  The formation of coherent three-dimensional islands in highly mismatched
epitaxy is discussed in terms of the traditional concept of wetting. It is
shown that the wetting layer and the 3D islands represent different phases
which cannot be in equilibrium with each other, and the transfer of matter from
the stable wetting layer to the 3D islands is thermodynamically unfavored. The
experimentally observed critical misfit for coherent 3D islanding to occur and
the coexistence of pyramids with discrete heights of two, three, four...
monolayers can be explained assuming that the thermodynamic driving force of
formation of coherent 3D islands on the surface of the wetting layer of the
same material is the reduced average adhesion of the islands to that layer and
that the islands height is a discrete variable.
