Structural, electronic structure, and band alignment properties at epitaxial NiO/Al₂O₃ heterojunction evaluated from synchrotron based X-ray techniques

The valence band offset value of 2.3 ± 0.2 eV at epitaxial NiO/Al₂O₃ heterojunction is determined from photoelectron spectroscopy experiments. Pulsed laser deposited thin film of NiO on Al₂O₃ substrate is epitaxially grown along [111] direction with two domain structures, which are in-plane rotated by 60° with respect to each other. Observation of Pendellosung oscillations around Bragg peak confirms high interfacial and crystalline quality of NiO layer deposited on Al₂O₃ substrate. Surface related feature in Ni 2p_3/2 core level spectra along with oxygen K-edge soft X-ray absorption spectroscopy results indicates that the initial growth of NiO on Al₂O₃ substrate is in the form of islands, which merge to form NiO layer for the larger coverage. The value of conduction band offset is also evaluated from the measured values of band gaps of NiO and Al₂O₃ layers. A type-I band alignment at NiO and Al₂O₃ heterojunction is also obtained. The determined values of band offsets can be useful in heterojunction based light emitting devices.