SEMICONDUCTOR SUBSTRATE, FIELD−EFFCT TRANSISTOR, AND THEIR MANUFACTURING METHODS

摘要

A semiconductor substrate comprises an Si substrate (1), a first SiGe layer (2) on the Si substrate, an a second SiGe layer (3) formed directly on the first SiGe layer (2) or with an Si layer being interposed therebetween. The thickness of the first SiGe layer is smaller than twice the critical thickness causing lattice relaxation due to dislocation caused when the film thickness is too large. The Ge composition ratio of the second SiGe layer is lower than the maximum value of the Ge composition ratio of the first SiGe layer at least at the contact surface in contact with the first SiGe layer or the Si layer. The second SiGe layer has at least a portion including a gradient composition region where the Ge composition ratio gradually increases toward the surface. As a result, the threading dislocation density and the surface roughness are reduced to practical levels.