Silicon germanium carbon with up to 1.5% of substitutional carbon can be grown epitaxially on Si(001) by rapid thermal chemical vapor deposition (RTCVD) using methylsilane as a precursor. The germanium and carbon atomic distributions are studied for a C-rich Si1-x-yGexCy heterostructure using high-resolution transmission electron microscopy (HREM), high-resolution X-ray diffraction and Raman spectrometry, HREM images show the formation of regularly spaced carbon-rich tilted sublattices and local germanium fluctuations. 2D reciprocal space mapping and Raman spectroscopy confirm this thin tilted sublayers formation. A new model for perpendicular lattice parameter reduction is proposed, The structure remains highly distorted and a statistical description of these strain fluctuations is exposed.