Scanning tunneling microscopy of the Si(111) surface interacting with LiF adsorbates

Submonolayers of LiF were thermally deposited on the Si(111)-7×7 surface. Scanning tunneling microscopy studies showed that there are no preferential sites in the 7×7 unit cell for adsorption of LiF. At coverages higher than 0.4 monolayer (ML), the 7×7 structure disappeared. After annealing the LiF-covered surface, most of the corner Si adatoms reappear at a temperature of 325°C, which essentially exhibits a 7×7 geometry, and the surface returned to 7×7 structure with a few atomic-level defects at 800°C. The adsorption of LiF on silicon can be understood via the interaction between the directional dangling bonds and the active dipole of the adsorbed LiF molecule or F pieces formed by dissociation of LiF. The annealing behavior of the covered surface can be understood by desorption of silicon fluorides and silicides formed following dissociation of LiF on the surface, as well as diffusion of silicon clusters.