Self-organized 3D nanostructured architectures including quasi-ordered concentric hexagonal structures generated during the growth of single crystalline n-GaN substrates by hydride vapor phase epitaxy (HVPE) are reported. The study of as-grown samples by using Kelvin Probe Force Microscopy shows that the formation of self-organized architectures can be attributed to fine modulation of doping related to the spatial distribution of impurities. The specific features of nanostructured architectures involved have been brought to light by using electrochemical and photoelectrochemical etching techniques which are highly sensitive to local doping. The analysis of the results shows that the formation of self-organized spatial architectures in the process of HVPE is caused by the generation of V-pits and their subsequent overgrowth accompanied by the growth in variable direction. It is demonstrated for the first time that the electrical and luminescence properties of HVPE-grown GaN are spatially modulated throughout, including islands between overgrown V-pit regions. The dependence of doping upon growth direction is confirmed by the micro-cathodoluminescence characterization of HVPE-grown pencil-like microcrystals exposing various crystallographic planes along the tip. These results are indicative of new possibilities for defect engineering in gallium nitride and for three-dimensional spatial nanostructuring of this important electronic material by controlling the growth direction. Gallium Nitride (GaN), a wide-bandgap semiconductor compound (E g = 3.4 eV at 300 K), is considered nowadays the second most important semiconductor material after Si. Over the past decades it has played a major role in the development of modern solid-state lighting industry.1-3 An intensive investigation of this compound started in 1970's, but with minimal success in the development of real applications. In the absence of native bulk material, GaN epilayers were grown on foreign substrates and, as a result of the large lattice mismatch and difference in thermal expansion coefficients, the overgrown films contained a high concentration of threading dislocations. The fascinating point, however, is that even in the presence of very high concentrations of dislocations, sometimes exceeding 10 10 cm −2 , gallium nitride exhibits intense luminescence, a property that makes the compound unique in comparison with other III-V materials, such as GaAs, GaP and InP. This particular feature along with other material characteristics were of paramount importance for the development and subsequent commercialization of GaN-based blue light emitting diodes by the mid-1990s. 4 This significant optoelectronic success resulted in the Nobel Prize for Physics being awarded to Shuji Nakamura, Isamu Akasaki and Hiroshi Amano, in 2014. There is no doubt that lighting technologies based on GaN and related nitrides will continue their impactful evolution, particularly in view of the recent demonstration of electrically pumped inversionless polariton lasing at room temper...

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Diamond-based thermionic emission devices could provide a means to produce clean and renewable energy through direct heat-to-electrical energy conversion. Hindering progress of the technology are the thermionic output current and threshold temperature of the emitter cathode. In this report, we study the effects on thermionic emission caused by in situ exposure of the diamond cathode to beta radiation. Nitrogen-doped diamond thin films were grown by microwave plasma chemical vapor deposition on molybdenum substrates. The hydrogen-terminated nanocrystalline diamond was studied using a vacuum diode setup with a 63 Ni beta radiation source-embedded anode, which produced a 2.7-fold increase in emission current compared to a 59 Ni-embedded control. The emission threshold temperature was also examined to further assess the enhancement of thermionic emission, with 63 Ni lowering the threshold temperature by an average of 58 ± 11 °C compared to the 59 Ni control. Various mechanisms for the enhancement are discussed, with a satisfactory explanation remaining elusive. Nevertheless, one possibility is discussed involving excitation of preexisting conduction band electrons that may skew their energy distribution toward higher energies.

General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/userguides/explore-bristol-research/ebr-terms/ Rev. Sci. Instrum. 90, 045110 (2019); https://doi.org/10.1063/1.5088150 90, 045110

Kelvin probe force microscopy (KPFM) and energy-filtered photoelectron emission microscopy (EF-PEEM) with vacuum UV (VUV) excitation have been used to study the work function of p-type diamond films treated to exhibit a negative electron affinity (NEA) surface. NEA was generated by a lithium-oxygen monolayer termination. This monolayer was achieved in two different ways: thermally evaporated films 50 nm thick were either treated by in situ vacuum annealing or by a subsequent water wash. The work function values obtained from these samples by EF-PEEM were compared with KPFM measurements to establish which of the two fabrication techniques was most effective in activating a NEA surface. The washing method was shown to be more effective and the work function values obtained by the two techniques were comparable, as they showed the same work function peaks at 4.54 eV in their respective histograms. It was found that neighbouring polycrystalline facets could show a large variation in work function of up to 400 meV.