An x-ray standing wave interference spectrometric analysis of chemisorption of selenium on silicon(111) and (220) surfaces

Study of location and coverage of selenium atoms adsorbed from weakly acidic methanol solution on chemically cleaned silicon (111) and (220) surfaces has been made by x-ray standing wave interference spectrometry (XSWIS). In addition to measurement of the adatom location normal to the crystal surface plane, the atom positions relative to the planes inclined to the surface were measured. For the (111) and (220) crystals, inclined (220) and (111) planes were used, respectively. We conclude from these measurements that on (111) surfaces selenium atoms bridge two nearest surface silicon atoms. On (220) surfaces selenium atoms are in an interchain bridged position. The substrate surface atoms appear to be in bulklike positions for (111) surfaces. For (220) surfaces the substrate atoms may have a multilayer inward relaxation. Based on minimum energy calculations for relaxed and unrelaxed surface models, the selenium-silicon bond lengths are assumed to be similar (2.55 ± 0.05) A for both surfaces and is somewhat greater than the sum of the covalent radii of the respective atoms. Measurements on selenium-adsorbed cleaved silicon (111) surfaces show the same results as for the chemically cleaned counterpart within the experimental error.