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We present a study of the electrical, structural and chemical properties of Ti contacts on atomic layer deposited α-Ga2O3 film. Ti forms an ohmic contact with α-Ga2O3. The contact performance is highly dependent on the post-evaporation annealing temperature, where an improved conductivity is obtained when annealing at 450 °C, and a strong degradation when annealing at higher temperatures. Structural and chemical characterisation by transmission electron microscopy techniques reveal that the electrical improvement or degradation of the contact upon annealing can be attributed to oxidation of the Ti metallic layer by the Ga2O3 film in combination with the possibility for Ti diffusion into the Au layer. The results highlight that the grain boundaries and inclusions in the Ga2O3 film provide fast diffusion pathways for this reaction, leaving the α-Ga2O3 crystallites relatively unaffected—this result differs from previous reports conducted on β-Ga2O3. This study underlines the necessity for a phase-specific and growth method-specific study of contacts on Ga2O3 devices.

Single-photon emitters with high degrees of purity are required for photonic-based quantum technologies. InGaN/GaN quantum dots are promising candidates for the development of single-photon emitters but have typically exhibited emission with insufficient purity. Here, pulsed single-photon emission with high purity is measured from an InGaN quantum dot. A raw g(2)(0) value of 0.043 ± 0.009 with no corrections whatsoever is achieved under quasi-resonant pulsed excitation. Such a low value is, in principle, sufficient for use in quantum key distribution systems.

α-Ga2O3 is a metastable phase of Ga2O3 of interest for wide bandgap engineering since it is isostructural with α-In2O3 and α-Al2O3. α-Ga2O3 is generally synthesised under high pressure (several GPa) or relatively high temperature (∼500 °C). In this study, we report the growth of α-Ga2O3 by low temperature atomic layer deposition (ALD) on sapphire substrate. The film was grown at a rate of 0.48 A/cycle, and predominantly consists of α-Ga2O3 in the form of ( 0001 ) -oriented columns originating from the interface with the substrate. Some inclusions were also present, typically at the tips of the α phase columns and most likely comprising e-Ga2O3. The remainder of the Ga2O3 film – i.e. nearer the surface and between the α-Ga2O3 columns, was amorphous. The film was found to be highly resistive, as is expected for undoped material. This study demonstrates that α-Ga2O3 films can be grown by low temperature ALD and suggests the possibility of a new range of ultraviolet optoelectronic and power devices grown by ALD. The study also shows that scanning electron diffraction is a powerful technique to identify the different polymorphs of Ga2O3 present in multiphase samples.

© 2017 Author(s). Nitride quantum dots are well suited for the deterministic generation of single photons at high temperatures. However, this material system faces the challenge of large in-built fields, decreasing the oscillator strength and possible emission rates considerably. One solution is to grow quantum dots on a non-polar plane; this gives the additional advantage of strongly polarized emission along one crystal direction. This is highly desirable for future device applications, as is electrical excitation. Here, we report on electroluminescence from non-polar InGaN quantum dots. The emission from one of these quantum dots is studied in detail and found to be highly polarized with a degree of polarization of 0.94. Single-photon emission is achieved under excitation with a constant current giving a g(2)(0) correlation value of 0.18. The quantum dot electroluminescence persists up to temperatures as high as 130 K.

Low temperature atomic layer deposition was used to deposit α-Ga2O3 films, which were subsequently annealed at various temperatures and atmospheres. The α-Ga2O3 phase is stable up to 400 °C, which is also the temperature that yields the most intense and sharpest reflection by x-ray diffraction. Upon annealing at 450 °C and above, the material gradually turns into the more thermodynamically stable ε or β phase. The suitability of the materials for solar-blind photodetector applications has been demonstrated with the best responsivity achieved being 1.2 A W−1 under 240 nm illumination and 10 V bias, for the sample annealed at 400 °C in argon. It is worth noting however that the device performance strongly depends on the annealing conditions, with the device annealed in forming gas behaving poorly. Given that the tested devices have similar microstructure, the discrepancies in device performance are attributed to hydrogen impurities.

Atomic layer deposition (ALD) offers a low thermal budget method for producing α-Ga2O3 films on sapphire substrate. In this paper we review the recent progress on plasma-enhanced ALD growth of α-Ga2O3 and present the optical and photoconductive properties of the deposited films. We show that the deposited material exhibits an epitaxial relationship with the sapphire substrate, and with an atomically sharp film-substrate interface. The α-Ga2O3 films had an optical bandgap energy measured at 5.11 eV, and exhibited a broad luminescence spectrum dominated by ultraviolet, blue and green bands, in line with current literature. We finally demonstrate the suitability of the material for solar-blind photodetection.