Heteroepitaxial growth of single-phase ε-Ga2O3 thin films on c-plane sapphire by mist chemical vapor deposition using a NiO buffer layer

Abstract

In this study, single-phase ε-gallium oxide (Ga₂O₃) thin films were heteroepitaxially grown on c-plane sapphire substrates. When the Ga₂O₃ films were directly grown on c-plane sapphire substrates, they tended to grow with a mixture of α-, β-, and ε-Ga₂O₃. However, with the insertion of a cubic NiO buffer layer, single-phase ε-Ga₂O₃ thin films were successfully grown at temperatures from 400 °C to 800 °C. Furthermore, ε-Ga₂O₃ thin films grown at 750 °C exhibited a smooth surface. Transmission electron microscopy observations revealed that the (111) plane-oriented NiO buffer layer prevented the growth of both polymorphs other than ε-Ga₂O₃ and the intermediate layers. The direct bandgap was estimated to be 4.9 eV in thin films in which ε-Ga₂O₃ was predominant and 5.3 eV in thin films dominated by α-Ga₂O₃.