Influence of crystalline order and defects on the absolute work functions and electron affinities of TiO2 - and SrO-terminated n−SrTiO3(001)

We explore how the structure and composition of the terminal layer of $n\text{−}\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$ determine key surface electronic properties. We have measured and calculated from first principles the absolute work functions and electron affinities of bulk $\mathrm{SrN}{\mathrm{b}}_{0.01}\mathrm{T}{\mathrm{i}}_{0.99}{\mathrm{O}}_3\left(001\right)$ terminated along the $\mathrm{Ti}{\mathrm{O}}_2$ and SrO planes. The match between theory and experiment is quite satisfactory for the $\mathrm{Ti}{\mathrm{O}}_2$ termination if an ideal, bulk-truncated surface structure is assumed. In contrast, the ideal SrO termination leads to a calculated work function considerably lower than the experimental value. We show that this discrepancy can be associated with defects on the SrO surface that act as electron scavengers. These defects deplete the concentration of itinerant electrons in the subsurface region and increase the negative charge density on the surface, thus increasing the work function. Several different surface defect configurations were modeled; the ones that yield the best agreement with experiment involve Sr vacancies in the terminal layer along with ${\mathrm{O}}^{2-}$, $\mathrm{O}{\mathrm{H}}^{-}/{\mathrm{H}}^{-}$ pairs or ${{\mathrm{O}}_2}^{2-}$ occupying anion sites adjacent to the Sr vacancies.