A Ge/Cu/Ge Ohmic contact scheme has been applied to moderately-doped n-GaN, showing specific contact resistivity of 1.1 x 10(-5) Omega cm(2) and contact resistance equal to 0.26 Omega mm. SiCl4 RIE at a dc self-bias of -150 V is found to reduce the GaN RMS surface roughness (R-q) from 4.6 angstrom to 3.2 angstrom while a -350 V treatment slightly increases R-q to 4.1 angstrom. It is demonstrated by x-ray photoelectron spectroscopy that ion bombardment of n-GaN, in conjunction with a hydrochloric acid treatment, reduces surface oxidation and increases the concentration of nitrogen vacancies (V-N), a donorlike defect, at the surface. Auger electron spectroscopy depth profiling shows that a 600 degrees C anneal encourages interdiffusion of Ga and Ge such that the heavily-n-doped interfacial region caused by V-N is further enhanced. The results therefore suggest that the Ohmic behavior of the Cu3Ge Ohmic contact on n-GaN is due to increased tunneling current at the metal/semiconductor interface caused by the creation of donor-like V-N as well as Ge on V-Ga. (c) 2005 American Vacuum Society.