Chemical potential shift and gap-state formation in SrTiO3-delta 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 degrees C in an ultrahigh vacuum. Annealing induced oxygen vacancies (O-vac) impart considerable changes in the electronic structure of SrTiO3. Using core-level photoemission spectroscopy, we have studied the chemical potential shift (Delta mu) as a function of annealing temperature. The result shows that the chemical potential monotonously increases with electron doping in SrTiO3-delta. 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-delta. 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.