Carbon-implanted GaAs was annealed by encapsulating its surface with a highly arsenic-doped hydrogenated amorphous silicon film. The annealed samples had very similar distribution profiles of the implanted C atoms as the as-implanted sample, whereas carriers were measured only in a shallow region near the surface. The implant dose and annealing temperature dependences of the sheet carrier concentrations were approximately the same as that for photoluminescence originating in the electron transition from the conduction band into the C acceptor level.