Gallium oxide films 20 A in thickness were deposited onto GaAs substrates in ultra high vacuum (UHV) via e-beam evaporation from a monolithic high-purity source. The substrates were prepared by molecular-beam epitaxy and transferred to the oxide film deposition site in a wholly UHV environment. The Ga2O3-x films were probed by x-ray photoelectron spectroscopy (XPS). Chemical states were identified and stoichiometry was estimated. Metallic layers were deposited by e-beam evaporation in UHV after XPS analysis as caps and for future work. Film morphology and structure were probed by cross-sectional high-resolution transmission electron microscopy. The films were found to have x less than or equal to 0.3 and a metal/oxide interface roughness < 1 Angstrom.