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Thermodynamic analyses for the growth of group-III sesquioxides, including α-Al2O3, β-Ga2O3, and c-In2O3, by both ozone and plasma-assisted MBE were performed. In either case, under O-rich conditions, the driving force for III2O3 (III = Al, Ga, In) growth ( Δ P III 2 O 3 ) increased with increasing input partial pressure of the group-III metal ( P III o ), without generation of metal droplets. Conversely, under group-III-metal-rich conditions, Δ P III 2 O 3 decreased with increasing P III o and/or decreasing input partial pressure of O3 or O. This decrease was caused by the formation of Ga2O or In2O during growth of β-Ga2O3 and c-In2O3. The decrease of Δ P Al 2 O 3 was smaller because the equilibrium constant of α-Al2O3 formation reaction was very large. Ga and In droplets formed at low temperatures (2O3 < β-Ga2O3 << α-Al2O3.

The heteroepitaxial growth of In2O3 on a (0001) α-Al2O3 substrate was carried out using the mist chemical vapor deposition method. The concentrations of In2O3 powder in the source precursor solution varied between 0.025 and 0.250 mol l−1. An increase in the growth rate was observed with the increasing In2O3 powder concentration, and the highest growth rate of 4.2 μm h−1 was obtained at a powder concentration of 0.150 mol l−1. An improvement in the electrical properties was eventually observed, i.e. the lowest carrier concentration of 3.1 × 1017 cm−3 and the highest Hall mobility of 241 cm2 V−1 s−1 at room temperature were obtained at a powder concentration of 0.150 mol l−1. The temperature-dependent Hall effect measurement for the sample showed pronounced carrier freeze-out in a temperature range of 300–30 K, indicating that the α-In2O3 film had near-non-degenerate conductivity.

In metalorganic vapor phase epitaxy of β-Ga2O3 using triethylgallium (TEGa) and O2 as precursors and Ar as the carrier gas, the gases directly above the substrate were sampled and analyzed by time-of-flight mass spectrometry. TEGa was found to decompose at 400 °C–600 °C via β-hydrogen elimination reaction to generate gaseous Ga, hydrocarbons (C2H4, C2H2, C2H6), and H2. When β-Ga2O3 was grown at temperatures greater than 1000 °C and with input VI/III ratios greater than 100, the hydrocarbons and H2 were combusted and CO2 and H2O were generated. The C and H impurity concentrations measured by secondary-ion mass spectrometry in the β-Ga2O3(010) homoepitaxial layer grown under these conditions were less than their respective background levels. Thus, to grow β-Ga2O3 without C and H contamination, conditions that favor the complete combustion of hydrocarbons and H2 generated by the decomposition of TEGa should be used.

β-Ga2O3 substrates and homoepitaxial films were characterized using the multiphoton excitation photoluminescence (MPPL) method. MPPL emission peaks at 3.2 and 3.36 eV were obtained with broad shoulders on the low energy side. MPPL images showed the presence of nanopipes in the substrate with a unique contrast. Three-dimensional MPPL imaging revealed the distribution and spatial location of the nanopipes. All the nanopipes were elongated along the [010] direction with diameters of approximately 0.4–1.0 μm and lengths of 10–30 μm. The nanopipes were aligned in the [100] and [001] directions with sub-micron spacing in the (0 1 ¯ 0)-oriented substrate. The nanopipes were located under the hillocks at the homoepitaxial surface. The nanopipes are suggested to trigger the hillock growth by transforming into threading dislocations in the homoepitaxial film.

The electrical properties of beta-gallium oxide (β-Ga2O3) and gallium arsenide semiconductors were characterized using the emerging terahertz time-domain ellipsometry (THz-TDE) technique. The dielectric and conductivity properties were obtained from the complex ratio of the measured p- and s-polarized THz pulses reflected from the samples. The carrier concentration and mobility were then deduced using the Drude model, and the results showed good accuracy. This work demonstrates THz-TDE as a promising tool for characterizing semiconductors, especially those with high carrier concentrations and significant absorption in the THz region.

Thermodynamic analyses of β-Ga2O3 growth by both ozone and plasma-assisted molecular beam epitaxy (MBE) were performed. In either case, the growth mechanism was found to differ depending on whether the input VI/III ratio was above or below 1.5. Under O-rich conditions (VI/III > 1.5), the driving force for β-Ga2O3 growth ( Δ P Ga 2 O 3 ) was determined to increase linearly with increasing Ga input partial pressure ( P Ga o ) because almost all the supplied Ga was used for the growth of the β-Ga2O3. In contrast, Ga-rich conditions (VI/III Δ P Ga 2 O 3 to decrease. Etching of the β-Ga2O3 occurred with increasing P Ga o due to the formation of volatile Ga2O. This work also demonstrated that the use of ozone allowed growth at higher temperatures than the use of O radicals. The calculated results were in good agreement with experimental values, indicating that β-Ga2O3 growth by MBE can be explained by thermodynamics.

The influence of growth rate on the homoepitaxial growth of AlN at 1450 °C by hydride vapor phase epitaxy on bulk AlN(0001) substrates was studied. X-ray diffraction and Raman spectroscopy revealed that high structural quality comparable to that of the initial substrate can be achieved even when the growth rate is increased to over 150 μm h−1. Although the concentration of Si impurities increased with increasing growth rate, a freestanding AlN substrate prepared from a homoepitaxial layer grown at 155.6 μm h−1 showed a steep optical absorption edge at 207 nm and high optical transmittance at longer wavelengths.

This study presents vertical Ga2O3 Schottky barrier diodes (SBDs) with a staircase field plate on a deep trench filled with SiO2. It was clarified from device simulation that at high reverse voltage operation, the staircase field plate and the deep trench can effectively alleviate electric field concentration in the Ga2O3 drift layer and the SiO2 layer, respectively. The Ga2O3 SBDs successfully demonstrated superior device characteristics typified by an on-resistance of 7.6 mΩ cm2 and an off-state breakdown voltage of 1.66 kV. These results offer the availability of the trench staircase field plate as an edge termination structure for the development of Ga2O3 SBDs.

The influence of substrate orientation on homoepitaxial growth of beta-gallium oxide by halide vapor phase epitaxy was investigated. Substrates were cut at various angles Δb from the (001) plane ( Δb = 0°) to the (010) plane ( Δb = 90°) of bulk crystals grown by the edge-defined film-fed growth method. The growth rate increased with increasing absolute value of Δb near the (001). However, from the (001) to the (010), as Δb increased, the growth rate decreased sharply, and streaky grooves observed in the grown layer on the (001) substrate became triangular pits. The length of the pits decreased with increasing Δb, and a pit-free homoepitaxial layer grew at Δb ≈ 60°. The valley line of the pits was parallel to the [010] direction; therefore, the length of the pits decreased with increasing Δb. In addition, transmission electron microscopy observations of the deepest part of a pit revealed that the pits originate from dislocations propagating in the substrate at an angle of 60° with respect to the (001) plane. Therefore, pits are not formed on the grown layer surface when the Δb of the substrate is ∼60°, because its surface is substantially parallel to the dislocations. The homoepitaxial growth of a pit-free layer on the (011) substrate ( Δb = 61.7°) was demonstrated, and void defects and dislocations in the substrate were confirmed by the etch-pit method to not be inherited by the homoepitaxial layer.

The growth of twin-free single-crystal cubic-indium oxide (c-In2O3) layers was investigated by halide vapor phase epitaxy on c-plane sapphire substrates with various off-axis angles. The growth rate of the c-In2O3 layer increased and twin formation was suppressed as the off-axis angle of the substrate was increased. A single-crystal c-In2O3(111) layer grown on a sapphire substrate with a 5° off-axis angle showed a room temperature carrier density and mobility of 1.4 × 1016 cm−3 and 232 cm2 V−1 s−1, respectively. Temperature-dependent Hall measurements of the layer revealed that the mobility is dominated by optical phonon scattering.

A hydrogen-environment anisotropic thermal etching (HEATE) method, which is a top-down etching process with the advantages of high controllability, large-area adaptability, and low damage to the etched surface, was applied to c-In2O3 layer. The etching characteristics under the HEATE conditions using a H2 flow at various temperatures and various low total pressures were investigated using (1 1 1) c-In2O3 layers grown by halide vapor phase epitaxy on (0 0 0 1) sapphire substrates. The onset temperature of etching at a low pressure of 5.7 × 10−4 atm and the onset total pressure of etching at 800 °C were approximately 600 °C and 2.0 × 10−5 atm, respectively. Etching occurred without the formation of In droplets on the surfaces under the investigated conditions. The etching rate for c-In2O3 increased with increasing temperature and pressure and could be controlled with nanometer-scale order in the range from 1 to 75 nm min−1. The etching rate estimated from the thermodynamic analysis was in good agreement with the experimental results, indicating that the experimental results follow thermodynamics.