Structure change of TiO2-terminated SrTiO3(001) surfaces by annealing in O2 atmosphere and ultrahigh vacuum

The structure change of TiO2-terminated SrTiO3(STO)(001) surfaces after annealing in an O-2 atmosphere 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 a HF-NH4F buffer solution. 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 650 degrees C the surface state induced by O vacancies in the top TiO2 plane was observed by UPS, and the RHEED pattern showed a 2 x 2 structure. On the other hand, the STO surface annealed in the O, atmosphere (1-5 x 10(-6) Torr) has a 1 x 1 structure without O vacancies. The MEIS analysis revealed that annealing at 650 degrees C 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 simultaneously to reduce the surface energy and thus the newly ordered 2x2 structure appears. (C) 1999 Elsevier Science B.V. All rights reserved.