Island formation and critical thickness in heteroepitaxy
In a recent Letter Chen and Washburn  proposed a mechanism for island nucleation in large-mismatch heteroepitaxy. The predicted coverage sQd dependence of the 3D island density ri sQd reproduces the fast increase in the island density near the critical coverage Qc ø 1.6 ML . Here we show that the critical coverage predicted by Ref.  depends strongly on the growth rate, thus contradicting, among others, the experimental results of Refs. [2,3].