Join us! Jobs / PhD studentships available
The Semiconductor Spectroscopy and Devices research group is part of the Nanoscience division of the Department of Physics at the University of Strathclyde. The Department is a member of the Scottish Universities Physics Alliance (SUPA).

Research interests

Group news
  • August 2017: Elena Pascal was jointly awarded the Corbett Prize at the 29th ICDS conference, Matsue, Japan.
  • July 2017: The Group was well represented at the ICNS-12 conference in Strasbourg, presenting invited and contributed talks and several posters.
  • July 2017: Our new JEOL JXA-8530F field-emission electron probe microanalyser (EPMA) is now up and running.
Latest publications
A complete list of our papers can be found here.

  • 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, pp. 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.}
    }

  • 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, 2017.
    [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 = {2017},
    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/}
    }

  • 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, pp. 1-14, 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},
    pages = {1--14},
    month = {November},
    note = {The following article has been accepted by Applied Physics Letters. After it is published, it will be found at http://aip.scitation.org/journal/apl/.},
    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, 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},
    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/}
    }

  • 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/}
    }