The production of electrically active defect centers in molecular beam epitaxy (MBE) GaAs irradiated with low energy (50–500 eV) Sn ions during growth has been investigated as a function of ion energy. GaAs was dopedntype during growth with a specially designed Sn focused ion beam column mounted on a MBE growth chamber. The 77 and 300 K Hall mobility and carrier concentration of the GaAs depended strongly on the ion energy, thus providing a sensitive measure of the concentration of ion‐induced acceptorlike defect centers. The material was found to be nonconducting for ion energies greater than 200 eV, while a systematic decrease in the acceptor concentration, and consequent increase in the mobility, was observed as the ion energy was decreased below this value. A peak mobility of 90 000 cm2 V−1 s−1at a carrier concentration of 1×1014cm−3was achieved (at 60 K) which is in excess of that obtained in other reports of ion‐doped GaAs. A similar dependence on ion energy was found in the 4.2 K photoluminescence spectra of the ion‐doped GaAs, characterized by the appearance of a broad acceptor level peak and a decrease in the overall luminescent intensity with increasing ion energy. The lowest energy doped samples provided high quality spectra with narrow linewidths comparable with thermal (Si) doped material. Using the above data, a mechanism is proposed for the production of acceptor centers in this system.