Scanning tunneling microscope study of dynamic phenomena on clean Si(111) surfaces: Si magic clusters and their role

On Si(111) surfaces, we observed a special type of Si magic clusters with a variable-temperature scanning tunneling microscope (STM). At temperatures above 400°C, these clusters migrate on Si(111)-(7×7) surfaces as a whole. They can hop within a half-cell of Si(111)-(7×7), but sometimes they hop away from their original halves, leaving the 7×7 structure intact. When this happens, the magic cluster usually reappears at a site a few hundred Å away. We characterize its structure and derive path-specific hopping parameters using Arrhenius analysis. In the long hops, interestingly, magic clusters exhibit a strong bias for moving in the direction of the heating current. Effects of the directed motion in electromigration and those in thermal migration are determined separately and quantitatively. We also observed fluctuations of step edges through detachment and attachment of magic clusters. The filling of two-dimensional (2-D) craters and the decay of 2-D islands are also found to occur preferentially at the cathode side. These observations provide important clues for understanding the atomic processes in epitaxial growth and in electromigration on Si(111) surfaces. Based on our observations, the phase transition of 7×7?1×1 is also discussed.