A quasi-equilibrium model for the uptake kinetics of hydrogen atoms on Si(100)

Abstract

A quasi-equilibrium model has been developed to describe the uptake kinetics of atomic hydrogen on Si(100) at 373 and 635 K. A new model is required because a simple consideration of only adsorption at dangling bond sites and Eley-Rideal abstraction cannot be reconciled with a saturation coverage of one monolayer (ML) at 635 K. We argue that although diffusion at low temperatures can be explained by hot precursor dynamics, slow vibrational relaxation in the chemisorption potential does not lead to an almost coverage independent sticking probability. Instead, we propose that a high sticking probability is maintained by reaction with doubly occupied dimers to give dihydride species which then migrate across the surface by an isomerization reaction. Subsequently, dihydride units either react with unoccupied dimer sites or molecular hydrogen is lost by desorption from two dihydride units (the β 2 desorption channel). At 635 K the dihydride species may be regarded as a mobile chemisorbed precursor, although only the bonding arrangement is propagated by isomerization. When the atomic hydrogen source is turned off, the small steady-state dihydride coverage is rapidly lost to leave a saturated monohydride phase. A saturation coverage of 1.5 ML is obtained when abstraction and adsorption reactions are in dynamic equilibrium at 373 K. It is shown that the rate constant for abstraction from monohydride is essentially the same as for abstraction from dihydride. The measured initial rate constant for loss of adsorbed hydrogen during exposure to atomic deuterium is (1.8 ± 0.1) times larger than that for loss of adsorbed deuterium during atomic hydrogen exposure at 635 K. At this temperature, the initial rate constant for loss of deuterium during exposure to atomic hydrogen is found to be the same regardless of initial coverage, but the rate decreases more slowly than expected for first-order kinetics as the reaction proceeds. The uptake model also implies that a small amount of deuterium is lost as D 2 and HD via the β 2 thermal desorption channel at 635 K.