We have investigated the role of surface-active species (surfactants) in heteroepitaxial growth. In general, the growth mode is determined by the balance between surface, interface, and film free energies. Thus, if A wets B, B will not wet A. Any attempt at growing an A/B/A heterostructure must overcome this fundamental obstacle. We propose the use of a segregating surfactant to reduce the surface free energies of A and B and suppress island formation, as demonstrated in the growth of Si/Ge/Si(001) with a monolayer of As. Control of growth by amnipulation of surface energetics provides a new avenue to achieve high-quality man-made microstructures against thermodynamic odds. © 1989 The American Physical Society.