GaAs(100) single crystal substrates have been implanted with 1.00 MeVC+ and 2.00 MeVC2+ at various ion doses. The surface topography and roughness of the implanted surfaces are studied by atomic force microscopy and x-ray reflectometry. Above a threshold dose, production of surface defects is observed, which results in an overall increase of the rms roughness parameter. The enhanced rms roughnesses induced by monomer and by dimer ions at a given dose are comparable. The influence of C+ and C-2(+) irradiations on the change of lattice parameter in GaAs, and the effect of thermal annealing, are examined by high-resolution x-ray diffraction. For the as-implanted specimens, an, increase of the lattice constant, caused by the accumulation of implantation-induced point defects, has been observed and quantified. Thermal treatment of the implantation damage. stimulates an almost complete recovery of the primary defects, leaving the crystal practically free from any lattice strain. The electrical characteristics of the C+ and C-2(+) implantations are also briefly presented and. discussed. (C) 2003 American Vacuum Society.