Temperature dependence of Ga2O3 growth by halide vapor phase epitaxy on sapphire and β-Ga2O3 substrates

The influence of growth temperature on Ga2O3 growth by atmospheric-pressure halide vapor phase epitaxy was investigated on sapphire and beta-Ga2O3 substrates. In the growth-temperature range of 700-1000 degrees 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 degrees 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 alpha-Ga2O3(0001) grew on the sapphire (0001) substrate, whereas the stable phase beta-Ga2O3 grew homoepitaxially on a beta-Ga2O3(001) substrate. However, under thermal equilibrium, the growth of the stable phase beta-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 beta-Ga2O3 homoepitaxial layer grown under nonthermal equilibrium, crystal twinning occurred in the homoepitaxial layer, presumably due to an insufficient growth temperature.