Highly reactive dissociative adsorption of hydrogen molecules on partially H-covered Si-(001) surfaces: A density-functional study

Despite intensive research the atomistic reaction path leading to the dissociative adsorption of hydrogen molecules on a Si(001) substrate is still debated. In this paper density-functional calculations employing a generalized gradient approximation for the exchange-correlation functional will be presented for the interdimer reaction paths on both clean and H-precovered Si(001) surfaces. It turns out that silicon surface dimers with one of their two dangling bonds saturated with a hydrogen atom result in very reactive H-2-adsorption sites. The high reactivity is explained to be due to dangling-bond states close to the Fermi level, which can very efficiently hybridize with the antibonding 1s Sigma (u)* state of the H-2 molecule. This is analogous to the high reactivity of step sites on Si(001) vicinal surfaces discussed by Kratzer et nl. On the clean surface the calculations produce an adsorption-energy barrier for the interdimer reaction path which is even slightly smaller than the respective barrier for the common intradimer reaction path. Thus, provided its prefactor is not too small, the interdimer reaction path might be relevant for the sticking of H-2 on Si(001).