  The two- to three-dimensional growth transition in the InAs/GaAs(001)
heterostructure has been investigated by atomic force microscopy. The kinetics
of the density of three dimensional quantum dots evidences two transition
thresholds at 1.45 and 1.59 ML of InAs coverage, corresponding to two separate
families, small and large. Based on the scaling analysis, such families are
characterized by different mechanisms of aggregation, involving the change of
the critical nucleus size. Remarkably, the small ones give rise to a wealth of
"monomers" through the erosion of the step edges, favoring the explosive
nucleation of the large ones.
