Gallium oxide (Ga2O3) has attracted tremendous attention in power electronics and ultraviolet photodetectors because of the large bandgap of 4.9–5.3 eV available to all polymorphs, as well as its high electric breakdown voltage. Recently, there has been increasing research interest in thermodynamically metastable phases such as α-, ε- (or κ-), and γ-Ga2O3, because they are predicted to exhibit superior properties compared with β-Ga2O3, the most stable phase of Ga2O3. For example, α-Ga2O3 (bandgap, Eg = 5.3 eV; expected breakdown field, Ec = ∼10 MV/cm) is expected to be a better potential candidate in power electronics than β-Ga2O3 (Eg = 4.5–4.8 eV; Ec = 8 MV/cm) because of its larger bandgap and higher breakdown field. Because these thermodynamically metastable phases cannot be grown using melt-growth techniques, they are grown heteroepitaxially on foreign substrates. We extensively illustrate the growth of these metastable phases and their alloys by employing various growth techniques and then discuss their doping and electronic properties. Finally, we emphasize their applications in devices, including power devices and solar-blind ultraviolet photodetectors.

A simple CAD model based on cavity model analysis is proposed to compute accurately the resonant frequency, input impedance, and gain of radome loaded rectangular patch antenna. The computed values using present model for wide range of radome parameters are compared with different theoretical and experimental values available in open literature. To validate the present model, we have performed several sets of experiments. The present model shows very close agreements with the experiments compared to the other models. We have also used electromagnetic software (HFSS) to generate simulated data. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:2460–2468, 2013

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Surface states serve as additional charge-carrier-trapping centers and create an energy barrier at the semiconductor−electrolyte interface. This in turn may severely reduce the internal quantum efficiency of Al x Ga 1−x N nanowire ultraviolet light-emitting diodes (UV-LEDs) and solar-to-hydrogen energy conversion efficiency of photoelectrodes used in photoelectrochemical water splitting applications. These states also cause Fermi-level pinning and band bending, leading to Shockley−Read−Hall nonradiative recombination. Hence, surface states need to be passivated. In the present study, we used phosphoric acid to passivate the surface states in AlGaN nanowires. The internal quantum efficiency of the near-band-edge emission peak of the chemically treated nanowires was 7%, whereas that of the as-grown nanowires was 3%. Suppression of the oxide layers was achieved, as indicated by the reduced intensity of the O 1s peak. The higher carrier lifetime of 3.2 ns of the treated nanowires compared to the lifetime of 2.6 ns of the as-grown nanowires' directly evidenced passivation of the surface states. Crystallinity loss at the nanowire edges was caused by strain relaxation, resulting in broadening of the A 1 (LO) AlGaN phonon mode. The experiments and findings could be useful in the fabrication of UV-LEDs and photoelectrodes with improved performance for water splitting applications.

Epitaxial growth of (In,Ga)N films on O-face ZnO substrates was studied via plasma-assisted molecular beam epitaxy. Atomically smooth GaN films, showing step edges, were grown at low temperatures to suppress the interfacial reaction between nitrides and the ZnO substrate at elevated temperatures using metal-enhanced epitaxy. High-quality growth of ∼300 nm-thick (In,Ga)N films with the In content varying from 11% to 23% was demonstrated using ∼2 monolayer-thick low temperature GaN as the buffer layer. A clear redshift in (In,Ga)N photoluminescence was observed by decreasing the substrate temperature. For the first time, we achieved an atomically smooth surface on 300 nm-thick GaN grown on ZnO, showing step edges. The surface morphology, however, eventually degraded after exposure to the ambient due to strain, which was perhaps facilitated by the formation of an oxide layer. These results are promising for optoelectronics and electronics applications since the eventual degradation of the surface morphology can be mitigated via strain engineering or surface passivation.