Various structures containing Si1-yCy alloy layers have been prepared and characterized by x-ray diffraction , cross sectional transmission electron microscopy, photoluminescence (PL), Fourier transform infrared spectroscopy, and spectroscopic ellipsometry. A band gap reduction equal to 63 meV/% C has been estimated from PL when taking into account the quantum well (QW) confinement shift using an effective mass calculation. The QW-related emission observed from a multiple QW structure has a temperature quenching behavior with an activation energy equal to 8 meV. Carbon outdiffusion from the QWs has been evidenced by a blueshift of the PL peak and changes in the x-ray diffraction data after furnace annealing at 800 and 850 degrees C.