STRUCTURAL TRANSITIONS IN EPITAXIAL OVERLAYERS

We present a theory of epitaxial growth appropriate to overlayer materials that can adopt a bulk metastable crystal structure with a lattice constant significantly different from that of the stable ground state structure, as is common for metal epitaxy. This requires a generalization of the traditional Frank and van der Merwe theory to a regime where the overlayer responds nonlinearly to the large shear stress exerted by the substrate. New results are obtained for the commensurate-incommensurate transition (for thin layers) and the coherent-incoherent transition (for thick layers) that are presented in the form of a structural phase diagram as a function of epitaxial misfit and overlayer thickness. We predict that a crystallographic analog to spinodal decomposition can occur in certain ranges of large misfit.