Chemical control of the electrical surface properties in donor-doped transition metal oxides

Donor-doped transition metal oxides such as donor-doped strontium titanate ($n\ensuremath{-}{\mathrm{SrTiO}}_{3}$) are of fundamental importance for oxide electronic devices as well as for electronic surface and interface engineering. Here we quantitatively analyze the variable band alignment and the resulting space charge layer at the surface of $n\ensuremath{-}{\mathrm{SrTiO}}_{3}$, determined by its surface redox chemistry. Synchrotron-based ambient-pressure x-ray photoelectron spectroscopy conducted under applied thermodynamic bias is used to access electronic structure and chemistry of the surface. We find an electron depletion layer driven by cationic surface point defects that are controlled by adjusting the ambient atmosphere ($p{\mathrm{O}}_{2}$). We correlate the $p{\mathrm{O}}_{2}$ dependence to a response of the strontium sublattice, namely the precipitation of strontium oxide and the formation of charged strontium vacancies at the surface. We suggest the reversible conversion of surface-terminating strontium oxide into extended strontium oxide clusters as the responsible process by resolving chemical dynamics in situ. As we show, atomic control of these subtle changes in the surface redox chemistry allows us to tailor electrical transport properties along the $n\ensuremath{-}{\mathrm{SrTiO}}_{3}$ surface. Our study thereby gives access to engineering electronic band bending in transition metal oxides by the control of the surface chemistry.