We studied the substitutional carbon incorporation in Si1-yCy layers grown by molecular beam epitaxy (MBE) on Si(100). We grew a series of pseudomorphic Si1-yCy/Si superlattices (SL) with carbon concentrations between 0 and 2% and growth temperatures varying between 400 and 650 degrees C. From a comparison between measured and simulated X-ray rocking curves of our samples, we determined the substitutional carbon content as a function of the growth temperature. We observed a decrease of the substitutional carbon incorporation with increasing growth temperature and increasing carbon flux. Reflection high energy electron diffraction (RHEED) showed a 3D pattern of those layers, which had not incorporated all the offered C atoms substitutionally. This result is confirmed by atomic force microscopy investigations of Si1-yCy epilayers grown at different temperatures. To check the crystal quality we performed photoluminescence (PL) investigations of a Si/Si1-yCy SL and of Angstrom 1100 Angstrom Si0.99C0.01 epilayer. Before and after annealing the epilayer was analyzed by PL and X-ray diffraction. We observed a blueshift of the NP and TO line, which is caused by a homogenization of potential fluctuations in the band edge energies.