Ab initio H 2 desorption pathways for H/Si(100): the role of SiH 2(a)

Abstract

We present ab initio calculations examining two previously proposed mechanisms for H 2 desorption from the Si(100)-2 × 1 monohydride phase: (i) the “prepairing” mechanism, where H 2 desorbs directly in a one-step process via two hydrogen atoms paired on one silicon dimer and (ii) a stepwise mechanism in which H 2 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 H 2 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 H 2 desorption. In particular, our calculations suggest two constraints on the mechanism: (i) H 2 must desorb from a SiH 2(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 SiH 2(a) plays a key role in determining the reaction orders on different Si surfaces. Our results indicate that H 2 always desorbs solely from SiH 2(a) independent of θ H or surface structure. At low coverages (θ H ⩽ 1 ML), where the monohydride phase is present, the activation barrier for H 2, desorption from SiH 2(a), is predicted to be 55 kcal mol . We predict that the activation barrier decreases with increasing θ H, reaching a minimum of 38 kcal mol at θ H = 2 ML, in good agreement with experiment.