Stability and electronic structure of the (1×1) SrTiO₃(110) polar surfaces by first-principles calculations

The electronic and atomic structures of several (1×1) terminations of the (110) polar orientation of the SrTiO₃ surface are systematically studied by first-principles calculations. The electronic structure of the two stoichiometric SrTiO and O₂ terminations are characterized by marked differences with respect to the bulk, as a consequence of the polarity compensation. In the former, the Fermi level is located at the bottom of the conduction band, while in the latter the formation of a peroxo bond between the two surface oxygens results in a small-gap insulating surface with states in the gap of the bulk projected band structure. We also consider three nonstoichiometric terminations with TiO, Sr, and O compositions, respectively, in the outermost atomic layer, which automatically allows the surface to be free from any macroscopic polarization. They are all insulating. The cleavage and surface energies of the five terminations are computed and compared, taking into account the influence of the chemical environment as a function of the relative richness in O and Sr. From our calculations, it appears that some (110) faces can even compete with the TiO₂ and SrO terminations of the (100) cleavage surface: in particular, the (110)-TiO termination is stable in Sr-poor conditions, the (110)-Sr one in simultaneously O- and Sr-rich environments. The available experimental data are compared to the outcomes of our calculations and discussed.