STRAINED-LAYER EPITAXY - HOW DO CAPPING LAYERS AND OPPOSITELY STRAINED INTERMEDIATE LAYERS ENHANCE THE CRITICAL THICKNESS

Within a valence force field framework, we calculate the critical thic kness of a film that is lattice mismatched to the substrate on which i t is epitaxially deposited. A capping layer that is lattice matched to the substrate is known to enhance the critical thickness. We calculat e the efficiency of the capping layer as a function of its thickness. We also demonstrate that this efficiency can be improved by using a ca pping layer that is not lattice matched to the substrate. When the cap ping layer is undesirable, another way of enhancing the critical thick ness is to use an oppositely strained intermediate (OSI) layer between the substrate and the film. To maximize efficiency, the OSI layer lat tice constant should be larger (respectively, smaller) than that of th e substrate if the film is in tension (respectively, compression). We demonstrate that a single OSI layer has no significant effect on the c ritical thickness. A microscopic description of the strain on all of s ystem is provided, and the mechanism of strain compensation is explain ed.