Computational studies of heterogeneous reactions of SiH2 on Si(111) surfaces
ABSTRACT The dynamics of sticking and decomposition of SiH2 on Si(111) surfaces have been investigated using classical trajectories on a potential-energy surface fitted to ab initio and semiempirical results for bond energies, structure, and barrier heights and to the available experimental data for SiH bond energies and barrier heights. SiH2 sticking probabilities are found to be unity for all temperatures and incident translational energies investigated. The reaction exothermicity of chemisorption is primarily dissipated into the phonon modes of the lattice. Chemisorption results in a tetrahedrally bonded structure. Direct hydrogen abstraction by the surface prior to chemi-sorption is never observed although it is an energetically open reaction channel. SiH2 decomposition on the Si(111) surfaces occurs via two modes: direct molecular hydrogen elimination and a two-step sequence in which successive hydrogen atoms dissociate to surface binding sites in a plane perpendicular to the ∗-Si-∗ plane. First-order rate coefficients for each elementary reaction in the two decomposition modes are obtained from decay plots of the trajectory data. Direct elimination of molecular hydrogen is the slow step with rate coefficients in the range (0.35–0.62) × 1012s−1. The sequential decomposition steps have rate coefficients 3–6 times larger with the ratio of the first to the second step coefficient lying in the range 0.77–1.73 depending upon the temperature and the initial translational energy. The potential barrier height for direct hydrogen elimination is estimated to be 1.5 eV. Upper limits for the barrier heights for the sequential steps are 0.51 and 0.24 eV for the first and second steps, respectively. Desorption of molecular hydrogen or silicon atoms from the Si(111) surface were not observed although both of these channels are energetically open.