The electrical activation and carrier concentration profiles in high-dose Si-implanted (120 keV, 1 × 1014 cm-2) GaAs layers are reported. The highest activation efficiency observed is 44% both for SiO2 and for SiOxNy, encapsulation during rapid thermal annealing. For SiOxNy, capping the carrier concentration profile is within the theoretical implantation profile, for SiO2 capping the carrier concentration profiles are diffusion broadened. Defect studies by deep-level transient spectroscopy revealed the midgap level EL2 as a dominant electron trap after implantation and annealing. The EL2 concentration in as-grown homogeneously Si-doped GaAs layers is very low, but increases strongly during annealing. The increase of the EL2 concentration depends on the cap layer, being much stronger for SiO2 encapsulation than for SiOxNy capping. The implantation-induced trap concentration can be annealed out to about a value as arising from the capping and annealing procedure at that temperature.