Highly Mismatched, Dislocation-Free SiGe/Si Heterostructures

An experiment has demonstrated that truly dislocation free SiGe crystals with widths of at least 5μm can be epitaxially grown on Si substrates by applying the compositional grading developed for layers to 3D structures. At such crystal widths, the epitaxial growth of SiGe alloys with a constant high Ge content inevitably leads to the injection of dislocations. The reason is that at dimension far above the nanoscale the low aspect ratio of the SiGe/Si system during the initial stages of the growth does not allow for sufficient elastic strain relaxation. The findings suggest that there is no limit to the final Ge content, and that the grading rate can be suitably tailored in order to give access to even larger dislocation-free structures. Indeed, preliminary experimental results indicate similar absence of dislocations in SiGe crystals graded to a fi nal Ge content of 80%. Combining this method with layer transfer techniques makes it possible to form dislocation-free SiGe patches many micrometers in size acting as virtual substrates with lattice parameters intermediate to those of Si and Ge on an Si substrate.