CHARACTERIZATION OF SAPPHIRE SURFACES BY ELECTRON ENERGY-LOSS SPECTROSCOPY

By combining four surface analysis methods, AES, LEED, EELS and XPS, the stoichiometry and the crystallographic structure of alpha-Al2O3 surfaces can be related to specific band gap levels. A preparation process is described which permits one to obtain ordered alumina surfaces in two orientations, (0001) and (1012BAR). Thermal heating under ultrahigh vacuum induces stoichiometry changes. Equilibrium-reduced structures, characterized by LEED patterns and modifications in the band gap structure, are identified. It is shown that intrinsic surface states (ISS) are specific of clean stoichiometric surfaces. These levels, located at 5.7 and 4.1 eV below the conduction band minimum (CBm), are shifted and broadened on reduced surfaces decreasing the band gap value.