  Several defect configurations including oxygen vacancies have been
investigated as possible origins of the reported room-temperature
ferroelectricity of strontium titanate (STO) thin films [Appl. Phys. Letts. 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 in agreement with
electronic structural changes determined from optical transmission and X-ray
absorption measurements. This work opens the way to exploiting oxygen vacancies
and their complexes as a source of ferroelectricity in perovskite oxide thin
films, including STO.
