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 mechanisms by which misfitting systems can relax the strain caused by pseudomorphic growth. We treat elastic, plastic and diffusive relaxation and their interdependence. As an extension we discuss additional relaxation mechanisms, though possibly not very efficient, that are brought about or forced by a lower 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 polymorphs with the idea that the high electrical fields due to spontaneous polarization in the wurtzite polymorph are at least partly reduced by piezoelectric fields caused by the strained incorporation of the cubic polymorph.