Carbon-doped GaAs and InP grown at low temperatures by molecular-beam epitaxy contain a high concentration of antisite defects which gives rise to ultrafast carrier trapping time and desirable radiation-hard properties. The use of CBr4 as the dopant source introduced significant bromine incorporation during low-temperature (LT) growth. Incomplete dissociation of the CBr4 molecules gives rise to the formation of C-Br complexes and results in a reduction of electrically active carbon concentration, In this work, we present our studies on the incorporation mechanism of C and Br in LT-GaAs and report on the effect of carbon and bromine incorporation on carrier lifetime and concentration of arsenic antisite defects. Preliminary results on LT-InP:C characterization are also presented.