C+ ion-implantation was carried out for extremely pure GaAs gown by moleplar Ieam epitaxy. The dose was very widely varied from 1×1015 to 1×1020 cm−3. Photoluminescence and Raman scattering measurements were performed at 2 K and room temperature, respectively, both as functions of dose and annealing temperature. The results revealed that for a dose less than 5×1017 cm−3, where the dominant damages are point defects, annealing at 200°C is sufficient to attain a damage-free lattice. When the dose is above that value, where the principal damage is a highly disordered lattice, annealing, at least above 550°C, is required. Photoluminescence spectra showed that the well-defined near-band-gap emission, [g-g], (exclusively inherent to acceptors), was a dominant emission in the above mentioned low dose region, but was strongly suppressed in the higher region. These observations indicate that ion-implanted C atoms in GaAs behave as amphoteric impurities when the dose exceeds a critical value, of around 5×1017 cm−3.