The thermal motion of Si ad-dimers on (001) was studied by means of scanning tunnelling microscopy. At room temperature, the ad-dimers which resided on top of the substrate dimer rows performed a 1-dimensional random walk along the substrate dimer rows. The activation barrier to the diffusion process was estimated to be 0.83eV. Although the preferred diffusion direction was along the substrate dimer rows, diffusion across the rows was also observed. The latter process involved 2 separate events. One was the jump of a Si ad-dimer from an on-top position to a position in the trough between the substrate dimer rows. The other was a hop from a trough position to an on-top position. The frequency of dimer hops from the on-top position to the trough position, at room temperature, was about 10-4 hops per second; with an activation barrier of about 1eV (assuming an attempt factor of 1013Hz). Ad-dimers which resided on top of substrate dimer rows at room temperature exhibited a rotational mode between 2 configurations in which the dimer bond was aligned along, or parallel to, the substrate dimer bond direction. The rate-limiting activation barrier to this process was estimated to be 0.7eV; assuming a pre-exponential factor of 1013Hz.

Diffusion mechanisms and the nature of Si ad-dimers on Ge(001) E.Zoethout, H.J.W.Zandvliet, W.Wulfhekel, G.Rosenfeld, B.Poelsema: Physical Review B, 1998, 58[24], 16167-71