Mechanisms of epitaxial growth

Abstract

‘Epitaxy’ means order in the relative orientation of identical crystals nucleated and grown on a large single-crystal face. Every crystal of the deposited material is oriented in such a way that there is coincidence of some vectors of its reciprocal lattice with vectors of the reciprocal lattice of the substrate surface. Depending on the length of the coincident vectors, one distinguishes between epitaxy of first order (coincidence of basis vectors), second order, and so on. In this paper, selected epitaxial systems (metals on metal, semiconductor and insulator substrates, semiconductors on semiconductors) are used to illustrate the influence of the lattice mismatch, interatomic forces and experimental parameters on the mode of film growth. The interaction across the epitaxial interface induces homogeneous strain in ultra-thin films and inhomogeneous strain in thicker deposits. The periodic strain is usually described in terms of misfit dislocations or static distortion waves, which are mobile at elevated temperature (misfit dislocations vibrate like interacting quasiparticles). Molecular dynamics studies suggest that a resonant coupling of the epitaxial film with an external field of appropriate frequency can result in a dramatic decrease of the misfit dislocations density. The growth of epitaxial films is an example of a first-order phase transition. As such the basic features of its thermodynamics and kinetics had been clarified long before it became of interest as a high technology. For this reason, 3 begins with a brief historical survey covering the classical results of Gibbs, Volmer, Stranski and Kaischew, Stranski and Krastanov. As with any small phase, the ultra-thin epitaxial films have a chemical potential that strongly depends on film thickness. This circumstance provides the thermodynamic basis for three modes of epitaxial growth: island growth, layer growth and their combination, namely islands growing on one or two completely built up monolayers. The mode of growth under near-to-equilibrium conditions can be predicted by a thermodynamic criterion based on an analysis of the μ(n) dependence, which accounts for both the interaction across the epitaxial interface and the strains in the film. The film morphology under far-from-equilibrium conditions can be predicted by a kinetic criterion based on an analysis of the surface transport arising from the thermodynamic driving force μ(n)- μ(n - 1).