Ge/Si heterostructures were grown on Si (001) by Sn-submonolayer-mediated molecular beam epitaxy (MBE) and characterized by a variety of techniques, in order to study the behavior of Sn surfactant during Ge and Si growth and its influence on Ge/Si interface quality. It was found that Sn strongly segregates to the growing surface of both Ge and Si and that the presence of Sn surfactant can effectively suppress Ge segregation into a Si overlayer and enhance the surface mobility of adatoms. These results suggest that Sn-mediated epitaxy can be used as a viable method to produce Ge/Si superlattices, with an interface quality superior to those grown either by conventional MBE or with other types of surfactants.