Adsorption processes of hydrogen molecules on SiC(001), Si(001) and C(001) surfaces

Adsorption processes of hydrogen molecules on the Si(001)-(2 x 1) and C(001)-(2 x 1) surfaces are discussed in light of our previous studies of H-2 adsorption on the related SiC(001)-c(4 x 2) surface. Very amazingly, there are pathways above the latter on which hydrogen molecules can adsorb dissociatively at room temperature. One of these pathways has not been considered before for adsorption of H2 on the Si(001)-(2 x 1) or C(001)-(2 x 1) surface. Therefore, we report first-principles investigations of the reaction of molecular hydrogen with the Si(001)-(2 x 1) and C(001)-(2 x 1) surfaces on this new adsorption pathway in addition to those that have been studied before. In spite of a number of similarities, the three surfaces show distinct differences as well, giving rise to spectacularly different reactivities with hydrogen molecules. This is due to the fact that the reaction of H-2 with semiconductor surfaces depends crucially on intricate combined effects of the arrangement of surface dimers, as well as the orientation of their dangling bond orbitals. In addition, the chemical nature of the surface atoms has a pronounced effect on the spatial extent of dangling bond orbitals which influences the adsorption behaviour markedly as well. In agreement with experiments, our results show that Si(001)(2 x 1) and C(001)-(2 x 1) are inert to H-2 adsorption at room temperature for all investigated pathways which exhibit substantial energy barriers. For the two reaction pathways that have been investigated before, our results are in good accord with those of previous density functional and quantum Monte Carlo (QMC) calculations. As a matter of fact, the new reaction channel studied in this work for the first time turns out to have the lowest energy barrier for H-2 adsorption on the diamond surface and should thus be the most important channel for sticking of H-2 on C(001)-(2 x 1).