  A continuum dynamical model is developed to determine the morphological and
compositional instabilities on the free surface of heteroepitaxial alloy films
in the absence of growth. We use linear stability analysis to study the early
nonequilibrium processes of surface evolution, and calculate the stability
conditions and diagrams for different cases of material parameters. There are
two key considerations in our treatment: the coupling between top free surface
of the film and the bulk phase underneath, and the dependence of both Young's
and shear elastic moduli on local composition. The combination and interplay of
different elastic effects caused by lattice misfit between film and substrate
(misfit strain), composition dependence of film lattice parameter
(compositional strain) and of film elastic constants lead to complicated and
rich stability results, in particular the joint stability or instability for
morphological and compositional profiles, the asymmetry between tensile and
compressive layers, as well as the possible stabilization and suppression of
surface decomposition even below the effective critical temperature. We also
compare our results with the observations of some postdeposition annealing
experiments.
