The heteroepitaxial growth of pure Ge films on (100) Si by an ultrahigh vacuum, chemical vapor deposition technique is reported for the first time. The growth mode is found to be critically dependent on the substrate temperature during deposition. Two temperature regimes for growth are observed. Between 300 and 375 °C, growth occurs in a two-dimensional, layer-by-layer mode, with an activation energy of 1.46 eV. Above 375 °C, island formation is observed. In the low-temperature regime the growth rate is controlled by a surface decomposition reaction, whereas in the high-temperature regime the growth rate is controlled by diffusion and adsorption from the gas phase.