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 degreesC, these clusters migrate on Si(111)-(7 X 7) surfaces as a whole. They can hop within a half-cell of Si(111)-(7 X 7), but sometimes t hey hop away from their original halves, leaving the 7 X 7 structure intact . When this happens, the magic cluster usually reappears at a site a few hu ndred A away. We characterize its structure and derive path-specific hoppin g parameters using Arrhenius analysis. In the long hops, interestingly, mag ic clusters exhibit a strong bias for moving in the direction of the heatin g current. Effects of the directed motion in electromigration and those in thermal migration are determined separately and quantitatively. We also obs erved 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 processe s in epitaxial growth and in electromigration on Si(111) surfaces. Based on our observations, the phase transition of 7 X 7 <----> 1 X 1 is also discu ssed. (C) 2001 Elsevier Science Ltd. All rights reserved.