Characterization of single crystal uranium-oxide thin films grown via reactive-gas magnetron sputtering on yttria-stabilized zirconia and sapphire

Abstract

The microstructure and valence states of three single crystal thin film systems, UO2 on (11¯02) r-plane sapphire, UO2 on (001) yttria-stabilized zirconia, and U3O8 on (11¯02) r-plane sapphire, grown via reactive-gas magnetron sputtering are analyzed primarily with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). XRD analysis indicates the growth of single crystal domains with varying degrees of mosaicity. XPS and UPS analyses yield U-4f, U-5f, O-1s, and O-2p electron binding energies consistent with reported bulk values. A change from p-type to n-type semiconductor behavior induced by preferential sputtering of oxygen during depth profile analysis was observed with both XPS and UPS. Trivalent cation impurities (Nd and Al) in UO2 lower the Fermi level, shifting the XPS spectral weight. This observation is consistent with hole-doping of a Mott–Hubbard insulator. The uranium oxide-(11¯02) sapphire system is unstable with respect to Al interdiffusion across the film–substrate interface at elevated temperature.