Strain and strain relaxation in semiconductors

Single-crystal semiconductor layers can be grown with large coherency strains. This review covers their standard elasticity theory and methods of measuring the strain. High-quality strained layers are thermodynamically stable up to a critical thickness, and both theoretical and experimental determinations of critical thickness are considered. Above critical thickness there is a metastable regime, with thicknesses of a few tens of nanometres for a typical misfit epsilon(0) similar to 1%. A relaxation critical thickness is identified, above which compressive strain produces plastic relaxation so the strain in a layer is less than its misfit (tensile layers commonly experience cracking instead of plastic relaxation). Relaxing layers may have a misfit epsilon(0) similar to 1%, and thicknesses of a few hundred nanometres. In the high-mismatch regime, any strain severely perturbs the crystal growth; this occurs typically for misfits of 2% upwards. The review concludes with some unresolved questions about multilayer structures.