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Published online by Cambridge University Press: 17 March 2011
Silicon-germanium virtual substrates have been synthesized by low-pressure chemical vapor deposition. We obtained threading dislocation densities ranging from 105 to 106 cm−2, surface roughness ranging from 1.5 to 4 nm, and also cross-hatch pattern densities, depending on the grading rate and top layer germanium composition. For the typical sample, which has a linear-graded structure with a grading rate of 20%/[µm, and germanium composition of 30 % at the top layer, we obtained dislocation densities of about 106 cm−2 and root mean squared surface roughness of about 3 nm. The obtained dislocation densities are equivalent with the virtual substrates synthesized by ultra-high vacuum system. On the other hand the surface roughness is superior to the typical reported values. In this study three kinds of structures, i.e. linear-graded, stepwise, and graded-step structures, were considered. We found the defects are effectively reduced by introduction of an optimum number of steps in the graded layer.