Eight strained-Si (sSi) on SiGe heterostructures with 8, 13, 25, or 40 nm sSi on top of 300 or 600 nm Si0.77Ge0.23 buffer were prepared by chemical vapor deposition and examined by preferential defect etching and Raman spectroscopy. Defect etching revealed that threading dislocations in the supercritical thickness sSi samples were more evenly distributed, while they were severely trapped inside threading dislocation pile-ups in the sub-critical thickness sSi samples. It was proposed that relaxation of the supercritical sSi layer, which was realized by threading dislocation gliding under a tensile stress, helped to breakup those pile-ups formed under a compressive stress. Defect etching revealed a threading dislocation density of (3–5) x 106/cm2, and no dependence upon the sSi or SiGe thickness was observed. Raman spectroscopy revealed that the relaxation degree of the 300 nm SiGe layer decreased from 80 to 67% with the sSi layer increasing from 8 to 40nm. This suggests a continuous relaxation of the highly compressively strained, thin SiGe buffer during or even after sSi growth, and its gradual suppression by the presence of a tensile strained sSi layer. The 600nm SiGe buffer has an ~82% relaxation for all sSi thickness, suggesting that its relatively small residual strain cannot support any further relaxation after switching to sSi growth and consequently the absence of any dependence on sSi thickness. Based on these observations, it was suggested that in situ thermal annealing prior to sSi growth would help to enhance the strain relaxation of thin SiGe buffers.

Effect of Strained-Si Layer Thickness on Dislocation Distribution and SiGe Relaxation in Strained-Si/SiGe Heterostructures. J.Lu, G.Rozgonyi, M.Seacrist, M.Chaumont, A.Campion: Journal of Applied Physics, 2008, 104[7], 074904