A CLUSTER APPROACH FOR THE SNO2 (110) FACE

Some results are given from a cluster approach for the electronic structure of the SnO2 (110) face together with some oxygen vacancies and 'adsorbates'. Computations are based on ab initio methods, the local-density approximation and atomic orbitals as a basis set. Solutions were calculated self-consistently, but also using a composition of atomic potentials (for some smaller clusters). The atomic-orbital nature (origin) of the cluster levels was traced by projection onto the atomic basis set. The results here refer to a basic cluster [SnO2]13 with 17 surface atoms modelling the SnO2 (110) face and the other 22 atoms in the next five surface layers. The effect of oxygen 'adsorbates' and oxygen vacancies in the few uppermost subsurface layers on the electronic structure was considered. In particular, the focus was on the levels related to oxygen vacancies and originating from Sn 5s orbitals, which are well-known donor levels in the deep bulk, making SnO2 an n-type semiconductor. The results support some other theoretical and experimental predictions that oxygen vacancies behave as neutral defects at or near SnO2 surfaces.