The authors describe very low temperature (350–420°C) growth of atomically smooth Ge films (0.2–0.4nm roughness) directly on Si(100) via gas-source molecular beam epitaxy. A carefully tuned admixture of (GeH3)2CH2, possessing unique pseudosurfactant properties, and conventional Ge2H6 provides unprecedented control of film microstructure, morphology, and composition. Formation of edge dislocations at the interface ensures growth of virtually relaxed monocrystalline Ge films (∼40–1000nm thick) with a threading dislocation density less than 105cm−2 as determined by etch pit measurements. Secondary ion mass spectroscopy showed no measurable carbon incorporation indicating that C desorbs as CH4, consistent with calculated chemisorption energies.