In situ study of redox processes on the surface of SrTiO3 single crystals

Highlights: • A systematic study of redox processes on the macro- and nanoscale is presented. • Bulk conductivity is metallic after reduction but does not change upon oxidation. • Annealing at reducing conditions changes reconstruction and surface becomes Ti-rich. • Oxidation hinders nanoscale conductivity, yet inhomogeneity is preserved. • Oxygen chemisorption increases average surface potential by up to 0.55 eV. In this paper, we report on surface transformations under high-temperature (up to 1000 °C) annealing of SrTiO₃(100) single crystals under reducing conditions and in situ oxidation. We compare macroscale electrical measurements with nanoscale investigations of as-reduced and oxidized surfaces. On the nanoscale, annealing in ultra-high-vacuum (UHV) conditions causes a restoration of the long-range atomic order of the (1 × 1) pattern. However, above annealing temperatures of 900 °C, a complex reconstruction of ($\sqrt{13}$×$\sqrt{13}$)R 33.7° and subsequently ($\sqrt{5}$×$\sqrt{5}$)R 26.6° appears. The surface becomes Ti-rich and residual carbon desorbs. Electrical surface conductivity increases with the annealing temperature, revealing an inhomogeneous spot-like structure on the nanoscale. Mapping of the surface potential also reveals comparable spatial variations, marking exits of dislocations. The estimated surface work function is increased upon reoxidation by 0.55 eV in the case of annealing at 900 °C, when ($\sqrt{13}$×$\sqrt{13}$)R 33.7° dominates. Our results show that in contrast to the macroscopic resistance of the crystal, the nanoscale surface conductivity and surface potential are significantly influenced by redox processes at room temperature.