Self-diffusion on Si(001) mono-hydride surfaces revisited: The role of   adatom clustering
  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. 
Gefei Qian, Xuan Luo, S. B. Zhang, Yia-Chung Chang, Dehuan Huang,   Shang-Fen Ren