DYNAMICS OF HYDROGEN INTERACTIONS WITH SI(100) AND SI(111) SURFACES

Experimental and theoretical work probing the dynamics of dissociative adsorption and recombinative desorption of hydrogen at Si(100) and Si(111) surfaces is reviewed. Whereas molecular beam experiments demonstrate that molecular excitations do aid in overcoming a substantial activation barrier toward adsorption, desorbed molecules are found to have a total energy content only slightly above the equilibrium expectation at the surface temperature. A consistent interpretation of the ad/desorption dynamics is arrived at which requires neither a violation of microscopic reversibility nor defect-mediated processes. An essential element of this model is that surface atom relaxations play an essential role in the dynamics such that different portions of the potential energy hypersurface govern the results of adsorption and desorption experiments. The 'lost' energy, i.e. that portion of the activation energy not evident in the total energy of the desorbed molecules, is deposited in the surface coordinates where it is inaccessible to experiments that probe the desorbates final state.