Brittle-to-ductile transition and strain relaxation in Si${}_{1-x}$Ge${}_x$ linearly graded buffers

Abstract

The strain-relaxation mechanism of a set of Si${}_{0.6}$Ge${}_{0.4}$ linearly graded buffers (LGBs), grown following different temperature profiles, has been investigated by means of defect-etching and variable-temperature high-resolution X-ray diffraction (VT-HRXRD). Defect-etching experiments demonstrate that a sharp increase of threading dislocation density (TDD) from $3\times 10^5$\,cm${}^{-2}$ to $1.2\times 10^6$\,cm${}^{-2}$ takes place when the final growth temperature exceeds a critical value T${}_c\approx 530^{\circ }$C. VT-HRXRD measurements show that in low TDD samples extra relaxation takes place for annealing temperatures larger than T${}_c$, thanks to the nucleation of new dislocations. These results indicate that, below T${}_c$, strain relaxation is driven by the gliding of existing dislocations while above T${}_c$ new dislocations are nucleated, suggesting a link with our results and the brittle-to-ductile transition in Si${}_{1-x}$Ge${}_x$ alloys.