The surface composition and structure of GaN films grown on sapphire substrates by organometallic vapor-phase epitaxy (OMVPE) have been determined through the use of time-of-flight scattering and recoiling spectrometry (TOF-SARS), classical ion trajectory simulations, and low-energy electron diffraction (LEED). TOF-SARS spectra of scattered and recoiled ions plus fast neutrals were collected using 4 keV Ar+ primary ions. The scattering results were simulated by means of the three-dimensional scattering and recoiling imaging code (SARIC). This data leads to the conclusions that both N-terminated {00 (1) over bar}-(1x1) and Ga-terminated {0001}-(1x1) surfaces occur, however no evidence was obtained for mixed terminations. No relaxation or reconstruction was detected on either surface, although both surfaces exhibited two structural domains. The {000 (1) over bar} surfaces are well-ordered and contained hydrogen atoms bound to the N atoms of the outermost layer. The (0001) surfaces are highly reactive towards adsorption of carbon and oxygen from residual gases, however unlike the {000 (1) over bar} surfaces, they adsorb very little hydrogen. These Ga-terminated surfaces are stabilized and obtain more ordered structures as a result of the contamination. (C) 1997 American Institute of Physics.