We have measured using x-ray photoelectron spectroscopy the surface composition, chemical state, and band bending of a GaAs{110} surface exposed to medium energy (0.5 and 3.0 keV) reactive N+2 ion bombardment at room temperature as a function of ion dose. Thermally stable, thin GaN films are formed at a rate is limited by ion-induced physical desorption of the surface layer. After annealing the sample, a small band bending shift toward higher kinetic energy (0.15 eV) is observed which results from the ion-induced formation of midgap charge acceptor states. The implication of these results on the use of GaN thin films for surface passivation of GaAs in device applications is described.