Several defect configurations including oxygen vacancies have been investigated as possible origins of the reported room-temperature ferroelectricity of strontium titanate (STO) thin films [Y. S. Kim et al, Appl. Phys. Lett.91, 042908 (2007)]. First-principles calculations revealed that the Sr–O–O vacancy complexes create deep localized states in the band gap of SrTiO3 without affecting its insulating property. These results are consistent with electronic structural changes determined from optical transmission and x-ray absorption measurements. Our work suggests importance of oxygen vacancies and their complexes in understanding of electronic properties of perovskite oxide thin films, including STO.

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