• D. Cameron, P. Coulon, S. Fairclough, G. Kusch, P. R. Edwards, N. Susilo, T. Wernicke, M. Kneissl, R. A. Oliver, P. A. Shields, and R. W. Martin, “Core-shell nanorods as ultraviolet light emitting diodes,” Nano Letters, 2023. doi:10.1021/acs.nanolett.2c04826

Existing barriers to efficient deep UV LEDs may be reduced or overcome by moving away from conventional planar growth and towards three dimensional nanostructuring. Nanorods have the potential for enhanced doping, reduced dislocation densities, improved light extraction efficiency and quantum wells free from the quantum confined Stark effect. Here, we demonstrate a hybrid top-down/bottom-up approach to creating highly uniform AlGaN core-shell nanorods on sapphire repeatable on wafer scales. Our GaN-free design avoids self-absorption of the quantum well emission, while preserving electrical functionality. The effective junctions formed by doping of both the n-type cores and p-type caps were studied using nanoprobing experiments where we find low turn on voltages, strongly rectifying behaviours and significant electron beam induced currents. Timeresolved cathodoluminescence measurements find short carrier liftetimes consistent with reduced polarisation fields. Our results show nanostructuring to be a promising route to deep-UV emitting LEDs, achievable using commercially compatible methods.

@article{strathprints84031,
month = {February},
title = {Core-shell nanorods as ultraviolet light emitting diodes},
year = {2023},
doi = {10.1021/acs.nanolett.2c04826},
journal = {Nano Letters},
keywords = {nanorods, LEDs, UV LED, nanowire, core-shell, AIGaN, semiconductors, electron microscopy, Optics. Light, Atomic and Molecular Physics, and Optics},
url = {https://doi.org/10.1021/acs.nanolett.2c04826},
issn = {1530-6992},
abstract = {Existing barriers to efficient deep UV LEDs may be reduced or overcome by moving away from conventional planar growth and towards three dimensional nanostructuring. Nanorods have the potential for enhanced doping, reduced dislocation densities, improved light extraction efficiency and quantum wells free from the quantum confined Stark effect. Here, we demonstrate a hybrid top-down/bottom-up approach to creating highly uniform AlGaN core-shell nanorods on sapphire repeatable on wafer scales. Our GaN-free design avoids self-absorption of the quantum well emission, while preserving electrical functionality. The effective junctions formed by doping of both the n-type cores and p-type caps were studied using nanoprobing experiments where we find low turn on voltages, strongly rectifying behaviours and significant electron beam induced currents. Timeresolved cathodoluminescence measurements find short carrier liftetimes consistent with reduced polarisation fields. Our results show nanostructuring to be a promising route to deep-UV emitting LEDs, achievable using commercially compatible methods.},
author = {Cameron, Douglas and Coulon, Pierre-Marie and Fairclough, Simon and Kusch, Gunnar and Edwards, Paul R. and Susilo, Norman and Wernicke, Tim and Kneissl, Michael and Oliver, Rachel A. and Shields, Philip A. and Martin, Robert W.}
}

• N. Gunasekar, P. Edwards, R. Martin, T. Batten, C. Trager-Cowan, B. Hourahine, B. Starosta, M. Nouf-Allehiani, P. A. Shields, E. D. Le Boulbar, A. Wilkinson, and A. Vilalta-Clemente, “Non-destructive imaging of residual strains in GaN and their effect on optical and electrical properties using correlative light-electron microscopy,” Journal of Applied Physics, vol. 131, iss. 7, 2022. doi:10.1063/5.0080024

We demonstrate a non-destructive approach to understanding the growth modes of a GaN thin film and simultaneously quantify its residual strains and their effect on optical and electrical properties using correlative scanning electron microscopy techniques and Raman microscopy. Coincident strain maps derived from electron backscatter diffraction, cathodoluminescence and confocal Raman techniques reveal strain variations with similar magnitude and directions, especially in the proximity of dislocations. Correlating confocal Raman imaging with electron channelling contrast imaging suggests that the dislocations organise themselves to form a distinctive pattern as a result of the underlying growth mask, where some of them align along the [0001] growth direction and some are inclined. The methodology presented in this work can be adopted to investigate any heteroepitaxial growth, in particular those using selective masks on the growth substrates, where the morphology influences the subsequent growth.

@article{strathprints79640,
volume = {131},
number = {7},
month = {February},
title = {Non-destructive imaging of residual strains in GaN and their effect on optical and electrical properties using correlative light-electron microscopy},
year = {2022},
doi = {10.1063/5.0080024},
journal = {Journal of Applied Physics},
keywords = {SEM, thin films, GaN LED, strain analysis, diffraction and scattering, cathodoluminescence hyperspectral imaging, Physics, Physics and Astronomy(all)},
url = {https://doi.org/10.1063/5.0080024},
issn = {0021-8979},
abstract = {We demonstrate a non-destructive approach to understanding the growth modes of a GaN thin film and simultaneously quantify its residual strains and their effect on optical and electrical properties using correlative scanning electron microscopy techniques and Raman microscopy. Coincident strain maps derived from electron backscatter diffraction, cathodoluminescence and confocal Raman techniques reveal strain variations with similar magnitude and directions, especially in the proximity of dislocations. Correlating confocal Raman imaging with electron channelling contrast imaging suggests that the dislocations organise themselves to form a distinctive pattern as a result of the underlying growth mask, where some of them align along the [0001] growth direction and some are inclined. The methodology presented in this work can be adopted to investigate any heteroepitaxial growth, in particular those using selective masks on the growth substrates, where the morphology influences the subsequent growth.},
author = {Gunasekar, Naresh and Edwards, Paul and Martin, Robert and Batten, Tim and Trager-Cowan, Carol and Hourahine, Ben and Starosta, Bohdan and Nouf-Allehiani, M. and Shields, Philip A. and Le Boulbar, Emmanuel D. and Wilkinson, Angus and Vilalta-Clemente, Arantxa}
}

• 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.

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/},
}

• 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

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.}
}

• 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

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}
}

• 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

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.}
}

• 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.

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/},
}

• 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.

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/},
}

• 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.

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.}
}

• 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.

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/}
}

• 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.

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. -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.

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.}
}

• 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.

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/}
}

• 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.

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/}
}

• 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.

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/}
}

• 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.

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/}
}

• 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.

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/}
}

• 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.

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/}
}

• 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.

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/}
}

• 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.

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/}
}

• 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.

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/}
}

• 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.

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/}
}

• 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.
@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/}
}

• 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.

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},
journal = {Journal of Applied Physics},
title = {Strong photonic crystal behavior in regular arrays of core-shell and quantum disc InGaN/GaN nanorod light-emitting diodes},
year = {2014},
number = {4},
pages = {044305},
volume = {116},
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.

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.}
}

• 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.

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},
journal = {Journal of Applied Physics},
title = {Bias dependence and correlation of the cathodoluminescence and electron beam induced current from an InGaN/GaN light emitting diode},
year = {2014},
number = {3},
pages = {033105},
volume = {116},
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/},
}

• 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.

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},
journal = {Japanese Journal of Applied Physics},
title = {Optical properties of GaN nanorods containing a single or multiple InGaN quantum wells},
year = {2013},
month = {May},
pages = {08JE11},
volume = {52},
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. 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.

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},
journal = {Journal of Applied Physics},
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},
year = {2013},
pages = {094302},
volume = {114},
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/},
}

• 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.

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. 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.

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},
journal = {Semiconductor Science and Technology},
title = {Cathodoluminescence studies of GaN coalesced from nanopyramids selectively grown by MOVPE},
year = {2012},
month = {June},
number = {8},
pages = {085010},
volume = {27},
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/},
}

• C. Liu, A. Satka, L. K. Jagadamma, P. R. Edwards, D. Allsopp, R. W. Martin, P. Shields, J. Kovac, F. Uherek, and W. Wang, “Light emission from InGaN quantum wells grown on the facets of closely spaced GaN nano-pyramids formed by nano-imprinting,” Applied Physics Express, vol. 2, p. 121002, 2009.

InxGa1-xN/GaN quantum wells have been grown on the \{1011\} facets of dense arrays of self-assembled GaN nano-pyramids formed by selective area growth and characterised by high spatial resolution cathodoluminescence. The pyramids are shown to have significantly reduced defect (green-yellow) band emission and the quantum well luminescence is correspondingly intense. The peak energy of this luminescence is shown to blue-shift as the sampled region is moved up the pyramid facets, revealing that InN incorporation in such closely spaced epitaxial nanostructures differs from that in widely spaced micron-size pyramidal structures decreasing rather than increasing towards the nano-pyramid tips.

@Article{strathprints26816,
author = {Chaowang Liu and Alexander Satka and L.K. Jagadamma and P.R. Edwards and D. Allsopp and R.W. Martin and Philip Shields and Jaroslav Kovac and Frantisek Uherek and Wang Wang},
journal = {Applied Physics Express},
title = {Light emission from InGaN quantum wells grown on the facets of closely spaced GaN nano-pyramids formed by nano-imprinting},
year = {2009},
month = {December},
pages = {121002},
volume = {2},
abstract = {InxGa1-xN/GaN quantum wells have been grown on the \{1011\} facets of dense arrays of self-assembled GaN nano-pyramids formed by selective area growth and characterised by high spatial resolution cathodoluminescence. The pyramids are shown to have significantly reduced defect (green-yellow) band emission and the quantum well luminescence is correspondingly intense. The peak energy of this luminescence is shown to blue-shift as the sampled region is moved up the pyramid facets, revealing that InN incorporation in such closely spaced epitaxial nanostructures differs from that in widely spaced micron-size pyramidal structures decreasing rather than increasing towards the nano-pyramid tips.},
keywords = {light emission, luminescence, quantum wells, cathodoluminescence, Physics, Physics and Astronomy(all), Engineering(all)},
url = {http://strathprints.strath.ac.uk/26816/},
}