The incorporation of carbon in Si1-yCy alloys grown using silane and methylsilane by low-pressure rapid thermal chemical vapor deposition is investigated. Substitutional carbon content determined by x-ray diffraction analysis is compared to total carbon concentration measured by secondary ion mass spectrometry. Lower growth temperatures (≪600 °C) and higher silane partial pressures are observed to significantly improve substitutional carbon incorporation. At 550 °C, to within experimental error, fully substitutional carbon incorporation is observed over the range of compositions studied (0–1.8 at. % carbon). Fourier transform infrared spectroscopy is also used to verify the presence of substitutional carbon. © 1997 American Institute of Physics.