Diffusion of dimers on silicon and germanium (001) surfaces

Atomic resolved imaging techniques have provided us with an exciting view on how atoms or clusters of atoms diffuse on surfaces. Here the stability and diffusion of Si and Ge dimers on elementary semiconductor (001) surfaces is briefly reviewed. It will be shown that, for these systems, in principle three diffusion pathways are active: diffusion of on-top dimers over the substrate rows, diffusion across the substrate rows, and diffusion in the troughs. Furthermore, we will report on a heretofore unknown phenomenon, namely, diffusion-driven concerted motion of substrate atoms. During diffusion of a dimer in the trough or across the substrate rows the substrate atoms in the proximity of the diffusing dimer exhibit a collective rearrangement. In retrospect, the occurrence of this concerted motion is not surprising at all, but is a direct consequence of the rearrangement of substrate atoms in the proximity of an adsorbed trough dimer.