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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 δ-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 δ-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. © 2000 American Vacuum Society.