Low-temperature(∼200 °C)molecular beam epitaxy ofGe1−xCxalloys grown on Si(100) have been extensively investigated byin situreflection high-energy electron diffraction,ex situx-ray diffraction, transmission electron microscopy, and Raman spectroscopy. Carbon concentrations were nominally varied from 0 up to ∼10 at. &percent;. Selected samples were annealed in an Ar ambient at 750 °C to evaluate the stability of the thin films. A few films were also grown on Ge substrates. With increasing C concentration, the epitaxial growth mode changes from two dimensional layer growth to three dimensional island growth. The surface has a tendency to facet along {311} planes under certain growth conditions. The microstructure shows an increase in planar defect density with increasing C concentration. The x-ray diffraction data show that the lattice parameter decreases with increasing C concentration and that a maximum of 1 at. &percent; C is incorporated substitutionally in Ge. Raman spectroscopy shows no clear Ge–C signal though extra intensity is measured at the energies where Ge–C modes may be expected. Films with nominal C concentrations greater than 2 to 3 at. &percent; show clear evidence for amorphous C. We propose that under our growth conditions, nominal C in excess of about 2 to 3 at. &percent; remains on the surface as amorphous C and plays an important role in 3D islanding, defect formation, and {311} faceting during epitaxial growth. ©1997 American Institute of Physics.