Instability and Surface Potential Modulation of Self-Patterned (001)SrTiO₃ Surfaces

Lucía Aballe, Sonia Matencio, Michael Foerster, Esther Barrena, Florencio Sánchez, Josep Fontcuberta, and Carmen Ocal*;  
Chem. Mater., 2015, 27 (18), pp 6198–6204;  
DOI: 10.1021/acs.chemmater.5b00661

The (001)SrTiO3 crystal surface can be engineered to display a self-organized pattern of well-separated and nearly pure single-terminated SrO and TiO2 regions by high temperature annealing in oxidizing atmosphere. By using surface sensitive techniques we have obtained evidence of such a surface chemical self-structuration in as-prepared crystals and unambiguously identified the local composition. The contact surface potential at regions initially consisting of majority single terminations (SrO and TiO2) is determined to be Φ(SrO) < Φ(TiO2), in agreement with theoretical predictions, although the measured difference ΔΦ ≤ 100 meV is definitely below calculations for ideally pure single-terminated SrO and TiO2 surfaces. These relative values are maintained if samples are annealed in UHV up to 200 °C. Annealing in UHV at higher temperature (400 °C) preserves the surface morphology of self-assembled TiO2 and SrO rich regions, although a non-negligible chemical intermixing is observed. The most dramatic consequence is that the surface potential contrast is reversed. It thus follows that electronic and chemical properties of (001)SrTiO3 surfaces, widely used in oxide thin film growth, can largely vary before growth starts in a manner strongly dependent on temperature and pressure conditions.