A theoretical trend of oxygen vacancy levels in typical metal oxides

Abstract

Oxygen vacancies are very common in metal oxides, which typically serve as donors. Electrons may be trapped at these vacancy sites, or thermal excitation may cause electron de-trapping to enhance the conduction. The depth of such trap levels is crucial for the performance of the oxide when applied as electronic materials. For instance, in some compounds such as TiO₂, the trap level is very close to the conduction band; in others like Ga₂O₃, the trap level is deep within the forbidden band. There are many parameters that can influence the oxygen vacancy-induced trap level. However, in this work, we show that the reducibility of the metal element is directly related to the Kohn–Sham defect level height with respect to the conduction band but not the valence band. HfO₂, ZrO₂, TiO₂, Al₂O₃ and Ga₂O₃ are selected as typical examples. The theory is validated through several “thought experiments” in terms of practical calculation and may be applied to predict the oxygen vacancy-induced electron trap level in oxides. Upon structural relaxation, an interesting “Matthew effect” is also discovered for the shift of these trap levels.