The effects of post-oxidation on the leakage current of high-voltage AlGaN/GaN Schottky barrier diodes (SBDs) on Si(111) substrates were investigated. Auger electron spectroscopy was used to study post-oxidation mechanisms. The group-Ill oxides, AlOx, or GaOx, were formed as indicated the oxygen on the AlGaN layer after post-oxidation. The formation of group-Ill oxides resulted in passivation. The diffusion of Ni into the AlGaN layer was also detected owing to the post-oxidation annealing shallow states near the Schottky contact. When post-oxidation was performed at 600 degrees C, all Ni at the Schottky contact was combined with oxygen. This high post-oxidation temperature was therefore unsuitable because the Schottky interface changed from Ni/AlGaN to Au/AlGaN. Leakage current until breakdown was considerably decreased after post-oxidation at 500 degrees C. The breakdown voltage of GaN SBD was increased from 351 to 524 V at a drift length of 20 mu m by the post-oxidation. The figure-of-merit (BV2/R-on,R-sp) of identical device was also improved from 29.6 to 53.1 by the post-oxidation. Our results suggest that post-oxidation is suitable for GaN power switch fabrication. (C) 2012 Elsevier Ltd. All rights reserved.