  The electronic structure of SrTiO3(001) surfaces was studied using scanning
tunneling spectroscopy and density-functional theory. With high dynamic range
measurements, an in-gap transition level was observed on SrO-terminated
surfaces, at 2.7 eV above the valence band maximum. The density of centers
responsible for this level was found to increase with surface segregation of
oxygen vacancies and decrease with exposure to molecular oxygen. Based on these
finding, the level is attributed to surface O vacancies. A level at a similar
energy is predicted theoretically on SrO-terminated surfaces. For
TiO2-terminated surfaces, no discrete in-gap state was observed, although one
is predicted theoretically. This lack of signal is believed to be due to the
nature of defect wavefunction involved, as well as the possible influence of
transport limitations in the tunneling spectroscopy measurements.
