We have investigated two-dimensional electron gases (2DEGs) in inverted selectively doped GaAs/AlxGa1-xAs heterostructures fabricated by molecular-beam epitaxy (MBE) overgrowth of focused-ion-beam (FIB)-doped AlxGa1-xAs layers. In a first MBE step, the AlxGa1-xAs barrier was grown, before the sample was transferred to the FIB system. There, Si was implanted with 60 keV employing doses between 1×1012 and 1×1014 cm-2 and thereafter the sample was transferred back to the MBE system where the AlxGa1-xAs spacer as well as the GaAs top layer were grown. To protect the surface during the growth interruption, an amorphous As layer was used. Either an in situ annealing step before regrowth (30 s at 730 °C) or an ex situ thermal processing (30 s at 750 °C) after regrowth was used to remove the crystal damage due to the implantation. For the ex situ annealing step, we obtained mobilities up to 1.2×105 cm2/V s at 4.2 K after illumination whereas we observed mobilities up to 1.5×106 cm2/V s employing the in situ annealing step. © 2003 American Institute of Physics.