Surfaces of reduced and oxidizedSrTiO3from atomic force microscopy

Measurements by atomic force microscopy are reported for (100) and (110) surfaces of ${\mathrm{SrTiO}}_3$ monocrystals prepared with different oxidizing and reducing conditions at elevated temperatures (800–1000 °C). The morphology of the surfaces turns out to be drastically altered for both oxidized and reduced crystals in comparison with the original stoichiometric surfaces. The observed changes on the surface of ${\mathrm{SrTiO}}_3$ due to the applied extensive thermal treatment cannot be explained by the formation of point defects, relaxation of the uppermost surface layer, rumpling, or reconstruction due to vacancy ordering. Instead, the results have to be interpreted in terms of segregation processes and solid-state reactions at elevated temperatures which cause the formation of new chemical phases on the surface and in the region underneath. On the surface of oxygen-annealed ${\mathrm{SrTiO}}_3,$ this leads to the growth of steps perpendicular to the surface with step heights larger than the unit cell of the perovskite structure. Crystals prepared above 900 °C are shown to exhibit a step height of 11.8 Å which is attributed to the formation of a Ruddlesden-Popper phase ${\mathrm{SrO}}^{*}({\mathrm{SrTiO}}_3{)}_n$ with $n=1\mathrm{}$ on the surface. In the case of reduced crystals, the topographic changes on the surface are caused by the formation of Ti-rich phases such as TiO and ${\mathrm{Ti}}_2\mathrm{O}$ on the surface above 900 °C. The complex interplay of the processes at the surface for different temperatures, in particular its dependence on the details of the heat treatment, is discussed. The induced chemical heterogeneity on the surface and in the near-surface region are interpreted in terms of a kinetic demixing. The potential driving forces for this behavior are discussed.