The role of a low temperature Si buffer layer (LT-Si) in the process of plastic relaxation of molecular-beam epitaxy grown GeSi/Si(001) is studied. Probable sources and mechanisms of generation of misfit dislocations (MD) are discussed. Transmission electron microscopic and x-ray diffraction techniques are used for studying 100 nm GexSi1-x films with LT-Si and those free of such a buffer layer. The MD density is found to be much lower in the former than in the latter, and the level of the film plastic relaxation is not higher than 20% in both as-grown and annealed films with LT-Si. As the thickness of the solid solution layer reaches 300-400 nm, the plastic relaxation of the films increases to almost 100%. Therefore, the determining role of the MD multiplication is supposed. We assume the double role of the LT-Si buffer layer. First, the diffusion flux of vacancies from the LT-Si layer to the GeSi/Si interface may cause erosion of the interface and, as a result, a decrease in the rate of MD generation at the early stages of epitaxy. Second, generation of intrinsic defect clusters in the LT-Si, which are potential sources of MDs, occurs in the field of mechanical stresses of the growing pseudomorphic layer. This process is thought to be the key feature of the plastic relaxation of GeSi/LT-Si/Si(100) films which promotes MD self-organization. © 2002 American Institute of Physics.