The electrical properties of carbon doped GaAs and AlGaAs were studied as a function of substrate temperature and CBr4 flux for the doping range ∼1018–1020 cm-3. Hall measurements indicate a strong reduction in the free carrier concentration of GaAs films grown with the same CBr4 flux at substrate temperatures above 620 °C. Secondary ion mass spectroscopy measurements, however, show no reduction of chemical carbon concentration. The electrical properties of GaAs:C epilayers grown on (n11)A and B surfaces, where n=2–5, show strong dependence on crystallographic orientation. Based on these measurements, the model of free carrier concentration reduction in GaAs:C based on formation of electrically inactive C–C pairs has been proposed. In contrast, no anomalous carbon incorporation in AlGaAs has been detected for the doping range ∼1018–1020 cm-3 and the substrate temperature range 550–700 °C. The resulting material exhibits excellent transport and optical properties. © 1999 American Vacuum Society.