Effects of oxidized Cu and Co layers on the formation of Au ohmic contacts to p-GaN were explored. Upon annealing at a temperature of 400-500 degrees C in air, Au/Cu/p-GaN and Cu/Au/p-GaN samples transformed to the CuO/Au/p-GaN structure and showed ohmic behavior. The specific contact resistance as low as 5 X 10(-3) Omega cm(2) could be reached. The formation of oxidized Au/Cu and Cu/Au ohmic contacts to p-GaN can be mainly attributed to removal of carbon contamination at the GaN surface and favored outdiffusion of Ga atoms to the Au contact layer due to the formation of Au-Ga solid solution and Ga-O, both of which are promoted by the formation of CuO. For the air-annealed Au/Co/p-GaN and Co/Au/p-GaN samples the failure in the formation of Au ohmic contacts to p-GaN can be ascribed to formation of Co oxide at the interface of Au/p-GaN. (c) 2005 The Electrochemical Society.