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

• Semiconductor materials issues, including:
• Group III nitrides: AlN, GaN, InN and their alloys
• Materials doped with rare earth ions such as Eu3+, Er3+ and Tb3+
• Chalcopyrites and kesterites for solar cells, such as Cu(In,Ga)(S,Se)2 and CZTS
• Spatially and spectrally resolved optical luminescence characterisation, including:
• Advanced scanning electron microscopy techniques, including:
• Theoretical studies, including:
Latest publications
A complete list of our papers can be found here.

• 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, pp. 30-38, 2017.

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/}
}

• 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.

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/}
}

• 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, pp. 1-5, 2017.

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},
pages = {1--5},
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/}
}

• 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, 2017.

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,
month = {March},
title = {Diffraction effects and inelastic electron transport in angle-resolved microscopic imaging applications},
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},
year = {2017},
journal = {Journal of Microscopy},
keywords = {electron diffraction, electron microscope, cathodoluminescence, Physics, Physics and Astronomy(all)},
url = {http://strathprints.strath.ac.uk/60424/},
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.}
}

• 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, 2017.

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},
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/}
}