Real-time spectroscopic ellipsometry was used to monitor the nucleation behavior of Ge grown on H-terminated Si(l 00) at 500 degreesC and H-terminated Si(111) with <0.3degrees and similar to 4degrees miscut angles at 570 degreesC. To model the dielectric functions of layers with space-separated 3D islands the Tauc-Lorentz parametrization (TLP) was used. This parametrization was found to be much better than the generally used Bruggemann effective media approximation. It is shown that the wetting layer (WL) in the system of Ge on flat Si(111) is of nearly constant thickness during island nucleation, whereas the nucleation of Ge islands on stepped Si(111) and Si(100) is accompanied by a strong decrease of the wetting-layer thickness. It is concluded that in the first case 3D islands grow at the expense of atoms adsorbed on the wetting layer surface, whereas in the latter cases 3D islands grow at the expense of atoms of the wetting layer itself, which corresponds to stress-driven nucleation. (C) 2004 Elsevier B.V. All rights reserved.