  The observed height distribution of clusters obtained in strained epitaxy has
been often interpreted in terms of electronic effects. We show that some
aspects can be explained classically by the interplay of strain and edge
energies. We find that soft materials can transform directly from monolayer
into thicker islands by two-dimensional (2D) multilayer nucleation and growth.
There is a critical thickness decreasing with the force constant. Thinner
islands are thermodynamically forbidden, due to the insufficient stress
relaxation upon clustering particularly under tensile stress. At sufficiently
large misfits the barrier for 2D multilayer nucleation is significantly smaller
than the barrier for subsequent single-layer nucleation. The effects are found
to be quantitatively reasonable and offer a plausible explanation for the
absence of thin islands and 2D growth of flattop islands usually attributed to
quantum size effects.

Materials Science