The thermal stability of molecular beam epitaxy grownSi1−x−yGexCy/Siheterostructures (0⩽x<0.30,y∼0.008) was studied using infrared absorption spectroscopy. The local vibrational mode of C in Si andSi1−x−yGexwas used to quantify the loss of C atoms from substitutional sites with high temperature annealing. The activation energy(Ea=4.9 eV)for the loss of substitutional C achieved a maximum for the strain compensated alloy(x∼0.1).An additional increase of Ge content resulted in a rapid decrease inEa,which was found to be 3.4 eV forx∼0.27.The nonmonotonic behavior ofEaon Ge content is explained by the effect of the interface strain between the epitaxial layer and Si substrate. ©1998 American Institute of Physics.