Effect of the regrowth temperature of the cap layer grown successively on a Si focused ion beam (FIB) implanted GaAs surface on the dopant activation was investigated using a FIB/molecular beam epitaxy combined system. Indication of the reevaporation of the implanted Si was observed at high regrowth temperature and the fabrication process was improved by using low regrowth temperature. A high doping efficiency was obtained for the ion dose at about 1 x 10(13) cm(-2). Present results indicate the importance of controlling the regrowth condition to obtain high doping efficiency.