The hetero growth of Ge on Si results in formation of 3D clusters with an uncontrolled defect structure. Introduction of a monolayer of a surfactant completely changes the growth mode to a 2D-layer growth (Frankvan der Merwe) with a continuous and smooth Ge film on Si(ll 1). The surfactant is not incorporated but segregates and floates on the growing Ge film. The saturation of the dangling bonds of the semiconductor reduces the surface free energy and drives the strong segregation. The effect on the growth process is the selective change of activation energies which are important for diffusion and the mobility of the Ge. Up to a thickness of 8 MLs (MonoLayers) the misfit-related strain of the pseudomorphic Ge film is relaxed by formation of a micro rough surface. This allows a partial relaxation of the Ge towards its bulk lattice constant which would not be possible for a flat and continuous film. For thicker Ge films the misfit of 4.2 % is relieved by a periodic dislocation network, which is confined to the Si-Ge interface. Ge-films thicker than 20 MLs are free of defects and completely relaxed to the Ge bulk lattice constant: a model system for perfect heteroepitaxial growth.