FIG. 4: Growth velocity dM dν$_{evap}$t (panel a) and roughness (panel b) as functions of the hit rate for various choices of the critical size, n$_{c}$ in the presence of a spiral defect. rate of increase of the solid is basically a measure of the nucleation rate and is not directly relevant to the rate of step growth. However, the same qualitative effects are seen with and without defects. Intuitively, one expects that if ν$_{hit}$ << ν$_{evap}$ then nucleation will be rare and growth will be dominated by attachment to existing super-critical clusters, i.e. smooth step growth. When the hit rate increases, nucleation becomes more probable until at some point nucleation can occur easily and growth will be a combination of growth of existing clusters and nucleation of new clusters. Simulations were performed for a system consisting of 1500 sites and the velocity, defined as the rate of change of the total mass, was averaged over 1000 cycles. Figure 4 shows the rate of growth of the surface as a function of the hit rate for various choices of the size of the critical cluster when a spiral defect is present. For a small critical cluster, n$_{c}$ = 2, nucleation is almost instantaneous for all but the lowest values of ν$_{hit}$/ν$_{evap}$ : growth is dominated by nucleation and the growth velocity increases more or less linearly with the hit rate. For larger critical sizes, two different growth regimes can be distinguished in the Figure. At small hit rates, growth is dominated by heterogeneous nucleation at the defect with the subsequent growth of steps away from the defect. This growth by aggregation is analogous to the NY → C$_{N}$ scenario in the mean-field model. However, at high hit rates, nucleation becomes important and the growth rate increases due to the 11