The annealing behavior of pseudomorphic Si0.603Ge0.397/Si and Si0.597Ge0.391C0.012/Si superlattice structures was studied in the temperature range between 750 °C and 900 °C. Carbon incorporation of 1.2&percent; changes the thermal stability of SiGe structures significantly. It suppresses plastic relaxation due to an effective dislocation pinning. No relaxation of the SiGeC sample was observed during annealing up to 875 °C for 3 h. For comparison the SiGe sample relaxed at 800 °C. Carbon strongly increases the interdiffusion of Ge and Si. The activation energy of this diffusion process for a Ge content of 40&percent; decreases from about 4.8 eV for the pure SiGe to about 2.0 eV with an additional C content of 1.2&percent; assuming a neglectable diffusion of the carbon. This leads to a distinct modification of the Ge profile in the investigated temperature range. ©1997 American Institute of Physics.