Polycrystalline silicon layers deposited without doping and afterwards implanted with arsenic or phosphorus show a strong increase of the sheet resistivity due to reactions between dopants and residual radiation damage during scanned electron beam annealing. In contrast, boron implanted specimens exhibit only a small shoulder due to annealing in the profile tail. The change of the dopant concentrations is modelled by numerical simulations, assuming the grain boundaries to be initially completely empty of dopants and only acting as weak sinks for dopants diffusing inside the microcrystallites.