C-doped GaAs films were prepared by novely a developed, combined ion beam and molecular beam method (CIBMBE) as a function of hyperthermal (30–500 eV) energies (EC +) of carbon ion (C+) beam. Ion beams of a fixed beam current density were impinged during molecular beam epitaxy growth of GaAs at substrate temperature of 550 °C. Low temperature (2 K) photoluminescence (PL) has been used to characterize the samples together with Hall effects measurements at room temperature. Through the spectral evolution of an emission denoted by [g-g]β which is a specific emission relevant to acceptor-acceptor pairs, the activation rate was confirmed to increase with increasing EC + for EC + lower than 170 eV. It was explicitly demonstrated that the most effective Ec+ to establish highest activation rate is located at ~170 eV. This growing activation rate was suggested to be attributed to the enhanced migration of both impinged C and host constituent atoms with increasing EC +. This surmise was supported also by Hall effect measurements which revealed the maximum net hole concentration ( |NA-ND| ) for EC +=170 eV. For EC + higher than ~170 eV, increasing EC + was found to induce the reduction of activation rate. It was suggested that this observation is ascribed not to the formation of C donors but to the enhanced sputtering effect of impinged C+ ions with increasing EC +.