  With scanning tunneling microscopy, we study the very early stage of Si
deposition on a Pb monolayer covered Si(111) surface. We find a special type of
Si magic clusters which are highly mobile on the defect-free surface but may be
trapped temporarily at certain boundary or defect sites. They may also
aggregate and a special aggregate arrangement is found to be stable for one to
two days at room temperature. Adding more Si on the cluster aggregate can
transform it into a metastable structure. A scenario based on magic clusters
and their chemical reactions is proposed to describe the nucleation and growth
processes of covalent materials.
