Raman scattering and cathodoluminescence experiments have been performed to investigate the effect of dislocations on the spatial distribution of point defects and on the free electron concentration in n-type GaAs:Si. An experimentally extended increase of the free electron and (Si_GaV_Ga)^2- complex concentrations from the matrix to the dislocation is explained as resulting from the formation of arsenic precipitates around the dislocation by means of computer simulations based on a diffusion-aggregation model.