INTERACTION OF OXYGEN VACANCIES WITH O-2 ON A REDUCED SRTIO3(100)ROOT-5X-ROOT-5-R26.6-DEGREES SURFACE OBSERVED BY STM

We have performed real space and real time observations of oxygen adsorption on a reduced SrTiO3(100)root 5 x root 5-R26.6 degrees surface by scanning tunneling microscopy (STM) in ultrahigh vacuum. The STM image of adsorbed oxygen is observed as a hole on the root 5 x root 5 surface. Two adsorption phases have been observed. The initial phase is for the coverage up to about 0.01 ML (oxygen pressure (P-O2) = 1 x 10(-9) Torr): STM images have revealed that adsorbed oxygen migrate and/or desorb without oxidized oxygen vacancy defects in this surface, suggesting that oxygen is adsorbed as an O-2 molecule with adsorption energy E(a) = 0.9 eV. The STM image can be understood by electron transfer from an oxygen vacancy to adsorbed oxygen. The second phase (P-O2 > 1 X 10(-9) Torr) is for the coverage greater than 0.01 ML, in which adsorbed oxygen form islands and irreversibly oxidize the oxygen vacancy defects.