Surface structure of TiO2-terminated SrTiO3(001) studied by medium energy ion scattering and ultraviolet photoelectron spectroscopy

The structure change of TiO2-terminated SrTiO3(STO)(001) surfaces after annealing in an O2 atmosphere (1-5x10-6 Torr) and ultrahigh vacuum (UHV) was studied by ultraviolet photoelectron spectroscopy (UPS), reflection high energy electron diffraction (RHEED) and medium energy ion scattering (MEIS). The TiO2-terminated SrTiO3 (001) surfaces were prepared by chemical treatment with an HF-NH4F buffer solution (pH: 3.5 - 3.8). The fraction of the TiO2 face was estimated to be 88 ± 2% from the MEIS analysis. For the STO surface annealed in UHV at temperatures higher than 650degC, the surface state induced by O vacancies in the top TiO2 plane was observed by UPS and the RHEED pattern shows a 2 x 2 structure. On the other hand, the STO surface annealed in the has a 1 x 1 structure without O vacancies. The MEIS analysis revealed that annealing at 650degC in UHV decreased the fraction of the TiO2 face by about 5% and the O vacancy fraction in the TiO2 face was about 7%. The present results indicate that O vacancies are initiated in the random sites of the top TiO2 plane by UHV annealing and rearrangement of the atomic configuration occurs simulta-neously to reduce the surface energy and thus the newly ordered 2 x 2 structure appears. (author)