Chemisorption of bromine on cleaved silicon (111) surfaces: an x-ray standing wave interference spectrometric analysis

Abstract

The coverage and location of bromine adsorbed from dilute methanol solution on cleaved silicon (111) surfaces have been determined by X-ray standing wave interference spectrometry. Two different locations of bromine atoms have been observed. One position, similar to that observed previously for chemically cleaned silicon (111) crystals, is directly over the first monolayer silicon atoms (atop position). The second position is much more loosely bound and may be in the open surface interstitial hollow over-the-fourth-layer atoms. In contrast to the chemically cleaned surface, where surface silicon atoms are bulk-like, we conclude that for a cleaved crystal, in the presence of bonded broine atoms, the surface silicon atoms are relaxed outward by . 0.13 ± 0.06 Å. It is suggested that this relaxation may open access to the interstitial site. Adsorption on chemically cleaned surfaces gave correlated coverage in the atop position of 0.3 monolayer (ML) or less. For cleaved surfaces, coverage of bromine in the strongly bound atop position never exceeded 0.24 ML which implies possible steric hindrance to bromine adsorption on adjacent sites The weak bonding site was observed with coverage up to 1 ML and could be obtained even for samples with saturation oxygen coverage before bromine adsorption. The cases of coadsorption of bromine were analyzed with a general model which allows difference of population at individual sites.