Secondary epitaxy of high Sn fraction GeSn layer on strain-relaxed GeSn virtue substrate by molecular beam epitaxy

Strain relaxation is critical for GeSn alloys transforming from indirect to direct bandgap nature with Sn fraction above 6.5%, but difficult for them grown by molecular beam epitaxy (MBE), in which low temperature has to be set up to avoid Sn segregation. In this work, compressively strained Ge0.935Sn0.065 thin films grown on Si with a Ge buffer layer by MBE are firstly treated by ex-situ rapid thermal annealing, rendering partially strain relaxation in the Ge0.935Sn0.065 by generation of misfit dislocation networks without Sn segregation. Then, secondary epitaxy of Ge0.905Sn0.095 layer is carried out on the thermally annealed Ge0.935Sn0.065 virtual substrate. The secondary epitaxial GeSn layers exhibit partial strain relaxation and strong photoluminescence with red-shift of peak position, compared to that of fully compressive strained GeSn thin films with the same structure grown primary epitaxially. Those results manifest that secondary epitaxy, combining with ex-situ rapid thermal annealing for strain relaxed GeSn virtual substrate, is a practical way to achieve strain relaxed GeSn thin films with direct bandgap nature by MBE.