Selectively doped layers buried in GaAs were formed by low (50 and 200 eV) and high (30 keV) energy focused Si ion implantation and successive overlayer growth using a focused ion beam/ molecular beam epitaxy (MBE) combined system to investigate the possibility to form patterned delta -doped layers. Carrier distribution profiles were measured by means of a capacitance-voltage profiling technique at room temperature. It was found for the low-energy implantation that the width of the depth profiles of the carrier distribution decreased with increasing sheet carrier density and was roughly in agreement with a theoretical estimation obtained by solving the Poisson equation. The width was decreased when the sheet carrier density increased by annealing. This indicates that the width is determined by a sheet carrier density and not by Si dopant profiles, and that narrower carrier profiles can be formed by optimizing annealing parameters, although the widths were 2-5 times wider than those observed for the MBE-grown delta -doped GaAs. The same doping efficiency as for the low-energy implantation was achieved but the distribution width was close to that of the dopant distribution.