DENSITY-FUNCTIONAL STUDY OF H-2 DESORPTION FROM MONOHYDRIDE AND DIHYDRIDE SI(100) SURFACES

We present a density functional study of H-2 adsorption on and desorption from Si(100)2 x 1 monohydride and Si(100)1 x 1 dihydride surfaces, carried out using a slab geometry with large supercells. For the monohydride surface we find that two distinct mechanisms - i.e. the recombination of two H atoms sitting either on the same dimer or on adjacent dimers along a row - yield desorption barriers in agreement with experiment. For the dihydride surface, the preferred pathway involves the desorption of two H atoms belonging to the same SiH2 unit. This is followed by a local structural rearrangement, leading to the formation of a surface dimer, and making the overall reaction slightly exothermic. The corresponding adsorption barrier, approximate to 2 eV, is large with respect to that inferred from desorption experiments.