The chemical bonding state and atomic composition at the surface of p-type GaN were studied by synchrotron radiation photo-emission spectroscopy. Ga-related oxides existed even after surface treatment using HCl solution, causing pinning of the surface Fermi level (E-F) at 0.62 eV above valence band maximum (E-V). Meanwhile, aqua regia treatment is effective in removing the surface oxide, resulting in the movement of the Fermi level toward E-V (E-F - E-V = 0.30 eV). This resulted from the existence of Ga vacancies underneath the surface oxide. As a result, the slope in the plot of Schottky barrier height with the metal work function was increased. (C) 2003 The Electrochemical Society.