  Stochastic simulation results, appropriate for Molecular Beam Epitaxy,
involving ballistic deposition and thermally activated Arrhenius diffusion of
adatoms are presented for one- and two-dimensional substrates, allowing for
overhangs and bulk vacancies. The asymptotic Kardar-Parisi- Zhang universality
is found to be triggered by a sudden nucleation of large-scale defect formation
in the growing film that shows a distinct dependence on dimensionality. The
pre-nucleation transient behavior, which may be of experimental relevance due
to the low defect content, is associated with standard solid-on-solid
universality classes.
