A reduction of Ni/Au ohmic contact on p-type GaN is obtained with surface treatment on GaN films using Cl-2 inductively coupled plasma (ICP). X-ray photoelectron spectroscopy (XPS) shows the modifying of the surface atomic ratio after the 0O(2) ICP treatment. The atomic ratio of nitrogen to gallium becomes larger after the Cl-2 ICP treats samples for 5 s. It suggests that GaClx and/or GaOx is formed and then removed in the boiling HCl solution. The Ga vacancies at the p-type GaN surface are therefore produced and act as acceptors for holes. It leads to the reduction of the contact resistance through the decrease of the resistivity for the conduction of holes. (C) 2004 American Vacuum Society.