Single crystal SrTiO3 layers were grown on GaAs using both molecular and atomic oxygen and the interfaces were probed using room temperature photoluminescence and in-situ X-ray photoemission spectroscopy. The increase in PL intensity for the oxide/GaAs structure in which molecular oxygen was used suggest a decrease in the density of interfacial defects resulting in some level of Fermi level unpinning. When atomic oxygen was used during growth the interface appears to have an increase in the density of defects and a pinned Fermi level similar to that of a GaAs layer with a native oxide. X-ray photoemission measurements indicate an increase in the Ga-O bonding at the interface when the plasma source was used with an addition of As-As bonding not present in the sample grown using molecular oxygen. These results suggest the possibility of using a crystalline oxide as a gate dielectric in III-V MOSFET devices. (c) 2013 Elsevier B.V. All rights reserved.