Threading Dislocation Reduction of Ge by Introducing a SiGe/Ge Superlattice

Abstract

The influence of introducing a SiGe / Ge superlattice (SL) between Ge layers and Si substrate for the sake of the reduction of the threading dislocation density (TDD) without additional annealing is investigated. In the case of 2.8 μm thick Ge directly grown on Si, the TDD at the surface is 7.6×10-8 cm-2. A slight TDD reduction is observed by introducing a Si0.2Ge0.8 / Ge SL between the Si substrate and the Ge layer. By inserting 5, 10 and 20 cycles of Si0.2Ge0.8 / Ge, the TDD is reduced to 7.1×10-8, 5.9×10,-8 and 5.3×10-8 cm-2, respectively. The lateral lattice parameters of these SLs are ~5.656Å, which is a smaller value compared to that of bulk Ge, indicating plastic relaxation by misfit dislocation (MD) formation. Further TDD reduction is realized with increasing Si concentration in the SiGe / Ge SL without changing the cycle of the SL. However, surface roughening due to pit formation occurs if the Si concentration in the SL is higher than 50% because of increased strain at the interfaces between SiGe and Ge. With increasing SiGe and Ge thickness ratio in the SL layer and maintaining periodicity and cycles, the TDD is reduced to 2.8×10-8 cm-2 without degrading the surface roughness. This improvement is related to a relaxation of the SiGe/Ge SL by plastic deformation.