The aggregation of in-diffused copper or sulfur at freshly introduced dislocations was studied by the combination of cathodoluminescence microscopy and analytical transmission electron microscopy. The occurrence of bright cathodoluminescence contrasts of dislocations could be related to the agglomeration of the impurities at dislocations and the formation of defect complexes. A Cottrell atmosphere of dissolved impurities around dislocations cannot explain the extended defect zone. The formation of extended defects such as precipitates is the reason for the change in the material properties up to a distance of several micrometers from the dislocation. The simulation of the diffusion-drift behavior of sulfur and copper at dislocations provides in accordance with the experiment a non-equilibrium atmosphere. The formation of an extended defect zone around dislocations in GaAs doped with other impurities such as tellurium may be explained in a similar way.