In this paper spectroscopic ellipsometry (SE) was applied for thickness and composition characterization of C and B doped SiGe layers grown on Si (100) by RPCVD at 600 degrees C. We present the effect of B doping (up to 1.2x10(21) cm(-3)) and C incorporation (up to 0.7%) on optical constants dispersions of Si(1-x)Ge(x) (0x<0.3) layers in the energy range 1.5-5.2 eV and their impact on thickness evaluation. We observed the changes of the positions of critical points and their broadening and found systematic monotonic changes for C incorporation in SiGe and nonmonotonic behaviour for B doped SiGe. The correlation between SE results and active boron concentration in the case of B incorporation was observed. The decoupling of the effect of C (or B) and Ge incorporation by SE was performed that can be applied in multi-dimensional lookup models for thickness and composition estimation and thus used for nondestructive characterization of SiGe-based materials. (c) 2008 Elsevier B.V. All rights reserved.