Density functional study of H₂ desorption from monohydride and dihydride Si(100) surfaces

We present a density functional study of H₂ adsorption on and desorption from Si(100)2×1 monohydride and Si(100)1×1 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, ≈ 2 eV, is large with respect to that inferred from desorption experiments.