Critical epinucleation on reconstructured surfaces and first-principle calculation of homonucleation on Si(100).

We introduce the concept of "critical epinucleation to distinguish nucleation on surfaces with and without reconstruction. On a reconstructed surface, the critical classical nucleus is stable against dissociation, but may not yet break the underlying surface reconstruction. Consequently, there must exist a "critical epinucleus" that is not only stable but also has established the epiconfiguration by unreconstructing the underlying substrate. We illustrate this concept by first-principle calculation of homonucleation on reconstructed Si(001) surface where the critical epinucleus consists of six adatoms.