Epitaxial growth of heterolayers has in the past relied on optimization of growth conditions to overcome thermodynamic obstacles. When there is an intrinsic tendency of a heterolayer to either island or interdiffuse, a common strategy is to lower the growth temperature and increase the growth rate to reduce surface mobility. An alternative strategy is to introduce a surface-active species (surfactant) that modifies the growth mode without significant levels of incorporation. This paper discusses the application of As and Sb surfactants to the growth of Ge/Si(001) and Si/GeSi(001). Results from analysis by medium-energy ion scattering, x-ray photoemission, and ultraviolet photoemission are reported. By using a surfactant, island formation is suppressed in the growth of both Ge/Si(001) and Si/Ge/Si(001), resulting in thick, epitaxial films.