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.
The influence of growth temperature on Ga2O3 growth by atmospheric-pressure halide vapor phase epitaxy was investigated on sapphire and β-Ga2O3 substrates. In the growth-temperature range of 700–1000 °C, the growth rate of Ga2O3 was in agreement with that estimated by thermodynamic analysis under the assumption of growth under thermal equilibrium. However, when the growth temperature was lower than 700 °C, the growth rate, which decreased as the growth temperature decreased, deviated from that estimated by thermodynamic analysis, reflecting growth behavior under nonthermal equilibrium. X-ray diffraction and optical absorption measurements of the grown layers revealed that the Ga2O3 growth under nonthermal equilibrium was constrained by the crystal structure of the substrate, i.e., the metastable phase α-Ga2O3(0001) grew on the sapphire (0001) substrate, whereas the stable phase β-Ga2O3 grew homoepitaxially on a β-Ga2O3(001) substrate. However, under thermal equilibrium, the growth of the stable phase β-Ga2O3 occurred irrespective of the substrate and the constraint from the crystal structure of the substrate was no longer observed. We also observed that in the β-Ga2O3 homoepitaxial layer grown under nonthermal equilibrium, crystal twinning occurred in the homoepitaxial layer, presumably due to an insufficient growth temperature.
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