First-principles study of bulk and surface oxygen vacancies in SrTiO ₃ crystal

The structural and electronic properties of the neutral and positively charged oxygen vacancies (F and F ⁺ centres) in the bulk and on the (001) surfaces of SrTiO ₃ crystal are examined within the hybrid Hartree-Fock and density functional theory (HF-DFT) method based upon the linear combination of atomic orbital (LCAO) approach. A comparison of the formation energy for surface and bulk defects indicates a perceptible propensity for the segregation of neutral and charged vacancies to both SrO and TiO ₂ surface terminations with a preference in the latter case which is important for interpretation of space charge effects at ceramic interfaces. It is found that the vacancies reveal more shallow energy levels in the band gap on surfaces rather than in the bulk, in particular, on the TiO ₂ surface. The charged F ⁺ centre has significantly deeper energy levels both in bulk and on the surfaces, as compared with the neutral F centre.