Epitaxial MgO(100) Film Grown on a NiO(100) Single Crystal Surface, Studied by XPS and UPS

Abstract

MgO and NiO have the rocksalt structure with a less than 1% lattice mismatch. Therefore, they are excellent materials for studies of epitaxial oxide film growth. This article reports comparison XPS and UPS spectra from both a clean stoichiometric NiO(100) single crystal substrate and a 5 nm thick MgO(100) epitaxially grown film, measured using a double-pass CMA. The NiO(100) surface was prepared by cleaving a NiO single crystal along the (100) plane in air, inserting it in UHV, and treating the surface with an Ar+ bombardment and oxidation treatment. The MgO film was grown by thermally evaporating Mg on the NiO(100) substrate at 523 K at an O2 partial pressure of 7 × 10−5 Pa. Epitaxial (100) structure of the MgO film was confirmed by LEED and in situ RHEED. Steady state sample charging was evident in UPS and XPS spectra from both the NiO surface and the MgO film. The charging was minimized by acquiring spectra at 523 K, rather than 298 K, due to the increased bulk conductivity of NiO. To correct for charging, all XPS spectra reported here are charge referenced to the measured Ni 2p3/2 of NiO, which is assigned a binding energy of 854.2 eV. The UPS spectra are referenced to the top of the valence band.