In this paper we characterize strain in Si1-xGex and Si1-x-yGexCy layers grown on silicon by rapid thermal chemical vapor deposition exploiting the phenomenon of strain-induced birefringence. The technique used is multiple angle of incidence ellipsometry at a wavelength of 670 mn to measure the ordinary and extraordinary refractive index of the layers. We demonstrate that the incorporation of carbon in Si1-xGex layers reduces strain and increases the critical thickness.