Ruthenium oxide and iridium oxide were used as the Schottky barrier materials of GaN metal-semiconductor-metal (MSM) ultraviolet photodetector. After annealing Ru and Ir contacts at 500 degreesC under O-2 ambient, the reverse leakage current density at -5 V drastically decreased by four orders of magnitude. Furthermore, the Schottky barrier height and optical transmittance at 360 nm simultaneously increased to 1.43 eV and 71.5% for the Ru, and to 1.48 eV and 74.8% for the Ir contact. The dramatic improvements originated from the formation of thermally stable oxide phases, RuO2 and IrO2, by the annealing, resulting in the increase in the responsivity of the GaN MSM photodetector by one order of magnitude in comparison with the photodetector with Pt Schottky contact. (C) 2004 The Electrochemical Society.