Thermal stability of α-(Al x Ga1–x )2O3 films grown on c-plane sapphire substrates with an Al composition up to 90%

The thermal stability of α-(Al x Ga1–x )2O3 films grown on c-plane sapphire substrates was investigated. The α-(Al x Ga1–x )2O3 epitaxial films grown by mist chemical vapor deposition were annealed at temperatures in the range of 600 °C–1100 °C in an atmospheric furnace, and then the crystal structures of the films were characterized using X-ray diffraction and transmission electron microscopy. When the Al composition was less than 0.5, the α-(Al x Ga1–x )2O3 films converted to the β-phase, which is the thermodynamically most stable phase for Ga2O3. The thermal stability was enhanced by increase in the Al composition, and α-(Al x Ga1–x )2O3 with x = 0.45 maintained the corundum structure after annealing at 950 °C. On the other hand, the α-(Al x Ga1–x )2O3 layers with Al contents higher than 0.6 were stable against the thermal treatment and did not show phase transformation to other phases upon high-temperature annealing at 1100 °C.