Thermal stability and relaxation mechanisms in compressively strained Ge_(0.94)Sn_(0.06) thin films grown by molecular beam epitaxy

摘要

Strained Ge_(1-x)Sn_x thin films have recently attracted a lot of attention as promising high mobility or light emitting materials for future micro- and optoelectronic devices. While they can be grown nowadays with high crystal quality, the mechanism by which strain energy is relieved upon thermal treatments remains speculative. To this end, we investigated the evolution (and the interplay) of composition, strain, and morphology of strained Ge_(0.94)Sn_(0.06) films with temperature. We observed a diffusion-driven formation of Sn-enriched islands (and their self-organization) as well as surface depressions (pits), resulting in phase separation and (local) reduction in strain energy, respectively. Remarkably, these compositional and morphological instabilities were found to be the dominating mechanisms to relieve energy, implying that the relaxation via misfit generation and propagation is not intrinsic to compressively strained Ge_(0.94)Sn_(0.06) films grown by molecular beam epitaxy.