Surface phase, morphology, and charge distribution transitions on vacuum and ambient annealed SrTiO_3(100)

摘要

The surface structures of SrTiO_3 (100) single crystals were examined as a function of annealing time and temperature in either oxygen atmosphere or ultrahigh vacuum (UHV) using noncontact atomic force microscopy (NC-AFM), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). Samples were subsequently analyzed for the effect the modulation of their charge distribution had on their surface potential. It was found that the evolution of the SrTiO_3 surface roughness, stoichiometry, and reconstruction depends on the preparation scheme. LEED revealed phase transitions from a (1 × 1) termination to an intermediate c(4 × 2) reconstruction to ultimately a (√13 × √13) - R33.7° surface phase when the surface was annealed in an oxygen flux, while the reverse transition from (√13 × √13) - R33.7° to c(4 × 2) was observed when samples were annealed in UHV. When the surface reverted to c(4 × 2), AES data indicated decreases in both the surface Ti and O concentrations. These findings were corroborated by NC-AFM imaging, where initially TiO_2-terminated crystals developed half-unit cell high steps following UHV annealing, which is typically attributed to a mix of SrO and TiO_2 terminations. Surface roughness evolved nonmonotonically with UHV annealing temperature, which is explained by electrostatic modulations of the surface potential caused by increasing oxygen depletion. This was further corroborated by experiments in which the apparent roughness tracked in NC-AFM could be correlated with changes in the surface charge distribution that were controlled by applying a bias voltage to the sample. Based on these findings, it is suggested that careful selection of preparation procedures combined with application of an electric field may be used to tune the properties of thin films grown on SrTiO_3.