The kinetics of heteroepitaxial island nucleation on the Si(111) surface during ultrahigh vacuum chemical vapor deposition with the precursor GeH4 was studied by scanning tunneling microscopy. The results can be described within the framework of rate-equation based nucleation theory, modified by an additional energetic barrier for the attachment of adatoms at steps. This barrier results from the passivation of steps by dissociation products from the GeH4 precursor. A critical nucleus size of 9 atoms is derived. Scanning tunneling microscopy images provide direct evidence for the existence of stable clusters consisting of 10 atoms and allow the unequivocal identification of their structure.