Ge/Si and Si1-xGex nanostructures were grown on Si(100) at 350 degrees C substrate temperature with a significant growth rate of about 10 Angstrom min(-1) using hot-wire decomposition of disilane and germane in an ultrahigh vacuum environment, A quartz crystal microbalance (QCM) was used to monitor growth rates and to study the influence of feed gas pressures. In-situ X-ray photoelectron spectroscopy measurements allowed us to calibrate alloy stoichiometry monitored by QCM. Ex-situ high-resolution transmission electron microscopy observations demonstrated the 2D epitaxial growth of partly relaxed nanolayers. These promising results may be closely related to a surfactant-like role of atomic hydrogen, which appears as a beneficial sub-product of the hot-wire decomposition process of hydride sources.