Anomalous strain relaxation in SiGe thin films and superlattices

We describe anomalous strain relaxation in graded SiGe superlattices and thin films. This relaxation is characterized by the presence of dislocations in the Si substrate, as well as in the lower part of the film or superlattice, and results in a dislocation-free top layer. This challenges the basic assumption, dating back to Van der Merwe, that the substrate does not participate in the strain-relief process. We show that this phenomenon is due to the paucity of nucleation sites, and controlled by the specific Ge concentration profile in the film. Relaxed, defect-free ${\mathrm{Si}}_{\mathit{x}}$ ${\mathrm{Ge}}_{1\mathrm{-}\mathit{x}}$ films containing up to 60% Ge have been grown in this manner.