First-principles study of resistance switching in rutile TiO2 with oxygen vacancy

The electronic properties of defective rutile TiO₂ with oxygen vacancy were studied by ab initio methods to understand resistance switching mechanism for non-volatile memory applications. Density functional theory calculations employing the Hubbard-U on-site correction to the local density approximation (LDA) were performed to determine the structural and electronic modifications introduced by oxygen vacancy formation in bulk TiO₂. It has been found that introducing on-site Coulomb interaction between 2p electrons (U^p) in addition to on-site Coulomb interaction between 3d electrons (U^d), a corrected description of the band gap state induced by the oxygen vacancy is obtained. The calculated band gap energy within the LDA+U^d+p method is in excellent agreement with the experimental value of 3.0 eV. We find that the oxygen vacancy produces a defect state within the band gap. This state is occupied by two electrons that are highly localized on Ti 3d orbitals of the nearest Ti atoms to the vacancy. The defect state is not a shallow donor, but a deep level at ~0.7 eV below the conduction band edge, indicating that the electron doping at room temperature is not favorable. The atomic relaxation effects are mainly on the Ti and O atoms in the immediate vicinity of the vacancy, however the induced displacements are rather small. The electrons localize on Ti after vacancy creation, and therefore a spatial electronic charge redistribution is observed. This change in electronic density around the vacancy increases the possibility of electron hopping in multi-vacancy systems that align to form a conductive path and ultimately lead to the low resistance state.