Current transport properties at Ni/n-GaN Schottky interfaces formed on oxide-etched or thermally oxidized surfaces are studied by current-voltage-temperature (I-V-T) and capacitance-voltage measurements. The results support existence of surface patches with low Schottky barrier height (SBH), which cause a leakage current of the Schottky structures. Based on "surface patch" model, the fraction of the total patch area of 10(-4)-10(-5) and the SBH lowering of 0.4 eV within patches are deduced for the oxide-etched Ni/n-GaN metalorganic chemical vapor deposition (MOCVD) samples. The influence of surface patches was found much larger for the present Ni/n-GaN molecular beam epitaxy (MBE) samples. The inclusion of N-polarity domains in the Ga-polarity layer seems to be main origin of the patches for the MBE-grown samples. Ni/n-GaN Schottky samples formed on dry-oxidized or wet-oxidized MOCVD layers represented reduction of the effective SBH with negligible increase of the n value, indicating that an intermediary native oxide between Ni and n-GaN degrades the SBH. I-V-T characteristics showed that the leakage current due to surface patches is comparable with that of the oxide-etched sample. Concerning thermal stability of the bulk GaN (MOCVD), the impurity concentration in the layer increased after wet oxidation above 500 degreesC, although little change was observed after wet oxidation at temperatures below 400 degreesC. Finally, a postfabrication annealing in nitrogen at 400 degreesC led to increase of the effective SBH even for the oxidized Ni/n-GaN Schottky samples. (C) 2004 American Vacuum Society.