We have fabricated GaN-based light-emitting diodes (LEDs) using Ni/Au with indium tin oxide (ITO) overlayer as a p-electrode. A Ni (20 Angstrom)/Au (30 Angstrom)/ITO (600 Angstrom) contact with pre-annealing at 500degreesC under an O-2 ambient before ITO deposition (O-annealed contact) showed lower contact resistivity compared to the contact with pre-annealing under a N-2 ambient (N-annealed contact) and without the pre-annealing (nonannealed contact). The pre-annealing under the O-2 ambient produced NiO, which acted as the diffusion barrier for out-diffusion of N and Ga atoms and in-diffusion of In during the subsequent postannealing. Thus, the formation of a Au-In solid solution was effectively suppressed, resulting in the decrease of contact resistivity and enhancement in thermal stability. The LED with the O-annealed contact as a p-electrode showed lower operation voltage at 20 mA, better thermal stability, and enhanced light output than the LED with the N-annealed or nonannealed contact. The low operation voltage and better thermal stability originated from the low contact resistivity and low sheet resistance of ITO. The refractive index of ITO is between GaN and air, reducing the total reflection at the interface of GaN, thus enhancing the light output. (C) 2004 American Vacuum Society.