Carbon-doped GaAs (GaAs:C) samples were grown using carbon tetrabromide (CBr4) as p-type dopant in a solid source molecular beam epitaxy (SSMBE) system. The effects of CBr4 flux, substrate temperature and growth rate were systematically studied. High CBr4 flux exceeding 2.6 x 10(-7) Torr and substrate temperature exceeding 620degreesC were found to induce the formation of dicarbon defects, resulting in decrease in hole concentration and increase in lattice parameter. Surface morphology images taken using atomic force microscopy (AFM) suggest signs of possible relationship between high surface roughness and dicarbon defects. The optimum growth conditions deduced from our experiment provide a useful baseline for application of GaAs:C as base layer in heterojunction bipolar transistor (HBT). (C) 2003 Elsevier B.V. All rights reserved.