Thermal stability of α-Ga2O3 films grown on c-plane sapphire substrates via mist-CVD

The thermal stability of α-Ga2O3 films grown on c-plane sapphire substrates was investigated. A strong correlation was found between thermal stability and film thickness: the more the α-Ga2O3 films maintained the α-phase upon heating at higher annealing temperature, the thinner they were. Transmission electron microscopy observations revealed that the phase transition of the α-Ga2O3 film to the thermodynamically most stable β-phase had the orientation relationship of β-Ga2O3 2̄01 || sapphire [0001]. High-temperature x-ray diffraction measurement for the α-Ga2O3 film showed the relationship of β-Ga2O3 4̄01/[301] || sapphire [0001] as well. The dependence of the stability boundary on the film thickness originates from a thermal stress caused by a larger thermal expansion coefficient of α-Ga2O3 than that of sapphire. Relaxation of residual stress by introducing a selective area growth technique enhanced the thermal stability of α-Ga2O3 so that α-Ga2O3 maintained the corundum structure upon heating at 800 °C, although a small diffraction peak from β-Ga2O3 was detected by x-ray diffraction measurement. The enhanced thermal stability of α-Ga2O3 widens device process windows as well as growth windows.