A thin Si-doped layer was fabricated over a laterally selected area on a molecular beam epitaxy (MBE)-grown GaAs surface by 200 eV or 30 keV Si focused ion beam (FIB) implantation and successive overlayer regrowth using an FIB/MBE combined system. Characteristics of the buried doped layer were investigated by means of capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) methods. It was found that irradiation damages can be reduced by lowering the implantation energy, and large amount of carriers were observed in the 200 eV Si implanted region without annealing. It was also found that the growth interruption induces electron traps that are located below the midgap but has fewer effects on the activation of implanted dopants.