Native SrTiO3 (001) surface layer from resonant Ti L2,3 reflectance spectroscopy

We quantitatively model resonant Ti L-2,L-3 reflectivity R-s,R-p(q, hv) from several SrTiO3 (001) single crystals having different initial surface preparations and stored in ambient conditions before and between measurements. All samples exhibit unexpected 300 K R-s(hv)-R-p(hv) anisotropy corresponding to weak linear dichroism and tetragonal distortion of the TiO6 octahedra indicating a surface layer with properties different from cubic SrTiO3. Oscillations in R-s(q) confirm a ubiquitous surface layer 2-3 nm thick that evolves over a range of time scales. Resonant optical constant spectra derived from R-s,R-p(hv) assuming a uniform sample are refined using a single surface layer to fit measured R-s(q). Differences in surface layer and bulk optical properties indicate that the surface is significantly depleted in Sr and enriched in Ti and O. While consistent with the tendency of SrTiO3 surfaces toward nonstoichiometry, this layer does not conform simply to existing models for the near surface region and apparently forms via room-temperature surface reactions with the ambient. This quantitative spectral modeling approach is generally applicable and has potential to study near-surface properties of a variety of systems with unique chemical and electronic sensitivities.