  First-principles total-energy calculations of the H/Si(001)-2x1 surfaces
reveals a dual diffusion process for the Si adatoms: single along the dimer row
while pairing up across the row. The calculated diffusion barrier along the
dimer row is 1.1 eV, which is, however, too small to account for the
hydrogen-induced growth disruption seen by experiments. Instead, we find that
the adatom diffusion, in the presence of H, leads to the formation of immobile
fourfold-ring Si tetramers which are difficult to break. This could explain the
adverse effects of H on Si homoepitaxy.
