VACANCY CREATION ON THE SI(111)-7X7 SURFACE DUE TO SULFUR DESORPTION STUDIED BY SCANNING-TUNNELING-MICROSCOPY

Ultra high vacuum scanning tunneling microscopy (STM) has been used to study the adsorption and subsequent thermal desorption of a sulfur ov erlayer deposited in situ, at room temperature, on the Si(111)-7 x 7 s urface. Little ordering is visible in the STM images of this overlayer 1-2 monolayers thick with the underlying silicon surface retaining th e (7 x 7) reconstruction with weakened low energy electron diffraction (LEED) spot intensity. STM images of this surface following a thermal anneal at 375-degrees-C revealed the presence of a number of monolaye r deep ''holes'' or voids in the (7 x 7) surface. The appearance of th ese voids is consistent with a coalescence of vacancy defects induced by the sulfur desorption process as also observed for oxygen induced e tching of the silicon surface. In addition, we have observed small reg ions of the (square-root 3 x square-root 3)R30-degrees and c(4 x 2) re constructions within areas exhibiting a high degree of surface disorde r, with the metastable (5 x 5) reconstruction also being found. Both r econstructions and disorder are attributed to vacancy diffusion to ste p edges causing a redistribution of surface silicon atoms.