  In this study, we report on investigations of the electronic structure of
SrTiO$_3$ annealed at temperature ranging between 550 and 840$^\circ$C in an
ultrahigh vacuum. Annealing induced oxygen vacancies (O$_{vac}$) impart
considerable changes in the electronic structure of SrTiO$_3$. 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
SrTiO$_{3-\delta}$. 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 SrTiO$_{3-\delta}$. The gap-states
were observed by spectral weight transfer over a large energy scale of the
stoichiometric band gap of SrTiO$_3$ system leading finally to an insulator -
metal transition. We have interpreted our results from the point of structural
distortions induced by oxygen vacancies.
