Alloy decomposition and surface instabilities in thin films

We show that in the presence of substrate misfit and compositional stresses, static or growing films that undergo surface spinodal decomposition are always unstable to perturbations around the planar surface. For sufficiently rapid deposition processes, the planar surface can be stabilized due to a suppression of the alloy decomposition. Films grown outside of the miscibility gap can become unstable due to the mismatch with the substrate and compositionally generated stresses. We also demonstrate that the instability is independent of the sign of the misfit when the elastic moduli of the alloy constituents are equal, and the existence of a maximum misfit above which the film is always unstable, even at high growth rates. The symmetry under sign reversal of the misfit can be broken by composition-dependent elastic constants. [S0163-1829(98)05508-8].