The adsorption and reactions of water on Si(100)-2x1 and Si(111)-7x7 surfaces

The dissociative adsorption of D2O on single crystal silicon surfaces and the subsequent desorption of D-2 and SiO have been investigated using temperature programmed desorption. Adsorption across a single dimer of the Si(100)-2x1 reconstruction involves a mobile and long-lived precursor at low surface temperatures and leads to first order Langmuir kinetics at higher temperatures. Occasionally, two diagonally adjacent dangling bonds are occupied and consequently the saturation coverage is (0.41+/-0.02) ML with 0.18 ML of unreacted but isolated dangling bonds remaining. Approximately 17% of the adsorbed OD can be abstracted as D2O by reaction with atomic deuterium. The initial sticking probability for adsorption across a rest-atom-adatom dangling bond pair of the Si(111)-7x7 reconstruction is 0.23+/-0.08. Although adsorption at the remaining isolated adatom sites occurs with a very much reduced probability, a saturation coverage of (0.22+/-0.02) ML is eventually achieved. Desorption of 90% of the adsorbed deuterium on Si(100) and 85% of the adsorbed deuterium on Si(111) occurs via the beta(1) channel. The activation energy for desorption via the beta(1) channel increases with initial D2O coverage due to an extended and cumulative interaction with adsorbed oxygen. The remaining coverage independent fraction is adsorbed at sites with one or two oxygen nearest neighbours and is more directly associated with SiO desorption. The desorption of SiO from Si(100) can be simulated using first order kinetics with a pre-exponential factor of 2.5x10(17) s(-1) and an activation energy that increases with initial D2O coverage from 75 kcal mol(-1) at 0.05 ML to 80 kcal mol(-1) at 0.4 ML.