Strain relaxation and self-organization phenomena in heteroepitaxial systems

The plastic behavior of strained, compositionally graded Si1-xGex alloy layers grown on Si substrates has been studied by a combination of optical, atomic force, and transmission electron microscopy. Formation of ordered patterns of misfit dislocations has been found in films grown at low (similar to 500 degrees C) temperatures, and examined at different length scales. We demonstrate that the strain relaxation in the thick metastable layers is an evolutionary propagative process, which is heterogenous from the very beginning and localized in narrow shear bands. It is shown that the relaxation process is characterized by dissipation of large amounts of the elastic internal energy accumulated during film growth and takes place far away from thermodynamic equilibrium. These results strongly suggest that the dislocation patterning in graded layers involves self-organization of dislocation populations. Striking analogies between the Seeger-Frank treatment of dislocation ordering in solids and the model of self-adjustment of misfit dislocations are elaborated. The graded Si1-xGex/Si system will be shown to constitute a convenient model case for studying self-organization in crystalline material.