We present ab initio density functional calculations for the surfactant effect of Te on the Ge/Si(001) surface. Direct deposition of a Ge layer on top of the Si substrate is found to be energetically favorable by 0.50 eV than the Ge-interdiffused case. We do not find any significant energy difference when we used 1 monolayer (ML) Te as surfactant within (1 x 1). For Te adsorption within the (2 x 2) surface reconstruction, we have considered two models for each of 1 ML Ge and 2 ML Ge case: (i) bridge model (Te atom is at the top of Ge-Ge dimer) and (ii) anti-bridge model (two Te atoms saturate the four dangling bonds at two neighboring Ge-Ge dimers in a row). For the 1 ML Ge case we have found that the anti-bridge model is energetically favorable by 0.70 eV than the bridge-model. However, when we have considered the same models for the 2 ML Ge case, this energy difference increases to 0.90 eV. (C) 2004 Elsevier B.V. All rights reserved.