Specific features of plastic relaxation of a metastable Ge$_x$Si$_{1-x}$ layer buried between a silicon substrate and a relaxed germanium layer

Heterostructures Ge/Ge$_x$Si$_{1-x}$/Si(001) grown by molecular beam epitaxy have been investigated using atomic scale high-resolution electron microscopy. A germanium film (with a thickness of 0.5–1.0 $\mu$m) grown at a temperature of 500$^\circ$C is completely relaxed. An intermediate Ge$_{0.5}$Si$_{0.5}$ layer remains in a strained metastable state, even though its thickness is 2–4 times larger than the critical value for the introduction of 60$^\circ$ misfit dislocations. It is assumed that the Ge/GeSi interface is a barrier for the penetration of dislocations from a relaxed Ge layer into the GeSi layer. This barrier is overcome during annealing of the heterostructures for 30 min at a temperature of 700$^\circ$C, after which dislocation networks having different degrees of ordering and consisting predominantly of edge misfit dislocations are observed in the Ge/GeSi and GeSi/Si(001) heteroboundaries.