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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.}
    }

  • 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,
    month = {October},
    title = {Spectroscopic and electrical signatures of acceptor states in solution processed Cu2ZnSn(S,Se)4 solar cells},
    author = {Devendra Tiwari and Ekaterina Skidchenko and Jake Bowers and M. V. Yakushev and Robert Martin and David J. Fermin},
    year = {2017},
    journal = {Journal of Materials Chemistry. C},
    keywords = {thin films, acceptor states, spectroscopy, Physics, Physics and Astronomy(all)},
    url = {https://strathprints.strath.ac.uk/62349/},
    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.}
    }

  • 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, 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},
    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) Al_xGa_(1-x)N 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) Al\_xGa\_(1-x)N 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/}
    }