Papers

@article{strathprints92845,

title = {Strain-induced modifications of thin film silicon membranes by physical bending},

author = {Eleni Margariti and Jochen Bruckbauer and Aimo Winkelmann and Benoit Guilhabert and Naresh-Kumar Gunasekar and Carol Trager-Cowan and Robert Martin and Michael Strain},

url = {https://doi.org/10.3390/ma18102335},

issn = {1996-1944},

year  = {2025},

date = {2025-05-01},

journal = {Materials},

volume = {18},

number = {10},

abstract = {Silicon, being the fundamental material for modern semiconductor devices, has seen continuous advancements to enhance its electrical and mechanical properties. Strain engineering is a well-established technique for improving the performance of silicon-based devices. In this paper, we propose a simple method for inducing and permanently maintaining strain in silicon through pure physical bending. By subjecting the silicon substrate to a controlled bending process, we demonstrate the generation of strain levels that persist even after the removal of external stress, with a maximum strain value of 0.4%. We present a comprehensive study of the mechanics behind this phenomenon, a full finite element mechanical model, and experimental verification of the bending-induced strain in Si membranes using electron backscatter diffraction measurements. Our findings show the potential of this approach for strain engineering in high-performance silicon-based technologies without resorting to complex and expensive fabrication techniques.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints92350,

title = {Strain and luminescence properties of hexagonal hillocks in N-polar GaN},

author = {Jochen Bruckbauer and Grzegorz Cios and Andrei Sarua and Peng Feng and Tao Wang and Ben Hourahine and Aimo Winkelmann and Carol Trager-Cowan and Robert Martin},

url = {https://doi.org/10.1063/5.0259840},

issn = {0021-8979},

year  = {2025},

date = {2025-04-01},

journal = {Journal of Applied Physics},

volume = {137},

number = {13},

abstract = {Owing to its unique properties, N-polar GaN offers several advantages over Ga-polar GaN, particularly for applications in high power electronics. However, the growth of high-quality N-polar material is challenging. One dominant issue is the increased surface roughness, due to the occurrence of hexagonal-shaped hillocks, referred to as hexagons, on the material's surface. Although, there are different methods to reduce the density of these hillocks, such as the use of vicinal substrates or optimum growth conditions, the properties of such hillocks are not extensively studied. Here, we investigate the crystallographic and luminescence properties of these hexagonal features using the techniques of electron backscatter diffraction (EBSD) and cathodoluminescence (CL) hyperspectral imaging in the scanning electron microscope combined with micro-Raman mapping. CL revealed increased light emission from the top of the hexagons compared with the surrounding material. Additionally, dark spots in intensity images, associated with non-radiative recombination at threading dislocations, could be resolved on top of the hexagons, but not in the surrounding area, implying improved material quality of the hexagons. Extensive strain analysis using EBSD revealed that the hexagons are composed of equivalent triangular segments with tensile strain along symmetrically equivalent ensuremath<11-20ensuremath> directions. As the hexagons become larger, this strain increases with distance from the centre. This was confirmed by mapping the Raman E2 (high) mode. Overall this provides crucial insight into the strain state of these hexagonal features},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints92316,

title = {Role of the electron transport layer in dictating the nanoscale heterogeneity in all-inorganic perovskite absorbers - correlating the optoelectronic and crystallographic properties},

author = {Stefan Nicholson and Jochen Bruckbauer and Paul R. Edwards and Carol Trager-Cowan and Robert W. Martin and Aruna Ivaturi},

url = {https://doi.org/10.1039/D4TA07152B},

issn = {2050-7488},

year  = {2025},

date = {2025-04-01},

journal = {Journal of Materials Chemistry A},

volume = {13},

number = {15},

pages = {11003–11014},

abstract = {Within the field of perovskite photovoltaics, there has been heavy focus on either improving the conductivity/mobility of the charge transport layers [electron transport (ETL) or hole transport layers (HTL)] or tuning their energy alignment with the perovskite absorber for optimising the device efficiency, with little attention paid to the impact of the underlying charge transport layer on the structural and optoelectronic properties of the perovskite overlayer. For example, in the n-i-p device architecture, the ETL provides a key surface upon which the perovskite film grows. In this work, electron backscatter diffraction (EBSD) and cathodoluminescence (CL) spectroscopy are used to show a direct correlation between optical emission and structural properties of all-inorganic CsPbI2Br perovskite absorber thin films with a selection of inorganic underlying ETLs, giving insights into the vital role of the ETL. Comparisons are drawn between the effect of three commonly used electron transport layers (zinc oxide, titanium dioxide and tin oxide) on the optical emission and crystallographic properties of the CsPbI2Br perovskite thin films processed at two different annealing temperatures. Among the ETLs, zinc oxide is found to promote perovskite films with enhanced grain size and preferred growth along the [100] orientation, and relatively uniform light emission for the high temperature processed layer, showing its strong potential as a low-cost electron transport layer for the development of perovskite solar cells. Titanium dioxide is found to result in a high level of heterogeneity in the light emission when the perovskite is processed at low temperature, while tin oxide is found not to promote large grain growth. The observed variations are understood in terms of the differences in thermal expansion coefficient of the perovskite as compared to those of the ETLs as well as the leading strain in the lattice. The results from the study show the importance of considering perovskite growth effects when selecting an underlayer.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints91679,

title = {Investigation of (mis-)orientation in zincblende GaN grown on micro-patterned Si(001) using electron backscatter diffraction},

author = {Dale M. Waters and Bethany Thompson and Gergely Ferenczi and Ben Hourahine and Grzegorz Cios and Aimo Winkelmann and Christoph J. M. Stark and Christian Wetzel and Carol Trager-Cowan and Jochen Bruckbauer},

url = {https://doi.org/10.1063/5.0244438},

issn = {0021-8979},

year  = {2025},

date = {2025-01-01},

journal = {Journal of Applied Physics},

volume = {137},

number = {4},

abstract = {We present the application of electron backscatter diffraction (EBSD) as a technique for characterizing wurtzite (wz) and zincblende (zb) polytypes of GaN grown upon micropatterned Si (001) substrates. The Si substrate is etched to create parallel V-shaped grooves with opposing 111 facets before the deposition of GaN. EBSD revealed that wz-GaN growth fronts initially form on the 111 Si facets before undergoing a transition from a wurtzite to zincblende structure as the two growth fronts meet. Orientation analysis of the GaN structures revealed that the wz-GaN growth fronts had different growth orientations but shared the same crystallographic relationship with the zb-GaN such that $perp$303?8wz$parallel$$łangle$110$rangle$zb, $łangle$112?0$rangle$wz$parallel$$łangle$110$rangle$zb, and $perp$303?4wz$parallel$$łangle$001$rangle$zb. Furthermore, the crystallographic relationship, 0001wz-GaN$parallel$111zb-GaN$parallel$111Si, and alignment of the wz- and zb-GaN with respect to the Si substrate was investigated. The two wz-GaN $łangle$0001$rangle$ growth directions were expected to coalesce at an angle of 109.5o; however, measurements revealed an angle of 108o. The resultant misalignment of 1.5o induces misorientation in the zb-GaN crystal lattice. While the degree of misorientation within the zb-GaN lattice is low, <1o, the zb-GaN lattice is deformed and bends toward the wz-GaN interfaces about the specimen direction parallel to the length of the V-groove. Further EBSD measurements over larger areas of the sample revealed that these results were consistent across the sample. However, it was also revealed that additional factors induce changes in the orientation of the zb-GaN lattice, which may relate to the initial growth conditions of the zb-GaN.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints89976,

title = {Unravelling the chloride dopant induced film improvement in all-inorganic perovskite absorbers},

author = {Stefan Nicholson and Jochen Bruckbauer and Paul R. Edwards and Carol Trager-Cowan and Robert W. Martin and Aruna Ivaturi},

url = {https://doi.org/10.1039/D4TA01259C},

issn = {2050-7488},

year  = {2024},

date = {2024-10-01},

journal = {Journal of Materials Chemistry A},

volume = {12},

number = {37},

pages = {25131–25139},

abstract = {CsPbI2Br perovskite material has been the focus of much recent research, thanks to its improved stability over CsPbI3, useful bandgap of 1.9 eV and enhanced thermal stability over hybrid perovskite materials with volatile organic components. It has great potential for both single junction solar cells for indoor applications, and implementation in tandem cells. However, moisture stability has remained an issue. In order to overcome this roadblock towards commercialisation, metal chloride dopants have been widely investigated to improve film quality and reduce damage from humidity. Most of the studies report that the metal cation in the dopant plays a greater role in the improvement of the film properties than the chloride anions, which are thought to be removed during annealing in some studies. The majority of the research to date on this topic has focussed on investigating device performance and bulk film characteristics, with limited attention paid to grain-level crystallinity and whether the dopant is proportionally incorporated into the film. In the present work, cathodoluminescence (CL) and electron backscatter diffraction (EBSD) are utilised to investigate the effects of a lead chloride dopant, both on emission and crystal structure at a grain level, with the findings supported by X-ray diffraction (XRD). Confirmation of proportional incorporation of the dopant into the final prepared films is provided by wavelength dispersive X-ray (WDX) spectroscopy. This work provides valuable insight into the impact chloride dopants have on all-inorganic perovskite absorbers, helping to influence future dopant strategies.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints86829,

title = {Imaging threading dislocations and surface steps in nitride thin films using electron backscatter diffraction},

author = {Kieran P Hiller and Aimo Winkelmann and Ben Hourahine and Bohdan Starosta and Aeshah Alasmari and Peng Feng and Tao Wang and Peter J Parbrook and Vitaly Z Zubialevich and Sylvia Hagedorn and Sebastian Walde and Markus Weyers and Pierre-Marie Coulon and Philip A Shields and Jochen Bruckbauer and Carol Trager-Cowan},

url = {https://doi.org/10.1093/micmic/ozad118},

doi = {10.1093/micmic/ozad118},

issn = {1431-9276},

year  = {2023},

date = {2023-12-01},

journal = {Microscopy and Microanalysis},

volume = {29},

number = {6},

pages = {1879–1888},

abstract = {Extended defects, like threading dislocations, are detrimental to the performance of optoelectronic devices. In the scanning electron microscope, dislocations are traditionally imaged using diodes to monitor changes in backscattered electron intensity as the electron beam is scanned over the sample, with the sample positioned so the electron beam is at, or close to the Bragg angle for a crystal plane/planes. Here we use a pixelated detector instead of single diodes, specifically an electron backscatter diffraction (EBSD) detector. We present post-processing techniques to extract images of dislocations and surface steps, for a nitride thin film, from measurements of backscattered electron intensities and intensity distributions in unprocessed EBSD patterns. In virtual diode (VD) imaging, the backscattered electron intensity is monitored for a selected segment of the unprocessed EBSD patterns. In center of mass (COM) imaging, the position of the center of the backscattered electron intensity distribution is monitored. Additionally, both methods can be combined (VDCOM). Using both VD and VDCOM, images of only threading dislocations, or dislocations and surface steps can be produced, with VDCOM images exhibiting better signal-to-noise. The applicability of VDCOM imaging is demonstrated across a range of nitride semiconductor thin films, with varying surface step and dislocation densities.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints86657,

title = {Electroluminescence hyperspectral imaging of light-emitting diodes using a liquid crystal tunable filter},

author = {Paul R. Edwards and Jochen Bruckbauer and Douglas Cameron and Robert W. Martin},

url = {https://doi.org/10.1063/5.0165060},

doi = {10.1063/5.0165060},

issn = {0003-6951},

year  = {2023},

date = {2023-09-01},

journal = {Applied Physics Letters},

volume = {123},

number = {11},

abstract = {We demonstrate the use of a low-cost liquid-crystal-based wavelength-tunable filter and CMOS video camera to add hyperspectral imaging capabilities to a probe station equipped with a simple optical microscope. The resultant setup is used to rapidly resolve the spectral and spatial variations in electroluminescence typically observed for InxGa1?xN/GaN light-emitting diodes. Applying standard statistical analyses of variation within the multivariate datasets, such as moments and principal components, we observe inhomogeneities on a spectral scale significantly smaller than the bandwidth of the tunable filter. The resultant tool offers an alternative to scanning beam luminescence techniques for high-throughput hyperspectral analysis of optoelectronic devices.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints86094,

title = {A cross-linkable, organic down-converting material for white light emission from hybrid LEDs},

author = {Hao Yang and Jochen Bruckbauer and Lyudmila Kanibolotskaya and Alexander L. Kanibolotsky and Joseph Cameron and David J. Wallis and Robert W. Martin and Peter J. Skabara},

url = {https://doi.org/10.1039/D2TC05139G},

doi = {10.1039/D2TC05139G},

issn = {2050-7526},

year  = {2023},

date = {2023-07-01},

journal = {Journal of Materials Chemistry. C},

volume = {11},

number = {29},

pages = {9984–9995},

abstract = {The use of organic materials and the replacement of rare-earth elements in the making of light-emitting devices has been increasingly popular over the last decades. Herein, the synthesis and characterisation of a novel organic green-emitting material (GreenCin), based on a fluorene-benzothiadiazole-fluorene (Flu-BT-Flu) core structure, and its performance as a down-converting layer in tandem with commercial blue light-emitting diodes (LEDs) for white light emission are reported. This material has been functionalised with cinnamate-groups to enable the emissive material to react with the cross-linker tetra(cinnamoyloxymethyl)methane (TCM), to produce stable films with high performance in hybrid LEDs. The hybrid devices can generate white light with a good colour rendering index (CRI) of 69. The hybrid devices also have $times$2.6 increased luminous efficacy (107 lm W?1) and $times$2.4 increased radiant flux (24 mW) when compared with hybrid devices using non-cross-linked analogues of GreenCin. Additionally, the hybrid devices containing GreenCin have a high blue-to-white efficacy value (defined by dividing the luminous flux of a hybrid device by the radiant flux of the underlying blue LED), of 213 lm W?1, for which inorganic phosphors have values in the range of 200-300 lm W?1.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints85633,

title = {Influence of polyvinylpyrrolidone (PVP) in the synthesis of luminescent NaYF4:Yb,Er upconversion nanoparticles},

author = {Ross Birch and Jochen Bruckbauer and Marta Gajewska and Grzegorz Cios and Robert Pal and Lewis E MacKenzie},

url = {https://doi.org/10.1088/2050-6120/acd837},

doi = {10.1088/2050-6120/acd837},

issn = {2050-6120},

year  = {2023},

date = {2023-05-01},

journal = {Methods and Applications in Fluorescence},

volume = {11},

number = {3},

abstract = {Polyvinylpyrrolidone (PVP) can be used to produce upconversion nanoparticles (UCNPs) in an advantageous manner, i.e. at modest temperatures in open-to-air conditions with simple hotplate and flask apparatus. However, the influence of PVP parameters on the formation of UCNPs has not been previously investigated. In this exploratory study, we establish that PVP molecular weight and relative amount of PVP can greatly influence the morphology and diameter of NaYF4:Yb,Er UCNPs produced via the PVP-assisted route. At nominal amounts of PVP, varying the molecular weight of PVP in synthesis between 10,000 g/mol (PVP10), 40,000 g/mol (PVP40), and 55,000 g/mol (PVP55), had minimal effect on UCNP morphology, whereas reducing the quantity of PVP10 and PVP40 in the reaction to 10% of the nominal amount resulted in two notable effects: (1) the generation of a greater range of UCNP diameters and (2) the production of an unexpected sub-population of rhombus-shaped UCNPs. Bulk and individual nanoparticle analysis indicates that all UCNP morphologies were cubic (ensuremathalpha-phase) crystal structure and consisted of NaYF4:Yb,Er. Optical emission properties exhibited only modest green and red luminescence emission ratio when PVP parameters were varied. However, separately produced PVP40 NaYF4:Yb,Tm UCNPs exhibited a much more intense and dual-band blue /red emission. This exploratory work demonstrates that tailoring PVP content in synthesis of UCNPs can greatly alter morphology of UCNPs produced and should be carefully considered in experimental design. However, the underlying mechanisms of action of the role PVP plays in this synthesis remain unclear. Ultimately, significant further work is still required to fully elucidate the relevant chemistry to achieve full control of PVP-UCNP synthesis.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints79652,

title = {Crystalline grain engineered CsPbIBr2 films for indoor photovoltaics},

author = {Paheli Ghosh and Jochen Bruckbauer and Carol Trager-Cowan and Lethy Krishnan Jagadamma},

url = {https://doi.org/10.1016/j.apsusc.2022.152865},

doi = {10.1016/j.apsusc.2022.152865},

issn = {0169-4332},

year  = {2022},

date = {2022-08-01},

journal = {Applied Surface Science},

volume = {592},

abstract = {Indoor photovoltaic devices have garnered profound research attention in recent years due to their prospects of powering 'smart' electronics for the Internet of Things (IoT). Here it is shown that all-inorganic Cs-based halide perovskites are promising for indoor light harvesting due to their wide bandgap matched to the indoor light spectra. Highly crystalline and compact CsPbIBr2 perovskite based photovoltaic devices have demonstrated a power conversion efficiency (PCE) of 14.1% under indoor illumination of 1000 lux and 5.9% under 1 Sun. This study revealed that a reduction in grain misorientation, as well as suppression of defects in the form of metallic Pb in the perovskite film are crucial for maximising the photovoltaic properties of CsPbIBr2 based devices. It was demonstrated that a pinhole free CsPbIBr2/Spiro-OMeTAD interface preserves the perovskite alpha phase and enhances the air stability of the CsPbIBr2 devices. These devices, despite being unencapsulated, retained ensuremath>55% of the maximum PCE even when stored under 30% relative humidity for a shelf-life duration of 40 days and is one of the best stability data reported so far for CsPbIBr2 devices.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints79640,

title = {Non-destructive imaging of residual strains in GaN and their effect on optical and electrical properties using correlative light-electron microscopy},

author = {G. Naresh-Kumar and P. R. Edwards and T. Batten and M. Nouf-Allehiani and A. Vilalta-Clemente and A. J. Wilkinson and E. Le Boulbar and P. A. Shields and B. Starosta and B. Hourahine and R. W. Martin and C. Trager-Cowan},

url = {https://doi.org/10.1063/5.0080024},

issn = {0021-8979},

year  = {2022},

date = {2022-02-01},

journal = {Journal of Applied Physics},

volume = {131},

number = {7},

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 channeling contrast imaging suggests that the dislocations organize 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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints78647,

title = {Kikuchi pattern simulations of backscattered and transmitted electrons},

author = {Aimo Winkelmann and Gert Nolze and Grzegorz Cios and Tomasz Tokarski and Piotr Bała and Ben Hourahine and Carol Trager-Cowan},

url = {https://doi.org/10.1111/jmi.13051},

issn = {0022-2720},

year  = {2021},

date = {2021-11-01},

journal = {Journal of Microscopy},

volume = {284},

number = {2},

pages = {157–184},

abstract = {We discuss a refined simulation approach which treats Kikuchi diffraction patterns in electron backscatter diffraction (EBSD) and transmission Kikuchi diffraction (TKD). The model considers the result of two combined mechanisms: (a) the dynamical diffraction of electrons emitted coherently from point sources in a crystal and (b) diffraction effects on incoherent diffuse intensity distributions. Using suitable parameter settings, the refined simulation model allows to reproduce various thickness- and energy-dependent features which are observed in experimental Kikuchi diffraction patterns. Excess-deficiency features are treated by the effect of gradients in the incoherent background intensity. Based on the analytical two-beam approximation to dynamical electron diffraction, a phenomenological model of excess-deficiency features is derived, which can be used for pattern matching applications. The model allows to approximate the effect of the incident beam geometry as a correction signal for template patterns which can be reprojected from pre-calculated reference data. As an application, we find that the accuracy of fitted projection centre coordinates in EBSD and TKD can be affected by changes in the order of 10-3-10-2 if excess-deficiency features are not considered in the theoretical model underlying a best-fit pattern matching approach. Correspondingly, the absolute accuracy of simulation-based EBSD strain determination can suffer from biases of a similar order of magnitude if excess-deficiency effects are neglected in the simulation model.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints74054,

title = {A systematic comparison of polar and semipolar Si-doped AlGaN alloys with high AlN content},

author = {Lucia Spasevski and Gunnar Kusch and Pietro Pampili and Vitaly Z Zubialevich and Duc V Dinh and Jochen Bruckbauer and Paul R Edwards and Peter J Parbrook and Robert W Martin},

url = {https://doi.org/10.1088/1361-6463/abbc95},

doi = {10.1088/1361-6463/abbc95},

issn = {0022-3727},

year  = {2021},

date = {2021-01-01},

journal = {Journal of Physics D: Applied Physics},

volume = {54},

number = {3},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints73997,

title = {Influence of micro-patterning of the growth template on defect reduction and optical properties of non-polar (11-20) GaN},

author = {Jochen Bruckbauer and Yipin Gong and Ling Jiu and Michael J Wallace and Anja Ipsen and Sebastian Bauer and Raphael Müller and Jie Bai and Klaus Thonke and Tao Wang and Carol Trager-Cowan and Robert W Martin},

url = {https://doi.org/10.1088/1361-6463/abbc37},

doi = {10.1088/1361-6463/abbc37},

issn = {1361-6463},

year  = {2020},

date = {2020-10-01},

journal = {Journal of Physics D: Applied Physics},

volume = {54},

number = {2},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints74728,

title = {Advances in electron channelling contrast imaging and electron backscatter diffraction for imaging and analysis of structural defects in the scanning electron microscope},

author = {C. Trager-Cowan and A. Alasmari and W. Avis and J. Bruckbauer and P. R. Edwards and B. Hourahine and S. Kraeusel and G. Kusch and B. M. Jablon and R. Johnston and R. W. Martin and R. McDermott and G. Naresh-Kumar and M. Nouf-Allehiani and E. Pascal and D. Thomson and S. Vespucci and K. Mingard and P. J. Parbrook and M. D. Smith and J. Enslin and F. Mehnke and M. Kneissl and C. Kuhn and T. Wernicke and A. Knauer 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. Gong and R. M. Smith and T. Wang and A. Winkelmann},

url = {https://doi.org/10.1088/1757-899X/891/1/012023},

doi = {10.1088/1757-899X/891/1/012023},

issn = {1757-899X},

year  = {2020},

date = {2020-08-01},

journal = {IOP Conference Series: Materials Science and Engineering},

volume = {891},

number = {1},

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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints73784,

title = {Nanomechanical behaviour of individual phases in WC-Co cemented carbides, from ambient to high temperature},

author = {F. De Luca and H. Zhang and K. Mingard and M. Stewart and B. M. Jablon and C. Trager-Cowan and M. G. Gee},

url = {https://doi.org/10.1016/j.mtla.2020.100713},

issn = {2589-1529},

year  = {2020},

date = {2020-08-01},

journal = {Materialia},

volume = {12},

abstract = {The dependence of the mechanical behaviour of individual phases in WC-Co on microstructural parameters such as grain size and orientation were investigated by means of nanoindentation and electron microscopy. A broad range of WC grain dimensions, from about 1 to 1000 um2, were selected and subsequently indented to investigate any size effect. A decrease in hardness as a function of grain dimensions was observed, due to an increase in dislocation mobility in larger grains. Whilst the binder phase only exhibits a hardness of about 11 GPa, the hardness of WC grains was measured about 29 and 53 GPa for the prismatic and basal orientations, respectively, in ambient conditions. All WC orientations exhibited a similar decrease in hardness with temperature, up to 700 ?C. Damage mechanisms occurring in WC-Co during nanoindentation were investigated for the different grain orientations at various temperatures. The damage was visualised using electron microscopy near the residual indent coupled with Focused Ion Beam (FIB) sectioning across the indent. The three-dimensional distribution of plastic deformation across multiple grains in the vicinity of an indent was examined using Electron Channelling Contrast Imaging (ECCI). ECCI micrographs enabled the observation of crystal defects, especially dislocations, and slip lines as well as the entire plastic zone. The defect density and spatial distribution in the deformed WC grains were compared to that of an untested WC grain to identify the type of deformation originating from spherical indentation. The work provides important information on the relationship between WC-Co microstructure and performance at operating temperatures.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints73474,

title = {Influence of an InGaN superlattice pre-layer on the performance of semi-polar (11-22) green LEDs grown on silicon},

author = {X. Zhao and K. Huang and J. Bruckbauer and S. Shen and C. Zhu and P. Fletcher and P. Feng and Y. Cai and J. Bai and C. Trager-Cowan and R. W. Martin and T. Wang},

url = {https://doi.org/10.1038/s41598-020-69609-4},

doi = {10.1038/s41598-020-69609-4},

issn = {2045-2322},

year  = {2020},

date = {2020-07-01},

journal = {Scientific Reports},

volume = {10},

number = {1},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints72016,

title = {Metrology of crystal defects through intensity variations in secondary electrons from the diffraction of primary electrons in a scanning electron microscope},

author = {G. Naresh-Kumar and A. Alasamari and G. Kusch and P. R. Edwards and R. W. Martin and K. P. Mingard and C. Trager-Cowan},

url = {https://doi.org/10.1016/j.ultramic.2020.112977},

issn = {0304-3991},

year  = {2020},

date = {2020-06-01},

journal = {Ultramicroscopy},

volume = {213},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints71512,

title = {Structural and luminescence imaging and characterisation of semiconductors in the scanning electron microscope},

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},

url = {https://doi.org/10.1088/1361-6641/ab75a5},

doi = {10.1088/1361-6641/ab75a5},

issn = {0268-1242},

year  = {2020},

date = {2020-03-01},

journal = {Semiconductor Science and Technology},

volume = {35},

number = {5},

abstract = {The scanning electron microscopy techniques of electron backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI) and cathodoluminescence (CL) hyperspectral imaging 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 material's 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 = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints71224,

title = {A poly(urethane)-encapsulated benzo[2,3-d:6,7-d']diimidazole organic down-converter for green hybrid LEDs},

author = {Alan A. Wiles and Jochen Bruckbauer and Nabeel Mohammed and Michele Cariello and Joseph Cameron and Neil J. Findlay and Elaine Taylor-Shaw and David J. Wallis and Robert W. Martin and Peter J. Skabara and Graeme Cooke},

url = {https://doi.org/10.1039/C9QM00771G},

doi = {10.1039/C9QM00771G},

year  = {2020},

date = {2020-03-01},

journal = {Materials Chemistry Frontiers},

volume = {4},

number = {3},

pages = {1006–1012},

abstract = {The development of organic down-converting materials continues to attract attention in hybrid LED technology by obviating the need for non-sustainable rare-earth elements. In this work, a benzodiimidazole-based system (TPA-BDI) has been employed as a down-converting layer in a hybrid organic-inorganic LED device. A commercially available poly(urethane)-based resin is used as the encapsulating material, providing a dilute layer of TPA-BDI that is deposited on top of the GaN-based LED. Crucially, the solution-state emissive performance is generally maintained when encapsulated at low concentrations within this resin. A maximum luminous efficacy of 87 lm W -1 was demonstrated using a 1.0 mg ml -1 concentration of TPA-BDI in the resin. The suitability of using organic down-converters to produce green light from hybrid devices was demonstrated by the excellent repeatability of the device characteristics across a series of encapsulated LEDs.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints70604,

title = {Subgrain structure and dislocations in WC-Co hard metals revealed by electron channelling contrast imaging},

author = {B. M. Jablon and K. Mingard and A. Winkelmann and G. Naresh-Kumar and B. Hourahine and C. Trager-Cowan},

url = {https://doi.org/10.1016/j.ijrmhm.2019.105159},

issn = {0263-4368},

year  = {2020},

date = {2020-02-01},

journal = {International Journal of Refractory Metals and Hard Materials},

volume = {87},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints71326,

title = {Improving EBSD precision by orientation refinement with full pattern matching},

author = {A. Winkelmann and B. M. Jablon and V. S. Tong and C. Trager-Cowan and K. P. Mingard},

url = {https://doi.org/10.1111/jmi.12870},

issn = {0022-2720},

year  = {2020},

date = {2020-02-01},

journal = {Journal of Microscopy},

volume = {277},

number = {2},

pages = {79–92},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints70583,

title = {AlN overgrowth of nano-pillar-patterned sapphire with different offcut angle by metalorganic vapor phase epitaxy},

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},

url = {https://doi.org/10.1016/j.jcrysgro.2019.125343},

issn = {0022-0248},

year  = {2020},

date = {2020-02-01},

journal = {Journal of Crystal Growth},

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 um 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 0.2o 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 0.1o 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 = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints70977,

title = {Luminescence behavior of semipolar (10-11) InGaN/GaN "bow-tie" structures on patterned Si substrates},

author = {Jochen Bruckbauer and Carol Trager-Cowan and Ben Hourahine and Aimo Winkelmann and Philippe Vennéguès and Anja Ipsen and Xiang Yu and Xunming Zhao and Michael J. Wallace and Paul R. Edwards and G. Naresh-Kumar and Matthias Hocker and Sebastian Bauer and Raphael Müller and Jie Bai and Klaus Thonke and Tao Wang and Robert W. Martin},

url = {https://doi.org/10.1063/1.5129049},

doi = {10.1063/1.5129049},

issn = {0021-8979},

year  = {2020},

date = {2020-01-01},

journal = {Journal of Applied Physics},

volume = {127},

number = {3},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints69913,

title = {Scanning electron microscopy as a flexible tool for investigating the properties of UV-emitting nitride semiconductor thin films},

author = {C. Trager-Cowan and A. Alasmari and W. Avis and J. 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 A. Knauer and V. Kueller and S. Walde and M. Weyers and P. -M. Coulon and P. A. Shields and Y. Zhang and L. Jiu and Yipin Gong and R. M. Smith and T. Wang and A. Winkelmann},

url = {https://doi.org/10.1364/PRJ.7.000B73},

doi = {10.1364/PRJ.7.000B73},

issn = {2327-9125},

year  = {2019},

date = {2019-10-01},

journal = {Photonics Research},

volume = {7},

number = {11},

pages = {B73–B82},

abstract = {In this paper we describe the scanning electron microscopy techniques of electron backscatter diffraction, electron channeling contrast imaging, wavelength dispersive X-ray spectroscopy, and cathodoluminescence 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 = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints73805,

title = {Two beam toy model for dislocation contrast in ECCI},

author = {Elena Pascal and Ben Hourahine and Carol Trager-Cowan and Marc De Graef},

url = {https://doi.org/10.1017/S1431927619010572},

year  = {2019},

date = {2019-08-01},

journal = {Microscopy and Microanalysis},

volume = {25},

number = {S2},

pages = {1968–1969},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints67670,

title = {Determining GaN nanowire polarity and its influence on light emission in the scanning electron microscope},

author = {G. Naresh-Kumar and J. Bruckbauer and A. Winkelmann and X. Yu and B. Hourahine and P. R. Edwards and T. Wang and C. Trager-Cowan and R. W. Martin},

url = {https://doi.org/10.1021/acs.nanolett.9b01054},

doi = {10.1021/acs.nanolett.9b01054},

issn = {1530-6992},

year  = {2019},

date = {2019-06-01},

journal = {Nano Letters},

volume = {19},

number = {6},

pages = {3863–3870},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints66811,

title = {Implementing fluorescent MOFs as down-converting layers in hybrid light-emitting diodes},

author = {Enrico Angioni and Ross J. Marshall and Neil J. Findlay and Jochen Bruckbauer and Ben Breig and David J. Wallis and Robert W. Martin and Ross S. Forgan and Peter J. Skabara},

url = {https://doi.org/10.1039/C9TC00067D},

doi = {10.1039/C9TC00067D},

issn = {2050-7526},

year  = {2019},

date = {2019-02-01},

journal = {Journal of Materials Chemistry. C},

volume = {7},

number = {8},

pages = {2394–2400},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints66339,

title = {Monolithic multiple colour emission from InGaN grown on patterned non-polar GaN},

author = {Y. Gong and L. Jiu and J. Bruckbauer and J. Bai and R. W. Martin and T. Wang},

url = {https://doi.org/10.1038/s41598-018-37575-7},

doi = {10.1038/s41598-018-37575-7},

issn = {2045-2322},

year  = {2019},

date = {2019-01-01},

journal = {Scientific Reports},

volume = {9},

number = {1},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints65206,

title = {You do what in your microprobe?! The EPMA as a multimode platform for nitride semiconductor characterization},

author = {Paul R. Edwards and G. Naresh-Kumar and Gunnar Kusch and Jochen Bruckbauer and Lucia Spasevski and Catherine G. Brasser and Michael J. Wallace and Carol Trager-Cowan and Robert W. Martin},

url = {https://doi.org/10.1017/S1431927618010619},

doi = {10.1017/S1431927618010619},

issn = {1431-9276},

year  = {2018},

date = {2018-08-01},

journal = {Microscopy and Microanalysis},

volume = {24},

number = {S1},

pages = {2026–2027},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints64858,

title = {Imaging basal plane stacking faults and dislocations in (11-22) GaN using electron channelling contrast imaging},

author = {G. Naresh-Kumar and David Thomson and Y. Zhang and J. Bai and L. Jiu and X. Yu and Y. P. Gong and Richard Smith Martin and Tao Wang and Carol Trager-Cowan},

url = {https://doi.org/10.1063/1.5042515},

issn = {0021-8979},

year  = {2018},

date = {2018-08-01},

journal = {Journal of Applied Physics},

volume = {124},

number = {6},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints63048,

title = {Dislocation contrast in electron channelling contrast images as projections of strain-like components},

author = {E. Pascal and B. Hourahine and G. Naresh-Kumar and K. Mingard and C. Trager-Cowan},

url = {https://doi.org/10.1016/j.matpr.2018.03.057},

issn = {2214-7853},

year  = {2018},

date = {2018-06-01},

journal = {Materials Today: Proceedings},

volume = {5},

number = {Issue},

pages = {14652-14661},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints63662,

title = {Cathodoluminescence studies of chevron features in semi-polar (11-22) InGaN/GaN multiple quantum well structures},

author = {C. Brasser and J. Bruckbauer and Y. P. Gong and L. Jiu and J. Bai and M. Warzecha and P. R. Edwards and T. Wang and R. W. Martin},

url = {https://doi.org/10.1063/1.5021883},

doi = {10.1063/1.5021883},

issn = {0021-8979},

year  = {2018},

date = {2018-05-01},

journal = {Journal of Applied Physics},

volume = {123},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints62987,

title = {Energy-weighted dynamical scattering simulations of electron diffraction modalites in the scanning electron microscope},

author = {Elena Pascal and Saransh Singh and Patrick G. Callahan and Ben Hourahine and Carol Trager-Cowan and Marc De Graef},

url = {https://doi.org/10.1016/j.ultramic.2018.01.003},

issn = {0304-3991},

year  = {2018},

date = {2018-04-01},

journal = {Ultramicroscopy},

volume = {187},

pages = {98–106},

abstract = {Transmission Kikuchi diffraction (TKD) has been gaining momentum as a high resolution alternative to electron back-scattered diffraction (EBSD), adding to the existing electron diffraction modalities in the scanning electron microscope (SEM). The image simulation of any of these measurement techniques requires an energy dependent diffraction model for which, in turn, knowledge of electron energies and diffraction distances distributions is required. We identify the sample-detector geometry and the effect of inelastic events on the diffracting electron beam as the important factors to be considered when predicting these distributions. However, tractable models taking into account inelastic scattering explicitly are lacking. In this study, we expand the Monte Carlo (MC) energy-weighting dynamical simulations models used for EBSD [1] and ECP [2] to the TKD case. We show that the foil thickness in TKD can be used as a means of energy filtering and compare band sharpness in the different modalities. The current model is shown to correctly predict TKD patterns and, through the dictionary indexing approach, to produce higher quality indexed TKD maps than conventional Hough transform approach, especially close to grain boundaries.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints62078,

title = {Practical application of direct electron detectors to EBSD mapping in 2D and 3D},

author = {K. P. Mingard and M. Stewart and M. G. Gee and S. Vespucci and C. Trager-Cowan},

url = {https://doi.org/10.1016/j.ultramic.2017.09.008},

issn = {0304-3991},

year  = {2018},

date = {2018-01-01},

journal = {Ultramicroscopy},

volume = {184},

number = {Part A},

pages = {242–251},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints61607,

title = {Spatially-resolved optical and structural properties of semi-polar (11-22) AlxGa1-xN with x up to 0.56},

author = {Jochen Bruckbauer and Zhi Li and G. Naresh-Kumar and Monika Warzecha and Paul R. Edwards and Ling Jiu and Yipin Gong and Jie Bai and Tao Wang and Carol Trager-Cowan and Robert W. Martin},

url = {https://doi.org/10.1038/s41598-017-10923-9},

doi = {10.1038/s41598-017-10923-9},

issn = {2045-2322},

year  = {2017},

date = {2017-09-01},

journal = {Scientific Reports},

volume = {7},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints61621,

title = {Quantitative imaging of anti-phase domains by polarity sensitive orientation mapping using electron backscatter diffraction},

author = {G. Naresh-Kumar and A. Vilalta-Clemente and H. Jussila and A. Winkelmann and G. Nolze and S. Vespucci and S. Nagarajan and A. J. Wilkinson and C. Trager-Cowan},

url = {https://doi.org/10.1038/s41598-017-11187-z},

issn = {2045-2322},

year  = {2017},

date = {2017-09-01},

journal = {Scientific Reports},

volume = {7},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints63158,

title = {Dynamical simulations of transmission Kikuchi diffraction (TKD) patterns},

author = {Elena Pascal and Saransh Singh and Ben Hourahine and Carol Trager-Cowan and Marc De Graef},

url = {https://doi.org/10.1017/S1431927617003385},

issn = {1431-9276},

year  = {2017},

date = {2017-08-01},

journal = {Microscopy and Microanalysis},

volume = {23},

number = {S1},

pages = {540–541},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints60304,

title = {Design and fabrication of enhanced lateral growth for dislocation reduction in GaN using nanodashes},

author = {E. D. Le Boulbar and J. Priesol and M. Nouf-Allehiani and G. Naresh-Kumar and S. Fox and C. Trager-Cowan and A. vSatka and D. W. E. Allsopp and P. A. Shields},

url = {https://doi.org/10.1016/j.jcrysgro.2017.02.047},

issn = {0022-0248},

year  = {2017},

date = {2017-05-01},

journal = {Journal of Crystal Growth},

pages = {30–38},

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 = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints60424,

title = {Diffraction effects and inelastic electron transport in angle-resolved microscopic imaging applications},

author = {A. Winkelmann and G. Nolze and S. Vespucci and N. Gunasekar and C. Trager-Cowan and A. Vilalta-Clemente and A. J. Wilkinson and M. Vos},

url = {https://doi.org/10.1111/jmi.12571},

issn = {0022-2720},

year  = {2017},

date = {2017-05-01},

journal = {Journal of Microscopy},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints60196,

title = {Diffractive triangulation of radiative point sources},

author = {S. Vespucci and G. Naresh-Kumar and C. Trager-Cowan and K. P. Mingard and D. Maneuski and V. O'Shea and A. Winkelmann},

url = {https://doi.org/10.1063/1.4978858},

issn = {0003-6951},

year  = {2017},

date = {2017-03-01},

journal = {Applied Physics Letters},

volume = {110},

number = {12},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints59864,

title = {Optical investigation of semi-polar (11-22) AlxGa1-xN with high Al composition},

author = {Z. Li and L. Wang and L. Jiu and J. Bruckbauer and Y. Gong and Y. Zhang and J. Bai and R. W. Martin and T. Wang},

url = {https://doi.org/10.1063/1.4977428},

doi = {10.1063/1.4977428},

issn = {0003-6951},

year  = {2017},

date = {2017-02-01},

journal = {Applied Physics Letters},

volume = {110},

number = {9},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints59555,

title = {Exploring transmission Kikuchi diffraction using a Timepix detector},

author = {S. Vespucci and A. Winkelmann and K. Mingard and D. Maneuski and V. O'Shea and C. Trager-Cowan},

url = {https://doi.org/10.1088/1748-0221/12/02/C02075},

issn = {1748-0221},

year  = {2017},

date = {2017-02-01},

journal = {Journal of Instrumentation},

volume = {12},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints59588,

title = {Cross-correlation based high resolution electron backscatter diffraction and electron channelling contrast imaging for strain mapping and dislocation distributions in InAlN thin films},

author = {A. Vilalta-Clemente and G. Naresh-Kumar and M. Nouf-Allehiani and P. Gamarra and M. A. Forte-Poisson and C. Trager-Cowan and A. J. Wilkinson},

url = {https://doi.org/10.1016/j.actamat.2016.11.039},

issn = {1359-6454},

year  = {2017},

date = {2017-02-01},

journal = {Acta Materialia},

volume = {125},

pages = {125–135},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints59478,

title = {Nanoscale fissure formation in AlxGa1-xN/GaN heterostructures and their influence on Ohmic contact formation},

author = {M. D. Smith and D. Thomson and V. Z. Zubialevich and H. Li and G. Naresh-Kumar and C. Trager-Cowan and P. J. Parbrook},

url = {https://doi.org/10.1002/pssa.201600353},

issn = {1862-6300},

year  = {2017},

date = {2017-01-01},

journal = {Physica Status Solidi A},

volume = {214},

number = {1},

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

note = {This is the peer reviewed version of the following article: Smith, M. D., Thomson, D., Zubialevich, V. Z., Li, H., Naresh-Kumar, G., Trager-Cowan, C., & Parbrook, P. J. (2017). Nanoscale fissure formation in AlxGa1-xN/GaN heterostructures and their influence on Ohmic contact formation. Physica Status Solidi A, [1600353]. , which has been published in final form at https://doi.org/10.1002/pssa.201600353. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints58749,

title = {Cool to warm white light emission from hybrid inorganic/organic light-emitting diodes},

author = {Elaine Taylor-Shaw and Enrico Angioni and Neil J. Findlay and Benjamin Breig and Anto R. Inigo and Jochen Bruckbauer and David J. Wallis and Peter J. Skabara and Robert W. Martin},

url = {https://doi.org/10.1039/C6TC03585J},

doi = {10.1039/C6TC03585J},

issn = {2050-7526},

year  = {2016},

date = {2016-11-01},

journal = {Journal of Materials Chemistry. C},

volume = {4},

number = {48},

pages = {11499–11507},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints72242,

title = {Reprint of : Electron channelling contrast imaging for III-nitride thin film structures},

author = {G. Naresh-Kumar and D. Thomson and M. Nouf-Allehiani and J. Bruckbauer and P. R. Edwards and B. Hourahine and R. W. Martin and C. Trager-Cowan},

url = {https://doi.org/10.1016/j.mssp.2016.09.025},

doi = {10.1016/j.mssp.2016.09.025},

issn = {1369-8001},

year  = {2016},

date = {2016-11-01},

journal = {Materials Science in Semiconductor Processing},

volume = {55},

pages = {19–25},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints57454,

title = {Colour tuning in white hybrid inorganic/organic light-emitting diodes},

author = {Jochen Bruckbauer and Catherine Brasser and Neil J. Findlay and Paul R. Edwards and David J. Wallis and Peter J. Skabara and Robert W. Martin},

url = {https://doi.org/10.1088/0022-3727/49/40/405103},

doi = {10.1088/0022-3727/49/40/405103},

issn = {0022-3727},

year  = {2016},

date = {2016-10-01},

journal = {Journal of Physics D: Applied Physics},

volume = {49},

number = {40},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints55555,

title = {Electron channelling contrast imaging for III-nitride thin film structures},

author = {G. Naresh-Kumar and D. Thomson and M. Nouf-Allehiani and J. Bruckbauer and P. R. Edwards and B. Hourahine and R. W. Martin and C. Trager-Cowan},

url = {https://doi.org/10.1016/j.mssp.2016.02.007},

doi = {10.1016/j.mssp.2016.02.007},

issn = {1369-8001},

year  = {2016},

date = {2016-06-01},

journal = {Materials Science in Semiconductor Processing},

volume = {47},

pages = {44–50},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{strathprints54220,

title = {Digital direct electron imaging of energy-filtered electron backscatter diffraction patterns},

author = {S. Vespucci and A. Winkelmann and G. Naresh-Kumar and K. P. Mingard and D. Maneuski and P. R. Edwards and A. P. Day and V. O'Shea and C. Trager-Cowan},

url = {https://doi.org/10.1103/PhysRevB.92.205301},

issn = {0163-1829},

year  = {2015},

date = {2015-11-01},

journal = {Physical Review B (Condensed Matter)},

volume = {92},

number = {20},

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

keywords = {},

pubstate = {published},

tppubtype = {article}

}