We present the results of double crystal X ray diffraction and infrared absorption measurements on C-doped low-temperature GaAs (LT-GaAs:C) layers grown by solid source molecular beam epitaxy at different growth temperatures T-G and C concentrations. Strain compensation is achieved at T-G = 220degreesC with a C concentration of 2.5 x 10(19)cm(-3). The density of As-Ga antisite point defects and the incorporation of C is studied by IRA. At T-G = 250degreesC and high C concentrations, we observe a drastic decrease in the intensity of the local vibrational mode of substitutionally incorporated C-As. Simultaneously, new local vibrational modes appear whose intensity increases by a further lowering of T-G. This result, which is neither observed at low C concentrations nor in the heavily doped GaAs:C grown at conventional T-G, is ascribed to the formation of C-C complexes in heavily doped LT-GaAs:C. (C) 2002 Elsevier Science B.V. All rights reserved.