Surfaces of reduced and oxidized SrTiO3 from atomic force microscopy

Measurements by atomic force microscopy are reported for (100) and (110) su rfaces of SrTiO3 monocrystals prepared with different oxidizing and reducin g conditions at elevated temperatures (800-1000 degrees C). The morphology of the surfaces turns out to be drastically altered for both oxidized and r educed crystals in comparison with the original stoichiometric surfaces. Th e observed changes on the surface of SrTiO3 due to the applied extensive th ermal treatment cannot be explained by the formation of point defects, rela xation of the uppermost surface layer, rumpling, or reconstruction due to v acancy ordering. Instead, the results have to be interpreted in terms of se gregation processes and solid-state reactions at elevated temperatures whic h cause the formation of new chemical phases on the surface and in the regi on underneath. On the surface of oxygen-annealed SrTiO3, 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 degr ees C are shown to exhibit a step height of 11.8 Angstrom which is attribut ed to the formation of a Ruddlesden-Popper phase SrO*(SrTiO3)(n) with n = 1 on the surface. In the case of reduced crystals, the topographic changes o n the surface are caused by the formation of Ti-rich phases such as TiO and Ti2O on the surface above 900 degrees C. The complex interplay of the proc esses at the surface for different temperatures, in particular its dependen ce on the details of the heat treatment, is discussed. The induced chemical heterogeneity on the surface and in the near-surface region are interprete d in terms of a kinetic demixing. The potential driving forces for this beh avior are discussed. [S0163-1829(99)09531-4].