We present ab initio calculations examining two previously proposed mechanisms for H2 desorption from the Si(100)-2 × 1 monohydride phase: (i) the "prepairing" mechanism, where H2 desorbs directly in a one-step process via two hydrogen atoms paired on one silicon dimer and (ii) a stepwise mechanism in which H2 desorbs from a dihydride intermediate formed via isomerization of the monohydride. Both pathways are predicted to be 66 kcal mol endothermic. A detailed search of the transition state region rules out the direct one-step mechanism, as only one saddle point was found and a search of the reaction path showed that it evolves from the dihydride intermediate rather than the monohydride. This saddle point for the second pathway corresponds to a desorption activation barrier of 94 kcal mol, which is much higher than those measured by thermal desorption experiments (45-66 kcal mol). Other prepairing desorption pathways involving H2 desorption from two neighboring hydrogen atoms on adjacent dimers are argued to be inconsistent with the observed first-order kinetics. Thus, no previously proposed mechanism appears consistent with both the observed barrier height and reaction order. We propose an alternative mechanism involving H atom diffusion prior to H2 desorption. In particular, our calculations suggest two constraints on the mechanism: (i) H2 must desorb from a SiH2(a) species that either has no memory of how it was formed or is formed by means of a step no more than ~ 10 kcal mol endothermic and (ii) H atom surface diffusion to form SiH2(a) plays a key role in determining the reaction orders on different Si surfaces. Our results indicate that H2 always desorbs solely from SiH2(a) independent of θH or surface structure. At low coverages (θH ≤ 1 ML), where the monohydride phase is present, the activation barrier for H2, desorption from SiH2(a), is predicted to be 55 kcal mol. We predict that the activation barrier decreases with increasing θH, reaching a minimum of 38 kcal mol at θH = 2 ML, in good agreement with experiment.
All Science Journal Classification (ASJC) codes
- Condensed Matter Physics
- Surfaces and Interfaces
- Surfaces, Coatings and Films
- Materials Chemistry