We report on unusually high concentration of carbon atoms in crystalline silicon films grown by chemical vapor deposition (CVD). Films with concentration higher than 10% are readily grown by supersonic free jet of disilane and organometallic molecules. The film quality varies from nearly single crystalline to polycrystalline depending on the growing conditions. Carbon atoms forced to exist in excess of its solubility in crystalline silicon seem to segregate to the surfaces by exhibiting abnormally high rate of diffusion compared to the reported values of bulk diffusion constants. This property is potentially useful in the fabrication of nanoscale modulations of carbon concentration profiles. These high carbon concentration regions consist of nanocrystalline particles of Si oriented a little off the host Si crystal directions. This phenomenon resembles nano-sized island formation in Ge/Si heterogeneous systems often used in quantum dot fabrication, and raises the possibility of quantum dot formation in this system.