We have investigated the temperature dependencies of charge carrier densities and Hall mobilities in tensile strainedSi1−yCyand in compressively strainedSi1−x−yGexCylayers. In both cases, the measured charge carrier densities at room temperature are not affected substantially by the addition of a small concentration of carbon (<1&percent;) under identical growth conditions and dopant fluxes. The measured Hall mobilities monotonically decrease with increasing carbon content for electrons inSi1−yCy,and for holes inSi1−x−yGexCy,respectively. Our results indicate that electrically active defects are formed with the addition of carbon. These defects are presumably connected with carbon/Si interstitials or other C-related complexes. It seems to be difficult to attribute the formation of those electrically active defects solely to contaminations originating from the used carbon evaporation source. We observed that donor- and acceptor-like defects are formed inSi1−yCyas well as inSi1−x−yGexCylayers with roughly a constant ratio, independent of source temperature. ©1997 American Institute of Physics.