Atomic and electronic structures of MgO/Ag(0 0 1) heterointerface

Thin film growth of MgO on Ag(0 0 1) was studied using reflection high-energy electron diffraction, Auger electron spectroscopy, electron energy loss spectroscopy (EELS), and ultraviolet photoemission spectroscopy (UPS), as a model system of the oxide/metal interface. At a substrate temperature of 450 K, a single-domain MgO film ([1 0 0]film//[1 0 0]substrate) grew heteroepitaxially on Ag(0 0 1). The in-plane lattice constant of the film changed continuously from the value of the substrate to that of the film, as is the case with the heteroepitaxial growth of alkali halides. The density functional calculations were performed for model clusters in order to investigate the initial adsorption structure. MgO adsorbed preferentially with the O atom above the Ag atom, and the Mg atom on the hollow site. EELS results showed that the band gap energy did not change for MgO thickness from 1 to 20 ML, indicating that the band gap energy of a 1 ML thick MgO film adjacent to a metal substrate was same as that of MgO surface. The UPS results, on the other hand, showed an upward shift of the valence band with decreasing film thickness. These UPS results could be explained by the image charge screening of a hole created by photoionization in the near presence of a metal.