The heteroepitaxial growth of Ge on (100) Si in a horizontal, atmospheric pressure metallorganic vapor-phase epitaxy reactor is reported using germane GeH4 (0.1% in H-2). A particularly crucial parameter for germanium deposition on silicon is the time for the onset of epitaxial growth, the incubation time. The time was measured at substrate temperatures between 450 and 600 degrees C. At a substrate temperature of 450 degrees C an incubation time of 520 s was found and for the subsequent epitaxy growth rates of 50 nm/min were determined by Nomarski microscopy and electron diffraction. The existence of residual oxide in the reactor chamber forming an in situ SiO2 layer was evaluated by x-ray photoemission spectroscopy. To obtain a more thorough understanding of the gas- and solid-phase composition of Ge, Si, and oxygen the Gibbs energy of the system was calculated for various growth temperatures. It was concluded that SiO2 molecules are reduced by GeH4 molecules during the incubation period.