Supersonic molecular beam techniques have been used to study the nucleation of Si and Si1-xGex thin films on Si and SiO2 surfaces, where Si2H6 and GeH4 have been used as sources. A particular emphasis of this study has been an examination of the effects of a coincident flux of atomic hydrogen. The time associated with formation of stable islands of Si or Si1-xGex on SiO2 surfaces—the incubation time—has been found to depend strongly on the kinetic energy of the incident molecular precursors (Si2H6 and GeH4) and the substrate temperature. After coalescence, thin film morphology has been found to depend primarily on substrate temperature, with smoother films being grown at substrate temperatures below 600 °C. Introduction of a coincident flux of atomic hydrogen has a large effect on the nucleation and growth process. First, the incubation time in the presence of atomic hydrogen has been found to increase, especially at substrate temperatures below 630 °C, suggesting that hydrogen atoms adsorbed on Si-like sites on SiO2 can effectively block nucleation of Si. Unfortunately, in terms of promoting selective area growth, coincident atomic hydrogen also decreases the rate of epitaxial growth rate, essentially offsetting any increase in the incubation time for growth on SiO2. Concerning Si1-x- - Gex growth, the introduction of GeH4 produces substantial changes in both thin film morphology and the rate nucleation of poly-Si1-xGex on SiO2. Briefly, the addition of Ge increases the incubation time, while it lessens the effect of coincident hydrogen on the incubation time. Finally, a comparison of the maximum island density, the time to reach this density, and the steady-state polycrystalline growth rate strongly suggests that all thin films [Si, Si1-xGex, both with and without H(g)] nucleate at special sites on the SiO2 surface, and grow primarily via direct deposition of adatoms on pre-existing islands. © 2004 American Vacuum Society.