The relaxation of ion beam synthesised SiGe alloys occurs during solid phase epitaxial growth (SPEG) by a roughening of the amorphous/crystalline (a/c) interface, leading to the formation of extended defects. SiGe/Si structures have been formed by implantation of Ge⁺ ions at energies of 70 keV and 400 keV and doses above the critical value for strain relaxation, followed by post-amorphisation to a depth of approx. 1 micro-m and regrowth at 700°C. TEM and RBS analysis of the regrown structures show that relaxation-induced stacking faults (SFs) are nucleated in the vicinity of the peak of the Ge concentration. For an implantation energy of 70 keV, SFs are the only defects observed and they extend up to the surface. For an implantation energy of 400 keV, the SFs terminate within the bulk far from the surface while “hairpin” dislocations are also formed and extend up to the surface. These results are explained in terms of the ratio between the depth at which SFs are nucleated and the roughness of the a/c interface at that depth. Varying the defect nucleation depth by removing a surface layer from the structures implanted at 400 keV prior to SPEG does not result in a change of the size of the SFs, while “hairpin” dislocations are still formed, suggesting that the roughness of the a/c interface during regrowth is only determined by the Ge content in the alloy.