta-Clemente and A. J. Wilkinson and P. J. Parbrook and M. J. Kappers and M. A. Moram and R. A. Oliver and C. J. Humphreys and P. Shields and E. D. Le Boulbar and D. Maneuski and V. O'Shea and K. P. Mingard},
    title = {Electron channeling contrast imaging of defects in {III}-nitride semiconductors},
    journal = {Microscopy and Microanalysis},
    year = {2014},
    volume = {20},
    number = {S3},
    pages = {1024--1025},
    month = {August},
    keywords = {Physics, Instrumentation},
    url = {http://strathprints.strath.ac.uk/49409/}
    }

  • I. A. Ajia, P. R. Edwards, Z. Liu, J. C. Yan, R. W. Martin, and I. S. Roqan, "Excitonic localization in AlN-rich AlxGa1-xN/AlyGa1-yN multi-quantum-well grain boundaries," Applied Physics Letters, vol. 105, iss. 12, p. 122111, 2014.
    [BibTeX] [Abstract] [Download PDF]

    AlGaN/AlGaN multi-quantum-wells (MQW) with AlN-rich grains have been grown by metal organic chemical vapor deposition. The grains are observed to have strong excitonic localization characteristics that are affected by their sizes. The tendency to confine excitons progressively intensifies with increasing grain boundary area. Photoluminescence results indicate that the MQW have a dominant effect on the peak energy of the near-bandedge emission at temperatures below 150 K, with the localization properties of the grains becoming evident beyond 150 K. Cathodoluminescence maps reveal that the grain boundary has no effect on the peak intensities of the AlGaN/AlGaN samples.

    @Article{strathprints49377,
    author = {Idris A. Ajia and P. R. Edwards and Z. Liu and J. C. Yan and R. W. Martin and I. S. Roqan},
    title = {Excitonic localization in {AlN}-rich {AlxGa1-xN/AlyGa1-yN} multi-quantum-well grain boundaries},
    journal = {Applied Physics Letters},
    year = {2014},
    volume = {105},
    number = {12},
    pages = {122111},
    month = {September},
    note = {. Copyright (2014) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.},
    abstract = {AlGaN/AlGaN multi-quantum-wells (MQW) with AlN-rich grains have been grown by metal organic chemical vapor deposition. The grains are observed to have strong excitonic localization characteristics that are affected by their sizes. The tendency to confine excitons progressively intensifies with increasing grain boundary area. Photoluminescence results indicate that the MQW have a dominant effect on the peak energy of the near-bandedge emission at temperatures below 150 K, with the localization properties of the grains becoming evident beyond 150 K. Cathodoluminescence maps reveal that the grain boundary has no effect on the peak intensities of the AlGaN/AlGaN samples.},
    keywords = {multi-quantum-wells, MQW, AlN-rich grains, excitonic localization, AlGaN/AlGaN MQW, Physics, Physics and Astronomy (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/49377/}
    }

  • C. J. Lewins, L. E. D. Boulbar, S. M. Lis, P. R. Edwards, R. W. Martin, P. A. Shields, and D. W. E. Allsopp, "Strong photonic crystal behavior in regular arrays of core-shell and quantum disc InGaN/GaN nanorod light-emitting diodes," Journal of Applied Physics, vol. 116, iss. 4, p. 44305, 2014.
    [BibTeX] [Abstract] [Download PDF]

    We show that arrays of emissive nanorod structures can exhibit strong photonic crystal behavior, via observations of the far-field luminescence from core-shell and quantum disc InGaN/GaN nanorods. The conditions needed for the formation of directional Bloch modes characteristic of strong photonic behavior are found to depend critically upon the vertical shape of the nanorod sidewalls. Index guiding by a region of lower volume-averaged refractive index near the base of the nanorods creates a quasi-suspended photonic crystal slab at the top of the nanorods which supports Bloch modes. Only diffractive behavior could be observed without this region. Slab waveguide modelling of the vertical structure shows that the behavioral regime of the emissive nanorod arrays depends strongly upon the optical coupling between the nanorod region and the planar layers below. The controlled crossover between the two regimes of photonic crystal operation enables the design of photonic nanorod structures formed on planar substrates that exploit either behavior depending on device requirements.

    @Article{strathprints49269,
    author = {C. J. Lewins and E. D. Le Boulbar and S. M. Lis and P. R. Edwards and R. W. Martin and P. A. Shields and D. W. E. Allsopp},
    title = {Strong photonic crystal behavior in regular arrays of core-shell and quantum disc InGaN/GaN nanorod light-emitting diodes},
    journal = {Journal of Applied Physics},
    year = {2014},
    volume = {116},
    number = {4},
    pages = {044305},
    abstract = {We show that arrays of emissive nanorod structures can exhibit strong photonic crystal behavior, via observations of the far-field luminescence from core-shell and quantum disc InGaN/GaN nanorods. The conditions needed for the formation of directional Bloch modes characteristic of strong photonic behavior are found to depend critically upon the vertical shape of the nanorod sidewalls. Index guiding by a region of lower volume-averaged refractive index near the base of the nanorods creates a quasi-suspended photonic crystal slab at the top of the nanorods which supports Bloch modes. Only diffractive behavior could be observed without this region. Slab waveguide modelling of the vertical structure shows that the behavioral regime of the emissive nanorod arrays depends strongly upon the optical coupling between the nanorod region and the planar layers below. The controlled crossover between the two regimes of photonic crystal operation enables the design of photonic nanorod structures formed on planar substrates that exploit either behavior depending on device requirements.},
    keywords = {nanorods, photonic crystals, light emitting diodes , III-V semiconductors, light diffraction, Solid state physics. Nanoscience, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/49269/}
    }

  • P. R. Edwards, L. E. D. Boulbar, P. A. Shields, D. W. E. Allsopp, and R. W. Martin, "Cathodoluminescence hyperspectral imaging of nitride core-shell structures," in Condensed Matter in Paris 2014 (CMD25-JMC14), 2014, p. 679–680.
    [BibTeX] [Abstract] [Download PDF]

    In this work, we demonstrate the use of hyperspectral CL in the evaluation of periodic arrays of GaN/InxGa1-xN core-shell nanorods. These were fabricated using a top-down approach, in which columns are formed from a GaN template using nano-imprint lithography and ICP etching, followed by MOCVD regrowth [2]. The formation of quantum wells (QWs) on the mplane sidewall facets offers a route to avoiding the detrimental electric fields associated with LEDs grown on the c-plane, while the use of periodic features has the potential to improve light extraction and directionality.

    @inproceedings{strathprints49238,
    booktitle = {Condensed Matter in Paris 2014 (CMD25-JMC14)},
    title = {Cathodoluminescence hyperspectral imaging of nitride core-shell structures},
    author = {P. R. Edwards and E. D. Le Boulbar and P. A. Shields and D. W. E. Allsopp and R. W. Martin},
    year = {2014},
    pages = {679--680},
    journal = {Condensed Matter in Paris 2014 (CMD25-JMC14)},
    keywords = {spectroscopic techniques, cathodoluminescence (CL), hyperspectral imaging (HSI), nitride nanostructures, Physics, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/49238/},
    abstract = {In this work, we demonstrate the use of hyperspectral CL in the evaluation of periodic arrays of GaN/InxGa1-xN core-shell nanorods. These were fabricated using a top-down approach, in which columns are formed from a GaN template using nano-imprint lithography and ICP etching, followed by MOCVD regrowth [2]. The formation of quantum wells (QWs) on the mplane sidewall facets offers a route to avoiding the detrimental electric fields associated with LEDs grown on the c-plane, while the use of periodic features has the potential to improve light extraction and directionality.}
    }

  • N. Findlay, J. Bruckbauer, J. Inigo, B. Breig, S. Arumugam, D. J. Wallis, R. Martin, and P. Skabara, "An organic down-converting material for white-light emission from hybrid LEDs," Advanced Materials, vol. 26, iss. 43, p. 7290–7294, 2014.
    [BibTeX] [Abstract] [Download PDF]

    A novel BODIPY-containing organic small molecule is synthesized and employed as a down-converting layer on a commercial blue light-emitting diode (LED). The resulting hybrid device demonstrates white light emission under low-current operation, with color coordinates of (0.34, 0.31) and an efficacy of 13.6 lm/W; four times greater than the parent blue LED.

    @Article{strathprints48999,
    author = {Neil Findlay and Jochen Bruckbauer and Jesuraj Inigo and Benjamin Breig and Sasikumar Arumugam and David J. Wallis and Robert Martin and Peter Skabara},
    title = {An organic down-converting material for white-light emission from hybrid {LED}s},
    journal = {Advanced Materials},
    year = {2014},
    volume = {26},
    number = {43},
    pages = {7290--7294},
    month = {November},
    abstract = {A novel BODIPY-containing organic small molecule is synthesized and employed as a down-converting layer on a commercial blue light-emitting diode (LED). The resulting hybrid device demonstrates white light emission under low-current operation, with color coordinates of (0.34, 0.31) and an efficacy of 13.6 lm/W; four times greater than the parent blue LED.},
    keywords = {hybrid LED technology, energy down-converter, BODIPY, colorimetry, organic light-emitting diodes, OLEDs, Chemistry, Chemical technology, Physics, Mechanics of Materials, Materials Science(all), Mechanical Engineering},
    url = {http://strathprints.strath.ac.uk/48999/}
    }

  • M. J. Wallace, P. R. Edwards, M. J. Kappers, M. A. Hopkins, F. Oehler, S. Sivaraya, D. W. E. Allsopp, R. A. Oliver, C. J. Humphreys, and R. W. Martin, "Bias dependence and correlation of the cathodoluminescence and electron beam induced current from an InGaN/GaN light emitting diode," Journal of Applied Physics, vol. 116, iss. 3, p. 33105, 2014.
    [BibTeX] [Abstract] [Download PDF]

    Micron-scale mapping has been employed to study a contacted InGaN/GaN LED using combined electroluminescence (EL), cathodoluminescence (CL), and electron beam induced current (EBIC). Correlations between parameters, such as the EBIC and CL intensity, were studied as a function of applied bias. The CL and EBIC maps reveal small areas, 2?10 {\ensuremath{\mu}}m in size, which have increased nonradiative recombination rate and/or a lower conductivity. The CL emission from these spots is blue shifted, by 30?40 meV. Increasing the reverse bias causes the size of the spots to decrease, due to competition between in-plane diffusion and drift in the growth direction. EL mapping shows large bright areas ({$\sim$}100 {\ensuremath{\mu}}m) which also have increased EBIC, indicating domains of increased conductivity in the p and/or n-GaN.

    @Article{strathprints48986,
    author = {M. J. Wallace and P. R. Edwards and M. J. Kappers and M. A. Hopkins and F. Oehler and S. Sivaraya and D. W. E. Allsopp and R. A. Oliver and C. J. Humphreys and R. W. Martin},
    title = {Bias dependence and correlation of the cathodoluminescence and electron beam induced current from an InGaN/GaN light emitting diode},
    journal = {Journal of Applied Physics},
    year = {2014},
    volume = {116},
    number = {3},
    pages = {033105},
    abstract = {Micron-scale mapping has been employed to study a contacted InGaN/GaN LED using combined electroluminescence (EL), cathodoluminescence (CL), and electron beam induced current (EBIC). Correlations between parameters, such as the EBIC and CL intensity, were studied as a function of applied bias. The CL and EBIC maps reveal small areas, 2?10 {\ensuremath{\mu}}m in size, which have increased nonradiative recombination rate and/or a lower conductivity. The CL emission from these spots is blue shifted, by 30?40 meV. Increasing the reverse bias causes the size of the spots to decrease, due to competition between in-plane diffusion and drift in the growth direction. EL mapping shows large bright areas ({$\sim$}100 {\ensuremath{\mu}}m) which also have increased EBIC, indicating domains of increased conductivity in the p and/or n-GaN.},
    keywords = {bias dependence , cathodoluminescence, electron beam induced current, InGaN/GaN , micron-scale mapping, light emitting diode, electroluminescence, Optics. Light, Atomic and Molecular Physics, and Optics},
    url = {http://strathprints.strath.ac.uk/48986/}
    }

  • M. V. Yakushev, P. Maiello, T. Raadik, M. J. Shaw, P. R. Edwards, J. Krustok, A. V. Mudryi, I. Forbes, and R. W. Martin, "Electronic and structural characterisation of Cu3BiS3 thin films for the absorber layer of sustainable photovoltaics," Thin Solid Films, vol. 562, p. 195–199, 2014.
    [BibTeX] [Abstract] [Download PDF]

    Abstract Thin films of p-type Cu3BiS3 with an orthorhombic wittichenite structure, a semiconductor with high potential for thin film solar cell absorber layers, were synthesised by thermal annealing of Cu and Bi precursors, magnetron sputtered on Mo/glass substrate, with a layer of thermo-evaporated S. The elemental composition, structural and electronic properties are studied. The Raman spectrum shows four modes with the dominant peak at 292 cm-1. Photoreflectance spectra demonstrate two band gaps EgX and EgY, associated with the X and Y valence sub-bands, and their evolution with temperature. Fitting the temperature dependencies of the band-gaps gives values of 1.24 and 1.53 eV for EgX and EgY at 0 K as well as the average phonon energy. Photoluminescence spectra at 5 K reveal two bright and broad emission bands at 0.84 and 0.99 eV, which quench with an activation energy of 40 meV. The photocurrent excitation measurements demonstrate a photoresponse and suggest a direct allowed nature of the band gap.

    @article{strathprints48646,
    volume = {562},
    title = {Electronic and structural characterisation of Cu3BiS3 thin films for the absorber layer of sustainable photovoltaics},
    author = {M.V. Yakushev and P. Maiello and T. Raadik and M.J. Shaw and P.R. Edwards and J. Krustok and A.V. Mudryi and I. Forbes and R.W. Martin},
    year = {2014},
    pages = {195--199},
    note = {{\copyright} 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license},
    journal = {Thin Solid Films},
    keywords = {photoluminescence, thin films, solar cells, semiconductors, electronic structure, raman spectroscopy, photoreflectance, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/48646/},
    abstract = {Abstract Thin films of p-type Cu3BiS3 with an orthorhombic wittichenite structure, a semiconductor with high potential for thin film solar cell absorber layers, were synthesised by thermal annealing of Cu and Bi precursors, magnetron sputtered on Mo/glass substrate, with a layer of thermo-evaporated S. The elemental composition, structural and electronic properties are studied. The Raman spectrum shows four modes with the dominant peak at 292 cm-1. Photoreflectance spectra demonstrate two band gaps EgX and EgY, associated with the X and Y valence sub-bands, and their evolution with temperature. Fitting the temperature dependencies of the band-gaps gives values of 1.24 and 1.53 eV for EgX and EgY at 0 K as well as the average phonon energy. Photoluminescence spectra at 5 K reveal two bright and broad emission bands at 0.84 and 0.99 eV, which quench with an activation energy of 40 meV. The photocurrent excitation measurements demonstrate a photoresponse and suggest a direct allowed nature of the band gap.}
    }

  • K. Imura, K. Ueno, H. Misawa, H. Okamoto, D. McArthur, B. Hourahine, and F. Papoff, "Plasmon modes in single gold nanodiscs," Optics Express, vol. 22, iss. 10, p. 12189–12199, 2014.
    [BibTeX] [Abstract] [Download PDF]

    Optical properties of single gold nanodiscs were studied by scanning near-field optical microscopy. Near-field transmission spectra of a single nanodisc exhibited multiple plasmon resonances in the visible to near-infrared region. Near-field transmission images observed at these resonance wavelengths show wavy spatial features depending on the wavelength of observation. To clarify physical pictures of the images, theoretical simulations based on spatial correlation between electromagnetic fundamental modes inside and outside of the disc were performed. Simulated images reproduced the observed spatial structures excited in the disc. Mode-analysis of the simulated images indicates that the spatial features observed in the transmission images originate mainly from a few fundamental plasmon modes of the disc.

    @article{strathprints48128,
    volume = {22},
    number = {10},
    title = {Plasmon modes in single gold nanodiscs},
    author = {Kohei Imura and Kosei Ueno and Hiroaki Misawa and Hiromi Okamoto and Duncan McArthur and Benjamin Hourahine and Francesco Papoff},
    year = {2014},
    pages = {12189--12199},
    journal = {Optics Express},
    keywords = {optical properties, single gold nanodiscs, plasmon modes, near-field optical microscopy, Physics, Solid state physics. Nanoscience, Optics. Light, Physics and Astronomy(all), Atomic and Molecular Physics, and Optics},
    url = {http://strathprints.strath.ac.uk/48128/},
    abstract = {Optical properties of single gold nanodiscs were studied by scanning near-field optical microscopy. Near-field transmission spectra of a single nanodisc exhibited multiple plasmon resonances in the visible to near-infrared region. Near-field transmission images observed at these resonance wavelengths show wavy spatial features depending on the wavelength of observation. To clarify physical pictures of the images, theoretical simulations based on spatial correlation between electromagnetic fundamental modes inside and outside of the disc were performed. Simulated images reproduced the observed spatial structures excited in the disc. Mode-analysis of the simulated images indicates that the spatial features observed in the transmission images originate mainly from a few fundamental plasmon modes of the disc.}
    }

  • V. V. Kachkanov, B. B. Leung, J. J. Song, J. J. Han, Y. Zhang, M. C. Tsai, G. Yuan, J. Han, and K. K. O'Donnell, "Structural dynamics of GaN microcrystals in evolutionary selection selective area growth probed by X-ray microdiffraction," Scientific Reports, vol. 4, p. 4651, 2014.
    [BibTeX] [Abstract] [Download PDF]

    A method to grow high quality, single crystalline semiconductor material irrespective of the substrate would allow a cost-effective improvement to functionality and performance of optoelectronic devices. Recently, a novel type of substrate-insensitive growth process called Evolutionary Selection Selective Area Growth (ES-SAG) has been proposed. Here we report the use of X-ray microdiffraction to study the structural properties of GaN microcrystals grown by ES-SAG. Utilizing high resolution in both direct and reciprocal spaces, we have unraveled structural dynamics of GaN microcrystals in growth structures of different dimensions. It has been found that the geometric proportions of the growth constrictions play an important role: 2.6â.Î 1/4m and 4.5â.Î 1/4m wide growth tunnels favor the evolutionary selection mechanism, contrary to the case of 8.6â.Î 1/4m growth tunnels. It was also found that GaN microcrystal ensembles are dominated by slight tensile strain irrespective of growth tunnel shape.

    @Article{strathprints47832,
    author = {Vyacheslav V. Kachkanov and Benjamin B. Leung and Jie J. Song and Jung J. Han and Y. Zhang and M.C. Tsai and G. Yuan and J. Han and Kevin K. O'Donnell},
    title = {Structural dynamics of GaN microcrystals in evolutionary selection selective area growth probed by {X}-ray microdiffraction},
    journal = {Scientific Reports},
    year = {2014},
    volume = {4},
    pages = {4651},
    month = {April},
    abstract = {A method to grow high quality, single crystalline semiconductor material irrespective of the substrate would allow a cost-effective improvement to functionality and performance of optoelectronic devices. Recently, a novel type of substrate-insensitive growth process called Evolutionary Selection Selective Area Growth (ES-SAG) has been proposed. Here we report the use of X-ray microdiffraction to study the structural properties of GaN microcrystals grown by ES-SAG. Utilizing high resolution in both direct and reciprocal spaces, we have unraveled structural dynamics of GaN microcrystals in growth structures of different dimensions. It has been found that the geometric proportions of the growth constrictions play an important role: 2.6{\^a}.{\^I} 1/4m and 4.5{\^a}.{\^I} 1/4m wide growth tunnels favor the evolutionary selection mechanism, contrary to the case of 8.6{\^a}.{\^I} 1/4m growth tunnels. It was also found that GaN microcrystal ensembles are dominated by slight tensile strain irrespective of growth tunnel shape.},
    keywords = {electronic devices, structure of solids, structure of liquids, GaN microcrystals, Physics, General},
    url = {http://strathprints.strath.ac.uk/47832/}
    }

  • K. P. O'Donnell, P. R. Edwards, M. J. Kappers, K. Lorenz, E. J. Alves, and M. X. Boćkowski, "Europium-doped GaN(Mg) : beyond the limits of the light-emitting diode," Physica Status Solidi C, vol. 11, iss. 3–4, p. 662–665, 2014.
    [BibTeX] [Abstract] [Download PDF]

    Rare-earth doped III-N semiconductors have been studied for decades on account of their possible application in visible light-emitting diodes (LED) with built-in utility as red (e.g. Eu), green (Er) and blue (Tm) monochromatic sources (O'Donnell and Dierolf (eds.), Topics in Applied Physics, Vol. 124 (Springer, Dordrecht, 2010) [1]). However, to date, no commercial devices have been introduced on the basis of these materials. Recently, we discovered thermally activated hysteresis in the emission spectrum of p-type GaN thin films that were co-doped with Mg and Eu (O'Donnell et al., Proc. ICPS31, Zurich, July 2012 [2]). We have also reported an unexpected Zeeman splitting and induced magnetic moment of Eu3+ ions in GaN (Kachkanov et al., Scientific Rep. 2, 969 (2012) and MRS Proc. 1290?i03?06 (2011) [3, 4]). These findings encourage speculation on taking the study of RE-doped III-N beyond the limited goal of improving LED efficiency into the realm of novel magneto-optic and quantum-optical devices. In particular we will describe in this presentation the spectroscopy of ion-implanted and annealed GaN(Mg): Eu samples and the possible exploitation of the Mg acceptor in GaN as a qubit.

    @Article{strathprints47513,
    author = {K.P. O'Donnell and P.R. Edwards and M.J. Kappers and K. Lorenz and E.J. Alves and M.X. Bo{\'c}kowski},
    title = {Europium-doped {GaN(Mg)} : beyond the limits of the light-emitting diode},
    journal = {Physica Status Solidi C},
    year = {2014},
    volume = {11},
    number = {3--4},
    pages = {662--665},
    month = {April},
    abstract = {Rare-earth doped III-N semiconductors have been studied for decades on account of their possible application in visible light-emitting diodes (LED) with built-in utility as red (e.g. Eu), green (Er) and blue (Tm) monochromatic sources (O'Donnell and Dierolf (eds.), Topics in Applied Physics, Vol. 124 (Springer, Dordrecht, 2010) [1]). However, to date, no commercial devices have been introduced on the basis of these materials. Recently, we discovered thermally activated hysteresis in the emission spectrum of p-type GaN thin films that were co-doped with Mg and Eu (O'Donnell et al., Proc. ICPS31, Zurich, July 2012 [2]). We have also reported an unexpected Zeeman splitting and induced magnetic moment of Eu3+ ions in GaN (Kachkanov et al., Scientific Rep. 2, 969 (2012) and MRS Proc. 1290?i03?06 (2011) [3, 4]). These findings encourage speculation on taking the study of RE-doped III-N beyond the limited goal of improving LED efficiency into the realm of novel magneto-optic and quantum-optical devices. In particular we will describe in this presentation the spectroscopy of ion-implanted and annealed GaN(Mg): Eu samples and the possible exploitation of the Mg acceptor in GaN as a qubit.},
    keywords = {GaN, rare earth doping, luminescence hysteresis, qubit, Physics, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/47513/}
    }

  • Z. Wei, S. Senthilarasu, M. V. Yakushev, R. W. Martin, and H. M. Upadhyaya, "Effect of mechanical compression on Cu(In,Ga)Se films : micro-structural and photoluminescence analysis," RSC Advances, vol. 4, iss. 10, p. 5141–5147, 2014.
    [BibTeX] [Abstract] [Download PDF]

    Cu(In,Ga)Se (CIGS) thin films were deposited by a two-step process on Mo-coated soda-lime glass substrates. The CuInGa (CIG) precursors were prepared in an in-line evaporation system at room temperature, and then selenised at 500 ?C. The two-step processed CIGS films were mechanically compressed at 25 MPa to improve their optoelectronic properties, which were verified by photoluminescence (PL). The surface and structural properties were compared before and after compression. The mechanical compression has brought changes in the surface morphology and porosity without changing the structural properties of the material. The PL technique has been used to reveal changes in the electronic properties of the films. PL spectra at different excitation laser powers and temperatures were measured for as-grown as well as compressed samples. The PL spectra of the as-grown films revealed three broad and intense bands shifting at a significant rate towards higher energies (j-shift) with the increase in excitation power suggesting that the material is highly doped and compensated. At increasing temperature, the bands shift towards lower energies, which is a characteristic of the band tails generated by spatial potential fluctuation. The compression increases the intensity of energy bands by an order of magnitude and reduces the j-shift, demonstrating an improvement of the electronic properties. {\copyright} 2014 The Royal Society of Chemistry.

    @article{strathprints47229,
    volume = {4},
    number = {10},
    month = {January},
    author = {Z. Wei and S. Senthilarasu and M.V. Yakushev and R.W. Martin and H.M. Upadhyaya},
    title = {Effect of mechanical compression on Cu(In,Ga)Se films : micro-structural and photoluminescence analysis},
    journal = {RSC Advances},
    pages = {5141--5147},
    year = {2014},
    keywords = {mechanical compression, Cu(In,Ga)Se films, micro-structural , photoluminescence analysis, Chemical engineering, Solid state physics. Nanoscience, Chemical Engineering(all), Chemistry(all)},
    url = {http://strathprints.strath.ac.uk/47229/},
    abstract = {Cu(In,Ga)Se (CIGS) thin films were deposited by a two-step process on Mo-coated soda-lime glass substrates. The CuInGa (CIG) precursors were prepared in an in-line evaporation system at room temperature, and then selenised at 500 ?C. The two-step processed CIGS films were mechanically compressed at 25 MPa to improve their optoelectronic properties, which were verified by photoluminescence (PL). The surface and structural properties were compared before and after compression. The mechanical compression has brought changes in the surface morphology and porosity without changing the structural properties of the material. The PL technique has been used to reveal changes in the electronic properties of the films. PL spectra at different excitation laser powers and temperatures were measured for as-grown as well as compressed samples. The PL spectra of the as-grown films revealed three broad and intense bands shifting at a significant rate towards higher energies (j-shift) with the increase in excitation power suggesting that the material is highly doped and compensated. At increasing temperature, the bands shift towards lower energies, which is a characteristic of the band tails generated by spatial potential fluctuation. The compression increases the intensity of energy bands by an order of magnitude and reduces the j-shift, demonstrating an improvement of the electronic properties. {\copyright} 2014 The Royal Society of Chemistry.}
    }

  • J. Bruckbauer, P. R. Edwards, S. Sahonta, F. C-P. Massabuau, M. J. Kappers, C. J. Humphreys, R. A. Oliver, and R. W. Martin, "Cathodoluminescence hyperspectral imaging of trench-like defects in InGaN/GaN quantum well structures," Journal of Physics D: Applied Physics, vol. 47, iss. 13, p. 135107, 2014.
    [BibTeX] [Abstract] [Download PDF]

    Optoelectronic devices based on the III-nitride system exhibit remarkably good optical efficiencies despite suffering from a large density of defects. In this work we use cathodoluminescence (CL) hyperspectral imaging to study InGaN/GaN multiple quantum well (MQW) structures. Different types of trench defects with varying trench width, namely wide or narrow trenches forming closed loops and open loops, are investigated in the same hyperspectral CL measurement. A strong redshift (90 meV) and intensity increase of the MQW emission is demonstrated for regions enclosed by wide trenches, whereas those within narrower trenches only exhibit a small redshift (10 meV) and a slight reduction of intensity compared with the defect-free surrounding area. Transmission electron microscopy (TEM) showed that some trench defects consist of a raised central area, which is caused by an increase of about 40\% in the thickness of the InGaN wells. The causes of the changes in luminescences are also discussed in relation to TEM results identifying the underlying structure of the defect. Understanding these defects and their emission characteristics is important for further enhancement and development of light-emitting diodes.

    @Article{strathprints47167,
    author = {Jochen Bruckbauer and Paul R Edwards and Suman-Lata Sahonta and Fabien C-P Massabuau and Menno J Kappers and Colin J Humphreys and Rachel A Oliver and Robert W Martin},
    title = {Cathodoluminescence hyperspectral imaging of trench-like defects in {InGaN/GaN} quantum well structures},
    journal = {Journal of Physics D: Applied Physics},
    year = {2014},
    volume = {47},
    number = {13},
    pages = {135107},
    month = {March},
    abstract = {Optoelectronic devices based on the III-nitride system exhibit remarkably good optical efficiencies despite suffering from a large density of defects. In this work we use cathodoluminescence (CL) hyperspectral imaging to study InGaN/GaN multiple quantum well (MQW) structures. Different types of trench defects with varying trench width, namely wide or narrow trenches forming closed loops and open loops, are investigated in the same hyperspectral CL measurement. A strong redshift (90 meV) and intensity increase of the MQW emission is demonstrated for regions enclosed by wide trenches, whereas those within narrower trenches only exhibit a small redshift (10 meV) and a slight reduction of intensity compared with the defect-free surrounding area. Transmission electron microscopy (TEM) showed that some trench defects consist of a raised central area, which is caused by an increase of about 40\% in the thickness of the InGaN wells. The causes of the changes in luminescences are also discussed in relation to TEM results identifying the underlying structure of the defect. Understanding these defects and their emission characteristics is important for further enhancement and development of light-emitting diodes.},
    keywords = {cathodoluminescence , hyperspectral imaging, trench-like defects, nGaN/GaN , quantum well structures, Physics, Solid state physics. Nanoscience, Surfaces, Coatings and Films, Acoustics and Ultrasonics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/47167/}
    }

  • S. M. C. Miranda, P. R. Edwards, K. P. O'Donnell, M. Boćkowski, E. Alves, I. S. Roqan, A. Vantomme, and K. Lorenz, "Sequential multiple-step europium ion implantation and annealing of GaN," Physica Status Solidi C, vol. 11, iss. 2, p. 253–257, 2014.
    [BibTeX] [Abstract] [Download PDF]

    Sequential multiple Eu ion implantations at low fluence (1{$\times$}1013 cm-2 at 300 keV) and subsequent rapid thermal annealing (RTA) steps (30 s at 1000 ?C or 1100 ?C) were performed on high quality nominally undoped GaN films grown by metal organic chemical vapour deposition (MOCVD) and medium quality GaN:Mg grown by hydride vapour phase epitaxy (HVPE). Compared to samples implanted in a single step, multiple implantation/annealing shows only marginal structural improvement for the MOCVD samples, but a significant improvement of crystal quality and optical activation of Eu was achieved in the HVPE films. This improvement is attributed to the lower crystalline quality of the starting material, which probably enhances the diffusion of defects and acts to facilitate the annealing of implantation damage and the effective incorporation of the Eu ions in the crystal structure. Optical activation of Eu3+ ions in the HVPE samples was further improved by high temperature and high pressure annealing (HTHP) up to 1400 ?C. After HTHP annealing the main room temperature cathodo- and photoluminescence line in Mg-doped samples lies at {$\sim$} 619 nm, characteristic of a known Mg-related Eu3+ centre, while after RTA treatment the dominant line lies at {$\sim$} 622 nm, typical for undoped GaN:Eu.

    @Article{strathprints47160,
    author = {S. M. C. Miranda and P. R. Edwards and K. P. O'Donnell and M. Bo{\'c}kowski and E. Alves and I. S. Roqan and A. Vantomme and K. Lorenz},
    title = {Sequential multiple-step europium ion implantation and annealing of {GaN}},
    journal = {Physica Status Solidi C},
    year = {2014},
    volume = {11},
    number = {2},
    pages = {253--257},
    month = {February},
    abstract = {Sequential multiple Eu ion implantations at low fluence (1{$\times$}1013 cm-2 at 300 keV) and subsequent rapid thermal annealing (RTA) steps (30 s at 1000 ?C or 1100 ?C) were performed on high quality nominally undoped GaN films grown by metal organic chemical vapour deposition (MOCVD) and medium quality GaN:Mg grown by hydride vapour phase epitaxy (HVPE). Compared to samples implanted in a single step, multiple implantation/annealing shows only marginal structural improvement for the MOCVD samples, but a significant improvement of crystal quality and optical activation of Eu was achieved in the HVPE films. This improvement is attributed to the lower crystalline quality of the starting material, which probably enhances the diffusion of defects and acts to facilitate the annealing of implantation damage and the effective incorporation of the Eu ions in the crystal structure. Optical activation of Eu3+ ions in the HVPE samples was further improved by high temperature and high pressure annealing (HTHP) up to 1400 ?C. After HTHP annealing the main room temperature cathodo- and photoluminescence line in Mg-doped samples lies at {$\sim$} 619 nm, characteristic of a known Mg-related Eu3+ centre, while after RTA treatment the dominant line lies at {$\sim$} 622 nm, typical for undoped GaN:Eu.},
    keywords = {crystal quality, europium, gallium nitride, ion implantation, multiple-step , GaN, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/47160/}
    }

  • G. Kusch, H. Li, P. R. Edwards, J. Bruckbauer, T. C. Sadler, P. J. Parbrook, and R. W. Martin, "Influence of substrate miscut angle on surface morphology and luminescence properties of AlGaN," Applied Physics Letters, vol. 104, iss. 9, p. 92114, 2014.
    [BibTeX] [Abstract] [Download PDF]

    The influence of substrate miscut on Al0.5Ga0.5 N layers was investigated using cathodoluminescence (CL) hyperspectral imaging and secondary electron imaging in an environmental scanning electron microscope. The samples were also characterized using atomic force microscopy and high resolution X-ray diffraction. It was found that small changes in substrate miscut have a strong influence on the morphology and luminescence properties of the AlGaN layers. Two different types are resolved. For low miscut angle, a crack-free morphology consisting of randomly sized domains is observed, between which there are notable shifts in the AlGaN near band edge emission energy. For high miscut angle, a morphology with step bunches and compositional inhomogeneities along the step bunches, evidenced by an additional CL peak along the step bunches, are observed.

    @Article{strathprints47137,
    author = {Gunnar Kusch and Haoning Li and Paul R. Edwards and Jochen Bruckbauer and Thomas C. Sadler and Peter J. Parbrook and Robert W. Martin},
    title = {Influence of substrate miscut angle on surface morphology and luminescence properties of AlGaN},
    journal = {Applied Physics Letters},
    year = {2014},
    volume = {104},
    number = {9},
    pages = {092114},
    month = {March},
    abstract = {The influence of substrate miscut on Al0.5Ga0.5 N layers was investigated using cathodoluminescence (CL) hyperspectral imaging and secondary electron imaging in an environmental scanning electron microscope. The samples were also characterized using atomic force microscopy and high resolution X-ray diffraction. It was found that small changes in substrate miscut have a strong influence on the morphology and luminescence properties of the AlGaN layers. Two different types are resolved. For low miscut angle, a crack-free morphology consisting of randomly sized domains is observed, between which there are notable shifts in the AlGaN near band edge emission energy. For high miscut angle, a morphology with step bunches and compositional inhomogeneities along the step bunches, evidenced by an additional CL peak along the step bunches, are observed.},
    keywords = {cathodoluminescence, III-V semiconductors, surface morphology, X-ray diffraction, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/47137/}
    }

  • G. Naresh-Kumar, J. Bruckbauer, P. R. Edwards, S. Kraeusel, B. Hourahine, R. W. Martin, M. J. Kappers, M. A. Moram, S. Lovelock, R. A. Oliver, C. J. Humphreys, and C. Trager-Cowan, "Coincident electron channeling and cathodoluminescence studies of threading dislocations in GaN," Microscopy and Microanalysis, vol. 20, iss. 1, p. 55–60, 2014.
    [BibTeX] [Abstract] [Download PDF]

    We combine two scanning electron microscopy techniques to investigate the influence of dislocations on the light emission from nitride semiconductors. Combining electron channeling contrast imaging and cathodoluminescence imaging enables both the structural and luminescence properties of a sample to be investigated without structural damage to the sample. The electron channeling contrast image is very sensitive to distortions of the crystal lattice, resulting in individual threading dislocations appearing as spots with black?white contrast. Dislocations giving rise to nonradiative recombination are observed as black spots in the cathodoluminescence image. Comparison of the images from exactly the same micron-scale region of a sample demonstrates a one-to-one correlation between the presence of single threading dislocations and resolved dark spots in the cathodoluminescence image. In addition, we have also obtained an atomic force microscopy image from the same region of the sample, which confirms that both pure edge dislocations and those with a screw component (i.e., screw and mixed dislocations) act as nonradiative recombination centers for the Si-doped c-plane GaN thin film investigated.

    @article{strathprints46655,
    volume = {20},
    number = {1},
    month = {February},
    author = {G. Naresh-Kumar and J. Bruckbauer and P. R. Edwards and S. Kraeusel and B. Hourahine and R. W. Martin and M. J. Kappers and M. A. Moram and S. Lovelock and R. A. Oliver and C. J. Humphreys and C. Trager-Cowan},
    title = {Coincident electron channeling and cathodoluminescence studies of threading dislocations in GaN},
    journal = {Microscopy and Microanalysis},
    pages = {55--60},
    year = {2014},
    keywords = {electron channeling, cathodoluminescence studies, threading, dislocations, GaN, Instrumentation},
    url = {http://strathprints.strath.ac.uk/46655/},
    abstract = {We combine two scanning electron microscopy techniques to investigate the influence of dislocations on the light emission from nitride semiconductors. Combining electron channeling contrast imaging and cathodoluminescence imaging enables both the structural and luminescence properties of a sample to be investigated without structural damage to the sample. The electron channeling contrast image is very sensitive to distortions of the crystal lattice, resulting in individual threading dislocations appearing as spots with black?white contrast. Dislocations giving rise to nonradiative recombination are observed as black spots in the cathodoluminescence image. Comparison of the images from exactly the same micron-scale region of a sample demonstrates a one-to-one correlation between the presence of single threading dislocations and resolved dark spots in the cathodoluminescence image. In addition, we have also obtained an atomic force microscopy image from the same region of the sample, which confirms that both pure edge dislocations and those with a screw component (i.e., screw and mixed dislocations) act as nonradiative recombination centers for the Si-doped c-plane GaN thin film investigated.}
    }

  • D. McArthur, B. Hourahine, and F. Papoff, "Evaluation of E. M. fields and energy transport in metallic nanoparticles with near field excitation," Physical Science International Journal, vol. 4, iss. 4, p. 564–575, 2014.
    [BibTeX] [Abstract] [Download PDF]

    We compare two ways of calculating the optical response of metallic nanoparticles illuminated by near field dipole sources. We develop tests to determine the accuracy of the calculations of internal and scattered fields of metallic nanoparticles at the boundary of the particles and in the far field. We verify the correct transport of energy by checking that the evaluation of the energy flux agrees at the surface of the particles and in the far field. A new test is introduced to check that the surface fields fulfill Maxwell's equations allowing evaluation of the validity of the internal field. Calculations of the scattering cross section show a faster rate of convergence for the principal mode theory. We show that for metallic particles the internal field is the most significant source of error.

    @article{strathprints46439,
    volume = {4},
    number = {4},
    month = {May},
    author = {Duncan McArthur and Benjamin Hourahine and Francesco Papoff},
    title = {Evaluation of E. M. fields and energy transport in metallic nanoparticles with near field excitation},
    journal = {Physical Science International Journal},
    pages = {564--575},
    year = {2014},
    keywords = {nanoparticles, electromagnetic scattering, energy flux, nanophotonics, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/46439/},
    abstract = {We compare two ways of calculating the optical response of metallic nanoparticles illuminated by near field dipole sources. We develop tests to determine the accuracy of the calculations of internal and scattered fields of metallic nanoparticles at the boundary of the particles and in the far field. We verify the correct transport of energy by checking that the evaluation of the energy flux agrees at the surface of the particles and in the far field. A new test is introduced to check that the surface fields fulfill Maxwell's equations allowing evaluation of the validity of the internal field. Calculations of the scattering cross section show a faster rate of convergence for the principal mode theory. We show that for metallic particles the internal field is the most significant source of error.}
    }

2013

  • A. Kaminska, P. Nowakowski, G. Staszczak, T. Suski, A. Suchocki, J. F. Carlin, N. Grandjean, R. Martin, and A. Yamamoto, "Peculiarities in the pressure dependence of photoluminescence in InAlN," Physica Status Solidi B, vol. 250, iss. 4, p. 677–682, 2013.
    [BibTeX] [Abstract] [Download PDF]

    Studies of ambient-pressure and high-pressure behavior of photoluminescence (PL) for series of InxAl1-xN layers are presented. The measured evolution of PL energy (EPL) with x is characterized by a clear decrease of EPL and exhibits a strong bowing. This dependence corresponds to the predictions of ab initio calculations of the band-gap energy changes EG with x. However, values of EPL are clearly lower than EG, for 0{\ensuremath{<}}x{\ensuremath{<}}0.3. For higher x, the measured EPL follows well the calculated EG. The experimentally determined pressure coefficient of PL energy (dEPL/dp) shows a complicated behavior for alloys with different In-content. We found a strong reduction of dEPL/dp for 0{\ensuremath{<}}x{\ensuremath{<}}0.3 and a relatively constant magnitude of this coefficient for higher x. Moreover, for the lower x region, we observed dEPL/dp that can differ even by a factor two in samples with nominally very similar In-content. The general tendency in dEPL/dp evolution with x corresponds to lower values than calculated dEG/dp for alloys with non-uniform indium distribution. We propose two not necessary independent explanations of these experimental findings. First, due to non-uniform In distribution (induced, e.g. by defects or non-homogeneous strain) both EPL and dEPL/dp are reduced. Second, a similar behavior results from an involvement of the localized states, whose formation and contribution to PL can be induced by strain and/or native defects. In both hypotheses, the strain/defect density can significantly change around x{$\approx$}0.18 where InxAl1-xN layers are lattice matched to GaN template. {\copyright} 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

    @article{strathprints48001,
    volume = {250},
    number = {4},
    month = {April},
    author = {Agata Kaminska and Piotr Nowakowski and Grzegorz Staszczak and Tadeusz Suski and Andrzej Suchocki and Jean Fran{\c c}ois Carlin and Nicolas Grandjean and Robert Martin and Akio Yamamoto},
    title = {Peculiarities in the pressure dependence of photoluminescence in InAlN},
    journal = {Physica Status Solidi B},
    pages = {677--682},
    year = {2013},
    keywords = {high-pressure spectroscopy, photoluminescence, pressure coefficients, ternary nitride alloys, InAlN, Physics, Condensed Matter Physics, Electronic, Optical and Magnetic Materials},
    url = {http://strathprints.strath.ac.uk/48001/},
    abstract = {Studies of ambient-pressure and high-pressure behavior of photoluminescence (PL) for series of InxAl1-xN layers are presented. The measured evolution of PL energy (EPL) with x is characterized by a clear decrease of EPL and exhibits a strong bowing. This dependence corresponds to the predictions of ab initio calculations of the band-gap energy changes EG with x. However, values of EPL are clearly lower than EG, for 0{\ensuremath{<}}x{\ensuremath{<}}0.3. For higher x, the measured EPL follows well the calculated EG. The experimentally determined pressure coefficient of PL energy (dEPL/dp) shows a complicated behavior for alloys with different In-content. We found a strong reduction of dEPL/dp for 0{\ensuremath{<}}x{\ensuremath{<}}0.3 and a relatively constant magnitude of this coefficient for higher x. Moreover, for the lower x region, we observed dEPL/dp that can differ even by a factor two in samples with nominally very similar In-content. The general tendency in dEPL/dp evolution with x corresponds to lower values than calculated dEG/dp for alloys with non-uniform indium distribution. We propose two not necessary independent explanations of these experimental findings. First, due to non-uniform In distribution (induced, e.g. by defects or non-homogeneous strain) both EPL and dEPL/dp are reduced. Second, a similar behavior results from an involvement of the localized states, whose formation and contribution to PL can be induced by strain and/or native defects. In both hypotheses, the strain/defect density can significantly change around x{$\approx$}0.18 where InxAl1-xN layers are lattice matched to GaN template. {\copyright} 2013 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim.}
    }

  • S. Schulz, M. A. Caro, L. -T. Tan, P. J. Parbrook, R. W. Martin, and E. P. O'Reilly, "Composition-dependent band gap and band-edge bowing in AIInN : a combined theoretical and experimental study," Applied Physics Express, vol. 6, iss. 12, p. 121001, 2013.
    [BibTeX] [Abstract] [Download PDF]

    A combined experimental and theoretical study of the band gap of AllnN is presented, which confirms the breakdown of the virtual crystal approximation (VCA) for the conduction and valence band edges. Composition-dependent bowing parameters for these quantities are extracted. Additionally, composition-dependent band offsets for GaN/AllnN systems are provided. We show that local strain and built-in fields affect the band edges significantly, leading to optical polarization switching at a much lower In composition than expected from a VCA approach.

    @Article{strathprints47231,
    author = {S. Schulz and M.A. Caro and L.-T. Tan and P.J. Parbrook and R.W. Martin and E.P. O'Reilly},
    title = {Composition-dependent band gap and band-edge bowing in AIInN : a combined theoretical and experimental study},
    journal = {Applied Physics Express},
    year = {2013},
    volume = {6},
    number = {12},
    pages = {121001},
    month = {December},
    abstract = {A combined experimental and theoretical study of the band gap of AllnN is presented, which confirms the breakdown of the virtual crystal approximation (VCA) for the conduction and valence band edges. Composition-dependent bowing parameters for these quantities are extracted. Additionally, composition-dependent band offsets for GaN/AllnN systems are provided. We show that local strain and built-in fields affect the band edges significantly, leading to optical polarization switching at a much lower In composition than expected from a VCA approach.},
    keywords = {composition-dependent, band gap, band-edge, bowing, AlInN, experimental study, Physics, Physics and Astronomy(all), Engineering(all)},
    url = {http://strathprints.strath.ac.uk/47231/}
    }

  • S. V. Novikov, K. M. Yu, A. Levander, D. Detert, W. L. Sarney, Z. Liliental-Weber, M. Shaw, R. W. Martin, S. P. Svensson, W. Walukiewicz, and C. T. Foxon, "Molecular beam epitaxy of highly mismatched N-rich GaNSb and InNAs alloys," Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures, vol. 31, iss. 3, p. 03C102, 2013.
    [BibTeX] [Abstract] [Download PDF]

    GaN materials alloyed with group V anions form the so-called highly mismatched alloys (HMAs). Recently, the authors succeeded in growing N-rich GaNAs and GaNBi alloys over a large composition range by plasma-assisted molecular beam epitaxy (PA-MBE). Here, they present first results on PA-MBE growth and properties of N-rich GaNSb and InNAs alloys and compare these with GaNAs and GaNBi alloys. The enhanced incorporation of As and Sb was achieved by growing the layers at extremely low growth temperatures. Although layers become amorphous for high As, Sb, and Bi content, optical absorption measurements show a progressive shift of the optical absorption edge to lower energy. The large band gap range and controllable conduction and valence band positions of these HMAs make them promising materials for efficient solar energy conversion devices.

    @Article{strathprints47230,
    author = {S.V. Novikov and K.M. Yu and A. Levander and D. Detert and W.L. Sarney and Z. Liliental-Weber and M. Shaw and R.W. Martin and S.P. Svensson and W. Walukiewicz and C.T. Foxon},
    title = {Molecular beam epitaxy of highly mismatched N-rich GaNSb and InNAs alloys},
    journal = {Journal of Vacuum Science and Technology B: Microelectronics and Nanometer Structures},
    year = {2013},
    volume = {31},
    number = {3},
    pages = {03C102},
    month = {May},
    abstract = {GaN materials alloyed with group V anions form the so-called highly mismatched alloys (HMAs). Recently, the authors succeeded in growing N-rich GaNAs and GaNBi alloys over a large composition range by plasma-assisted molecular beam epitaxy (PA-MBE). Here, they present first results on PA-MBE growth and properties of N-rich GaNSb and InNAs alloys and compare these with GaNAs and GaNBi alloys. The enhanced incorporation of As and Sb was achieved by growing the layers at extremely low growth temperatures. Although layers become amorphous for high As, Sb, and Bi content, optical absorption measurements show a progressive shift of the optical absorption edge to lower energy. The large band gap range and controllable conduction and valence band positions of these HMAs make them promising materials for efficient solar energy conversion devices.},
    keywords = {molecular beam epitaxy, mismatched, N-rich GaNSb, InN1xAsx , alloys, Electrical engineering. Electronics Nuclear engineering, Physics, Electrical and Electronic Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/47230/}
    }

  • K. P. O'Donnell, R. W. Martin, P. R. Edwards, K. Lorenz, E. Alves, and M. Bockowski, "Temperature-dependent hysteresis of the emission spectrum of Eu-implanted, Mg-doped HVPE GaN," in The Physics of Semiconductors, T. Ihn, C. Rössler, and A. Kozikov, Eds., AIP Conference Proceedings, 2013, vol. 1566, p. 63.
    [BibTeX] [Abstract] [Download PDF]

    A red emission site (hereafter, Eu0), with its main 5D0 to 7F2 peak at 619 nm, is observed by photoluminescence (PL) spectroscopy of Eu-implanted, Mg-doped GaN, in samples annealed at high temperature and pressure (up to 1400 ?C, 1 GPa) in order to remove lattice damage. The PL spectrum is strongly temperature-hysteretic between room temperature and {$\sim$}20 K: below 30 K, photochromic switching occurs between Eu0 and the usually dominant Eu1 center; upon warming the sample, the Eu0 signal does not recover until the temperature reaches {$\sim$}150 K. Photobleaching of Eu1 takes place at low temperatures after cooling, while photo-enhancement of Eu0 takes place at high temperatures after re-warming. These observations suggest a microscopic model of charge-driven defect interconversion in p-type GaN:Eu, Mg.

    @InCollection{strathprints46604,
    author = {K. P. O'Donnell and R. W. Martin and P. R. Edwards and K. Lorenz and E. Alves and M. Bockowski},
    title = {Temperature-dependent hysteresis of the emission spectrum of Eu-implanted, Mg-doped HVPE GaN},
    booktitle = {The Physics of Semiconductors},
    publisher = {AIP Conference Proceedings},
    year = {2013},
    editor = {Thomas Ihn and Clemens R{\"o}ssler and Aleksey Kozikov},
    volume = {1566},
    pages = {63},
    month = {December},
    abstract = {A red emission site (hereafter, Eu0), with its main 5D0 to 7F2 peak at 619 nm, is observed by photoluminescence (PL) spectroscopy of Eu-implanted, Mg-doped GaN, in samples annealed at high temperature and pressure (up to 1400 ?C, 1 GPa) in order to remove lattice damage. The PL spectrum is strongly temperature-hysteretic between room temperature and {$\sim$}20 K: below 30 K, photochromic switching occurs between Eu0 and the usually dominant Eu1 center; upon warming the sample, the Eu0 signal does not recover until the temperature reaches {$\sim$}150 K. Photobleaching of Eu1 takes place at low temperatures after cooling, while photo-enhancement of Eu0 takes place at high temperatures after re-warming. These observations suggest a microscopic model of charge-driven defect interconversion in p-type GaN:Eu, Mg.},
    keywords = {temperature dependent hysteresis, emission spectrum, eu-implanted, mg-doped, hvpe gan, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/46604/}
    }

  • I. Nikiforov, B. Hourahine, B. Aradi, T. Frauenheim, and T. D. u a, "Ewald summation on a helix : a route to self-consistent charge density-functional based tight-binding objective molecular dynamics," Journal of Chemical Physics, vol. 139, p. 94110, 2013.
    [BibTeX] [Abstract] [Download PDF]

    We explore the generalization to the helical case of the classical Ewald method, the harbinger of all modern self-consistent treatments of waves in crystals, including ab initio electronic structure methods. Ewald-like formulas that do not rely on a unit cell with translational symmetry prove to be numerically tractable and able to provide the crucial component needed for coupling objective molecular dynamics with the self-consistent charge density-functional based tight-binding treatment of the inter-atomic interactions. The robustness of the method in addressing complex hetero-nuclear nano- and bio-systems is demonstrated with illustrative simulations on a helical boron nitride nanotube, a screw dislocated zinc oxide nanowire, and an ideal DNA molecule.

    @Article{strathprints44729,
    author = {Ilia Nikiforov and Benjamin Hourahine and B. Aradi and Th. Frauenheim and Traian Dumitric{\u a}},
    title = {Ewald summation on a helix : a route to self-consistent charge density-functional based tight-binding objective molecular dynamics},
    journal = {Journal of Chemical Physics},
    year = {2013},
    volume = {139},
    pages = {094110},
    month = {September},
    abstract = {We explore the generalization to the helical case of the classical Ewald method, the harbinger of all modern self-consistent treatments of waves in crystals, including ab initio electronic structure methods. Ewald-like formulas that do not rely on a unit cell with translational symmetry prove to be numerically tractable and able to provide the crucial component needed for coupling objective molecular dynamics with the self-consistent charge density-functional based tight-binding treatment of the inter-atomic interactions. The robustness of the method in addressing complex hetero-nuclear nano- and bio-systems is demonstrated with illustrative simulations on a helical boron nitride nanotube, a screw dislocated zinc oxide nanowire, and an ideal DNA molecule.},
    keywords = {helical electrostratics dispersion electronic structure, ewald summation, tight-binding objective, molecular dynamics, Physics, Physical and Theoretical Chemistry, Condensed Matter Physics, Materials Science (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/44729/}
    }

  • E. D. Le Boulbar, I. Gîrgel, C. Lewins, P. R. Edwards, R. W. Martin, A. Satka, D. W. E. Allsopp, and P. A. Shields, "Facet recovery and light emission from GaN/InGaN/GaN core-shell structures grown by metal organic vapour phase epitaxy on etched GaN nanorod arrays," Journal of Applied Physics, vol. 114, p. 94302, 2013.
    [BibTeX] [Abstract] [Download PDF]

    The use of etched nanorods from a planar template as a growth scaffold for a highly regular GaN/InGaN/GaN core-shell structure is demonstrated. The recovery of m-plane non-polar facets from etched high-aspect-ratio GaN nanorods is studied with and without the introduction of a hydrogen silsesquioxane passivation layer at the bottom of the etched nanorod arrays. This layer successfully prevented c-plane growth between the nanorods, resulting in vertical nanorod sidewalls ({$\sim$}89.8?) and a more regular height distribution than re-growth on unpassivated nanorods. The height variation on passivated nanorods is solely determined by the uniformity of nanorod diameter, which degrades with increased growth duration. Facet-dependent indium incorporation of GaN/InGaN/GaN core-shell layers regrown onto the etched nanorods is observed by high-resolution cathodoluminescence imaging. Sharp features corresponding to diffracted wave-guide modes in angle-resolved photoluminescence measurements are evidence of the uniformity of the full core-shell structure grown on ordered etched nanorods.

    @Article{strathprints44721,
    author = {Le Boulbar, E D and I G{\^i}rgel and C Lewins and P R Edwards and R W Martin and A Satka and D W E Allsopp and P A Shields},
    title = {Facet recovery and light emission from GaN/InGaN/GaN core-shell structures grown by metal organic vapour phase epitaxy on etched GaN nanorod arrays},
    journal = {Journal of Applied Physics},
    year = {2013},
    volume = {114},
    pages = {094302},
    abstract = {The use of etched nanorods from a planar template as a growth scaffold for a highly regular GaN/InGaN/GaN core-shell structure is demonstrated. The recovery of m-plane non-polar facets from etched high-aspect-ratio GaN nanorods is studied with and without the introduction of a hydrogen silsesquioxane passivation layer at the bottom of the etched nanorod arrays. This layer successfully prevented c-plane growth between the nanorods, resulting in vertical nanorod sidewalls ({$\sim$}89.8?) and a more regular height distribution than re-growth on unpassivated nanorods. The height variation on passivated nanorods is solely determined by the uniformity of nanorod diameter, which degrades with increased growth duration. Facet-dependent indium incorporation of GaN/InGaN/GaN core-shell layers regrown onto the etched nanorods is observed by high-resolution cathodoluminescence imaging. Sharp features corresponding to diffracted wave-guide modes in angle-resolved photoluminescence measurements are evidence of the uniformity of the full core-shell structure grown on ordered etched nanorods.},
    keywords = {facet recovery, light emission, GaN/InGaN/GaN , core-shell structures, metal organic vapour phase epitaxy, GaN nanorod arrays, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/44721/}
    }

  • B. Hourahine and F. Papoff, "Optical control of scattering, absorption and lineshape in nanoparticles," Optics Express, vol. 21, iss. 17, p. 20322–20333, 2013.
    [BibTeX] [Abstract] [Download PDF]

    We find exact conditions for the enhancement or suppression of internal and/or scattered fields in any smooth particle and the determination of their spatial distribution or angular momentum through the combination of simple fields. The incident fields can be generated by a single monochromatic or broad band light source, or by several sources, which may also be impurities embedded in the nanoparticle. We can design the lineshape of a particle introducing very narrow features in its spectral response.

    @article{strathprints44539,
    volume = {21},
    number = {17},
    month = {August},
    author = {Benjamin Hourahine and Francesco Papoff},
    note = {9 pages, 5 figures},
    title = {Optical control of scattering, absorption and lineshape in nanoparticles},
    year = {2013},
    journal = {Optics Express},
    pages = {20322--20333},
    keywords = {physics, optics, optical control, scattering , absorption, lineshape, nanoparticles, Physics, Atomic and Molecular Physics, and Optics},
    url = {http://strathprints.strath.ac.uk/44539/},
    abstract = {We find exact conditions for the enhancement or suppression of internal and/or scattered fields in any smooth particle and the determination of their spatial distribution or angular momentum through the combination of simple fields. The incident fields can be generated by a single monochromatic or broad band light source, or by several sources, which may also be impurities embedded in the nanoparticle. We can design the lineshape of a particle introducing very narrow features in its spectral response.}
    }

  • J. Bruckbauer, P. R. Edwards, J. Bai, T. Wang, and R. W. Martin, "Probing light emission from quantum wells within a single nanorod," Nanotechnology, vol. 24, iss. 36, p. 365704, 2013.
    [BibTeX] [Abstract] [Download PDF]

    Significant improvements in the efficiency of optoelectronic devices can result from the exploitation of nanostructures. These require optimal nanocharacterization techniques to fully understand and improve their performance. In this study we employ room temperature cathodoluminescence hyperspectral imaging to probe single GaN-based nanorods containing multiple quantum wells (MQWs) with a simultaneous combination of very high spatial and spectral resolution. We have investigated the strain state and carrier transport in the vicinity of the MQWs, demonstrating the high efficiencies resulting from reduced electric fields. Power-dependent photoluminescence spectroscopy of arrays of these nanorods confirms that their fabrication results in partial strain relaxation in the MQWs. Our technique allows us to interrogate the structures on a sufficiently small length scale to be able to extract the important information.

    @Article{strathprints44537,
    author = {Jochen Bruckbauer and Paul R Edwards and Jie Bai and Tao Wang and Robert W Martin},
    title = {Probing light emission from quantum wells within a single nanorod},
    journal = {Nanotechnology},
    year = {2013},
    volume = {24},
    number = {36},
    pages = {365704},
    month = {August},
    abstract = {Significant improvements in the efficiency of optoelectronic devices can result from the exploitation of nanostructures. These require optimal nanocharacterization techniques to fully understand and improve their performance. In this study we employ room temperature cathodoluminescence hyperspectral imaging to probe single GaN-based nanorods containing multiple quantum wells (MQWs) with a simultaneous combination of very high spatial and spectral resolution. We have investigated the strain state and carrier transport in the vicinity of the MQWs, demonstrating the high efficiencies resulting from reduced electric fields. Power-dependent photoluminescence spectroscopy of arrays of these nanorods confirms that their fabrication results in partial strain relaxation in the MQWs. Our technique allows us to interrogate the structures on a sufficiently small length scale to be able to extract the important information.},
    keywords = {probing light emission, quantum wells, single nanorod, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/44537/}
    }

  • S. Nagarajan, O. Svensk, M. Ali, G. Naresh-Kumar, C. Trager-Cowan, S. Suihkonen, M. Sopanen, and H. Lipsanen, "Stress distribution of GaN layer grown on micro-pillar patterned GaN templates," Applied Physics Letters, vol. 103, iss. 1, p. 12102, 2013.
    [BibTeX] [Abstract] [Download PDF]

    High-resolution Raman mapping of the stress distribution in an etched GaN micro-pillar template and a 5 {\ensuremath{\mu}}m thick GaN layer grown on a micro-pillar patterned GaN template is investigated. Raman mapping of the E2 (high) phonon shows differences in stress between the coalescing boundary, the top surface of the pillar region and around the GaN micro-pillar. Increased compressive stress is observed at the coalescing boundary of two adjacent GaN micro-pillars, when compared to the laterally grown GaN regions. The electron channeling contrast image reveals the reduction of threading dislocation density in the GaN layer grown on the micro-pillar patterned GaN template.

    @Article{strathprints44402,
    author = {S. Nagarajan and O. Svensk and M. Ali and G. Naresh-Kumar and Carol Trager-Cowan and S. Suihkonen and M. Sopanen and H. Lipsanen},
    title = {Stress distribution of GaN layer grown on micro-pillar patterned GaN templates},
    journal = {Applied Physics Letters},
    year = {2013},
    volume = {103},
    number = {1},
    pages = {012102},
    abstract = {High-resolution Raman mapping of the stress distribution in an etched GaN micro-pillar template and a 5 {\ensuremath{\mu}}m thick GaN layer grown on a micro-pillar patterned GaN template is investigated. Raman mapping of the E2 (high) phonon shows differences in stress between the coalescing boundary, the top surface of the pillar region and around the GaN micro-pillar. Increased compressive stress is observed at the coalescing boundary of two adjacent GaN micro-pillars, when compared to the laterally grown GaN regions. The electron channeling contrast image reveals the reduction of threading dislocation density in the GaN layer grown on the micro-pillar patterned GaN template.},
    keywords = {channelling, wide band gap semiconductors, stress analysis, semiconductor epitaxial layers, Raman spectra, phonons, III-V semiconductors, gallium compounds, dislocation density, compressive strength, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/44402/}
    }

  • Y. D. Zhuang, S. Lis, J. Bruckbauer, S. E. J. O'Kane, P. A. Shields, P. R. Edwards, J. Sarma, R. W. Martin, and D. W. E. Allsopp, "Optical properties of GaN nanorods containing a single or multiple InGaN quantum wells," Japanese Journal of Applied Physics, vol. 52, p. 08JE11, 2013.
    [BibTeX] [Abstract] [Download PDF]

    Measurements of light emission from GaN nanorods of diameter between 80 and 350 nm, containing either a three-well multiple InGaN quantum well or a single quantum well, have been performed by photoluminescence (PL) and cathodoluminescence (CL) hyperspectral imaging. The PL underwent a Stark shift to the blue as the nanorod diameter was reduced, indicating substantial relaxation of the compressive strain in the quantum wells. The intensity of the nanorod emission per unit area can exceed that of the planar starting material. The CL measurements revealed that the wavelength of the quantum well emission varied with radial position in the nanorod. Simulations by a modal expansion method revealed that the light extraction efficiency varies with radial position and the variation is dependent on nanorod diameter. Finite difference time domain simulations showed that Bloch mode formation in the buffer layer below the nanorods impacts on the light extraction.

    @Article{strathprints43862,
    author = {Yi D. Zhuang and Szymon Lis and Jochen Bruckbauer and Simon E. J. O'Kane and Philip A. Shields and Paul R. Edwards and Jayanta Sarma and Robert W. Martin and Duncan W. E. Allsopp},
    title = {Optical properties of GaN nanorods containing a single or multiple InGaN quantum wells},
    journal = {Japanese Journal of Applied Physics},
    year = {2013},
    volume = {52},
    pages = {08JE11},
    month = {May},
    abstract = {Measurements of light emission from GaN nanorods of diameter between 80 and 350 nm, containing either a three-well multiple InGaN quantum well or a single quantum well, have been performed by photoluminescence (PL) and cathodoluminescence (CL) hyperspectral imaging. The PL underwent a Stark shift to the blue as the nanorod diameter was reduced, indicating substantial relaxation of the compressive strain in the quantum wells. The intensity of the nanorod emission per unit area can exceed that of the planar starting material. The CL measurements revealed that the wavelength of the quantum well emission varied with radial position in the nanorod. Simulations by a modal expansion method revealed that the light extraction efficiency varies with radial position and the variation is dependent on nanorod diameter. Finite difference time domain simulations showed that Bloch mode formation in the buffer layer below the nanorods impacts on the light extraction.},
    keywords = {optical properties, GaN nanorods, InGaN quantum wells, photoluminescence , cathodoluminescence, Physics, Atomic and Molecular Physics, and Optics},
    url = {http://strathprints.strath.ac.uk/43862/}
    }

  • E. Taylor, F. Fang, F. Oehler, P. R. Edwards, M. J. Kappers, K. Lorenz, E. Alves, C. McAleese, C. J. Humphreys, and R. W. Martin, "Composition and luminescence studies of InGaN epilayers grown at different hydrogen flow rates," Semiconductor Science and Technology, vol. 28, iss. 6, p. 65011, 2013.
    [BibTeX] [Abstract] [Download PDF]

    Indium gallium nitride (In(x)Ga(1-x)N) is a technologically important material for many optoelectronic devices, including LEDs and solar cells, but it remains a challenge to incorporate high levels of InN into the alloy while maintaining sample quality. A series of InGaN epilayers was grown with different hydrogen flow rates (0-200 sccm) and growth temperatures (680-750 ?C) to obtain various InN fractions and bright emission in the range 390-480 nm. These 160-nm thick epilayers were characterized through several compositional techniques (wavelength dispersive x-ray spectroscopy, x-ray diffraction, Rutherford backscattering spectrometry) and cathodoluminescence hyperspectral imaging. The compositional analysis with the different techniques shows good agreement when taking into account compositional gradients evidenced in these layers. The addition of small amounts of hydrogen to the gas flow at lower growth temperatures is shown to maintain a high surface quality and luminescence homogeneity. This allowed InN fractions of up to {\texttt{\char126}}16\% to be incorporated with minimal peak energy variations over a mapped area while keeping a high material quality.

    @Article{strathprints43861,
    author = {E Taylor and F Fang and F Oehler and P R Edwards and M J Kappers and K Lorenz and E Alves and C McAleese and C J Humphreys and R W Martin},
    title = {Composition and luminescence studies of InGaN epilayers grown at different hydrogen flow rates},
    journal = {Semiconductor Science and Technology},
    year = {2013},
    volume = {28},
    number = {6},
    pages = {065011},
    month = {May},
    abstract = {Indium gallium nitride (In(x)Ga(1-x)N) is a technologically important material for many optoelectronic devices, including LEDs and solar cells, but it remains a challenge to incorporate high levels of InN into the alloy while maintaining sample quality. A series of InGaN epilayers was grown with different hydrogen flow rates (0-200 sccm) and growth temperatures (680-750 ?C) to obtain various InN fractions and bright emission in the range 390-480 nm. These 160-nm thick epilayers were characterized through several compositional techniques (wavelength dispersive x-ray spectroscopy, x-ray diffraction, Rutherford backscattering spectrometry) and cathodoluminescence hyperspectral imaging. The compositional analysis with the different techniques shows good agreement when taking into account compositional gradients evidenced in these layers. The addition of small amounts of hydrogen to the gas flow at lower growth temperatures is shown to maintain a high surface quality and luminescence homogeneity. This allowed InN fractions of up to {\texttt{\char126}}16\% to be incorporated with minimal peak energy variations over a mapped area while keeping a high material quality.},
    keywords = {composition and luminescence studies, InGaN epilayers, hydrogen flow rates, optoelectronic devices, Physics, Materials Chemistry, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/43861/}
    }

  • N. G. Kumar, C. Mauder, K. R. Wang, S. Kraeusel, J. Bruckbauer, P. R. Edwards, B. Hourahine, H. Kalisch, A. Vescan, C. Giesen, M. Heuken, A. Trampert, A. P. Day, and C. Trager-Cowan, "Electron channeling contrast imaging studies of nonpolar nitrides using a scanning electron microscope," Applied Physics Letters, vol. 102, iss. 14, p. 142103, 2013.
    [BibTeX] [Abstract] [Download PDF]

    Threading dislocations, stacking faults, and associated partial dislocations significantly degrade the optical and electrical properties of materials such as non-polar III-nitride semiconductor thin films. Stacking faults are generally difficult to detect and quantify with existing characterization techniques. We demonstrate the use of electron channeling contrast imaging in the scanning electron microscope to non-destructively reveal basal plane stacking faults terminated by partial dislocations in m-plane GaN and InGaN/GaN multiple quantum well structures grown on {\ensuremath{\gamma}}-LiAlO2 by metal organic vapor phase epitaxy.

    @Article{strathprints43763,
    author = {G. Naresh Kumar and C Mauder and K.R. Wang and Simon Kraeusel and Jochen Bruckbauer and P. R. Edwards and Benjamin Hourahine and H. Kalisch and A. Vescan and C. Giesen and M Heuken and A. Trampert and A.P. Day and Carol Trager-Cowan},
    title = {Electron channeling contrast imaging studies of nonpolar nitrides using a scanning electron microscope},
    journal = {Applied Physics Letters},
    year = {2013},
    volume = {102},
    number = {14},
    pages = {142103},
    month = {April},
    abstract = {Threading dislocations, stacking faults, and associated partial dislocations significantly degrade the optical and electrical properties of materials such as non-polar III-nitride semiconductor thin films. Stacking faults are generally difficult to detect and quantify with existing characterization techniques. We demonstrate the use of electron channeling contrast imaging in the scanning electron microscope to non-destructively reveal basal plane stacking faults terminated by partial dislocations in m-plane GaN and InGaN/GaN multiple quantum well structures grown on {\ensuremath{\gamma}}-LiAlO2 by metal organic vapor phase epitaxy.},
    keywords = {channelling, gallium compounds, indium compounds, scanning electron microscopy, wide band gap semiconductors, non-polar III-nitride semiconductor thin films, Physics, Physics and Astronomy (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/43763/}
    }

  • N. J. Findlay, C. Orofino-Pena, J. Bruckbauer, S. E. T. Elmasly, S. Arumugam, A. R. Inigo, A. L. Kanibolotsky, R. W. Martin, and P. J. Skabara, "Linear oligofluorene-BODIPY structures for fluorescence applications," Journal of Materials Chemistry. C, vol. 1, iss. 11, p. 2249–2256, 2013.
    [BibTeX] [Abstract] [Download PDF]

    A family of linear oligofluorene-BODIPY structures, containing either a ter- or quaterfluorene unit, have been prepared, in which the attachment of the oligofluorene chain to the BODIPY unit is switched between the meso-and beta-positions. Each member of this family was investigated by UV-vis absorption and photoluminescence spectroscopy, cyclic voltammetry and thermal studies (TGA and DSC) to determine their suitability as emissive layers in hybrid luminescent devices. One candidate was then successfully deployed as a down converter to convert UV to visible light.

    @article{strathprints43346,
    volume = {1},
    number = {11},
    title = {Linear oligofluorene-BODIPY structures for fluorescence applications},
    author = {Neil J. Findlay and Clara Orofino-Pena and Jochen Bruckbauer and Saadeldin E. T. Elmasly and Sasikumar Arumugam and Anto R. Inigo and Alexander L. Kanibolotsky and Robert W. Martin and Peter J. Skabara},
    year = {2013},
    pages = {2249--2256},
    journal = {Journal of Materials Chemistry. C},
    keywords = {oligofluorene-BODIPY structures, cyclic voltammetry, thermal studies, Chemistry},
    url = {http://strathprints.strath.ac.uk/43346/},
    abstract = {A family of linear oligofluorene-BODIPY structures, containing either a ter- or quaterfluorene unit, have been prepared, in which the attachment of the oligofluorene chain to the BODIPY unit is switched between the meso-and beta-positions. Each member of this family was investigated by UV-vis absorption and photoluminescence spectroscopy, cyclic voltammetry and thermal studies (TGA and DSC) to determine their suitability as emissive layers in hybrid luminescent devices. One candidate was then successfully deployed as a down converter to convert UV to visible light.}
    }

  • V. Kachkanov, I. Dobnya, K. O'Donnell, K. Lorenz, S. M. de Sousa Pereira, I. Watson, T. Sadler, H. Li, V. Zubialevich, and P. Parbrook, "Characterisation of III-nitride materials by synchrotron X-ray microdiffraction reciprocal space mapping," Physica Status Solidi C, vol. 10, iss. 3, p. 481–485, 2013.
    [BibTeX] [Abstract] [Download PDF]

    X-ray Reciprocal Space Mapping (RSM) is a powerful tool to explore the structure of semiconductor materials. However, conventional lab-based RSMs are usually measured in two dimensions (2D) ignoring the third dimension of diffraction-space volume. We report the use of a combination of X-ray microfocusing and state-of-the-art 2D area detectors to study the full volume of diffraction?space while probing III-nitride materials on the microscale.

    @Article{strathprints42828,
    author = {V. Kachkanov and Igor Dobnya and Kevin O'Donnell and Katharina Lorenz and de Sousa Pereira, Sergio Manuel and Ian Watson and Thomas Sadler and Haoning Li and Vitaly Zubialevich and Peter Parbrook},
    title = {Characterisation of III-nitride materials by synchrotron X-ray microdiffraction reciprocal space mapping},
    journal = {Physica Status Solidi C},
    year = {2013},
    volume = {10},
    number = {3},
    pages = {481--485},
    month = {March},
    abstract = {X-ray Reciprocal Space Mapping (RSM) is a powerful tool to explore the structure of semiconductor materials. However, conventional lab-based RSMs are usually measured in two dimensions (2D) ignoring the third dimension of diffraction-space volume. We report the use of a combination of X-ray microfocusing and state-of-the-art 2D area detectors to study the full volume of diffraction?space while probing III-nitride materials on the microscale.},
    keywords = {X-ray microdiffraction, eciprocal space mapping, synchrotron radiation, nitride materials, Physics, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/42828/}
    }

2012

  • K. P. K. P. O'Donnell, "Photoluminescence of Eu-doped GaN," MRS Online Proceedings Library, vol. 1342, p. 101–109, 2012.
    [BibTeX] [Abstract] [Download PDF]

    This talk reviews work on the optical properties of Eu-doped GaN at the Semiconductor Spectroscopy laboratory of the University of Strathclyde. The principal experimental technique used has been lamp-based Photoluminescence/ Excitation (PL/E) spectroscopy on samples produced mainly by high-energy ion implantation and annealing, either at low or high pressures of nitrogen, as described by Lorenz et al. [1]. These have been supplemented by samples doped in-situ either by Molecular Beam Epitaxy or Metallorganic Vapour Phase Epitaxy. Magneto-optic experiments on GaN:Eu were carried out in collaboration with the University of Bath.

    @article{strathprints47951,
    volume = {1342},
    month = {January},
    title = {Photoluminescence of Eu-doped GaN},
    author = {Kevin Peter K.P. O'Donnell},
    year = {2012},
    pages = {101--109},
    journal = {MRS Online Proceedings Library},
    keywords = {semiconductor spectroscopy, Eu-doped GaN, high energy, vapour phase epitaxy, Physics, Mechanics of Materials, Materials Science(all), Mechanical Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/47951/},
    abstract = {This talk reviews work on the optical properties of Eu-doped GaN at the Semiconductor Spectroscopy laboratory of the University of Strathclyde. The principal experimental technique used has been lamp-based Photoluminescence/ Excitation (PL/E) spectroscopy on samples produced mainly by high-energy ion implantation and annealing, either at low or high pressures of nitrogen, as described by Lorenz et al. [1]. These have been supplemented by samples doped in-situ either by Molecular Beam Epitaxy or Metallorganic Vapour Phase Epitaxy. Magneto-optic experiments on GaN:Eu were carried out in collaboration with the University of Bath.}
    }

  • C. Trager-Cowan, N. Gunasekar, B. Hourahine, P. Edwards, J. Bruckbauer, R. Martin, C. Mauder, A. Day, G. England, A. Winkelmann, P. Parbrook, and A. Wilkinson, "Applications of electron channeling contrast imaging for characterizing nitride semiconductor thin films," Microscopy and Microanalysis, vol. 18, iss. S2, p. 684–685, 2012.
    [BibTeX] [Abstract] [Download PDF]

    We are now all familiar with the bright blue, green and white LEDs that light up our electronic appliances; decorate our streets and buildings and illuminate airport runways. However, the ultimate performance of these nitride semiconductor based LEDs is limited by extended defects such as threading dislocations (TDs), partial dislocations (PDs) and stacking faults (SFs). If we want to develop LEDs to be an effective replacement for the light bulb, or have sufficient power to purify water; we need to eliminate these defects as they act as scattering centres for light and charge carriers and give rise to nonradiative recombination and to leakage currents, severely limiting device performance. The capability to rapidly detect and analyze TDs, PDs and SFs, with negligible sample preparation, represents a real step forward in the development of more efficient nitride-based semiconductor devices

    @article{strathprints44510,
    volume = {18},
    number = {S2},
    author = {Carol Trager-Cowan and Naresh Gunasekar and Benjamin Hourahine and Paul Edwards and Jochen Bruckbauer and Robert Martin and Christof Mauder and Austin Day and Gordon England and Aimo Winkelmann and Peter Parbrook and Anjus Wilkinson},
    note = {Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 ? August 2, 2012.},
    title = {Applications of electron channeling contrast imaging for characterizing nitride semiconductor thin films},
    journal = {Microscopy and Microanalysis},
    pages = {684--685},
    year = {2012},
    keywords = {semiconductor thin films , electron channeling, nanotechnology, Microbiology, Physics, Biotechnology},
    url = {http://strathprints.strath.ac.uk/44510/},
    abstract = {We are now all familiar with the bright blue, green and white LEDs that light up our electronic appliances; decorate our streets and buildings and illuminate airport runways. However, the ultimate performance of these nitride semiconductor based LEDs is limited by extended defects such as threading dislocations (TDs), partial dislocations (PDs) and stacking faults (SFs). If we want to develop LEDs to be an effective replacement for the light bulb, or have sufficient power to purify water; we need to eliminate these defects as they act as scattering centres for light and charge carriers and give rise to nonradiative recombination and to leakage currents, severely limiting device performance. The capability to rapidly detect and analyze TDs, PDs and SFs, with negligible sample preparation, represents a real step forward in the development of more efficient nitride-based semiconductor devices}
    }

  • K. O'Donnell, M. A. D. Maur, A. D. Carlo, and K. Lorenz, "It's not easy being green : strategies for all-nitrides, all-colour solid state lighting," Physica Status Solidi (RRL) - Rapid Research Letters, vol. 6, iss. 2, p. 49–52, 2012.
    [BibTeX] [Abstract] [Download PDF]

    The design strategy presently employed to obtain ?white? light from semiconductors combines the emission of an InGaN blue or UV light-emitting diode (LED) with that of one or more yellow-orange phosphors. While commercially successful, this approach achieves good colour rendering only by increasing the number and spectral range of the phosphors used; compared to the alternative of combining ?true? red, green and blue (RGB) sources, it is intrinsically inefficient. The two major roadblocks to the RGB approach are 1) the green gap in the internal quantum efficiency (IQE) of LEDs; (2) the diode droop in the efficiency of LEDs at higher current densities. The physical origin of these effects, in the case of III-nitrides, is generally thought to be a combination of Quantum Confined Stark Effect (QCSE) and Auger Effect (AE). These effects respectively reduce the electron-hole wavefunction overlap of In-rich InGaN quantum wells (QW), and provide a non-radiative shunt for electron-hole recombination, particularly at higher excitation densities. SORBET, a novel band gap engineering strategy based upon quantum well intermixing (QWIM), offers solutions to both of the roadblocks mentioned above. In this introduction to SORBET, its great potential is tested and confirmed by the results of simulations of green InGaN diodes performed using the TiberCAD device modelling suite, which calculates the macroscopic properties of real-world optoelectronic and electronic devices in a multiscale formalism. An alternative approach to the realisation of RGB GaN-based LEDs through doping of an active layer by rare earth (RE) ions will also be briefly described.

    @article{strathprints43970,
    volume = {6},
    number = {2},
    month = {February},
    author = {Kevin O'Donnell and Matthias Auf Der Maur and Aldo Di Carlo and Katharina Lorenz},
    title = {It's not easy being green : strategies for all-nitrides, all-colour solid state lighting},
    journal = {Physica Status Solidi (RRL) - Rapid Research Letters},
    pages = {49--52},
    year = {2012},
    keywords = {solid state lighting, band engineering, nitrides, Solid state physics. Nanoscience, Materials Science(all), Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/43970/},
    abstract = {The design strategy presently employed to obtain ?white? light from semiconductors combines the emission of an InGaN blue or UV light-emitting diode (LED) with that of one or more yellow-orange phosphors. While commercially successful, this approach achieves good colour rendering only by increasing the number and spectral range of the phosphors used; compared to the alternative of combining ?true? red, green and blue (RGB) sources, it is intrinsically inefficient. The two major roadblocks to the RGB approach are 1) the green gap in the internal quantum efficiency (IQE) of LEDs; (2) the diode droop in the efficiency of LEDs at higher current densities. The physical origin of these effects, in the case of III-nitrides, is generally thought to be a combination of Quantum Confined Stark Effect (QCSE) and Auger Effect (AE). These effects respectively reduce the electron-hole wavefunction overlap of In-rich InGaN quantum wells (QW), and provide a non-radiative shunt for electron-hole recombination, particularly at higher excitation densities. SORBET, a novel band gap engineering strategy based upon quantum well intermixing (QWIM), offers solutions to both of the roadblocks mentioned above. In this introduction to SORBET, its great potential is tested and confirmed by the results of simulations of green InGaN diodes performed using the TiberCAD device modelling suite, which calculates the macroscopic properties of real-world optoelectronic and electronic devices in a multiscale formalism. An alternative approach to the realisation of RGB GaN-based LEDs through doping of an active layer by rare earth (RE) ions will also be briefly described.}
    }

  • D. Massoubre, P. R. Edwards, E. Xie, E. Richardson, I. M. Watson, E. Gu, R. W. Martin, and M. D. Dawson, "Individually-addressed planar nanoscale InGaN-based light emitters," in 2012 IEEE Photonics Conference (IPC), 2012, p. 754–755.
    [BibTeX] [Abstract] [Download PDF]

    We report on a new fabrication approach to create individually-addressable InGaN-based nanoscale-LEDs. It is based on the creation by LEEBI of a spatially confined sub-micron-size charge injection path within the p-GaN of an LED structure.

    @inproceedings{strathprints43328,
    booktitle = {2012 IEEE Photonics Conference (IPC)},
    month = {September},
    title = {Individually-addressed planar nanoscale InGaN-based light emitters},
    author = {D. Massoubre and P. R. Edwards and E. Xie and E. Richardson and I. M. Watson and E. Gu and R. W. Martin and M. D. Dawson},
    year = {2012},
    pages = {754--755},
    journal = {2012 IEEE Photonics Conference (IPC)},
    keywords = {InGaN-based light emitters , nanoscale-LEDs , photonics, optics, Optics. Light},
    url = {http://strathprints.strath.ac.uk/43328/},
    abstract = {We report on a new fabrication approach to create individually-addressable InGaN-based nanoscale-LEDs. It is based on the creation by LEEBI of a spatially confined sub-micron-size charge injection path within the p-GaN of an LED structure.}
    }

  • V. V. Kachkanov, M. J. M. J. Wallace, G. V. D. G. Laan, S. S. S. S. Dhesi, S. A. S. A. Cavill, Y. Y. Fujiwara, and K. P. K. P. O'Donnell, "Induced magnetic moment of Eu³⁺ ions in GaN," Scientific Reports, vol. 2, p. 969, 2012.
    [BibTeX] [Abstract] [Download PDF]

    Magnetic semiconductors with coupled magnetic and electronic properties are of high technological and fundamental importance. Rare-earth elements can be used to introduce magnetic moments associated with the uncompensated spin of 4f-electrons into the semiconductor hosts. The luminescence produced by rare-earth doped semiconductors also attracts considerable interest due to the possibility of electrical excitation of characteristic sharp emission lines from intra 4f-shell transitions. Recently, electroluminescence of Eu-doped GaN in current-injection mode was demonstrated in p-n junction diode structures grown by organometallic vapour phase epitaxy. Unlike most other trivalent rare-earth ions, Eu3+ ions possess no magnetic moment in the ground state. Here we report the detection of an induced magnetic moment of Eu3+ ions in GaN which is associated with the 7F2 final state of 5D0{$\rightarrow$}7F2 optical transitions emitting at 622 nm. The prospect of controlling magnetic moments electrically or optically will lead to the development of novel magneto-optic devices.

    @Article{strathprints42840,
    author = {Vyacheslav V. Kachkanov and M. J. M.J. Wallace and Gerrit G. Van Der Laan and Sarnjeet S. S.S. Dhesi and Stuart A. S.A. Cavill and Yasufumi Y. Fujiwara and Kevin Peter K.P. O'Donnell},
    title = {Induced magnetic moment of Eu³⁺ ions in GaN},
    journal = {Scientific Reports},
    year = {2012},
    volume = {2},
    pages = {969},
    month = {December},
    abstract = {Magnetic semiconductors with coupled magnetic and electronic properties are of high technological and fundamental importance. Rare-earth elements can be used to introduce magnetic moments associated with the uncompensated spin of 4f-electrons into the semiconductor hosts. The luminescence produced by rare-earth doped semiconductors also attracts considerable interest due to the possibility of electrical excitation of characteristic sharp emission lines from intra 4f-shell transitions. Recently, electroluminescence of Eu-doped GaN in current-injection mode was demonstrated in p-n junction diode structures grown by organometallic vapour phase epitaxy. Unlike most other trivalent rare-earth ions, Eu3+ ions possess no magnetic moment in the ground state. Here we report the detection of an induced magnetic moment of Eu3+ ions in GaN which is associated with the 7F2 final state of 5D0{$\rightarrow$}7F2 optical transitions emitting at 622 nm. The prospect of controlling magnetic moments electrically or optically will lead to the development of novel magneto-optic devices.},
    keywords = {materials science, optical materials, applied physics, Physics, General},
    url = {http://strathprints.strath.ac.uk/42840/}
    }

  • K. K. Lorenz, S. M. C. S. M. C. Miranda, J. E. E. J. Alves, I. S. I. S. Roqan, K. P. K. P. O'Donnell, and M. ł, "High pressure annealing of Europium implanted GaN," in Proceedings of SPIE - The International Society for Optical Engineering, SPIE, 2012, vol. 8262, p. 82–87.
    [BibTeX] [Abstract] [Download PDF]

    GaN epilayers were implanted with Eu to fluences of 1{$\times$}1013 Eu/cm2 and 1{$\times$}1015 Eu/cm2. Post-implant thermal annealing was performed in ultra-high nitrogen pressures at temperatures up to 1450 ?C. For the lower fluence effective structural recovery of the crystal was observed for annealing at 1000 ?C while optical activation could be further improved at higher annealing temperatures. The higher fluence samples also reveal good optical activation; however, some residual implantation damage remains even for annealing at 1450 ?C which leads to a reduced incorporation of Eu on substitutional sites, a broadening of the Eu luminescence lines and to a strongly reduced fraction of optically active Eu ions. Possibilities for further optimization of implantation and annealing conditions are discussed.

    @incollection{strathprints42833,
    booktitle = {Proceedings of SPIE - The International Society for Optical Engineering},
    volume = {8262},
    title = {High pressure annealing of Europium implanted GaN},
    author = {Katharina K. Lorenz and S. M C S.M.C. Miranda and E. Jorge E.J. Alves and Iman S. I.S. Roqan and Kevin Peter K.P. O'Donnell and Micha{\l} X. M.X. Bo{\'c}kowski},
    publisher = {SPIE},
    year = {2012},
    pages = {82--87},
    keywords = {Europium implanted GaN , GaN epilayers , crystals, Physics, Condensed Matter Physics, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Applied Mathematics, Computer Science Applications},
    url = {http://strathprints.strath.ac.uk/42833/},
    abstract = {GaN epilayers were implanted with Eu to fluences of 1{$\times$}1013 Eu/cm2 and 1{$\times$}1015 Eu/cm2. Post-implant thermal annealing was performed in ultra-high nitrogen pressures at temperatures up to 1450 ?C. For the lower fluence effective structural recovery of the crystal was observed for annealing at 1000 ?C while optical activation could be further improved at higher annealing temperatures. The higher fluence samples also reveal good optical activation; however, some residual implantation damage remains even for annealing at 1450 ?C which leads to a reduced incorporation of Eu on substitutional sites, a broadening of the Eu luminescence lines and to a strongly reduced fraction of optically active Eu ions. Possibilities for further optimization of implantation and annealing conditions are discussed.}
    }

  • M. V. Yakushev, F. Luckert, A. V. Rodina, C. Faugeras, A. V. Karotki, A. V. Mudryi, and R. Martin, "Anisotropy of effective masses in CuInSe₂," Applied Physics Letters, vol. 101, p. 262101, 2012.
    [BibTeX] [Abstract] [Download PDF]

    Anisotropy of the valence band is experimentally demonstrated in CuInSe2, a key component of the absorber layer in one of the leading thin-film solar cell technology. By changing the orientation of applied magnetic fields with respect to the crystal lattice, we measure considerable differences in the diamagnetic shifts and effective g-factors for the A and B free excitons. The resulting free exciton reduced masses are combined with a perturbation model for non-degenerate independent excitons and theoretical dielectric constants to provide the anisotropic effective hole masses, revealing anisotropies of 5.5 (4.2) for the A (B) valence bands.

    @Article{strathprints42671,
    author = {Michael V Yakushev and Franziska Luckert and A.V. Rodina and C. Faugeras and A.V. Karotki and A.V. Mudryi and Robert Martin},
    title = {Anisotropy of effective masses in CuInSe₂},
    journal = {Applied Physics Letters},
    year = {2012},
    volume = {101},
    pages = {262101},
    month = {December},
    abstract = {Anisotropy of the valence band is experimentally demonstrated in CuInSe2, a key component of the absorber layer in one of the leading thin-film solar cell technology. By changing the orientation of applied magnetic fields with respect to the crystal lattice, we measure considerable differences in the diamagnetic shifts and effective g-factors for the A and B free excitons. The resulting free exciton reduced masses are combined with a perturbation model for non-degenerate independent excitons and theoretical dielectric constants to provide the anisotropic effective hole masses, revealing anisotropies of 5.5 (4.2) for the A (B) valence bands.},
    keywords = {anisotropy , CuInSe2, effective masses, valence band, thin-?lm solar cell technology, applied magnetic ?elds, crystal lattice, Physics, Physics and Astronomy (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/42671/}
    }

  • T. V. Kuznetsova, V. I. Grebennikov, H. Zhao, C. Derks, C. Taubitz, M. Neumann, C. Persson, M. V. Kuznetsov, I. V. Bodnar, R. W. Martin, and M. V. Yakushev, "A photoelectron spectroscopy study of the electronic structure evolution in CuInSe₂-related compounds at changing copper content," Applied Physics Letters, vol. 101, iss. 11, p. 111607, 2012.
    [BibTeX] [Abstract] [Download PDF]

    Evolution of the valence-band structure at gradually increasing copper content has been analysed by x-ray photoelectron spectroscopy (XPS) in In2Se3, CuIn5Se8, CuIn3Se5, and CuInSe2 single crystals. A comparison of these spectra with calculated total and angular-momentum resolved density-of-states (DOS) revealed the main trends of this evolution. The formation of the theoretically predicted gap between the bonding and non-bonding states has been observed in both experimental XPS spectra and theoretical DOS.

    @Article{strathprints42666,
    author = {T. V. Kuznetsova and V. I. Grebennikov and H. Zhao and C. Derks and C. Taubitz and M. Neumann and C. Persson and M. V. Kuznetsov and I. V. Bodnar and R. W. Martin and M. V. Yakushev},
    title = {A photoelectron spectroscopy study of the electronic structure evolution in CuInSe₂-related compounds at changing copper content},
    journal = {Applied Physics Letters},
    year = {2012},
    volume = {101},
    number = {11},
    pages = {111607},
    month = {September},
    abstract = {Evolution of the valence-band structure at gradually increasing copper content has been analysed by x-ray photoelectron spectroscopy (XPS) in In2Se3, CuIn5Se8, CuIn3Se5, and CuInSe2 single crystals. A comparison of these spectra with calculated total and angular-momentum resolved density-of-states (DOS) revealed the main trends of this evolution. The formation of the theoretically predicted gap between the bonding and non-bonding states has been observed in both experimental XPS spectra and theoretical DOS.},
    keywords = {photoelectron spectroscopy study, electronic structure evolution, CuInSe2-related compounds , changing copper content, CUINSE2, Physics, Physics and Astronomy (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/42666/}
    }

  • P. R. Edwards, L. K. Jagadamma, J. Bruckbauer, C. Liu, P. Shields, D. Allsopp, T. Wang, and R. W. Martin, "High-resolution cathodoluminescence hyperspectral imaging of nitride nanostructures," Microscopy and Microanalysis, vol. 18, iss. 6, p. 1212–1219, 2012.
    [BibTeX] [Abstract] [Download PDF]

    Hyperspectral cathodoluminescence imaging provides spectrally and spatially resolved information on luminescent materials within a single dataset. Pushing the technique toward its ultimate nanoscale spatial limit, while at the same time spectrally dispersing the collected light before detection, increases the challenge of generating low-noise images. This article describes aspects of the instrumentation, and in particular data treatment methods, which address this problem. The methods are demonstrated by applying them to the analysis of nanoscale defect features and fabricated nanostructures in III-nitride-based materials.

    @article{strathprints42421,
    volume = {18},
    number = {6},
    month = {December},
    author = {Paul R. Edwards and Lethy Krishnan Jagadamma and Jochen Bruckbauer and Chaowang Liu and Philip Shields and Duncan Allsopp and Tao Wang and Robert W. Martin},
    title = {High-resolution cathodoluminescence hyperspectral imaging of nitride nanostructures},
    journal = {Microscopy and Microanalysis},
    pages = {1212--1219},
    year = {2012},
    keywords = {cathodoluminescence, hyperspectral imaging, gallium nitride, principal component analysis, multivariate statistical analysis, SEM, Physics, Instrumentation},
    url = {http://strathprints.strath.ac.uk/42421/},
    abstract = {Hyperspectral cathodoluminescence imaging provides spectrally and spatially resolved information on luminescent materials within a single dataset. Pushing the technique toward its ultimate nanoscale spatial limit, while at the same time spectrally dispersing the collected light before detection, increases the challenge of generating low-noise images. This article describes aspects of the instrumentation, and in particular data treatment methods, which address this problem. The methods are demonstrated by applying them to the analysis of nanoscale defect features and fabricated nanostructures in III-nitride-based materials.}
    }

  • K. P. O'Donnell, A. D. M. Maur, D. A. Carlo, K. Lorenz, and S. Consortium, "It's not easy being green : strategies for all-nitrides, all-colour solid state lighting," Physica Status Solidi (RRL) - Rapid Research Letters, vol. 6, iss. 2, p. 49–52, 2012.
    [BibTeX] [Abstract] [Download PDF]

    The design strategy presently employed to obtain 'white' light from semiconductors combines the emission of an InGaN blue or UV light-emitting diode (LED) with that of one or more yellow-orange phosphors. While commercially successful, this approach achieves good colour rendering only by increasing the number and spectral range of the phosphors used; compared to the alternative of combining 'true' red, green and blue (RGB) sources, it is intrinsically inefficient. The two major roadblocks to the RGB approach are 1. the green gap in the internal quantum efficiency (IQE) of LEDs; 2. the diode droop in the efficiency of LEDs at higher current densities. The physical origin of these effects, in the case of III-nitrides, is generally thought to be a combination of Quantum Confined Stark Effect (QCSE) and Auger Effect (AE). These effects respectively reduce the electron-hole wave-function overlap of In-rich InGaN quantum wells (QW), and provide a non-radiative shunt for electron-hole recombination, particularly at higher excitation densities. SORBET, a novel band gap engineering strategy based upon quantum well intermixing (QWIM), offers solutions to both of the roadblocks mentioned above. In this introduction to SORBET, its great potential is tested and confirmed by the results of simulations of green InGaN diodes performed using the TiberCAD device modelling suite, which calculates the macroscopic properties of real-world optoelectronic and electronic devices in a multiscale formalism. An alternative approach to the realisation of RGB GaN-based LEDs through doping of an active layer by rare earth (RE) ions will also be briefly described.

    @Article{strathprints42405,
    author = {K. P. O'Donnell and M. Auf Der Maur and A. Di Carlo and K. Lorenz and SORBET Consortium},
    journal = {Physica Status Solidi (RRL) - Rapid Research Letters},
    title = {It's not easy being green : strategies for all-nitrides, all-colour solid state lighting},
    year = {2012},
    month = {February},
    number = {2},
    pages = {49--52},
    volume = {6},
    abstract = {The design strategy presently employed to obtain 'white' light from semiconductors combines the emission of an InGaN blue or UV light-emitting diode (LED) with that of one or more yellow-orange phosphors. While commercially successful, this approach achieves good colour rendering only by increasing the number and spectral range of the phosphors used; compared to the alternative of combining 'true' red, green and blue (RGB) sources, it is intrinsically inefficient. The two major roadblocks to the RGB approach are 1. the green gap in the internal quantum efficiency (IQE) of LEDs; 2. the diode droop in the efficiency of LEDs at higher current densities. The physical origin of these effects, in the case of III-nitrides, is generally thought to be a combination of Quantum Confined Stark Effect (QCSE) and Auger Effect (AE). These effects respectively reduce the electron-hole wave-function overlap of In-rich InGaN quantum wells (QW), and provide a non-radiative shunt for electron-hole recombination, particularly at higher excitation densities. SORBET, a novel band gap engineering strategy based upon quantum well intermixing (QWIM), offers solutions to both of the roadblocks mentioned above. In this introduction to SORBET, its great potential is tested and confirmed by the results of simulations of green InGaN diodes performed using the TiberCAD device modelling suite, which calculates the macroscopic properties of real-world optoelectronic and electronic devices in a multiscale formalism. An alternative approach to the realisation of RGB GaN-based LEDs through doping of an active layer by rare earth (RE) ions will also be briefly described.},
    keywords = {III-nitrides, light emitting diodes, bandgap engineering, solid state lighting, Physics, Materials Science(all), Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/42405/},
    }

  • F. Elfituri and B. Hourahine, "Simple models for InGaN alloys," Physica Status Solidi A, vol. 209, iss. 1, p. 79–82, 2012.
    [BibTeX] [Abstract] [Download PDF]

    The properties of InGaN alloys are important for many applications in optoelectronics, since their fundamental gap spans the visible range. Calculating properties, particularly for InN, is theoretically challenging, especially to obtain accurate values for the band gap. We have developed a semi-empirical parametrization of (In,Ga) N using the density functional based tight binding method (DFTB), where the band gaps of InN and GaN have been empirically corrected to experiment. We demonstrate the performance of this method by calculating a range of properties for the two materials, including elastic constants and carrier effective masses. There are several methods to model alloys of these material, one of the simplest being the virtual crystal approximation, which we apply to derive electronic properties over the whole composition range for InGaN.

    @Article{strathprints42202,
    author = {Fathi Elfituri and Benjamin Hourahine},
    title = {Simple models for InGaN alloys},
    journal = {Physica Status Solidi A},
    year = {2012},
    volume = {209},
    number = {1},
    pages = {79--82},
    abstract = {The properties of InGaN alloys are important for many applications in optoelectronics, since their fundamental gap spans the visible range. Calculating properties, particularly for InN, is theoretically challenging, especially to obtain accurate values for the band gap. We have developed a semi-empirical parametrization of (In,Ga) N using the density functional based tight binding method (DFTB), where the band gaps of InN and GaN have been empirically corrected to experiment. We demonstrate the performance of this method by calculating a range of properties for the two materials, including elastic constants and carrier effective masses. There are several methods to model alloys of these material, one of the simplest being the virtual crystal approximation, which we apply to derive electronic properties over the whole composition range for InGaN.},
    keywords = {InGaN alloys, optoelectronics , crystal approximation, Physics, Materials Chemistry, Surfaces, Coatings and Films, Surfaces and Interfaces, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/42202/}
    }

  • S. Kraeusel and B. Hourahine, "Global search for stable screw dislocation cores in III-N semiconductors," Physica Status Solidi A, vol. 209, iss. 1, p. 71–74, 2012.
    [BibTeX] [Abstract] [Download PDF]

    The promise of the broad range of direct band gaps of the \{Al,Ga,In\}N system is limited by the crystal quality of current material. As grown defect densities of InN, when compared with the more mature GaN, are extremely high and InN is strongly influenced by these defects. This is particularly important due to the unusual position of the charge neutrality level of InN, leading to both the well known surface charge accumulation and difficulties in p-type doping. While impurities and native defects clearly impact on the bulk carrier density in InN, the effects of threading dislocations on the electrical properties are still in dispute. Issues such as whether the dislocation line is charged or contains dangling bonds remain open. We present the results of a global search for possible dislocation core reconstructions for a range of screw dislocations in wurtzite III-N material, utilizing empirical Stillinger-Weber inter-atomic potentials. In addition we investigate a wide range of non-stoichiometric core structures.

    @article{strathprints42201,
    volume = {209},
    number = {1},
    month = {January},
    author = {Simon Kraeusel and Benjamin Hourahine},
    title = {Global search for stable screw dislocation cores in III-N semiconductors},
    journal = {Physica Status Solidi A},
    pages = {71--74},
    year = {2012},
    keywords = {semiconductors , stable screw dislocation , crystal structures, Solid state physics. Nanoscience, Materials Chemistry, Surfaces, Coatings and Films, Surfaces and Interfaces, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/42201/},
    abstract = {The promise of the broad range of direct band gaps of the \{Al,Ga,In\}N system is limited by the crystal quality of current material. As grown defect densities of InN, when compared with the more mature GaN, are extremely high and InN is strongly influenced by these defects. This is particularly important due to the unusual position of the charge neutrality level of InN, leading to both the well known surface charge accumulation and difficulties in p-type doping. While impurities and native defects clearly impact on the bulk carrier density in InN, the effects of threading dislocations on the electrical properties are still in dispute. Issues such as whether the dislocation line is charged or contains dangling bonds remain open. We present the results of a global search for possible dislocation core reconstructions for a range of screw dislocations in wurtzite III-N material, utilizing empirical Stillinger-Weber inter-atomic potentials. In addition we investigate a wide range of non-stoichiometric core structures.}
    }

  • B. Hourahine and F. Papoff, "The geometrical nature of optical resonances : from a sphere to fused dimer nanoparticles," Measurement Science and Technology, vol. 23, iss. 8, p. 84002, 2012.
    [BibTeX] [Abstract] [Download PDF]

    We study the electromagnetic response of smooth gold nanoparticles with shapes varying from a single sphere to two ellipsoids joined smoothly at their vertices. We show that the plasmonic resonance visible in the extinction and absorption cross sections shifts to longer wavelengths and eventually disappears as the mid-plane waist of the composite particle becomes narrower. This process corresponds to an increase of the numbers of internal and scattering modes that are mainly confined to the surface and coupled to the incident field. These modes strongly affect the near field, and therefore are of great importance in surface spectroscopy, but are almost undetectable in the far field.

    @Article{strathprints42181,
    author = {B. Hourahine and F. Papoff},
    title = {The geometrical nature of optical resonances : from a sphere to fused dimer nanoparticles},
    journal = {Measurement Science and Technology},
    year = {2012},
    volume = {23},
    number = {8},
    pages = {084002},
    month = {August},
    abstract = {We study the electromagnetic response of smooth gold nanoparticles with shapes varying from a single sphere to two ellipsoids joined smoothly at their vertices. We show that the plasmonic resonance visible in the extinction and absorption cross sections shifts to longer wavelengths and eventually disappears as the mid-plane waist of the composite particle becomes narrower. This process corresponds to an increase of the numbers of internal and scattering modes that are mainly confined to the surface and coupled to the incident field. These modes strongly affect the near field, and therefore are of great importance in surface spectroscopy, but are almost undetectable in the far field.},
    keywords = {geometrical nature , optical resonances , sphere , fused dimer , Physics, Instrumentation, Applied Mathematics},
    url = {http://strathprints.strath.ac.uk/42181/}
    }

  • B. Hourahine, K. Holms, and F. Papoff, "Accurate light scattering for non spherical particles from Mie-type theory," in 3rd Workshop on theory, modelling and computational methods for semiconductors (TMCSII), Bristol: IOP, 2012, vol. 367, p. 12010.
    [BibTeX] [Abstract] [Download PDF]

    We report a new approach for accurate calculation of optical cross sections and internal and scattered fields at any point in space for micro- and nanoparticles. Our approach is based on constructing the intrinsic optical modes of general smooth particles and hence optimised surface Green functions that, for any incident field, provide an a priori upper bound on the error and identify the class of incident fields with largest error.

    @InCollection{strathprints42176,
    author = {Benjamin Hourahine and Kenneth Holms and Francesco Papoff},
    title = {Accurate light scattering for non spherical particles from Mie-type theory},
    booktitle = {3rd Workshop on theory, modelling and computational methods for semiconductors (TMCSII)},
    publisher = {IOP},
    year = {2012},
    volume = {367},
    series = {Journal of Physics Conference Series},
    pages = {012010},
    address = {Bristol},
    abstract = {We report a new approach for accurate calculation of optical cross sections and internal and scattered fields at any point in space for micro- and nanoparticles. Our approach is based on constructing the intrinsic optical modes of general smooth particles and hence optimised surface Green functions that, for any incident field, provide an a priori upper bound on the error and identify the class of incident fields with largest error.},
    keywords = {accurate light scattering , non spherical particles , mie-type theory, Physics},
    url = {http://strathprints.strath.ac.uk/42176/}
    }

  • F. Luckert, M. V. Yakushev, C. Faugeras, A. V. Karotki, A. V. Mudryi, and R. W. Martin, "Excitation power and temperature dependence of excitons in CuInSe₂," Journal of Applied Physics, vol. 111, iss. 9, p. 93507, 2012.
    [BibTeX] [Abstract] [Download PDF]

    Excitonic recombination processes in high quality CuInSe2 single crystals have been studied by photoluminescence (PL) and reflectance spectroscopy as a function of excitation powers and temperature. Excitation power dependent measurements confirm the identification of well-resolved A and B free excitons in the PL spectra and analysis of the temperature quenching of these lines provides values for activation energies. These are found to vary from sample to sample, with values of 12.5 and 18.4meV for the A and B excitons, respectively, in the one showing the highest quality spectra. Analysis of the temperature and power dependent PL spectra from the bound excitonic lines, labelled M1, M2, and M3 appearing in multiplets points to a likely assignment of the hole involved in each case. The M1 excitons appear to involve a conduction band electron and a hole from the B valence band hole. In contrast, an A valence band hole appears to be involved for the M2 and M3 excitons. In addition, the M1 exciton multiplet seems to be due to the radiative recombination of excitons bound to shallow hydrogenic defects, whereas the excitons involved in M2 and M3 are bound to more complex defects. In contrast to the M1 exciton multiplet, the excitonic lines of M2 and M3 saturate at high excitation powers suggesting that the concentration of the defects involved is low. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4709448]

    @Article{strathprints40877,
    author = {F. Luckert and M. V. Yakushev and C. Faugeras and A. V. Karotki and A. V. Mudryi and R. W. Martin},
    title = {Excitation power and temperature dependence of excitons in CuInSe₂},
    journal = {Journal of Applied Physics},
    year = {2012},
    volume = {111},
    number = {9},
    pages = {093507},
    month = {May},
    abstract = {Excitonic recombination processes in high quality CuInSe2 single crystals have been studied by photoluminescence (PL) and reflectance spectroscopy as a function of excitation powers and temperature. Excitation power dependent measurements confirm the identification of well-resolved A and B free excitons in the PL spectra and analysis of the temperature quenching of these lines provides values for activation energies. These are found to vary from sample to sample, with values of 12.5 and 18.4meV for the A and B excitons, respectively, in the one showing the highest quality spectra. Analysis of the temperature and power dependent PL spectra from the bound excitonic lines, labelled M1, M2, and M3 appearing in multiplets points to a likely assignment of the hole involved in each case. The M1 excitons appear to involve a conduction band electron and a hole from the B valence band hole. In contrast, an A valence band hole appears to be involved for the M2 and M3 excitons. In addition, the M1 exciton multiplet seems to be due to the radiative recombination of excitons bound to shallow hydrogenic defects, whereas the excitons involved in M2 and M3 are bound to more complex defects. In contrast to the M1 exciton multiplet, the excitonic lines of M2 and M3 saturate at high excitation powers suggesting that the concentration of the defects involved is low. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4709448]},
    keywords = {band-edge photoluminescence, acceptors, CulnSe2, luminescence, emission, defect physics, radiative recombination, chalcopyrite semiconductor, single-crystals, excitation power , temperature dependence , excitons, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/40877/}
    }

  • S. V. Novikov, C. R. Staddon, F. Luckert, P. R. Edwards, R. W. Martin, A. J. Kent, and C. T. Foxon, "Zinc-blende and wurtzite AlₓGa₁₋ₓN bulk crystals grown by molecular beam epitaxy," Journal of Crystal Growth, vol. 350, iss. 1, p. 80–84, 2012.
    [BibTeX] [Abstract] [Download PDF]

    There is a significant difference in the lattice parameters of GaN and AlN and for many device applications AlxGa1-xN substrates would be preferable to either GaN or AlN. We have studied the growth of free-standing zinc-blende and wurtzite AlxGa1-xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE). Thick (similar to 10 mu m) zinc-blende and wurtzite AlxGa1-xN films were grown by PA-MBE on 2-in. GaAs (0 0 1) and GaAs (1 1 1)B substrates respectively and were removed from the GaAs substrate after the growth. We demonstrate that free-standing zinc-blende and wurtzite AlxGa1-xN wafers can be achieved by PA-MBE for a wide range of Al compositions. (C) 2011 Elsevier B.V. All rights reserved.

    @Article{strathprints40875,
    author = {S. V. Novikov and C. R. Staddon and F. Luckert and P. R. Edwards and R. W. Martin and A. J. Kent and C. T. Foxon},
    title = {Zinc-blende and wurtzite {AlₓGa₁₋ₓN} bulk crystals grown by molecular beam epitaxy},
    journal = {Journal of Crystal Growth},
    year = {2012},
    volume = {350},
    number = {1},
    pages = {80--84},
    month = {July},
    abstract = {There is a significant difference in the lattice parameters of GaN and AlN and for many device applications AlxGa1-xN substrates would be preferable to either GaN or AlN. We have studied the growth of free-standing zinc-blende and wurtzite AlxGa1-xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE). Thick (similar to 10 mu m) zinc-blende and wurtzite AlxGa1-xN films were grown by PA-MBE on 2-in. GaAs (0 0 1) and GaAs (1 1 1)B substrates respectively and were removed from the GaAs substrate after the growth. We demonstrate that free-standing zinc-blende and wurtzite AlxGa1-xN wafers can be achieved by PA-MBE for a wide range of Al compositions. (C) 2011 Elsevier B.V. All rights reserved.},
    keywords = {zinc-blende , AlxGa1-xN, wurtzite , bulk crystals , molecular beam epitaxy , substrates , semiconducting III-V materials, nitrides, Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/40875/}
    }

  • P. Vennegues, B. S. Diaby, H. Kim-Chauveau, L. Bodiou, H. P. D. Schenk, E. Frayssinet, R. W. Martin, and I. M. Watson, "Nature and origin of V-defects present in metalorganic vapor phase epitaxy-grown (InₓAl₁₋ₓ)N layers as a function of InN content, layer thickness and growth parameters," Journal of Crystal Growth, vol. 353, iss. 1, p. 108–114, 2012.
    [BibTeX] [Abstract] [Download PDF]

    Our study of samples grown in different metalorganic chemical vapor deposition reactors and with different growth conditions reveals that V-pits are always present in (InxAl1-x)N films whatever the layer thickness and the InN content. V-pits are empty inverted pyramids terminating threading dislocations. InN-rich triangular regions are present around the threading dislocations terminated by pits with a hexagonal 6-fold symmetry distribution in \{11 - 20\} planes. The nature of the facets of the V-pits depends on the growth conditions: pits with either \{11 - 2l\}, I being between 1 and 3, or \{1 - 101\} facets have been observed. Moreover, the nature of the threading dislocations terminated by pits also depends on the growth conditions. Our observations suggest that with a high V/III ratio only edge a + c-type dislocations are terminated by pits whereas with a low V/III ratio both edge a-type and mixed a + c-type dislocations are terminated by pits.

    @Article{strathprints40872,
    author = {P. Vennegues and B. S. Diaby and H. Kim-Chauveau and L. Bodiou and H. P. D. Schenk and E. Frayssinet and R. W. Martin and I. M. Watson},
    title = {Nature and origin of V-defects present in metalorganic vapor phase epitaxy-grown (InₓAl₁₋ₓ)N layers as a function of InN content, layer thickness and growth parameters},
    journal = {Journal of Crystal Growth},
    year = {2012},
    volume = {353},
    number = {1},
    pages = {108--114},
    month = {August},
    abstract = {Our study of samples grown in different metalorganic chemical vapor deposition reactors and with different growth conditions reveals that V-pits are always present in (InxAl1-x)N films whatever the layer thickness and the InN content. V-pits are empty inverted pyramids terminating threading dislocations. InN-rich triangular regions are present around the threading dislocations terminated by pits with a hexagonal 6-fold symmetry distribution in \{11 - 20\} planes. The nature of the facets of the V-pits depends on the growth conditions: pits with either \{11 - 2l\}, I being between 1 and 3, or \{1 - 101\} facets have been observed. Moreover, the nature of the threading dislocations terminated by pits also depends on the growth conditions. Our observations suggest that with a high V/III ratio only edge a + c-type dislocations are terminated by pits whereas with a low V/III ratio both edge a-type and mixed a + c-type dislocations are terminated by pits.},
    keywords = {epitaxy-grown , nature and origin , V-defects , metalorganic vapor phase , layers, growth parameters , layer thickness , function of InN content, metalorganic chemical vapor deposition, semiconducting indium compounds, nitrides, crystal morphology, defects, Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/40872/}
    }

  • K. J. Lethy, P. R. Edwards, C. Liu, W. N. Wang, and R. W. Martin, "Cross-sectional and plan-view cathodoluminescence of GaN partially coalesced above a nanocolumn array," Journal of Applied Physics, vol. 112, iss. 2, p. 23507, 2012.
    [BibTeX] [Abstract] [Download PDF]

    The optical properties of GaN layers coalesced above an array of nanocolumns have important consequences for advanced optoelectronic devices. GaN nanocolumns coalesced using a nanoscale epitaxial overgrowth technique have been investigated by high resolution cathodoluminescence (CL) hyperspectral imaging. Plan-view microscopy reveals partially coalesced GaN layers with a sub-{\ensuremath{\mu}}m scale domain structure and distinct grain boundaries, which is mapped using CL spectroscopy showing high strain at the grain boundaries. Cross-sectional areas spanning the partially coalesced GaN and underlying nanocolumns are mapped using CL, revealing that the GaN bandedge peak shifts by about 25 meV across the partially coalesced layer of {$\sim$}2 {\ensuremath{\mu}}m thick. The GaN above the nanocolumns remains under tensile strain, probably due to Si out-diffusion from the mask or substrate. The cross-sectional data show how this strain is reduced towards the surface of the partially coalesced layer, possibly due to misalignment between adjacent partially coalesced regions. {\copyright} 2012 American Institute of Physics

    @Article{strathprints40509,
    author = {K. J. Lethy and P. R. Edwards and C. Liu and W. N. Wang and R. W. Martin},
    title = {Cross-sectional and plan-view cathodoluminescence of GaN partially coalesced above a nanocolumn array},
    journal = {Journal of Applied Physics},
    year = {2012},
    volume = {112},
    number = {2},
    pages = {023507},
    month = {July},
    abstract = {The optical properties of GaN layers coalesced above an array of nanocolumns have important consequences for advanced optoelectronic devices. GaN nanocolumns coalesced using a nanoscale epitaxial overgrowth technique have been investigated by high resolution cathodoluminescence (CL) hyperspectral imaging. Plan-view microscopy reveals partially coalesced GaN layers with a sub-{\ensuremath{\mu}}m scale domain structure and distinct grain boundaries, which is mapped using CL spectroscopy showing high strain at the grain boundaries. Cross-sectional areas spanning the partially coalesced GaN and underlying nanocolumns are mapped using CL, revealing that the GaN bandedge peak shifts by about 25 meV across the partially coalesced layer of {$\sim$}2 {\ensuremath{\mu}}m thick. The GaN above the nanocolumns remains under tensile strain, probably due to Si out-diffusion from the mask or substrate. The cross-sectional data show how this strain is reduced towards the surface of the partially coalesced layer, possibly due to misalignment between adjacent partially coalesced regions. {\copyright} 2012 American Institute of Physics},
    keywords = {cathodoluminescence, nanostructured materials, tensile strength, semiconductor growth, gallium compounds, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/40509/}
    }

  • E. Y. Xie, Z. Z. Chen, P. R. Edwards, Z. Gong, N. Y. Liu, Y. B. Tao, Y. F. Zhang, Y. J. Chen, I. M. Watson, E. Gu, R. W. Martin, G. Y. Zhang, and M. D. Dawson, "Strain relaxation in InGaN/GaN micro-pillars evidenced by high resolution cathodoluminescence hyperspectral imaging," Journal of Applied Physics, vol. 112, iss. 1, p. 13107, 2012.
    [BibTeX] [Abstract] [Download PDF]

    A size-dependent strain relaxation and its effects on the optical properties of InGaN/GaN multiple quantum wells (QWs) in micro-pillars have been investigated through a combination of high spatial resolution cathodoluminescence (CL) hyperspectral imaging and numerical modeling. The pillars have diameters (d) ranging from 2 to 150 {\ensuremath{\mu}}m and were fabricated from a III-nitride light-emitting diode (LED) structure optimized for yellow-green emission at {$\sim$}560 nm. The CL mapping enables us to investigate strain relaxation in these pillars on a sub-micron scale and to confirm for the first time that a narrow ({$\leq$}2 {\ensuremath{\mu}}m) edge blue-shift occurs even for the large InGaN/GaN pillars (d {\ensuremath{>}} 10 {\ensuremath{\mu}}m). The observed maximum blue-shift at the pillar edge exceeds 7 nm with respect to the pillar centre for the pillars with diameters in the 2?16 {\ensuremath{\mu}}m range. For the smallest pillar (d = 2 {\ensuremath{\mu}}m), the total blue-shift at the edge is 17.5 nm including an 8.2 nm ?global? blue-shift at the pillar centre in comparison with the unetched wafer. By using a finite element method with a boundary condition taking account of a strained GaN buffer layer which was neglected in previous simulation works, the strain distribution in the QWs of these pillars was simulated as a function of pillar diameter. The blue-shift in the QWs emission wavelength was then calculated from the strain-dependent changes in piezoelectric field, and the consequent modification of transition energy in the QWs. The simulation and experimental results agree well, confirming the necessity for considering the strained buffer layer in the strain simulation. These results provide not only significant insights into the mechanism of strain relaxation in these micro-pillars but also practical guidance for design of micro/nano LEDs.

    @Article{strathprints40400,
    author = {E. Y. Xie and Z. Z. Chen and P. R. Edwards and Z. Gong and N. Y. Liu and Y. B. Tao and Y. F. Zhang and Y. J. Chen and I. M. Watson and E. Gu and R. W. Martin and G. Y. Zhang and M. D. Dawson},
    title = {Strain relaxation in InGaN/GaN micro-pillars evidenced by high resolution cathodoluminescence hyperspectral imaging},
    journal = {Journal of Applied Physics},
    year = {2012},
    volume = {112},
    number = {1},
    pages = {013107},
    month = {July},
    abstract = {A size-dependent strain relaxation and its effects on the optical properties of InGaN/GaN multiple quantum wells (QWs) in micro-pillars have been investigated through a combination of high spatial resolution cathodoluminescence (CL) hyperspectral imaging and numerical modeling. The pillars have diameters (d) ranging from 2 to 150 {\ensuremath{\mu}}m and were fabricated from a III-nitride light-emitting diode (LED) structure optimized for yellow-green emission at {$\sim$}560 nm. The CL mapping enables us to investigate strain relaxation in these pillars on a sub-micron scale and to confirm for the first time that a narrow ({$\leq$}2 {\ensuremath{\mu}}m) edge blue-shift occurs even for the large InGaN/GaN pillars (d {\ensuremath{>}} 10 {\ensuremath{\mu}}m). The observed maximum blue-shift at the pillar edge exceeds 7 nm with respect to the pillar centre for the pillars with diameters in the 2?16 {\ensuremath{\mu}}m range. For the smallest pillar (d = 2 {\ensuremath{\mu}}m), the total blue-shift at the edge is 17.5 nm including an 8.2 nm ?global? blue-shift at the pillar centre in comparison with the unetched wafer. By using a finite element method with a boundary condition taking account of a strained GaN buffer layer which was neglected in previous simulation works, the strain distribution in the QWs of these pillars was simulated as a function of pillar diameter. The blue-shift in the QWs emission wavelength was then calculated from the strain-dependent changes in piezoelectric field, and the consequent modification of transition energy in the QWs. The simulation and experimental results agree well, confirming the necessity for considering the strained buffer layer in the strain simulation. These results provide not only significant insights into the mechanism of strain relaxation in these micro-pillars but also practical guidance for design of micro/nano LEDs.},
    keywords = {cathodoluminescence, light emitting diodes, spectral line shift, quantum well devices, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/40400/}
    }

  • K. J. Lethy, P. R. Edwards, C. Liu, P. A. Shields, D. W. E. Allsopp, and R. W. Martin, "Cathodoluminescence studies of GaN coalesced from nanopyramids selectively grown by MOVPE," Semiconductor Science and Technology, vol. 27, iss. 8, p. 85010, 2012.
    [BibTeX] [Abstract] [Download PDF]

    Coalescence of GaN over arrays of GaN nanopyramids has important device applications and has been achieved on nano-imprint lithographically patterned GaN/sapphire substrates using metal organic vapour phase epitaxy. Spatially and spectrally resolved cathdoluminescence (CL) from such coalesced layers are studied in detail. The observed redshift of the GaN band edge emission with increasing electron beam depth of maximum CL into the coalesced layer is discussed in relation to a carrier-induced peak shift, likely due to Si out-diffusion from the mask material into the GaN. Depth-resolved CL measurements are used to quantify the redshift in terms of bandgap renormalization and strain effects. CL maps showing the GaN near band edge peak energy distribution reveal micron-scale domain-like variations in peak energy and are attributed to the effects of local strain.

    @Article{strathprints40398,
    author = {K J Lethy and P R Edwards and C Liu and P A Shields and D W E Allsopp and R W Martin},
    title = {Cathodoluminescence studies of GaN coalesced from nanopyramids selectively grown by MOVPE},
    journal = {Semiconductor Science and Technology},
    year = {2012},
    volume = {27},
    number = {8},
    pages = {085010},
    month = {June},
    abstract = {Coalescence of GaN over arrays of GaN nanopyramids has important device applications and has been achieved on nano-imprint lithographically patterned GaN/sapphire substrates using metal organic vapour phase epitaxy. Spatially and spectrally resolved cathdoluminescence (CL) from such coalesced layers are studied in detail. The observed redshift of the GaN band edge emission with increasing electron beam depth of maximum CL into the coalesced layer is discussed in relation to a carrier-induced peak shift, likely due to Si out-diffusion from the mask material into the GaN. Depth-resolved CL measurements are used to quantify the redshift in terms of bandgap renormalization and strain effects. CL maps showing the GaN near band edge peak energy distribution reveal micron-scale domain-like variations in peak energy and are attributed to the effects of local strain.},
    keywords = {cathodoluminescence, ionoluminescence , nanoparticles, nanolithography, Physics, Materials Chemistry, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/40398/}
    }

  • V. Kachkanov, I. P. Dolbnya, K. P. O'Donnell, K. Lorenz, S. Pereira, R. W. Martin, P. R. Edwards, and I. M. Watson, "Characterization of InGaN and InAlN epilayers by microdiffraction X-Ray reciprocal space mapping," MRS Online Proceedings Library, vol. 1396, p. mrsf11-1396-o06-11, 2012.
    [BibTeX] [Abstract] [Download PDF]

    We report a study of InGaN and InAlN epilayers grown on GaN/Sapphire substrates by microfocused three-dimensional X-ray Reciprocal Space Mapping (RSM). The analysis of the full volume of reciprocal space, while probing samples on the microscale with a focused X-ray beam, allows us to gain uniquely valuable information about the microstructure of III-N alloy epilayers. It is found that ?seed? InGaN mosaic nanocrystallites are twisted with respect to the ensemble average and strain free. This indicates that the growth of InGaN epilayers follows the Volmer-Weber mechanism with nucleation of ?seeds? on strain fields generated by the a-type dislocations which are responsible for the twist of underlying GaN mosaic blocks. In the case of InAlN epilayer formation of composition gradient was observed at the beginning of the epitaxial growth.

    @Article{strathprints40397,
    author = {V. Kachkanov and I. P. Dolbnya and K. P. O'Donnell and K. Lorenz and S. Pereira and R. W. Martin and P. R. Edwards and I. M. Watson},
    title = {Characterization of InGaN and InAlN epilayers by microdiffraction X-Ray reciprocal space mapping},
    journal = {MRS Online Proceedings Library},
    year = {2012},
    volume = {1396},
    pages = {mrsf11-1396-o06-11},
    month = {February},
    abstract = {We report a study of InGaN and InAlN epilayers grown on GaN/Sapphire substrates by microfocused three-dimensional X-ray Reciprocal Space Mapping (RSM). The analysis of the full volume of reciprocal space, while probing samples on the microscale with a focused X-ray beam, allows us to gain uniquely valuable information about the microstructure of III-N alloy epilayers. It is found that ?seed? InGaN mosaic nanocrystallites are twisted with respect to the ensemble average and strain free. This indicates that the growth of InGaN epilayers follows the Volmer-Weber mechanism with nucleation of ?seeds? on strain fields generated by the a-type dislocations which are responsible for the twist of underlying GaN mosaic blocks. In the case of InAlN epilayer formation of composition gradient was observed at the beginning of the epitaxial growth.},
    keywords = {microstructure, nitride, x-ray diffraction (XRD), Physics, Mechanics of Materials, Materials Science(all), Mechanical Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/40397/}
    }

  • E. Taylor, P. Edwards, and R. Martin, "Colorimetry and efficiency of white LEDs : Spectral width dependence," Physica Status Solidi A: Applications and Materials Science, vol. 209, iss. 3, p. 461–464, 2012.
    [BibTeX] [Abstract] [Download PDF]

    The potential colour rendering capability and efficiency of white LEDs constructed by a combination of individual red, green and blue (RGB) LEDs are analysed. The conventional measurement of colour rendering quality, the colour rendering index (CRI), is used as well as a recently proposed colour quality scale (CQS), designed to overcome some of the limitations of CRI when narrow-band emitters are being studied. The colour rendering performance is maximised by variation of the peak emission wavelength and relative intensity of the component LEDs, with the constraint that the spectral widths follow those measured in actual devices. The highest CRI achieved is 89.5, corresponding to a CQS value of 79, colour temperature of 3800 K and a luminous efficacy of radiation (LER) of 365 lm/W. By allowing the spectral width of the green LED to vary the CRI can be raised to 90.9, giving values of 82.5 and 370 lm/W for the CQS and LER, respectively. The significance of these values are discussed in terms of optimising the possible performance of RGB LEDs.

    @article{strathprints39520,
    volume = {209},
    number = {3},
    month = {February},
    author = {Elaine Taylor and Paul Edwards and Robert Martin},
    title = {Colorimetry and efficiency of white LEDs : Spectral width dependence},
    journal = {Physica Status Solidi A: Applications and Materials Science},
    pages = {461--464},
    year = {2012},
    keywords = {spectral width, colorimetry, GaN, LED, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/39520/},
    abstract = {The potential colour rendering capability and efficiency of white LEDs constructed by a combination of individual red, green and blue (RGB) LEDs are analysed. The conventional measurement of colour rendering quality, the colour rendering index (CRI), is used as well as a recently proposed colour quality scale (CQS), designed to overcome some of the limitations of CRI when narrow-band emitters are being studied. The colour rendering performance is maximised by variation of the peak emission wavelength and relative intensity of the component LEDs, with the constraint that the spectral widths follow those measured in actual devices. The highest CRI achieved is 89.5, corresponding to a CQS value of 79, colour temperature of 3800 K and a luminous efficacy of radiation (LER) of 365 lm/W. By allowing the spectral width of the green LED to vary the CRI can be raised to 90.9, giving values of 82.5 and 370 lm/W for the CQS and LER, respectively. The significance of these values are discussed in terms of optimising the possible performance of RGB LEDs.}
    }

  • S. V. Novikov, K. M. Yu, A. X. Levander, Z. Liliental-Weber, R. dos Reis, A. J. Kent, A. Tseng, O. D. Dubon, J. Wu, J. Denlinger, W. Walukiewicz, F. Luckert, P. Edwards, R. Martin, and C. T. Foxon, "Molecular beam epitaxy of GaN₁₋ₓBiₓ alloys with high bismuth content," Physica Status Solidi A: Applications and Materials Science, vol. 209, iss. 3, p. 419–423, 2012.
    [BibTeX] [Abstract] [Download PDF]

    We have analysed bismuth incorporation into GaN layers using plasma-assisted molecular beam epitaxy (PA-MBE) at extremely low growth temperatures of less than {$\sim$}100 ?C under both Ga-rich and N-rich growth conditions. The formation of amorphous GaN1?xBix alloys is promoted by growth under Ga-rich conditions. The amorphous matrix has a short-range order resembling random crystalline GaN1?xBix alloys. We have observed the formation of small crystalline clusters embedded into amorphous GaN1?xBix alloys. Despite the fact that the films are pseudo-amorphous we observe a well defined optical absorption edges that rapidly shift to very low energy of {$\sim$}1 eV.

    @Article{strathprints39519,
    author = {S.V. Novikov and K.M. Yu and A.X. Levander and Z. Liliental-Weber and R. dos Reis and A.J. Kent and A. Tseng and O.D. Dubon and J. Wu and J. Denlinger and W. Walukiewicz and Franziska Luckert and Paul Edwards and Robert Martin and C.T. Foxon},
    title = {Molecular beam epitaxy of GaN₁₋ₓBiₓ alloys with high bismuth content},
    journal = {Physica Status Solidi A: Applications and Materials Science},
    year = {2012},
    volume = {209},
    number = {3},
    pages = {419--423},
    month = {January},
    abstract = {We have analysed bismuth incorporation into GaN layers using plasma-assisted molecular beam epitaxy (PA-MBE) at extremely low growth temperatures of less than {$\sim$}100 ?C under both Ga-rich and N-rich growth conditions. The formation of amorphous GaN1?xBix alloys is promoted by growth under Ga-rich conditions. The amorphous matrix has a short-range order resembling random crystalline GaN1?xBix alloys. We have observed the formation of small crystalline clusters embedded into amorphous GaN1?xBix alloys. Despite the fact that the films are pseudo-amorphous we observe a well defined optical absorption edges that rapidly shift to very low energy of {$\sim$}1 eV.},
    keywords = {semiconducting III?V materials, molecular beam epitaxy, nitrides, Physics, Electronic, Optical and Magnetic Materials, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/39519/}
    }

  • N. Gunasekar, B. Hourahine, P. Edwards, A. P. Day, A. Winkelmann, A. J. Wilkinson, P. J. Parbrook, G. England, and C. Trager-Cowan, "Rapid nondestructive analysis of threading dislocations in wurtzite materials using the scanning electron microscope," Physical Review Letters, vol. 108, iss. 13, p. 135503, 2012.
    [BibTeX] [Abstract] [Download PDF]

    We describe the use of electron channeling contrast imaging in the scanning electron microscope to rapidly and reliably image and identify threading dislocations (TDs) in materials with the wurtzite crystal structure. In electron channeling contrast imaging, vertical TDs are revealed as spots with black-white contrast. We have developed a simple geometric procedure which exploits the differences observed in the direction of this black-white contrast for screw, edge, and mixed dislocations for two electron channeling contrast images acquired from two symmetrically equivalent crystal planes whose g vectors are at 120? to each other. Our approach allows unambiguous identification of all TDs without the need to compare results with dynamical simulations of channeling contrast.

    @Article{strathprints39229,
    author = {Naresh Gunasekar and Benjamin Hourahine and Paul Edwards and A.P. Day and Aimo Winkelmann and A.J. Wilkinson and P.J. Parbrook and G. England and Carol Trager-Cowan},
    title = {Rapid nondestructive analysis of threading dislocations in wurtzite materials using the scanning electron microscope},
    journal = {Physical Review Letters},
    year = {2012},
    volume = {108},
    number = {13},
    pages = {135503},
    month = {March},
    abstract = {We describe the use of electron channeling contrast imaging in the scanning electron microscope to rapidly and reliably image and identify threading dislocations (TDs) in materials with the wurtzite crystal structure. In electron channeling contrast imaging, vertical TDs are revealed as spots with black-white contrast. We have developed a simple geometric procedure which exploits the differences observed in the direction of this black-white contrast for screw, edge, and mixed dislocations for two electron channeling contrast images acquired from two symmetrically equivalent crystal planes whose g vectors are at 120? to each other. Our approach allows unambiguous identification of all TDs without the need to compare results with dynamical simulations of channeling contrast.},
    keywords = {wurtzite , nondestructive analysis , scanning electron microscope, Solid state physics. Nanoscience, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/39229/}
    }

  • C. Rauch, F. Tuomisto, A. Vilalta-Clemente, B. Lacroix, P. Ruterana, S. Kraeusel, B. Hourahine, and W. J. Schaff, "Defect evolution and interplay in n-type InN," Applied Physics Letters, vol. 100, iss. 9, p. 91907, 2012.
    [BibTeX] [Abstract] [Download PDF]

    The nature and interplay of intrinsic point and extended defects in n-type Si-doped InN epilayers with free carrier concentrations up to 6.6{$\times$}1020 cm?3 are studied using positron annihilation spectroscopy and transmission electron microscopy and compared to results from undoped irradiated films. In as-grown Si-doped samples, mixed In-N vacancy complexes (VIn-VN) are the dominant III-sublattice related vacancy defects. An increase in the number of VN in these complexes toward the interface suggests high concentrations of additional isolated VN and VN-clusters near the GaN buffer layer and coincides with elevated dislocation densities in that area.

    @Article{strathprints38080,
    author = {Christian Rauch and Filip Tuomisto and Arantxa Vilalta-Clemente and Bertrand Lacroix and Pierre Ruterana and Simon Kraeusel and Benjamin Hourahine and William J. Schaff},
    title = {Defect evolution and interplay in n-type InN},
    journal = {Applied Physics Letters},
    year = {2012},
    volume = {100},
    number = {9},
    pages = {091907},
    month = {February},
    abstract = {The nature and interplay of intrinsic point and extended defects in n-type Si-doped InN epilayers with free carrier concentrations up to 6.6{$\times$}1020 cm?3 are studied using positron annihilation spectroscopy and transmission electron microscopy and compared to results from undoped irradiated films. In as-grown Si-doped samples, mixed In-N vacancy complexes (VIn-VN) are the dominant III-sublattice related vacancy defects. An increase in the number of VN in these complexes toward the interface suggests high concentrations of additional isolated VN and VN-clusters near the GaN buffer layer and coincides with elevated dislocation densities in that area.},
    keywords = {defect evolution, n-type InN , dislocation densities, Solid state physics. Nanoscience, Physics and Astronomy (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/38080/}
    }

  • G. Naresh-Kumar, B. Hourahine, A. Vilalta-Clemente, P. Ruterana, P. Gamarra, C. Lacam, M. Tordjman, M. A. di Forte-Poisson, P. J. Parbrook, A. P. Day, G. England, and C. Trager-Cowan, "Imaging and identifying defects in nitride semiconductor thin films using a scanning electron microscope," Physica Status Solidi A, vol. 209, iss. 3, p. 424–426, 2012.
    [BibTeX] [Abstract] [Download PDF]

    We describe the use of electron channelling contrast imaging (ECCI) ? in a field emission scanning electron microscope ? to reveal and identify defects in nitride semiconductor thin films. In ECCI changes in crystallographic orientation, or changes in lattice constant due to local strain, are revealed by changes in grey scale in an image constructed by monitoring the intensity of backscattered electrons as an electron beam is scanned over a suitably oriented sample. Extremely small orientation changes are detectable, enabling small angle tilt and rotation boundaries and dislocations to be imaged. Images with a resolution of tens of nanometres are obtainable with ECCI. In this paper we describe the use of ECCI with TEM to determine threading dislocation densities and types in InAlN/GaN heterostructures grown on SiC and sapphire substrates.

    @article{strathprints35178,
    volume = {209},
    number = {3},
    month = {March},
    author = {G. Naresh-Kumar and Benjamin Hourahine and A. Vilalta-Clemente and P. Ruterana and P. Gamarra and C. Lacam and M. Tordjman and M. A. di Forte-Poisson and P. J. Parbrook and A. P. Day and G. England and Carol Trager-Cowan},
    title = {Imaging and identifying defects in nitride semiconductor thin films using a scanning electron microscope},
    journal = {Physica Status Solidi A},
    pages = {424--426},
    year = {2012},
    keywords = {dislocations, electron channelling , SEM, Solid state physics. Nanoscience, Materials Chemistry, Surfaces, Coatings and Films, Surfaces and Interfaces, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/35178/},
    abstract = {We describe the use of electron channelling contrast imaging (ECCI) ? in a field emission scanning electron microscope ? to reveal and identify defects in nitride semiconductor thin films. In ECCI changes in crystallographic orientation, or changes in lattice constant due to local strain, are revealed by changes in grey scale in an image constructed by monitoring the intensity of backscattered electrons as an electron beam is scanned over a suitably oriented sample. Extremely small orientation changes are detectable, enabling small angle tilt and rotation boundaries and dislocations to be imaged. Images with a resolution of tens of nanometres are obtainable with ECCI. In this paper we describe the use of ECCI with TEM to determine threading dislocation densities and types in InAlN/GaN heterostructures grown on SiC and sapphire substrates.}
    }

2011

  • R. E. L. Powell, S. V. Novikov, F. Luckert, P. R. Edwards, A. V. Akimov, C. T. Foxon, R. W. Martin, and A. J. Kent, "Carrier localization and related photoluminescence in cubic AlGaN epilayers," Journal of Applied Physics, vol. 110, iss. 6, p. 63517, 2011.
    [BibTeX] [Abstract] [Download PDF]

    The steady state and time-resolved photoluminescence (PL) spectra of cubic AlxGa1-xN have been measured for 0 {\ensuremath{<}} x {\ensuremath{<}} 1. The intensity of the room temperature PL increases by an order of magnitude when the AlN content increases from x = 0 to x = 0.95. Additionally, the PL decay slows down with the decrease of temperature and increase of x. These results show that strong localization of carriers on alloy composition fluctuations plays a large role in determining the intensity and temporal evolution of the PL. The activation energy for the localized carriers increases with the increase of x and reaches the value of 55 meV at x = 0.95.

    @Article{strathprints44735,
    author = {R. E. L. Powell and S. V. Novikov and F. Luckert and P. R. Edwards and A. V. Akimov and C. T. Foxon and R. W. Martin and A. J. Kent},
    title = {Carrier localization and related photoluminescence in cubic AlGaN epilayers},
    journal = {Journal of Applied Physics},
    year = {2011},
    volume = {110},
    number = {6},
    pages = {063517},
    month = {September},
    abstract = {The steady state and time-resolved photoluminescence (PL) spectra of cubic AlxGa1-xN have been measured for 0 {\ensuremath{<}} x {\ensuremath{<}} 1. The intensity of the room temperature PL increases by an order of magnitude when the AlN content increases from x = 0 to x = 0.95. Additionally, the PL decay slows down with the decrease of temperature and increase of x. These results show that strong localization of carriers on alloy composition fluctuations plays a large role in determining the intensity and temporal evolution of the PL. The activation energy for the localized carriers increases with the increase of x and reaches the value of 55 meV at x = 0.95.},
    keywords = {solid-solutions, alloys, epilayers, spectra, Physics, Atomic and Molecular Physics, and Optics},
    url = {http://strathprints.strath.ac.uk/44735/}
    }

  • T. Raadik, J. Krustok, and M. V. Yakushev, "Photoreflectance study of AgGaTe₂ single crystals," Physica B: Condensed Matter, vol. 406, iss. 3, p. 418–420, 2011.
    [BibTeX] [Abstract] [Download PDF]

    The optical properties of ternary chalcopyrite AgGaTe2 were studied by photoreflectance spectorscopy (PR). Due to the optimal direct energy gap and high absorption coefficient AgGaTe2 is a promising material for solar energy conversion. Single crystals used in this work were grown by the vertical Bridgman technique. The PR temperature dependent spectra were measured in the range of 25?300 K. At room temperature two energy gaps in AgGaTe2 were detected: and , with temperature coefficients /dT=?2.1{$\times$}10?4 eV/K and /dT=?3.4{$\times$}10?4 eV/K. At low temperature (T=25 K) these bandgap energies were and . Temperature dependence of bandgap energies is maximum at about T=90 K.

    @Article{strathprints40447,
    author = {T. Raadik and J. Krustok and M. V. Yakushev},
    title = {Photoreflectance study of AgGaTe₂ single crystals},
    journal = {Physica B: Condensed Matter},
    year = {2011},
    volume = {406},
    number = {3},
    pages = {418--420},
    month = {February},
    abstract = {The optical properties of ternary chalcopyrite AgGaTe2 were studied by photoreflectance spectorscopy (PR). Due to the optimal direct energy gap and high absorption coefficient AgGaTe2 is a promising material for solar energy conversion. Single crystals used in this work were grown by the vertical Bridgman technique. The PR temperature dependent spectra were measured in the range of 25?300 K. At room temperature two energy gaps in AgGaTe2 were detected: and , with temperature coefficients /dT=?2.1{$\times$}10?4 eV/K and /dT=?3.4{$\times$}10?4 eV/K. At low temperature (T=25 K) these bandgap energies were and . Temperature dependence of bandgap energies is maximum at about T=90 K.},
    keywords = {ternary chalcopyrite AgGaTe2, photoreflectance, semiconductor, solar energy materials, semiconductors, shape, Physics, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/40447/}
    }

  • K. P. O'Donnell, I. S. Roqan, K. Wang, K. Lorenz, E. Alves, and M. Bockowski, "The photoluminescence/excitation (PL/E) spectroscopy of Eu-implanted GaN," Optical Materials, vol. 33, iss. 7, p. 1063–1065, 2011.
    [BibTeX] [Abstract] [Download PDF]

    Several distinct luminescent centres form in GaN samples doped with Eu. One centre, Eu2, recently identified as the isolated, substitutional Eu impurity, Eu(Ga), is dominant in ion-implanted samples annealed under very high pressures (1 GPa) of N(2). According to structural determinations, such samples exhibit an essentially complete removal of lattice damage caused by the implantation process. A second centre, Eu1, probably comprising Eu(Ga) in association with an intrinsic lattice defect, produces a more complex emission spectrum. In addition there are several unidentified features in the (5)D(0) to (7)F(2) spectral region near 620 nm. We can readily distinguish Eu1 and Eu2 by their excitation spectra, in particular through their different sensitivities to above-gap and below-gap excitation. The present study extends recent work on photoluminescence/excitation (PL/E) spectroscopy of Eu1 and Eu2 to arrive at an understanding of these mechanisms in terms of residual optically active defect concentrations. We also report further on the 'host-independent' excitation mechanism that is active in the case of a prominent minority centre. The relevance of this work to the operation of the red GaN:Eu light-emitting diode is discussed.

    @article{strathprints35792,
    volume = {33},
    number = {7},
    month = {May},
    author = {K. P. O'Donnell and I. S. Roqan and Ke Wang and K. Lorenz and E. Alves and M. Bockowski},
    title = {The photoluminescence/excitation (PL/E) spectroscopy of Eu-implanted GaN},
    journal = {Optical Materials},
    pages = {1063--1065},
    year = {2011},
    keywords = {ion implantation, excitation mechanism, europium, luminescence, photoluminescence, gallium nitride, photoluminescence/excitation , PL/E, spectroscopy, Eu-implanted GaN , Physics, Electronic, Optical and Magnetic Materials, Atomic and Molecular Physics, and Optics, Electrical and Electronic Engineering, Computer Science(all)},
    url = {http://strathprints.strath.ac.uk/35792/},
    abstract = {Several distinct luminescent centres form in GaN samples doped with Eu. One centre, Eu2, recently identified as the isolated, substitutional Eu impurity, Eu(Ga), is dominant in ion-implanted samples annealed under very high pressures (1 GPa) of N(2). According to structural determinations, such samples exhibit an essentially complete removal of lattice damage caused by the implantation process. A second centre, Eu1, probably comprising Eu(Ga) in association with an intrinsic lattice defect, produces a more complex emission spectrum. In addition there are several unidentified features in the (5)D(0) to (7)F(2) spectral region near 620 nm. We can readily distinguish Eu1 and Eu2 by their excitation spectra, in particular through their different sensitivities to above-gap and below-gap excitation. The present study extends recent work on photoluminescence/excitation (PL/E) spectroscopy of Eu1 and Eu2 to arrive at an understanding of these mechanisms in terms of residual optically active defect concentrations. We also report further on the 'host-independent' excitation mechanism that is active in the case of a prominent minority centre. The relevance of this work to the operation of the red GaN:Eu light-emitting diode is discussed.}
    }

  • V. Kachkanov, K. P. O'Donnell, C. Rice, D. Wolverson, R. W. Martin, K. Lorenz, E. Alves, and M. Bockowski, "Zeeman splittings of the ⁵D₀–⁷F₂ transitions of Eu³⁺ ions implanted into GaN," MRS Online Proceedings Library, vol. 1290, p. mrsf10-1290-i03-06, 2011.
    [BibTeX] [Abstract] [Download PDF]

    We report the magnetic field splittings of emission lines assigned to the 5D0?7F2 transitions of Eu3+ centres in GaN. The application of a magnetic field in the c-axis direction (B{\ensuremath{|}}{\ensuremath{|}}c) leads to a splitting of the major lines at 621 nm, 622 nm and 622.8 nm into two components. The Zeeman splitting is linear with magnetic field up to 5 Tesla for each line. In contrast, a magnetic field applied in the growth plane (B?c) does not influence the photoluminescence spectra. The estimated g-factors vary slightly from sample to sample with mean values of g{\ensuremath{|}}{\ensuremath{|}} {\texttt{\char126}}2.8, {\texttt{\char126}}1.5 and {\texttt{\char126}}2.0 for the emission lines at 621 nm, 622 nm and 622.8 nm respectively.

    @Article{strathprints35791,
    author = {V. Kachkanov and K. P. O'Donnell and C. Rice and D. Wolverson and R. W. Martin and K. Lorenz and E. Alves and M. Bockowski},
    title = {Zeeman splittings of the {⁵D₀}--{⁷F₂} transitions of Eu³⁺ ions implanted into GaN},
    journal = {MRS Online Proceedings Library},
    year = {2011},
    volume = {1290},
    pages = {mrsf10-1290-i03-06},
    abstract = {We report the magnetic field splittings of emission lines assigned to the 5D0?7F2 transitions of Eu3+ centres in GaN. The application of a magnetic field in the c-axis direction (B{\ensuremath{|}}{\ensuremath{|}}c) leads to a splitting of the major lines at 621 nm, 622 nm and 622.8 nm into two components. The Zeeman splitting is linear with magnetic field up to 5 Tesla for each line. In contrast, a magnetic field applied in the growth plane (B?c) does not influence the photoluminescence spectra. The estimated g-factors vary slightly from sample to sample with mean values of g{\ensuremath{|}}{\ensuremath{|}} {\texttt{\char126}}2.8, {\texttt{\char126}}1.5 and {\texttt{\char126}}2.0 for the emission lines at 621 nm, 622 nm and 622.8 nm respectively.},
    keywords = {Eu, ion-implantation, nitride, TIC - Bionanotechnology, Physics, Mechanics of Materials, Materials Science(all), Mechanical Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/35791/}
    }

  • V. Kachkanov, I. P. Dolbnya, K. O'Donnell, R. Martin, P. Edwards, and S. Pereira, "InGaN epilayer characterization by microfocused x-ray reciprocal space mapping," Applied Physics Letters, vol. 99, iss. 18, p. 181909, 2011.
    [BibTeX] [Abstract] [Download PDF]

    We report the use of microfocused three-dimensional x-ray reciprocal space mapping to study InGaN epilayers with average InN content 20\%-22\%. Analysis of the full volume of reciprocal space, while probing samples on the microscale with a focused x-ray beam, allowed us to gain valuable information about the nanostructure of InN-rich InGaN epilayers. It is found that ?seed? InGaN mosaic nanocrystallites are twisted with respect to the ensemble average and strain-free. The initial stages of InGaN-on-GaN epitaxial growth, therefore, conform to the Volmer-Weber growth mechanism with ?seeds? nucleated on strain fields generated by the a-type edge dislocations.

    @Article{strathprints35790,
    author = {V. Kachkanov and I.P. Dolbnya and Kevin O'Donnell and Robert Martin and Paul Edwards and S. Pereira},
    title = {InGaN epilayer characterization by microfocused x-ray reciprocal space mapping},
    journal = {Applied Physics Letters},
    year = {2011},
    volume = {99},
    number = {18},
    pages = {181909},
    abstract = {We report the use of microfocused three-dimensional x-ray reciprocal space mapping to study InGaN epilayers with average InN content 20\%-22\%. Analysis of the full volume of reciprocal space, while probing samples on the microscale with a focused x-ray beam, allowed us to gain valuable information about the nanostructure of InN-rich InGaN epilayers. It is found that ?seed? InGaN mosaic nanocrystallites are twisted with respect to the ensemble average and strain-free. The initial stages of InGaN-on-GaN epitaxial growth, therefore, conform to the Volmer-Weber growth mechanism with ?seeds? nucleated on strain fields generated by the a-type edge dislocations.},
    keywords = {epitaxial growth, gallium nitride, gallium compounds, wide band gap semiconductors , TIC - Bionanotechnology, Physics, Physics and Astronomy (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/35790/}
    }

  • F. Papoff and B. Hourahine, "Geometrical Mie theory for resonances in nanoparticles of any shape," Optics Express, vol. 19, iss. 22, p. 21432–21444, 2011.
    [BibTeX] [Abstract] [Download PDF]

    We give a geometrical theory of resonances in Maxwell?s equations that generalizes the Mie formulae for spheres to all scattering channels of any dielectric or metallic particle without sharp edges. We show that the electromagnetic response of a particle is given by a set of modes of internal and scattered fields that are coupled pairwise on the surface of the particle and reveal that resonances in nanoparticles and excess noise in macroscopic cavities have the same origin. We give examples of two types of optical resonances: those in which a single pair of internal and scattered modes become strongly aligned in the sense defined in this paper, and those resulting from constructive interference of many pairs of weakly aligned modes, an effect relevant for sensing. This approach calculates resonances for every significant mode of particles, demonstrating that modes can be either bright or dark depending on the incident field. Using this extra mode information we then outline how excitation can be optimized. Finally, we apply this theory to gold particles with shapes often used in experiments, demonstrating effects including a Fano-like resonance.

    @article{strathprints34708,
    volume = {19},
    number = {22},
    month = {October},
    author = {Francesco Papoff and Benjamin Hourahine},
    title = {Geometrical Mie theory for resonances in nanoparticles of any shape},
    journal = {Optics Express},
    pages = {21432--21444},
    year = {2011},
    keywords = {metals, scattering, nanomaterials, scattering theory, Physics, Atomic and Molecular Physics, and Optics},
    url = {http://strathprints.strath.ac.uk/34708/},
    abstract = {We give a geometrical theory of resonances in Maxwell?s equations that generalizes the Mie formulae for spheres to all scattering channels of any dielectric or metallic particle without sharp edges. We show that the electromagnetic response of a particle is given by a set of modes of internal and scattered fields that are coupled pairwise on the surface of the particle and reveal that resonances in nanoparticles and excess noise in macroscopic cavities have the same origin. We give examples of two types of optical resonances: those in which a single pair of internal and scattered modes become strongly aligned in the sense defined in this paper, and those resulting from constructive interference of many pairs of weakly aligned modes, an effect relevant for sensing. This approach calculates resonances for every significant mode of particles, demonstrating that modes can be either bright or dark depending on the incident field. Using this extra mode information we then outline how excitation can be optimized. Finally, we apply this theory to gold particles with shapes often used in experiments, demonstrating effects including a Fano-like resonance.}
    }

  • B. Hourahine, "Excited multiplets of Eu in GaN," MRS Online Proceedings Library, vol. 1290, p. 46–51, 2011.
    [BibTeX] [Abstract] [Download PDF]

    A method to calculate the multiplet states of lanthanide impurities in solids is presented. This approach is based on a semi-empirical density functional method which includes corrections to account for the correlation and spin-orbit coupling of the 4f electrons. Specific multiplet states of the rare earth are produced by constraining the system. This approach is then used to investigate some of the properties of substitutional europium impurities in gallium nitride, reproducing the relative energy of two multiplets, and discussing a potential excitation mechanism for these centers.

    @Article{strathprints34305,
    author = {Benjamin Hourahine},
    title = {Excited multiplets of Eu in GaN},
    journal = {MRS Online Proceedings Library},
    year = {2011},
    volume = {1290},
    pages = {46--51},
    abstract = {A method to calculate the multiplet states of lanthanide impurities in solids is presented. This approach is based on a semi-empirical density functional method which includes corrections to account for the correlation and spin-orbit coupling of the 4f electrons. Specific multiplet states of the rare earth are produced by constraining the system. This approach is then used to investigate some of the properties of substitutional europium impurities in gallium nitride, reproducing the relative energy of two multiplets, and discussing a potential excitation mechanism for these centers.},
    keywords = {Eu, GaN, electronic structure, Physics, Mechanics of Materials, Materials Science(all), Mechanical Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/34305/}
    }

  • M. V. Yakushev, F. Luckert, C. Faugeras, A. V. Karotki, A. V. Mudryi, and R. W. Martin, "Excited states of the A and B free excitons in CuInSe₂," Japanese Journal of Applied Physics, vol. 50, iss. 5, p. 05FC03, 2011.
    [BibTeX] [Abstract] [Download PDF]

    CuInSe2 single crystals, grown by the vertical Bridgman technique were studied using polarisation resolved photoluminescence (PL) at cryogenic temperatures. The emission lines related to the first (n = 2) excited states for the A and B free excitons were observed in the PL spectra at 1.0481 and 1.0516 eV, respectively. The spectral positions of these lines were used to estimate accurate values for the A and B exciton binding energies (8.5 and 8.4 meV, respectively), Bohr radii (7.5 nm), band gaps (E-g(A) = 1.050 eV and E-g(B) = 1.054 eV), and the static dielectric constant (11.3) assuming the hydrogenic model.

    @Article{strathprints34032,
    author = {Michael V. Yakushev and Franziska Luckert and Clement Faugeras and Anatoli V. Karotki and Alexander V. Mudryi and Robert W. Martin},
    title = {Excited states of the A and B free excitons in CuInSe₂},
    journal = {Japanese Journal of Applied Physics},
    year = {2011},
    volume = {50},
    number = {5},
    pages = {05FC03},
    month = {May},
    abstract = {CuInSe2 single crystals, grown by the vertical Bridgman technique were studied using polarisation resolved photoluminescence (PL) at cryogenic temperatures. The emission lines related to the first (n = 2) excited states for the A and B free excitons were observed in the PL spectra at 1.0481 and 1.0516 eV, respectively. The spectral positions of these lines were used to estimate accurate values for the A and B exciton binding energies (8.5 and 8.4 meV, respectively), Bohr radii (7.5 nm), band gaps (E-g(A) = 1.050 eV and E-g(B) = 1.054 eV), and the static dielectric constant (11.3) assuming the hydrogenic model.},
    keywords = {optical properties, single crystals, Physics},
    url = {http://strathprints.strath.ac.uk/34032/}
    }

  • F. Luckert, D. I. Hamilton, M. V. Yakushev, N. S. Beattie, G. Zoppi, M. Moynihan, I. Forbes, A. V. Karotki, A. V. Mudryi, M. Grossberg, J. Krustok, and R. W. Martin, "Optical properties of high quality Cu₂ZnSnSe₄ thin films," Applied Physics Letters, vol. 99, iss. 6, p. 62104, 2011.
    [BibTeX] [Abstract] [Download PDF]

    Cu2ZnSnSe4 thin films, fabricated on bare or molybdenum coated glass substrates by magnetron sputtering and selenisation, were studied by a range of techniques. Photoluminescence spectra reveal an excitonic peak and two phonon replicas of a donor-acceptor pair (DAP) recombination. Its acceptor and donor ionisation energies are 27 and 7 meV, respectively. This demonstrates that high-quality Cu2ZnSnSe4 thin films can be fabricated. An experimental value for the longitudinal optical phonon energy of 28 meV was estimated. The band gap energy of 1.01 eV at room temperature was determined using optical absorption spectra.

    @Article{strathprints34031,
    author = {F. Luckert and D. I. Hamilton and M. V. Yakushev and N. S. Beattie and G. Zoppi and M. Moynihan and I. Forbes and A. V. Karotki and A. V. Mudryi and M. Grossberg and J. Krustok and R. W. Martin},
    title = {Optical properties of high quality Cu₂ZnSnSe₄ thin films},
    journal = {Applied Physics Letters},
    year = {2011},
    volume = {99},
    number = {6},
    pages = {062104},
    month = {August},
    abstract = {Cu2ZnSnSe4 thin films, fabricated on bare or molybdenum coated glass substrates by magnetron sputtering and selenisation, were studied by a range of techniques. Photoluminescence spectra reveal an excitonic peak and two phonon replicas of a donor-acceptor pair (DAP) recombination. Its acceptor and donor ionisation energies are 27 and 7 meV, respectively. This demonstrates that high-quality Cu2ZnSnSe4 thin films can be fabricated. An experimental value for the longitudinal optical phonon energy of 28 meV was estimated. The band gap energy of 1.01 eV at room temperature was determined using optical absorption spectra.},
    keywords = {photoluminescence, selenisation, dependence, energy, copper compounds, excitons, phonons, photoionisation, Raman spectra, semiconductor thin films, sputter deposition, tin compounds, zinc compounds, TIC - Bionanotechnology, Optics. Light, Physics and Astronomy (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/34031/}
    }

  • H. Zachmann, S. Puttnins, M. V. Yakushev, F. Luckert, R. W. Martin, A. V. Karotki, V. F. Gremenok, and A. V. Mudryi, "Fabrication and characterisation of Cu(In,Ga)Se₂ solar cells on polyimide," Thin Solid Films, vol. 519, iss. 21, p. 7264–7267, 2011.
    [BibTeX] [Abstract] [Download PDF]

    Solar cells with the structure ZnO:Al/i-ZnO/CdS/Cu(In,Ga)Se-2/Mo/polyimide were examined using a range of techniques. The elemental composition of the Cu(InGa)Se-2 (CIGS) layers, their crystalline structure and optical properties were studied. Photoluminescence (PL) spectra of the CIGS absorber layers were studied as functions of temperature (4.2-240 K) and excitation power density. The band gap energy E-g of the CIGS layers was determined by employing photoluminescence excitation (PLE) spectroscopy. The influence of sodium incorporation on the PL properties of CIGS was analysed. Correlations of the optical properties of the CIGS absorber layers and the photovoltaic parameters of the solar cells were revealed.

    @Article{strathprints32471,
    author = {H. Zachmann and S. Puttnins and M. V. Yakushev and F. Luckert and R. W. Martin and A. V. Karotki and V. F. Gremenok and A. V. Mudryi},
    title = {Fabrication and characterisation of Cu(In,Ga)Se₂ solar cells on polyimide},
    journal = {Thin Solid Films},
    year = {2011},
    volume = {519},
    number = {21},
    pages = {7264--7267},
    month = {August},
    abstract = {Solar cells with the structure ZnO:Al/i-ZnO/CdS/Cu(In,Ga)Se-2/Mo/polyimide were examined using a range of techniques. The elemental composition of the Cu(InGa)Se-2 (CIGS) layers, their crystalline structure and optical properties were studied. Photoluminescence (PL) spectra of the CIGS absorber layers were studied as functions of temperature (4.2-240 K) and excitation power density. The band gap energy E-g of the CIGS layers was determined by employing photoluminescence excitation (PLE) spectroscopy. The influence of sodium incorporation on the PL properties of CIGS was analysed. Correlations of the optical properties of the CIGS absorber layers and the photovoltaic parameters of the solar cells were revealed.},
    keywords = {cu(in,ga)se-2, solar cells, polyimide, photoluminescence, thin-films, high-efficiency, layers, na, TIC - Bionanotechnology, Physics, Materials Chemistry, Surfaces and Interfaces, Electronic, Optical and Magnetic Materials, Surfaces, Coatings and Films, Metals and Alloys},
    url = {http://strathprints.strath.ac.uk/32471/}
    }

  • S. V. Novikov, C. R. Staddon, C. T. Foxon, K. M. Yu, R. Broesler, M. Hawkridge, Z. Liliental-Weber, J. Denlinger, I. Demchenko, F. Luckert, P. Edwards, R. Martin, and W. Walukiewicz, "Growth by molecular beam epitaxy of amorphous and crystalline GaNAs alloys with band gaps from 3.4 to 0.8 eV for solar energy conversion devices," Journal of Crystal Growth, vol. 323, iss. 1, p. 60–63, 2011.
    [BibTeX] [Abstract] [Download PDF]

    Using low temperature MBE, we have shown that it is possible to grow amorphous GaN1-xAsx layers with a variable As content (0 {\ensuremath{<}} x {\ensuremath{<}} 0.8) on both crystalline (sapphire and silicon) and amorphous (glass and Pyrex glass) substrates. Despite the fact that the samples with high As content are amorphous, we observe a gradual continuous decrease of bandgap from similar to 3.4 to similar to 0.8 eV with increase in As content. To the best of our knowledge this is the first demonstration of homogeneous amorphous GaN-based alloys over a wide composition range. The large band gap range of the amorphous phase of GaNAs covers much of the solar spectrum. The amorphous nature of the GaNAs alloys is particularly advantageous since low cost substrates such as glass and Pyrex glass can be used for solar cell fabrication.

    @article{strathprints32272,
    volume = {323},
    number = {1},
    month = {May},
    author = {S.V. Novikov and C.R. Staddon and C.T. Foxon and K.M. Yu and R. Broesler and M. Hawkridge and Z. Liliental-Weber and J. Denlinger and I. Demchenko and Franziska Luckert and Paul Edwards and Robert Martin and W. Walukiewicz},
    title = {Growth by molecular beam epitaxy of amorphous and crystalline GaNAs alloys with band gaps from 3.4 to 0.8 eV for solar energy conversion devices},
    journal = {Journal of Crystal Growth},
    pages = {60--63},
    year = {2011},
    keywords = {crystals, molecular beam epitaxy, amorphous, crystalline, GaNAs alloys , solar energy conversion devices, band gaps , Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/32272/},
    abstract = {Using low temperature MBE, we have shown that it is possible to grow amorphous GaN1-xAsx layers with a variable As content (0 {\ensuremath{<}} x {\ensuremath{<}} 0.8) on both crystalline (sapphire and silicon) and amorphous (glass and Pyrex glass) substrates. Despite the fact that the samples with high As content are amorphous, we observe a gradual continuous decrease of bandgap from similar to 3.4 to similar to 0.8 eV with increase in As content. To the best of our knowledge this is the first demonstration of homogeneous amorphous GaN-based alloys over a wide composition range. The large band gap range of the amorphous phase of GaNAs covers much of the solar spectrum. The amorphous nature of the GaNAs alloys is particularly advantageous since low cost substrates such as glass and Pyrex glass can be used for solar cell fabrication.}
    }

  • S. V. Novikov, C. R. Staddon, C. T. Foxon, F. Luckert, P. Edwards, R. Martin, and A. J. Kent, "Molecular beam epitaxy as a method for the growth of free-standing bulk zinc-blende GaN and AlGaN crystals," Journal of Crystal Growth, vol. 323, iss. 1, p. 80–83, 2011.
    [BibTeX] [Abstract] [Download PDF]

    We have studied the growth of zinc-blende GaN and AlxGa1-xN layers, structures and bulk crystals by molecular beam epitaxy (MBE). MBE is normally regarded as an epitaxial technique for growth of very thin layers with monolayer control of their thickness. However, we have used the MBE technique for bulk crystal growth and have produced GaN layers up to 100 mu m in thickness. Thick, undoped, cubic GaN films were grown on semi-insulating GaAs (0 0 1) substrates by a modified plasma-assisted molecular beam epitaxy (PA-MBE) method and were removed from the GaAs substrate after the growth. The resulting free-standing GaN wafers may be used as substrates for further epitaxy of cubic GaN-based structures and devices. We have demonstrated that the PA-MBE process, we had developed, also allows us to achieve free-standing zinc-blende AlxGa1-xN wafers.

    @article{strathprints32270,
    volume = {323},
    number = {1},
    month = {May},
    author = {S.V. Novikov and C.R. Staddon and C.T. Foxon and Franziska Luckert and Paul Edwards and Robert Martin and A.J. Kent},
    title = {Molecular beam epitaxy as a method for the growth of free-standing bulk zinc-blende GaN and AlGaN crystals},
    journal = {Journal of Crystal Growth},
    pages = {80--83},
    year = {2011},
    keywords = {substrates, molecular beam epitaxy, nitrides, semiconducting III?V materials, free-standing bulk , zinc-blende GaN , AlGaN crystals, Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/32270/},
    abstract = {We have studied the growth of zinc-blende GaN and AlxGa1-xN layers, structures and bulk crystals by molecular beam epitaxy (MBE). MBE is normally regarded as an epitaxial technique for growth of very thin layers with monolayer control of their thickness. However, we have used the MBE technique for bulk crystal growth and have produced GaN layers up to 100 mu m in thickness. Thick, undoped, cubic GaN films were grown on semi-insulating GaAs (0 0 1) substrates by a modified plasma-assisted molecular beam epitaxy (PA-MBE) method and were removed from the GaAs substrate after the growth. The resulting free-standing GaN wafers may be used as substrates for further epitaxy of cubic GaN-based structures and devices. We have demonstrated that the PA-MBE process, we had developed, also allows us to achieve free-standing zinc-blende AlxGa1-xN wafers.}
    }

  • P. Edwards, D. Sleith, A. Wark, and R. Martin, "Mapping localized surface plasmons within silver nanocubes using cathodoluminescence hyperspectral imaging," Journal of Physical Chemistry C, vol. 115, iss. 29, p. 14031?14035, 2011.
    [BibTeX] [Abstract] [Download PDF]

    Localized surface plasmons within silver nanocubes less than 50 nm in size are investigated using high resolution cathodoluminescence hyperspectral imaging. Multivariate statistical analysis of the multidimensional luminescence dataset allows both the identification of distinct spectral features in the emission and the mapping of their spatial distribution. These results show a 490 nm peak emitted from the cube faces, with shorter wavelength luminescence coming from the vertices and edges; this provides direct experimental confirmation of theoretical predictions.

    @article{strathprints31948,
    volume = {115},
    number = {29},
    month = {July},
    author = {Paul Edwards and David Sleith and Alastair Wark and Robert Martin},
    title = {Mapping localized surface plasmons within silver nanocubes using cathodoluminescence hyperspectral imaging},
    journal = {Journal of Physical Chemistry C},
    pages = {14031?14035},
    year = {2011},
    keywords = {nanocube, hyperspectral, cathodoluminescence, plasmon, surface, Chemistry, Physics, Physical and Theoretical Chemistry},
    url = {http://strathprints.strath.ac.uk/31948/},
    abstract = {Localized surface plasmons within silver nanocubes less than 50 nm in size are investigated using high resolution cathodoluminescence hyperspectral imaging. Multivariate statistical analysis of the multidimensional luminescence dataset allows both the identification of distinct spectral features in the emission and the mapping of their spatial distribution. These results show a 490 nm peak emitted from the cube faces, with shorter wavelength luminescence coming from the vertices and edges; this provides direct experimental confirmation of theoretical predictions.}
    }

  • S. V. Novikov, C. R. Staddon, R. E. L. Powell, A. V. Akimov, F. Luckert, P. R. Edwards, R. W. Martin, A. J. Kent, and C. T. Foxon, "Wurtzite AlₓGa₁₋ₓN bulk crystals grown by molecular beam epitaxy," Journal of Crystal Growth, vol. 322, iss. 1, p. 23–26, 2011.
    [BibTeX] [Abstract] [Download PDF]

    We have studied the growth of wurtzite GaN and AlxGa1-xN layers and bulk crystals by molecular beam epitaxy (MBE). MBE is normally regarded as an epitaxial technique for the growth of very thin layers with monolayer control of their thickness. However, we have used the MBE technique for bulk crystal growth and have produced 2 in diameter wurtzite AlxGa1-xN layers up to 10 [mu]m in thickness. Undoped wurtzite AlxGa1-xN films were grown on GaAs (1 1 1)B substrates by a plasma-assisted molecular beam epitaxy (PA-MBE) method and were removed from the GaAs substrate after the growth. The fact that free-standing ternary AlxGa1-xN wafers can be grown is very significant for the potential future production of wurtzite AlxGa1-xN substrates optimized for AlGaN-based device structures.

    @Article{strathprints30548,
    author = {S.V. Novikov and C.R. Staddon and R.E.L. Powell and A.V. Akimov and F. Luckert and P.R. Edwards and R.W. Martin and A.J. Kent and C.T. Foxon},
    title = {Wurtzite {AlₓGa₁₋ₓN} bulk crystals grown by molecular beam epitaxy},
    journal = {Journal of Crystal Growth},
    year = {2011},
    volume = {322},
    number = {1},
    pages = {23--26},
    month = {May},
    abstract = {We have studied the growth of wurtzite GaN and AlxGa1-xN layers and bulk crystals by molecular beam epitaxy (MBE). MBE is normally regarded as an epitaxial technique for the growth of very thin layers with monolayer control of their thickness. However, we have used the MBE technique for bulk crystal growth and have produced 2 in diameter wurtzite AlxGa1-xN layers up to 10 [mu]m in thickness. Undoped wurtzite AlxGa1-xN films were grown on GaAs (1 1 1)B substrates by a plasma-assisted molecular beam epitaxy (PA-MBE) method and were removed from the GaAs substrate after the growth. The fact that free-standing ternary AlxGa1-xN wafers can be grown is very significant for the potential future production of wurtzite AlxGa1-xN substrates optimized for AlGaN-based device structures.},
    keywords = {semiconducting III-V materials, substrates, molecular beam epitaxy, nitrides, wurtzite, Physics, Materials Chemistry, Inorganic Chemistry, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/30548/}
    }

  • J. Bruckbauer, P. R. Edwards, T. Wang, and R. W. Martin, "High resolution cathodoluminescence hyperspectral imaging of surface features in InGaN/GaN multiple quantum well structures," Applied Physics Letters, vol. 98, iss. 14, p. 141908, 2011.
    [BibTeX] [Abstract] [Download PDF]

    InGaN/GaN multiple quantum wells (MQWs) have been studied by using cathodoluminescence hyperspectral imaging with high spatial resolution. Variations in peak emission energies and intensities across trenchlike features and V-pits on the surface of the MQWs are investigated. The MQW emission from the region inside trenchlike features is redshifted by approximately 45 meV and more intense than the surrounding planar regions of the sample, whereas emission from the V-pits is blueshifted by about 20 meV and relatively weaker. By employing this technique to the studied nanostructures it is possible to investigate energy and intensity shifts on a 10 nm length scale.

    @Article{strathprints30547,
    author = {Jochen Bruckbauer and Paul R. Edwards and Tao Wang and Robert W. Martin},
    title = {High resolution cathodoluminescence hyperspectral imaging of surface features in InGaN/GaN multiple quantum well structures},
    journal = {Applied Physics Letters},
    year = {2011},
    volume = {98},
    number = {14},
    pages = {141908},
    month = {April},
    abstract = {InGaN/GaN multiple quantum wells (MQWs) have been studied by using cathodoluminescence hyperspectral imaging with high spatial resolution. Variations in peak emission energies and intensities across trenchlike features and V-pits on the surface of the MQWs are investigated. The MQW emission from the region inside trenchlike features is redshifted by approximately 45 meV and more intense than the surrounding planar regions of the sample, whereas emission from the V-pits is blueshifted by about 20 meV and relatively weaker. By employing this technique to the studied nanostructures it is possible to investigate energy and intensity shifts on a 10 nm length scale.},
    keywords = {cathodoluminescence, gallium compounds, III-V semiconductors, indium compounds, MOCVD, nanofabrication, nanostructured materials, red shift, semiconductor growth, semiconductor quantum wells, wide band gap semiconductors, Physics, Physics and Astronomy (miscellaneous)},
    url = {http://strathprints.strath.ac.uk/30547/}
    }

  • P. R. Edwards and R. W. Martin, "Cathodoluminescence nano-characterization of semiconductors," Semiconductor Science and Technology, vol. 26, iss. 6, p. 64005, 2011.
    [BibTeX] [Abstract] [Download PDF]

    We give an overview of the use of cathodoluminescence (CL) in scanning electron microscopy (SEM) for the nano-scale characterization of semiconducting materials and devices. We discuss the technical aspects of the measurement, such as factors limiting the spatial resolution and design considerations for efficient collection optics. The advantages of more recent developments in the technique are outlined, including the use of the hyperspectral imaging mode and the combination of CL and other SEM-based measurements. We illustrate these points with examples from our own experience of designing and constructing CL systems and applying the technique to the characterization of III-nitride materials and nanostructures.

    @article{strathprints30546,
    volume = {26},
    number = {6},
    month = {June},
    author = {Paul R Edwards and Robert W Martin},
    title = {Cathodoluminescence nano-characterization of semiconductors},
    journal = {Semiconductor Science and Technology},
    pages = {064005},
    year = {2011},
    keywords = {optics, condensed matter, electrical matter, magnetic matter, thin films, cathodoluminescence, ionoluminescence , Physics, Materials Chemistry, Electronic, Optical and Magnetic Materials, Electrical and Electronic Engineering, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/30546/},
    abstract = {We give an overview of the use of cathodoluminescence (CL) in scanning electron microscopy (SEM) for the nano-scale characterization of semiconducting materials and devices. We discuss the technical aspects of the measurement, such as factors limiting the spatial resolution and design considerations for efficient collection optics. The advantages of more recent developments in the technique are outlined, including the use of the hyperspectral imaging mode and the combination of CL and other SEM-based measurements. We illustrate these points with examples from our own experience of designing and constructing CL systems and applying the technique to the characterization of III-nitride materials and nanostructures.}
    }

2010

  • A. V. Mudryi, V. F. Gremenok, A. V. Karotki, V. B. Zalesski, M. V. Yakushev, F. Luckert, and R. Martin, "Structural and optical properties of thin films of Cu(In,Ga)Se₂ semiconductor compounds," Journal of Applied Spectroscopy, vol. 77, iss. 3, p. 371–377, 2010.
    [BibTeX] [Abstract] [Download PDF]

    The chemical composition of Cu(In,Ga)Se-2 (CIGS) semiconductor compounds is analyzed by local x-ray spectral microanalysis and scanning Auger electron spectroscopy. X-ray diffraction analysis reveals a difference in the predominant orientation of CIGS films depending on the technological conditions under which they are grown. The chemical composition is found to have a strong effect on the shift in the self-absorption edge of CIGS compounds. It is shown that a change in the relative proportion of Ga and In in CIGS semiconducting compounds leads to a change in the band gap E-g for this material in the 1.05-1.72 eV spectral range at 4.2 K.

    @Article{strathprints35401,
    author = {A. V. Mudryi and V. F. Gremenok and A. V. Karotki and V. B. Zalesski and M. V. Yakushev and F. Luckert and R. Martin},
    title = {Structural and optical properties of thin films of Cu(In,Ga)Se₂ semiconductor compounds},
    journal = {Journal of Applied Spectroscopy},
    year = {2010},
    volume = {77},
    number = {3},
    pages = {371--377},
    month = {July},
    abstract = {The chemical composition of Cu(In,Ga)Se-2 (CIGS) semiconductor compounds is analyzed by local x-ray spectral microanalysis and scanning Auger electron spectroscopy. X-ray diffraction analysis reveals a difference in the predominant orientation of CIGS films depending on the technological conditions under which they are grown. The chemical composition is found to have a strong effect on the shift in the self-absorption edge of CIGS compounds. It is shown that a change in the relative proportion of Ga and In in CIGS semiconducting compounds leads to a change in the band gap E-g for this material in the 1.05-1.72 eV spectral range at 4.2 K.},
    keywords = {chalcopyrite semiconductors, chemical composition, absorption coefficient, x-ray diffraction analysis, structure, CUINSE2 SINGLE-CRYSTALS, SOLAR-CELLS, EPITAXIAL LAYERS, CUIN1-XGAXSE2, QUALITY, PHOTOREFLECTANCE, PERFORMANCE, EFFICIENCY, Spectroscopy, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/35401/}
    }

  • R. Martin, "Microscopic characterisation of luminescent III-N:RE epilayers," in Rare Earth Doped III-Nitrides For Optoelectronic And Spintronic Applications, K. P. O'Donnell and V. Dierolf, Eds., Berlin: Springer-Verlag Berlin, 2010, p. 189–219.
    [BibTeX] [Abstract] [Download PDF]

    Study of the luminescence of Rare Earth ions introduced into III-nitride semiconductor hosts produces a wealth of important information. This chapter describes investigations of photoluminescence and cathodoluminescence spectroscopy on a range of RE-implanted samples, with hosts including GaN, AlGaN and AlInN. Raising the post-implantation annealing temperature is shown to lead to a dramatic increase in RE luminescence intensity, and methods to allow annealing well above the growth temperature of the host are discussed. The high-brightness samples that result enable the resolution of additional fine-structure in the luminescence from GaN:Eu and clarification of multiple sites for the RE. Measurements for AlGaN hosts covering the entire composition range point to the importance of core-excitons in the luminescence process. Adding in information from X-ray microanalysis and high spatial resolution luminescence mapping reveals further details of the effects resulting from annealing and of changes in host composition.

    @incollection{strathprints35400,
    author = {Robert Martin},
    series = {Topics in Applied Physics},
    booktitle = {Rare Earth Doped III-Nitrides For Optoelectronic And Spintronic Applications},
    editor = {Kevin Peter O'Donnell and Volkmar Dierolf},
    address = {Berlin},
    title = {Microscopic characterisation of luminescent III-N:RE epilayers},
    publisher = {Springer-Verlag Berlin},
    year = {2010},
    pages = {189--219},
    keywords = {rare earth ions, eu-implanted gan, electroluminescent Devices, optical-properties, light emission, blue emission, III-Nitrides, thin films, photoluminescence, ER, Physics},
    url = {http://strathprints.strath.ac.uk/35400/},
    abstract = {Study of the luminescence of Rare Earth ions introduced into III-nitride semiconductor hosts produces a wealth of important information. This chapter describes investigations of photoluminescence and cathodoluminescence spectroscopy on a range of RE-implanted samples, with hosts including GaN, AlGaN and AlInN. Raising the post-implantation annealing temperature is shown to lead to a dramatic increase in RE luminescence intensity, and methods to allow annealing well above the growth temperature of the host are discussed. The high-brightness samples that result enable the resolution of additional fine-structure in the luminescence from GaN:Eu and clarification of multiple sites for the RE. Measurements for AlGaN hosts covering the entire composition range point to the importance of core-excitons in the luminescence process. Adding in information from X-ray microanalysis and high spatial resolution luminescence mapping reveals further details of the effects resulting from annealing and of changes in host composition.}
    }

  • B. Hourahine, B. Aradi, and T. Frauenheim, "DFTB+ and lanthanides," Journal of Physics Conference Series, vol. 242, iss. 1, p. 12005, 2010.
    [BibTeX] [Abstract] [Download PDF]

    DFTB+ is a recent general purpose implementation of density-functional based tight binding. One of the early motivators to develop this code was to investigate lanthanide impurities in nitride semiconductors, leading to a series of successful studies into structure and electrical properties of these systems. Here we describe our general framework to treat the physical effects needed for these problematic impurities within a tight-binding formalism, additionally discussing forces and stresses in DFTB. We also present an approach to evaluate the general case of Slater-Koster transforms and all of their derivatives in Cartesian coordinates. These developments are illustrated by simulating isolated Gd impurities in GaN.

    @Article{strathprints34306,
    author = {B. Hourahine and B Aradi and T. Frauenheim},
    title = {DFTB+ and lanthanides},
    journal = {Journal of Physics Conference Series},
    year = {2010},
    volume = {242},
    number = {1},
    pages = {012005},
    abstract = {DFTB+ is a recent general purpose implementation of density-functional based tight binding. One of the early motivators to develop this code was to investigate lanthanide impurities in nitride semiconductors, leading to a series of successful studies into structure and electrical properties of these systems. Here we describe our general framework to treat the physical effects needed for these problematic impurities within a tight-binding formalism, additionally discussing forces and stresses in DFTB. We also present an approach to evaluate the general case of Slater-Koster transforms and all of their derivatives in Cartesian coordinates. These developments are illustrated by simulating isolated Gd impurities in GaN.},
    keywords = {crystals, semiconductors, crystal impurities, condensed matter, tight-binding approach, Physics, Physics and Astronomy(all)},
    url = {http://strathprints.strath.ac.uk/34306/}
    }

  • A. V. Karotki, A. V. Mudryi, M. V. Yakushev, F. Luckert, and R. Martin, "Structural and optical properties of CdS/Cu(In,Ga)Se₂ heterostructures irradiated by high energy electrons," Journal of Applied Spectroscopy, vol. 77, iss. 5, p. 668–674, 2010.
    [BibTeX] [Abstract] [Download PDF]

    Thin films of Cu(In, Ga)Se-2 (CIGS) with a Ga/(Ga + In) ratio of similar to 0.27 corresponding to the standard elemental composition for solar-energy transducers were grown on Mo-coated glass substrates by the Cu, In, Ga, and Se co-evaporation technique from different sources. Transmission (T), photoluminescence (PL), and photoluminescence excitation (PLE) spectra at 4.2 K were used to analyze electronic properties in the asgrown and electron-irradiated CIGS films. The band-gap energy (E-g) of the CIGS films measured using both transmission and PLE methods was found to be about 1.28 eV at 4.2 K. Two deep bands in the PL spectra of the irradiated CIGS films, P-1 at similar to 0.91 eV and P-2 at similar to 0.77 eV, have been detected. These bands are tentatively associated with copper atoms substituting indium (Cu-In) and indium vacancies V-In, respectively, as the simplest radiation-induced defects.

    @Article{strathprints32495,
    author = {A. V. Karotki and A.V. Mudryi and Michael V Yakushev and Franziska Luckert and Robert Martin},
    title = {Structural and optical properties of CdS/Cu(In,Ga)Se₂ heterostructures irradiated by high energy electrons},
    journal = {Journal of Applied Spectroscopy},
    year = {2010},
    volume = {77},
    number = {5},
    pages = {668--674},
    month = {November},
    abstract = {Thin films of Cu(In, Ga)Se-2 (CIGS) with a Ga/(Ga + In) ratio of similar to 0.27 corresponding to the standard elemental composition for solar-energy transducers were grown on Mo-coated glass substrates by the Cu, In, Ga, and Se co-evaporation technique from different sources. Transmission (T), photoluminescence (PL), and photoluminescence excitation (PLE) spectra at 4.2 K were used to analyze electronic properties in the asgrown and electron-irradiated CIGS films. The band-gap energy (E-g) of the CIGS films measured using both transmission and PLE methods was found to be about 1.28 eV at 4.2 K. Two deep bands in the PL spectra of the irradiated CIGS films, P-1 at similar to 0.91 eV and P-2 at similar to 0.77 eV, have been detected. These bands are tentatively associated with copper atoms substituting indium (Cu-In) and indium vacancies V-In, respectively, as the simplest radiation-induced defects.},
    keywords = {Cu(In,Ga)Se2 , photoluminescence, electron irradiation, defects, Therapeutics. Pharmacology, Spectroscopy, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/32495/}
    }

  • F. Luckert, M. V. Yakushev, A. V. Mudryi, and R. W. Martin, "Excitons in chalcopyrite solar cell materials: CuInSe₂ and CuInS₂," in 11th International Conference on Optics of Excitons in Confined Systems, 2010.
    [BibTeX] [Abstract] [Download PDF]

    A poster presentation on excitons in chalcopyrite solar cell materials

    @InProceedings{strathprints31043,
    author = {F. Luckert and M. V. Yakushev and A. V. Mudryi and R. W. Martin},
    title = {Excitons in chalcopyrite solar cell materials: CuInSe₂ and CuInS₂},
    booktitle = {11th International Conference on Optics of Excitons in Confined Systems},
    year = {2010},
    editor = {L Vina and C Tejedor and JM Calleja},
    month = {September},
    abstract = {A poster presentation on excitons in chalcopyrite solar cell materials},
    journal = {11th International Conference on Optics of Excitons in Confined Systems},
    keywords = {exciton, cell materials, CUINSE2, CUINS2, Physics},
    url = {http://strathprints.strath.ac.uk/31043/}
    }

  • C. Trager-Cowan, "Cafe scientifique: nobel laureate communicates science across the world," MRS Bulletin, vol. 35, iss. 1, p. 10–11, 2010.
    [BibTeX] [Abstract] [Download PDF]

    Communication of science across the world by a nobel laureate

    @article{strathprints30217,
    volume = {35},
    number = {1},
    month = {January},
    author = {Carol Trager-Cowan},
    title = {Cafe scientifique: nobel laureate communicates science across the world},
    journal = {MRS Bulletin},
    pages = {10--11},
    year = {2010},
    keywords = {nobel laureate, science, world, communication, Physics, Materials Science(all), Physical and Theoretical Chemistry, Condensed Matter Physics},
    url = {http://strathprints.strath.ac.uk/30217/},
    abstract = {Communication of science across the world by a nobel laureate}
    }

  • K. O'Donnell, "Summary and prospects for future work," in Rare-earth doped III-nitrides for optoelectronic and spintronic applications, K. O'Donnell and V. Dierolf, Eds., Springer, 2010, p. 343–345.
    [BibTeX] [Abstract] [Download PDF]

    This book provides a snapshot of recent progress in the field of rare-earth-doped group III-nitride semiconductors, especially GaN, but extending to AlN and the alloys AlGaN, AlInN and InGaN. This material class is currently enjoying an upsurge in interest due to its ideal suitability for both optoelectronic and spintronic applications. The text first introduces the reader to the historical background and the major theoretical challenges presented when 4f electron systems are embedded in a semiconductor matrix. It details the preparation of samples for experimental study, either by in-situ growth or ion implantation/annealing, and describes their microscopic structural characterisation. Optical spectroscopy is a dominant theme, complicated by site multiplicity, whether in homogeneous hosts or in heterostructures such as quantum dots, and enlivened by the abiding fascination of the energy transfer mechanism between the host material and the lumophore. Finally, the rapid progress towards prospective optoelectronic and spintronic devices is presented along with several examples

    @incollection{strathprints30174,
    author = {Kevin O'Donnell},
    series = {Topics in applied physics},
    booktitle = {Rare-earth doped III-nitrides for optoelectronic and spintronic applications},
    editor = {Kevin O'Donnell and Volkmar Dierolf},
    title = {Summary and prospects for future work},
    publisher = {Springer},
    pages = {343--345},
    year = {2010},
    keywords = {optical \& plasma physics , electronic materials, atomic, molecular, Physics},
    url = {http://strathprints.strath.ac.uk/30174/},
    abstract = {This book provides a snapshot of recent progress in the field of rare-earth-doped group III-nitride semiconductors, especially GaN, but extending to AlN and the alloys AlGaN, AlInN and InGaN. This material class is currently enjoying an upsurge in interest due to its ideal suitability for both optoelectronic and spintronic applications. The text first introduces the reader to the historical background and the major theoretical challenges presented when 4f electron systems are embedded in a semiconductor matrix. It details the preparation of samples for experimental study, either by in-situ growth or ion implantation/annealing, and describes their microscopic structural characterisation. Optical spectroscopy is a dominant theme, complicated by site multiplicity, whether in homogeneous hosts or in heterostructures such as quantum dots, and enlivened by the abiding fascination of the energy transfer mechanism between the host material and the lumophore. Finally, the rapid progress towards prospective optoelectronic and spintronic devices is presented along with several examples}
    }

  • K. Lorenz, E. Alves, I. S. Roqan, K. P. O'Donnell, A. Nishikawa, Y. Fujiwara, and M. Bockowsk