Joint experimental and theoretical study of low-energy ion-beam-assisted Ge growth on Si has been carried out. Pulsed ion-beam action results in the increase of Ge nanoislands density and decrease of average island size and size dispersion. The effect is interpreted in terms of ion-beam-induced formation of surface vacancies and bulk interstitial atoms. The vacancies play the role of effective traps for Ge adatoms, while interstitials create the local stretched regions at the surface. Both factors promote nucleation of 3D Ge nanoislands. Molecular dynamics and Monte Carlo simulations of ion impact and 3D-nanoisland growth, respectively, have been carried out. The nanoislands density is found to be 8 times less in phosphorus-doped substrate. This is in agreement with the proposed mechanism of ion-beam action, since impurities are known to serve as effective sinks for bulk interstitials. (C) 2010 Elsevier B.V. All rights reserved.