We show that C incorporation kinetics depend significantly on the carbon concentration. The carbon substitutionality (fraction of substitutional incorporated carbon atoms) is strongly influenced by the growth conditions, such as growth temperature and Si growth rate. In addition, reduction in the growth temperature and increase of the growth rate can both increase the substitutional carbon fraction. This behavior is well described by a kinetic model, with the energy barrier and preexponential frequency factor decreasing with increasing carbon concentration. Very low carbon concentrations (in the dopant range) can be predominantly incorporated substitutionally, independent of the growth temperature. At higher C concentrations (in the percentage range), the substitutional carbon fraction is shifted to lower values. We predict the existence of an upper limit for substitutional C incorporation, with its value depending on the specific growth conditions. © 1999 American Institute of Physics.