The effect of anharmonicity in epitaxial interfaces: II. Equilibrium structure of thin epitaxial films

Abstract

The influence-of anharmonicity on the equilibrium structure of thin epitaxial films is studied in the framework of the theoretical model, introduced in Part I. Using the conventional Frank and van der Merwe model as a reference basis, it is shown that allowing for anharmonicity of the interatomic forces leads to qualitatively new effects, such as: (i) a significant asymmetry of dislocation length, strains in the deposit, activation energy for surface diffusion of epitaxial islands with respect to the sign of natural misfit between substrate and deposit; (ii) a split of the critical stability limits for pseudomorphic growth as well as for spontaneous generation of misfit dislocations whereby the limits corresponding to negative misfit rapidly increase whereas the positive misfit limits decrease (in absolute terms) with growing degree of anharmonicity. Experimental evidence, concerning the pseudomorphic growth of metals, semiconductors and ferromagnetic garnets, is shown to agree favourably with these theoretical results. A dependence of structural properties of the overlayer, such as critical thickness of pseudomorphic growth, density of dislocations beyond the critical thickness and preferred epitaxial orientation, not only on absolute value but also on sign of the natural misfit is predicted. It appears important as a guideline for material research in the field of epitaxial growth.