Atomic structure of SrTiO3 (100) upon surface reduction
Due to concerns over problems with sample conductivity, the first atomic resolution STM images on oxides were obtained on severely reduced crystals and therefore produced surface structures that differed from the ideal termination of the lattice. The first oxide imaged was TiO2 (110) on which the periodic surface structures were attributed to stabilization of phases at the surface based on crystallography. Similarly, the first images of atomic scale structure on SrTiO3 were obtained on significantly reduced (001) surfaces, figure 8.Large area scans illustrate the row-like structure observed on polished and annealed surfaces. The structure consists of corrugations of several distinct periodicities, in this case 4 Å and 12 Å, but always multiples of 4 Å. Corrugation amplitudes vary between 0.5 Å and 3 Å and atomic detail is observed in some regions. A small region imaged with high resolution emphasizes the structural detail of the corrugations in a region containing 12 Å and 20 Å corrugations. The variations in corrugation periodicity on this surface cannot be described in terms of missing oxygen rows because it was found that not only had oxygen left the surface but the Sr:Ti ratio changed. The structures were described in terms of surface stabilized Ruddleson Popper phases, known to be stable in bulk perovskite, and occurring as coherent intergrowths of lamellae of reduced Srn+1TinO3n+1. The structures are related to perovskite as shown below. The corrugation periods for the structure in the figure are those of Sr2TiO4 and Sr3Ti2O7.These trends in stoichiometry were confirmed by AES, and Rutherford back scattering spectroscopy (RBS).