Morphological instability in coherent thin film structures is a well known phenomenon. This instability results when misfit strain energy relaxation at surface protuberances overcomes the increased surface energy of the nonplanar surface. Theoretical studies of this phenomenon have focused on single component films, although a significant fraction of experimental thin film growth consists of binary and higher order alloys. Alloy film systems are frequently assumed to be compositionally uniform, even in the presence of morphological instability. The lattice parameter of the alloy, however, is generally a function of composition. This property is used to carefully tailor the film-substrate lattice mismatch, but we find that it also has important implications for the film stability, both as a modification of the misfit strain energy and as a distinct source of compositional strain energy. We find in our linear stability analysis of alloy film growth that the stress generated by the composition gradients in the film affects the composition of newly deposited material and, under appropriate conditions, can destabilize even a nominally lattice-matched system. This novel instability is a result of our kinetic treatment of the growing alloy film and does not occur in pure materials or static systems. Although enhanced in films grown near the limit to metastability, this alloy instability occurs above both the chemical and coherent spinodals.
In order to test our theoretical predictions, we have deposited indium gallium arsenide films on different III-V substrates, using molecular beam epitaxy. This alloy was chosen for its large solute expansion coefficient (approximately 7%) and growth temperatures which are near the chemical spinodal (482°C). We have chosen film compositions and substrates such that we can examine the role of the sign of the misfit, as well as the evolution of lattice matched films. Analysis of in situ reflection high energy electron diffraction and ex situ atomic force microscopy results yields good qualitative agreement with our theoretical predictions. Notably, we find films grown under tension to have nonplanar surfaces of smaller amplitude and shorter wavelength than identical films grown under the same magnitude of compressive misfit.
*Present address: Semiconductor Electronics Division, NIST