THE STRUCTURE-SENSITIVE ADSORPTION OF H2O AND H2S ON FLAT AND STEPPED SI(001) SURFACES

The adsorption of H20 at 350 K and of H2S between 90 and 550 K has been investigated by photoelectron spectroscopy on a spherical lens shaped Si sample with (001) in its center. In this paper, the vicinal orientations in the [11̄0] and the [100] zone are compared up to a deviation of 14° from (001). On (001), H2O at 350 K and H2S at 150 K are adsorbed with a constant sticking probability near unity until saturation which occurs sharply at a coverage of 1 2 monolayer. This implies a kinetics with mobile precursors. The adsorbed species are OH + H and SH + H, respectively. The same kinetics and species are observed on stepped surfaces up to a deviation of about 9° from (001). The saturation coverage decreases slightly with increasing step density. This is in full agreement with a step structure with a rebonded row of adatoms which reduce the density of dangling bonds. The step related sites turn thus out to be as reactive as the dimers on (001). At 90 K, H2S adsorption shows the same kinetics. However, the adsorbate at the steps is different, probably molecular. Beyond about 9° from (001), the saturation coverage drops more steeply and the spectra for water adsorption change their shape. This indicates a change from purely stepped structures to structures with admixture of other facets. At 550 K, H2S adsorption is still fast, but no mobile precursor kinetics is observable. The adsorbate is fully dissociated. © 1990.