Chemical potential shift and gap-state formation in SrTiO3−δ revealed by photoemission spectroscopy

In this study, we report on investigations of the electronic structure of SrTiO3 annealed at temperature ranging between 550 and 840◦C in an ultrahigh vacuum. Annealing induced oxygen vacancies (Ovac) impart considerable changes in the electronic structure of SrTiO3. Using core-level photoemission spectroscopy, we have studied the chemical potential shift (∆μ) as a function of annealing temperature. The result shows that the chemical potential monotonously increases with electron doping in SrTiO3−δ. The monotonous increase of the chemical potential rules out the existence of electronic phase separation in the sample. Using valence band photoemission, we have demonstrated the formation of a low density of states at the near Fermi level electronic spectrum of SrTiO3−δ. The gap-states were observed by spectral weight transfer over a large energy scale of the stoichiometric band gap of SrTiO3 system leading finally to an insulator metal transition. We have interpreted our results from the point of structural distortions induced by oxygen vacancies.