Relaxation of strained silicon on 20% linear graded virtual substrates was quantified using high resolution x-ray diffraction and a defect etching technique. The thickness of strained silicon was varied between 10 and 180 nm. Relaxation was observed in layers below the critical thickness but increased to only 2% relaxation in the thickest layers even with annealings up to 950°C. Cross-sectional transmission electron microscopy revealed stacking faults present in layers thicker than 25 nm, and nucleated 90°Shockley partial dislocations forming microtwins in the thickest layer. These features are implicated in the impediment of the relaxation process.

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 μm. 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.

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