A buried conductive layer in GaAs was formed using a low-energy focused ion beam (FIB) and molecular beam epitaxy (MBE) combined system. The sheet carrier density of the structure was measured as a function of ion dose ranging from 2 x 10(12) to 5 x 10(13) cm(-2) and postrapid thermal annealing time al 800 degrees C. It was shown that implanted Si ions at an ion energy of 200 eV and an ion dose of 2 x 10(12) to 1 x 10(13) cm(-2) can be fully activated by choosing an optimum annealing condition. Present results suggest the possibility of fabricating a high quality two-dimensional electron gas (2DEG) layer in the laterally selected area in GaAs and GaAs/AlGaAs systems.