Heteroepitaxy and Strain: Applications to Electronic and Optoelectronic Materials
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The following articles emphasize these two couples as representative examples but there are many more systems of current technological interest such as GaN/AIN, GaN/sapphire, GaSb/InAs, and SiC/Si to name a few. To include a discussion of all these materials would not be possible or even reasonable since many of the features of strained heteroepitaxy are common to all of these systems. One key issue with heteroepitaxy is that the strain introduced by the lattice mismatch will always find a way to be relieved as the epitaxial film thickness is increased. Generally this will occur (1) by roughening the surface of the epitaxial layer, (2) by introducing defects such as dislocations into the epitaxial layer, or (3) through some combination of these two. Since flat layers with very low concentrations of defects are generally desired for most technological applications, controlling when and how strain relief occurs becomes a key issue in heteroepitaxy. This issue of the Bulletin reviews the effects of strain on the growth of the epitaxial layer and indicates some of the variety of ways that nature relieves strain. We have also tried to indicate some of the ways that the effects of strain relief can be controlled in a way to minimize their negative aspects on potential devices. For those more inclined to "go with the flow," we have also reviewed some of the attempts to take advantage of the effects of strain relief in heteroepitaxy to make potentially useful structures. The first article, by A.G. Cullis, deals
with the general issue of strain-induced modulations in the surface morphology of an epitaxial layer. This article explains, in general terms, why the surface of a heteroepitaxial layer would actually prefer to ripple, or form some other sort of modulation, to reduce the strain energy. The article that follows, by Christopher Roland, takes a more atomistic look at the effects of stress on step energies and surface roughness. In this article, Roland shows how realistic calculations of atomicstep energies of germanium, which is strained by epitaxial growth on silicon substrates, lead to surface-roughness models that are in excellent agreement with experimental results. The next two articles deal with how strain is relieved in the growth of heteroepitaxial layers of silicon-germanium alloys on silicon substrates (the Gei_^Si,/Si couple). Both articles emphasize the difficulty the growing epitaxial layer has in relieving strain and in reaching its minimum energy configuration. The article by D.E. Jesson, K.M. Chen, and SJ. Pennycook on kinetic pathways to stain relaxation in the Si-Ge system emphasizes the role of facet formation in determining the surface morphology and in the eventual nucleation of misfit dislocations. This article presents an approach that may help in understanding why a uniform size distribution of small islands may be obtained under proper growth conditions. (Assuming that is what you want. See the last article for reasons why these islands may be attractive.) On the other hand, in the article on
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