ELECTRONIC-STRUCTURE OF CLEAN INSULATING OXIDE SURFACES .2. MODIFICATIONS OF THE IONO-COVALENT BONDING

This paper aims at a microscopic description of the anion-cation bondi ng in mixed iono-covalent materials and focuses on its modifications a t the surfaces due to the competition between ionic and covalent proce sses. Systematic trends in the surface electronic structure of simple insulating oxides, previously obtained in a numerical way [J. Goniakow ski and C. Noguera, Surf. Sci. 319 (1994)] are interpreted thanks to a tight binding analytical model. This latter stresses the importance o f parameters such as the atomic level positions, the anion-cation hopp ing probabilities, the Madelung constant and the coordination number. The results of the numerical and analytical methods are fully consiste nt as far as the density of states positions and widths, the forbidden gap and the ionic charges are concerned. This allows us to assign a p recise microscopic origin to each effect and to firmly establish the c oncepts underlying the surface physics of these simple oxides.