Oxygen bombardment is used as the oxidation method to form an insulator on GaN substrate at room temperature. The surface of clean substrate and the Ga2O3/GaN interface are characterized in situ by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and low-energy electron diffraction (LEED). XPS spectra of the Ga 2p core level show a peak shift of similar to 1 eV from Ga-O to Ga-N bonding, demonstrating an oxide formation of Ga2O3. Electron affinity of the clean GaN surface amounts to 3.65 eV; the electron affinity of the Ga2O3 on GaN(0001) surface is 3.7 eV. Valence band offset (VBO) of the Ga2O3/GaN interface measured using XPS and UPS is 1.15 eV. After annealing the system at 500 degrees C the electron affinity and the VBO decrease to the values 3.5 and 0.35 eV, respectively. (C) 2013 Elsevier B.V. All rights reserved.