Islanding and misfit relaxation are obstacles for growth of heteroepitaxial films. Surfactants not only inhibit islanding, but also control defect structure. Growth of Ge on Si(111) was mediated by a monolayer of Sb floating on the surface. Upon exceeding the critical thickness, Shockley partial dislocations initially thread to the surface and then act as nucleation sites for complementary partial dislocations which glide down to the interface, leaving behind a fully relaxed, defect-free, epitaxial Ge film. Thus, the seemingly incompatible goals of strain relief and defect-free growth can be met by a surfactant-modified growth front.