We present a structural and chemical analysis of high-vacuum deposited Si films grown on clean or oxidized Si (001) wafers by low-temperature molecular-beam epitaxy. For growth on clean Si, we observed a limiting thickness for epitaxy that decreases with decreasing temperature with an activation energy of 0.47 eV. The onset of defect formation is correlated to a peak in the H impurity concentration. The transition to an amorphous phase is, however, observed beyond the depth where impurities are first observed pointing to surface disorder/roughening as a source of epitaxy breakdown. The 0 and C content in these films remains low until the film crystallinity has strongly deteriorated and reaches a saturation concentration of 2-4 at. % in the fully amorphous regions. The impurity profiles in amorphous-Si films grown on oxidized Si are similar to those obtained on clean Si when grown at the same temperature and indicate that the impurity uptake depends primarily on residual gas and surface condition. Raman scattering results show the structural changes and evolution of the Si bond configuration. (C) 2004 American Vacuum Society.