TABLE OF CONTENTS In the small misfit class we can expect the unit cell structure in the crystal planes of film and substrate that adjoin the common interface to be the same or that for the film to be deformable to the unit cell structure of the substrate. Further, the lattice parameters of the epilayer unit cell and substrate unit cell will be the same initially, as film deposition proceeds from the thinnest layer. The film having this characteristic is called pseudomorphic. It has a coherent (commensurate) interface with the substrate. The principle that governs deposition at this stage was first enunciated by Frank and van der Merwe.40 It is that thermodynamics controls to produce the equilibrium, minimum free energy, configuration of the substrate/film system. (The statement that the pseudomorphic film/substrate system is in its lowest free energy configuration at film thicknesses less than the critical thickness is not strictly true because it neglects a modifying restriction. The latter is that the temperature is low enough that atomic diffusion cannot occur.) Since any non-zero misfit introduces strain energy into the pseudomorphic film, which increases with increase in the thickness of the film, whereas the interface energy between a non-strained film and substrate is independent of the film thickness, it is apparent that there will be a critical thickness below which the pseudomorphically strained film configuration will have the lower free energy, but above it the strain-relaxed film, separated from the substrate by a...
