RELAXATION OF SIGE THIN-FILMS GROWN ON SI/SIO2 SUBSTRATES

The relaxation in Si/SiGe bilayers grown on top of SIMOX wafers has been studied. By judiciously choosing the thickness ratios of the Si and SiGe, it is possible to relax the bilayer through the glide of dislocations exclusively in the Si layer, leaving the top SiGe layer relaxed and (mostly) dislocation free. This approach is completely different from previously proposed ways of reducing the number of threading dislocations in SiGe films because at no stage during the relaxation process are new threads introduced in the top SiGe layer. It is shown that the Si/SiGe bilayer behaves as a free-floating foil constrained to remain flat by the substrate, even at temperatures as low as 700-degrees-C. The relaxation is shown to proceed until the strain left in the Si layer is too low for dislocations to glide. When the temperature is raised to 1050-degrees-C, interdiffusion between the two layers forces the dislocation network to move into the SiGe through glide. The original network of 60-degrees dislocations can then react to form a network of edge dislocations, which had never been observed before in this system. At such high temperature, glide is no longer the limiting factor for relaxation, and almost complete relaxation is attained.