Oxygen vacancies on SrO-terminated SrTi O 3 (001 ) surfaces studied by scanning tunneling spectroscopy

The electronic structure of $\mathrm{SrTi}{\mathrm{O}}_{3}(001)$ 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 findings, the level is attributed to surface O vacancies. A level at a similar energy is predicted theoretically on SrO-terminated surfaces. For $\mathrm{Ti}{\mathrm{O}}_{2}$-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 the defect wave function involved, as well as the possible influence of transport limitations in the tunneling spectroscopy measurements.