Effect of O-O bonds on p-type conductivity in Ag-doped ZnO twin grain boundaries

Based on density functional theory, first-principles calculation is applied to study the electronic properties of undoped and Ag-doped ZnO-Σ7 (1230) twin grain boundaries (GBs). The calculated result indicates that the twin GBs can facilitate the formation and aggregation of Ag substitution at Zn sites (Ag_Zn) due to the strain release. Meanwhile, some twin GBs can also lower the ionization energy of Ag_Zn. The density of state shows that the O-O bonds in GBs play a key role in the formation of a shallow acceptor energy level. When Ag_Zn bonds with one O atom in the O-O bond, the antibonding state of the O-O bond becomes partially occupied. As a result, a weak spin splitting occurs in the antibonding state, which causes a shallow empty energy level above the valence band maximum. Further, the model can be applied to explain the origin of p-type conductivity in Ag-doped ZnO.