We have performed first-principles total energy calculations to investigate the adsorption process of Ge on Si(001)/Te, and the surfactant action of Te during the epitaxial deposition of Ge on Si(001)/Te, based on an atomic exchange process, Ge <-> Te. The total energy calculation indicates that the adsorption of Ge atom in "vacancy" site is energetically more favourable than its adsorption in "bridge" site. The adsorption of Ge in "on-top" and "anti-bridge" sites can be ruled out. The segregation process of Te atoms, based on an atomic exchange mechanism, reduces the total energy of the surface, confirming the surfactant effect of Te atoms.