Relaxation of misfit-induced strain in semiconductor heterostructures

Using examples from SiGe solid solutions grown onto (001)Si substrates from metallic solutions (liquid phase epitaxy) we discuss the different mechani sms by which misfitting systems can relax the strain caused by pseudomorphi c growth. We treat elastic, plastic and diffusive relaxation and their inte rdependence. As an extension we discuss additional relaxation mechanisms, t hough possibly not very efficient, that are brought about or forced by a lo wer crystal symmetry. We use examples from wurtzite group-III nitrides. In a last section, based on transmission electron microscopy of AlGaN layers, we interpret an observed pseudomorphic mixture of wurtzite and cubic polymo rphs with the idea that the high electrical fields due to spontaneous polar ization in the wurtzite polymorph are at least partly reduced by piezoelect ric fields caused by the strained incorporation of the cubic polymorph.