Photo-induced in-gap states in SrTiO₃ probed by photoemission spectroscopy under ultraviolet illumination

We report on an electronic structure study of SrTiO₃ under ultraviolet illumination. Photoemission measurements at 20K under ultraviolet illumination show that the illumination induces electronic states within the band gap. The photo-induced in-gap states show the power-law energy distribution with exponent of 3.4- 3.7 and weakly depends on the strength of illumination. The photo-induced in-gap states indicate that most of the photo-excited electrons are trapped by local lattice distortions with various size scale. We argue that the free carriers in the Ti 3d band and the localized Ti 3d electrons and O 2p holes within the band gap contribute to the photoconductivity and the ferroelectricity under the ultraviolet illumination, respectively.