COUPLING OF THE MORPHOLOGICAL AND COMPOSITIONAL INSTABILITIES IN A STRESSED ALLOY WITH A FREE-SURFACE

We give a unified thermodynamic treatment of the morphological and com positional instabilities of a nonhydrostatically stressed alloy with a free surface, applying in particular to lattice-mismatched semiconduc tor epitaxial films. Provided its stress free unit cell volume depends on composition, any such alloy is unstable with respect to a range of joint surface undulations and composition modulations. The modulation s of lowest energy are exponentially attenuated normally to the free s urface. The domain of existence of the MC instability is determined as a function of temperature and modulation wavenumber. The critical tem perature for decomposition becomes infinite. The critical wavenumber i s larger than for the pure morphological instability. Quantitative pre dictions are given for various semiconductor alloys. (C) 1998 Elsevier Science B.V.