Using scanning tunneling microscopy, we have observed electromigration of S
i on Si(lll)-(7 X 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 pre
viously recognized.