An upper temperature limit of 450 °C has been established for growth of heteroepitaxial Si1-x-yGexCy solid solutions with substitutional C on Si(100) by combined ion and molecular beam deposition (CIMD). At 450 °C infrared absorption spectroscopy shows that C is on substitutional sites and no SiC precipitates are detected, whereas at 560 °C the substitutional C signal is much smaller but SiC precipitates are still not detected. High resolution transmission electron microscopy shows that Si1-x-yGexCy films deposited at 560 °C exhibit Ge deficient, coherent, secondary phase clusters in the cubic diamond matrix, which are not seen in films deposited at 450 °C. These observations suggest that the clusters are C‐rich, Ge‐deficient precursors to SiC, with a lattice which is distorted but free of extended defects. Ion channeling results indicate that the Si1-x-yGexCy films might have a distribution of different bond lengths. © 1996 American Institute of Physics.