Germanium thin films have been grown on silicon(100) substrates by ion beam sputter deposition (IBSD) in an ultrahigh vacuum system. The growth mode of the films as a function of the growth temperature and the ion incidence angle was investigated by in situ Auger electron spectrometry. The morphology of the film surface was examined by scanning electron microscopy. The effect of the deposition temperature on the interface roughness was investigated after selectively etching the Ge film. We have found that the growth mode of IBSD-Ge on silicon does not agree with a layer-by-layer growth. The decay of the Si signal as the growth proceeds may be interpreted in terms of either a Stransky-Krastanov growth mode with island formation after two-dimensional growth or Si segregation to the surface. We showed that IBSD leads to more abrupt interfaces than molecular beam epitaxy. This last result may be related to the energetic bombardment of the film during deposition. To support this model more quantitatively, computer calculations have been carried out by using an advanced version of the Monte Carlo "transport of ions in matter" code (trim).