In a scanning tunneling microscope (STM) study of Si(111) surfaces from roo
m temperature to 600 degrees C, we find a special type of clusters which ar
e not only stable with respect to surface diffusion, but are also the funda
mental unit in mass transport phenomena, step fluctuations in detachment an
d attachment of Si atoms at step edges, and epitaxial growth. Using Arrheni
us analysis, we derive path specific hopping rate parameters for these clus
ters. A concerted reaction model is proposed to explain the nucleation and
growth behavior of Si on Si(111).