Direct Observation of Electromigration of Si Magic Clusters on Si(111) Surfaces

Using scanning tunneling microscopy, we have observed electromigration of Si on $\mathrm{Si}\left(111\right)-(7\times 7)$ surfaces and have identified the diffusion species to be Si magic clusters. Effects of the directed motion along the direction of the heating current in electromigration and those in thermal migration are determined separately and quantitatively. We also observe the preferential filling of two-dimensional (2D) Si craters and the preferential detachment of Si magic clusters from the edges of 2D Si islands near the cathode side. The driving force for this anisotropic behavior is much stronger than previously recognized.