The materials and electronic properties of Si/Si1-x-yGexCy and Si/Si1-yCy heterojunctions grown by rapid thermal chemical vapor deposition are compared. Substitutional carbon incorporation is readily achieved in Si1-x-yGexCy alloys by the addition of a carbon precursor such as ethylene or methylsilane during growth, using germane and dichlorosilane as the germanium and silicon sources respectively. In contrast, a significant fraction of the carbon is not substitutional in Si1-yCy films grown using dichlorosilane in combination with either carbon source. A highly reactive silicon source, such as silane, enables the growth of high quality Si1-yCy. Good agreement between the substitutional carbon concentration extracted from X-ray diffraction and the total carbon concentration measured by secondary ion mass spectrometry is observed in Si1-yCy alloys grown at 550 degrees C with silane, for carbon contents up to about 1.8 at.%. Si/Si1-x-yGexCy and Si/Si1-yCy metal-oxide-semiconductor capacitors show well-behaved electrical characteristics. Analysis of the capacitance-voltage data indicates that the band offsets are primarily in the valence band for Si/Si1-yGexCy and the conduction band for Si/Si1-yCy heterojunctions. The conduction band is lowered as carbon is added to Si and the effect is larger than expected from strain alone.