Band alignment of p-type oxide/ε-Ga₂O₃ heterojunctions investigated by x-ray photoelectron spectroscopy

The ε-Ga₂O₃ p-n heterojunctions (HJ) have been demonstrated using typical p-type oxide semiconductors (NiO or SnO). The ε-Ga₂O₃ thin film was heteroepitaxial grown by metal organic chemical vapor deposition (MOCVD) with three-step growth method. The polycrystalline SnO and NiO thin films were deposited on the ε-Ga₂O₃ thin film by electron-beam evaporation and thermal oxidation, respectively. The valence band offsets (VBO) were determined by x-ray photoelectron spectroscopy (XPS) to be 2.17 eV at SnO/ε-Ga₂O₃ and 1.7 eV at NiO/ε-Ga₂O₃. Considering the bandgaps determined by ultraviolet-visible spectroscopy, the conduction band offsets (CBO) of 0.11 eV at SnO/ε-Ga₂O₃ and 0.44 eV at NiO/ε-Ga₂O₃ were obtained. The type-Ⅱ band diagrams have been drawn for both p-n HJs. The results are useful to understand the electronic structures at the ε-Ga₂O₃ p-n HJ interface, and design optoelectronic devices based on ε-Ga₂O₃ with novel functionality and improved performance.