We report experiments on the diffusion of n-type dopants in isotopically controlled Ge multilayer structures doped with carbon. The diffusion profiles reveal a strong aggregation of the dopants within the carbon-doped layers and a retarded penetration depth compared to dopant diffusion in high purity natural Ge. Dopant aggregation and diffusion retardation is strongest for Sb and similar for P and As. Successful modeling of the simultaneous self- and dopant diffusion is performed on the basis of the vacancy mechanism and additional reactions that take into account the formation of carbon-vacancy-dopant and dopant-vacancy complexes. The stability of these complexes is confirmed by density functional theory calculations. The overall consistency between experimental and theoretical results supports the stabilization of donor-vacancy complexes in Ge by the presence of carbon and the dopant deactivation via the formation of dopant-vacancy complexes. These results help to develop concepts to suppress the enhanced diffusion of n-type dopants and the donor deactivation in Ge. Both issues hamper the formation of ultra shallow donor profiles with high active dopant concentrations that are required for the fabrication of Ge-based n-type metal oxide semiconductor field effect transistors.