We have designed a promising contact scheme to p-GaN. Au/NiOx layers with a low concentration of O in NiOx are deposited on p-GaN by reactive dc magnetron sputtering and annealed in N-2 and in a mixture of O-2 + N-2 to produce low resistivity ohmic contacts. Annealing has been studied of NiOx layers with various contents of oxygen upon the electrical properties of Au/NiOx/p-GaN. It has been found that the Au/NiOx/p-GaN structure with a low content of oxygen in NiO, layer provides a low resistivity ohmic contact even after subsequent annealing in N-2 or O-2 + N, ambient at 500 degrees C for 2 min. Auger depth profiles and transmission electron microscopy (TEM) micrographs reveal that while annealing in O-2 + N-2 ambient results in reconstruction of the initial deposited Au/NiO/p-GaN contact structure into a Au/p-NiO/p-GaN structure, annealing in N-2 brings about reconstruction into Au/p-NiO/p-GaN and Ni/p-NiO/p-GaN structures. Hence, in both cases, after annealing in N-2 as well as in O-2 + N-2 ambient, the ohmic properties of the contacts are determined by creation of a thin oxide layer (p-NiO) on the metal/p-GaN interface. Higher contact resistivities in the samples annealed in O-2 + N-2 ambient are most likely caused by a smaller effective area of the contact due to creation of voids. (c) 2006 Elsevier B.V. All rights reserved.