Impact of process-related oxidation on the band alignment of Al₂O₃/diamond heterojunction investigated by X-ray photoelectron spectroscopy

Band alignment of Al₂O₃/diamond heterojunctions with varying pre-process oxidation (PPO) and process-induced oxidation (PIO) was investigated using X-ray photoelectron spectroscopy (XPS) and first-principles density functional theory (DFT) calculations. The material constant E CL − E VBM = 284.0 eV was adopted to accurately determine the valence band maximum (VBM) positions of diamond samples. The surface oxygen coverage of O-terminated diamond was found to depend non-monotonically on O₂ plasma reactive ion etching (RIE) duration, with shorter treatment producing higher oxygen coverage due to the competition between ion-bombardment-induced oxidation and progressive etching removal of oxidized species. For H-terminated diamond, high-temperature ALD introduces PIO that fully converts the hydrogen-terminated surface into an oxygen-rich interface, eliminating the negative electron affinity (NEA) and generating sp 2 carbon components. For O-terminated diamond, the interfacial oxygen coverage following ALD increases with decreasing PPO level, and the valence band offset (VBO) decreases correspondingly. Band diagram analysis reveals a transition between type-I and type-II alignment compared with other heterojunction reports in the literature, and DFT calculations confirm that higher interfacial oxygen coverage reduces the VBO. These findings demonstrate that interfacial oxygen content governed by the combined effect of PPO and PIO is the primary determinant of band alignment in Al₂O₃/diamond heterojunctions.