Terrace grading of SiGe for high-quality virtual substrates

Silicon germanium (SiGe) virtual substrates of final germanium composition x=0.50 have been fabricated using solid-source molecular beam epitaxy with a thickness of 2 mum. A layer structure that helps limit the size of dislocation pileups associated with the modified Frank-Read dislocation multiplication mechanism has been studied. It is shown that this structure can produce lower threading dislocation densities than conventional linearly graded virtual substrates. Cross-sectional transmission electron microscopy shows the superior quality of the dislocation network in the graded regions with a lower rms roughness shown by atomic force microscopy. X-ray diffractometry shows these layers to be highly relaxed. This method of Ge grading suggests that high-quality virtual substrates can be grown considerably thinner than with conventional grading methods. (C) 2002 American Institute of Physics.