The growth of Sn layers on Ge (001) surfaces was studied with respect to the formation of Stranski-Krastanov islands due to the large lattice mismatch. Atomic force microscopy shows the transition to Stranski-Krastanov growth at an Sn coverage of 2.33 atomic monolayers on the surface. The high Sn segregation causes crystalline defects by overgrowing the Sn islands. Using Sb as a surfactant, it is possible to suppress the Stranski-Krastanov growth.