The refractive index of fully strained Si1−xGexlayers, with compositionsx=0.20, 0.25, 0.30, and 0.33, has been measured using spectroscopic ellipsometry as a function of the wavelength in the 0.9–1.7 &mgr;m range. The dependence of the refractive index on the wavelength, for these compositions, is similar to that of crystalline Si. Its value for any wavelength is lower than that of relaxed Si1−xGexof the same composition. The experimental values of the refractive index of the fully strained Si1−xGexwere fitted to the expressionnSiGe(x,&lgr;)=nSi(&lgr;)+(1.16–0.26 &lgr;)x2. This expression is applicable for wavelengths from 0.9 to 1.7 &mgr;m and compositionsx≤0.33. ©1995 American Institute of Physics.