Buried Si-doped layers in GaAs were fabricated by low-energy focused Si2+ ion beam (Si FIB) implantation in GaAs grown by molecular beam epitaxy (MBE) and successive overlayer regrowth using an FIB/MBE combined system, Carrier profiles were measured by means of a capacitance-voltage profiling technique. It was found that doped layers with a carrier concentration of 1.2 x 10(12) cm(-2) and a width (full width at half maximum) of 23 nm were formed without postannealing for the 200 eV Si implantation at: a dose of 7 x 10(12) cm(-2). After postannealing, the doping efficiency was improved and the width became narrower. At a dose of 1.4 x 10(13) cm(-2), narrower carrier distribution with higher peak density was＇ observed although the total doping efficiency was decreased. In contrast a deeply depleted layer was formed and no carriers were observed for implantations at an energy higher than 400 eV before the postannealing. This may be because damage is significantly reduced for the lower implantation energy.