The carbon incorporation into lattice sites in Si1-yCy epilayers and Si1-yCy/Si multiple quantum wells grown by molecular beam epitaxy was investigated as a function of growth temperature in the range of 350-650 degrees C using Raman spectroscopy, double crystal X-ray diffraction (DCXD), and atomic force microscopy (AFM). The substitutional carbon concentration deduced from the simulation of DCXD results decreases with increasing growth temperature due to the strain relaxation process which occurred by the change of the growth mode from two-dimensional to three-dimensional island growth.