We have examined the role of carbon co‐implantation in the formation of secondary defects in self‐ion‐irradiated Si(100). Implantation of Si ions (540 keV energy, 1015ions/cm2at 1.3×1011ions/cm2/s,Ti=90 °C) followed by a 900 °C, 15 min anneal leads to the growth of an extended defect band at the end of range. Range matched‐carbon co‐implantation (300 keV energy, 1015ions/cm2plus 500 keV energy 1015ions/cm2of 1.5×1011ions/cm2/s,Ti=90 °C) can be used to modify this defect development dramatically. While direct co‐implantation of carbon and silicon ions to similar concentrations has no apparent effect on the formation of extended defects, such formation is suppressed when the implanted C is incorporated substitutionally into the silicon lattice. These results are discussed in the context of recent reports on C suppression of the transient enhanced diffusion of boron. ©1995 American Institute of Physics.