The influence of He implantation and annealing on the relaxation of Sio,72Geo.z$ layers on Si substrates is investigated. Proper choice of the implantation energy results in a narrow defect band underneath the substrate/epilayer interface. During annealing at 700°C to 1000°C, He-filled bubbles are created, which act as dislocation sources for misfit dislocation generation. Complete annihilation of the threading dislocations is theoretically predicted, if a certain bubble density with respect to the buffer layer thickness is maintained. The variation of the implantation dose and the annealing conditions changes the density and size of He bubbles resulting in characteristic differences of the dislocation structure. A strain relaxation of 70°/a of Si,_,,Gex layers with Ge fractions up to 30 at.°/a is obtained at temperatures as low as 850°C. Simultaneously low threading dislocation densities of 10' ctri 2 are achieved.