Oxygen surplus and oxygen vacancies on the surface of epitaxial MgO layersgrown on Ag(100)

Growth, stoichiometry, and the generation of characteristic oxygen vacancie s of epitaxial ultrathin films of MgO grown on Ag(100) single crystals have been investigated by photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS). We show that the surface of MgO is non-stoichiome tric and contains a species, identified spectroscopically as MgO2, which is strictly confined to the first monolayer. From XPS investigations of the s ubmonolayer regime of MgO, where systematic peak shifts of the O Is and Mg Is emissions similar to those measured during growth of thicker MgO layers have been found, we conclude that by thermal activation the metal-oxide int erface can be oxidized forming preferentially Mg-O-Ag bonds. By deposition of Mg layers and their incomplete evaporation, a monolayer with large oxyge n deficiency can be produced. In EELS this layer shows characteristic loss peaks at 2.1 and 3.3 eV which can be attributed to color centers due to oxy gen vacancies. (C) 1999 Elsevier Science B.V. All rights reserved.