Effect of the lattice model on the dynamics of dissociative chemisorption of H 2 on a Si(111) surface

Abstract

The effects of (a) surface relaxation to the “bath” modes of the bulk, (b) the number of lattice atoms explicitly considered in the primary reaction zone, (c) lattice force constants, and (d) the functional form of the lattice potential upon the dynamics of H2 chemisorption on a Si(111) surface have been investigated. The H2 sticking probabilities, energy transfer rates out of the newly formed Si−H bond, and instantaneous surface mobilities have been computed. The sticking probabilities are virtually independent of surface relaxation, the size of the lattice model, and the nature of the lattice potential. Si−H vibrational energy transfer rates are increased and surface mobilities are decreased by inclusion of relaxation. However, the magnitude of these effects is less than a factor of two. Variation of the lattice potential is likewise found to exert a small effect upon the Si−H energy transfer rates and surface mobilities. The origin of these effects appears to be related to resonance effects between the Si−H vibrational motion and the phonon modes of the lattice. The results suggest that a three-layer lattice model with 25 atoms in the primary reaction zone is sufficient for this system. When primary zones of this size are employed, surface relaxation to the “bath” modes of the bulk are not of major importance for semiconductor systems at low temperatures.