Surface defects and the electronic structure of SrTiO3surfaces

Abstract

The electronic properties of various types of SrTi${\mathrm{O}}_3$ surfaces have been studied by using ultraviolet-photoemission and electron-energy-loss spectroscopy, low-energy electron diffraction and Auger spectroscopy. Vacuum-fractured surfaces exhibit weak photoemission in the region of the bulk band gap, which is probably due to residual surface defects. When surface defects are produced by Ar-ion bombardment, a much stronger band of surface states appears in the band-gap region. These states arise from the creation of ${\mathrm{Ti}}^{3+}$-O-vacancy complexes and are predominantly of $d$-electron character. Two surface defect phases are seen, one due to surface disorder and the other to changes in surface composition. The latter phase is stable under annealing to at least 1100 K. Exposure to ${\mathrm{O}}_2$, however, depopulates the band-gap surface states on both vacuum-fractured and ion-bombarded surfaces. Models for the detailed structure of the defect surface states are discussed.