Influence of surface relaxation on the electronic states of the alpha-Al2O3(0001) surface: A self-consistent tight-binding approach

The electronic structure of the alpha-Al2O3 (0001) surface is calculated using a self-consistent tight-binding method, taking into account both the covalent and the electrostatic effects. For the ideal Al-terminated surface, a surface state is present in the band gap, 2 eV below the bottom of the conduction band (CBm). When the surface is relaxed, this surface state is pushed into the conduction band, 2.7 eV above CBm. We show that this 4.7 eV shift induced by the surface relaxation is strongly related to an enhancement of the Al-O charge transfer on the ideal surface, which shows an increased ionicity compared to the relaxed one. As regards the energy position of this surface state with respect to the bulk band gap, the relative importance of the bond breaking on covalent effects and on the effective atomic levels energy positions is discussed in comparison with previous published theoretical works.