We propose a physical model which explains the existence of finite thickness
wetting layers in epitaxially strained films. The finite wetting layer is shown
to be stable due to the variation of the non-linear elastic free energy with
film thickness. We show that anisotropic surface tension gives rise to a
metastable enlarged wetting layer. The perturbation amplitude needed to
destabilize this wetting layer decreases with increasing lattice mismatch. We
observe the development of faceted islands in unstable films.Comment: 4 pages, 3 eps figure