X-RAY PHOTOELECTRON SPECTROSCOPIC STUDY OF SI(111) AND SI(100) SURFACES WITH CHEMICALLY ADSORBED BROMINE

The chemical state and the desorption behaviour of bromine adsorbed on silicon surfaces have been studied by X-ray photoelectron spectroscopy (XPS). Br was adsorbed on hydrofluoric-acid(HF)-etched silicon surfaces from a weak bromine-methanol solution. Initial coverages of Br obtained on Si(lll) and Si(100) surfaces were 0.62 and 0.72 monolayers, respectively. On the (111) surface, immediately after sample preparation, the Br 3p(3/2) and 3p(1/2) XPS peaks appear at 182.9 and 187.5 eV, respectively. After a storage time of 45 h in ambient air these peaks show a chemical shift of 1.2 eV towards higher binding energy, which is attributed to the formation of the Si-O-Br complex species. Some of these features are not observed for the Si(100) surface, indicating that the chemical state of Br on the (100) surface is different from that on the (111) surface. As HF etching is involved in the sample preparation, there is also adsorbed fluorine on the surface. On the (111) surface, immediately after sample preparation, a F1s peak appears at 685.1 eV with a small shoulder at 687.5 eV. These are attributed to F chemisorption at different sites. The F1s chemical shift, upon storage in ambient air, also suggests formation of the Si-O-F species. The Br and F desorption rates and the oxide growth rates are, in general, higher for the (100) than for the (111) surfaces.