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

In this study, single-phase epsilon-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 -, -, and epsilon-Ga2O3. However, with the insertion of a cubic NiO buffer layer, single-phase epsilon-Ga2O3 thin films were successfully grown at temperatures from 400 degrees C to 800 degrees C. Furthermore, epsilon-Ga2O3 thin films grown at 750 degrees 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 epsilon-Ga2O3 and the intermediate layers. The direct bandgap was estimated to be 4.9 eV in thin films in which epsilon-Ga2O3 was predominant and 5.3 eV in thin films dominated by -Ga2O3.