Growth of highly strain-relaxed Ge_1-xSn_x/virtual Ge by a Sn precipitation controlled compositionally step-graded method

We have investigated Sn precipitation and strain relaxation behaviors in the growth of Ge_1-xSn_x layers on virtual Ge substrates (v-Ge) for strain engineering of Ge. By varying misfit strain at Ge_1-xSn_x/v-Ge and Ge_1-ySn_y/Ge_1-xSn_x interfaces, we found that a critical misfit strain controls the onset of Sn precipitation at a given thickness of the Ge_1-xSn_x layer. A compositionally step-graded method, in which the critical misfit strain is taken into account, was applied to the growth of strain-relaxed Ge_1-xSn_x layers on v-Ge. Postdeposition annealing at each growth step led to lateral propagation of threading dislocations preexisting in the layer and originating from v-Ge, which resulted in high degree of strain relaxation. An epitaxial Ge layer was grown on the strain-relaxed Ge_1-xSn_x layer and an in-plane tensile strain of 0.68% was achieved.