The initial growth process of ${\mathrm{Si}}_{\text{1−x−y}}{\mathrm{Ge}}_x{\mathrm{C}}_y$ thin films on Si(001) surfaces is examined by scanning tunneling microscopy. The surface morphology of the film critically depends on the C fraction in the film. Evidence is presented on an atomic scale that the epitaxial growth of ${\mathrm{Si}}_{\text{1−x−y}}{\mathrm{Ge}}_x{\mathrm{C}}_y$ films with large C fractions is dominated by phase separation between Si–C and Si–Ge, concomitant with C condensation on the surface of the growing films. We find that the addition of a thin (1–2 ML) SiGe interlayer between the ${\mathrm{Si}}_{\text{1−x−y}}{\mathrm{Ge}}_x{\mathrm{C}}_y$ film and the Si substrate drastically improves the film structure, leading to a planar morphology even with large C fractions present in the film.