Molecular beam epitaxy has been used to deposit Ga2O3 onto GaAs(001) to form a III-V/oxide interface. Photoluminescence is used to monitor the interface quality and to compare these films with samples known to have a low interface state density and an unpinned Fermi level. An additional flux of molecular oxygen has been used during oxide growth, and the impact on growth rate is reported. A rf plasma source is shown to produce mainly neutral atomic oxygen. Atomic oxygen has a significant impact on the oxide growth mechanism and interface quality. The performance of metal oxide semiconductor field effect transistors fabricated from GaAs structures with their surface. unpinned by Ga2O3 is discussed briefly. (C) 2007 American Vacuum Society.