The growth of semiconductor heteroepitaxial layers is assuming ever greater importance due to the demands of modern electronic-device fabrication. Furthermore although low-strain heterosystems such as AlGaAs/GaAs remain the basis of many device structures, there is an increasing trend also to use more highly strained combinations such as InGaAs/GaAs and SiGe/Si. However the growth of the latter epitaxial systems must be approached with great care in order to achieve the optimum layer structural quality. Of course for any given alloy-layer composition, interfacial misfit defects in general will be introduced when the layer thickness exceeds a critical value, as originally described by Frank and van der Merwe, and Jesser and Matthews. (See the article by F.K. LeGoues in this issue for more general considerations of misfit-defect introduction.) In addition the morphology of such strained layers must be given very careful attention. It is the purpose of this article to examine our current understanding in this latter area.
When any epitaxial layer is grown, initially it might be expected that a flat surface would result under ideal growth conditions when internal defects have been eliminated. This would be expected to minimize the surface step density and hence the surface energy. However nature has a way of confounding our simplest expectations and while for homoepitaxial layers in the absence of kinetic effects this expectation virtually can be realized, the presence of strain in a heteroepitaxial system can severely affect the observed layer morphology.