Epitaxial Si1-yCy layers were grown by gas-source molecular beam epitaxy using Si2H6 (100%) and C2H2 (1% C2H2 in 99% He) between substrate temperatures of 600 to 700degreesC. Epitaxial Si1-yCy films were obtained using C2H2 flow rates ranging from 0 to 1.0 sccm. Substitutional C atoms in the films were detected as the C local vibration mode (607 cm(-1)) in Raman and Fourier transform infrared absorption spectroscopy. The formation of the Si1-yCy alloys was also confirmed by theta/2theta measurements of X-ray diffractometry (XRD). The reciprocal lattice space map indicated the pseudomorphic growth of the Si1-yCy layer on Si(0 0 1). The substitutional C content (C-s) was estimated using Vegard's law and XRD data with a maximum Cs of 0.96% obtained in a sample grown at 675degreesC. From the characterization of Si1-yCy films annealed at temperatures between 700degreesC and 900degreesC, it was found that the thermal stability of the film strongly correlates with the initial Cs content. (C) 2002 Elsevier Science B.V. All rights reserved.