Energetics and electronic structure of point defects associated with oxygen excess at a tilt boundary of ZnO

The formation energies and electronic structure of zinc vacancies and oxygen interstitials at a tilt boundary of ZnO were investigated by a combination of static lattice and first-principles molecular orbital methods. For both of the defect species, the formation energies were lower than those of the bulk defects at certain sites in the grain boundary. The defects with low formation energies formed electronic states close to the top of the valence band. The interfacial electronic states observed experimentally in ZnO varistors cannot be explained solely by the point defects associated with the oxygen excess: the effects of impurities should be significant for the states.