ALPHA-AL2O3-(0001) SURFACES - ATOMIC AND ELECTRONIC-STRUCTURE

New results on the (root 31 X root 31)R+/-9 degrees reconstructed alpha-Al2O3 (0001) surface, which can be obtained after heating at high temperature (1400 degrees C) under vacuum, are presented. The atomic structure has been studied by combining low-energy electron diffractometry and grazing incidence X-ray scattering. The surface structure is found to be perfectly commensurable with the underlying bulk lattice. The surface consists of hexagonal zones of two, nearly perfect, close-packed Al (111) planes separated by a effect of hexagonal periodicity with a parameter of 26.44 Angstrom. This model is consistent with previous surface studies of this reconstruction. The electronic structure has been investigated using valence band photoemission spectroscopy, X-ray absorption spectroscopy at the O Kedge, electron energy loss spectroscopy, and X-ray-induced Auger electron spectroscopy. Interpretation of these experimental data in the frame of a self-consistent, tight-binding calculation leads to the conclusion that the (root 31 X root 31)R+/-9 degrees reconstructed surface is more covalent than the (1 X 1) surface, Significant changes in the Al-O hybridizations are observed; these are likely due to a difference in the interatomic distances along the [0001] axis (relaxations). The increase of covalent character is mainly due to a strong decrease of the Madelung field on the reconstructed surface.