  Accounting for the anharmonicity of the real interatomic potentials in a
model in 1+1 dimensions shows that coherent 3D islands can be formed on the
wetting layer in a Stranski-Krastanov growth mode predominantly in compressed
overlayers. Coherent 3D islanding in expanded overlayers could be expected as
an exception rather than as a rule. The thermodynamic driving force of
formation of coherent 3D islands on the wetting layer of the same material is
the weaker adhesion of the atoms near the islands edges. The average adhesion
gets weaker with increasing island's thickness but reaches a saturation after
several monolayers. A misfit greater than a critical value is a necessary
condition for coherent 3D islanding. Monolayer height islands with a critical
size appear as necessary precursors of the 3D islands. The 2D-3D transformation
from monolayer-high islands to three-dimensional pyramids takes place through a
series of stable intermediate 3D islands with discretely increasing thickness.
