Hydrogen diffusion across DB steps on Si(001) surfaces was investigated by means of variable-temperature scanning tunnelling microscopy and first-principles calculations. Experimentally, the hopping rate for diffusion from the step sites to the Si dimers of the upper terrace was found to be more than an order of magnitude higher than that for diffusion to the lower terrace. This clear preference, opposite to the trend in the respective binding energies, was explained by first-principles calculations that identified a metastable intermediate as benign responsible for an unexpected lowering of the energy barrier for upward diffusion.

Diffusion Pathways of Hydrogen across the Steps of a Vicinal Si(001) Surface. M.Lawrenz, P.Kratzer, C.H.Schwalb, M.Dürr, U.Höfer: Physical Review B, 2007, 75[12], 125424