Heteroepitaxy of 3C-SiC on Si(001) in gas source molecular beam epitaxy has been carried out by a combination of the formation of a SiGeC buffer layer and subsequent 3C-SiC growth. The SiGeC buffer layer was formed by the use of dimethylgermane ((CH₃)₂GeH₂) to convert the surface region chemically into 3C-SiC. It was found that single-crystalline 3C-SiC with a smooth surface could be obtained reproducibly at as low as 650^oC, and Ge atoms could be introduced successfully in the carbonized layer. Single crystalline 3C-SiC showing a single-domain (3×2) surface superstructure could be grown at as low as 910^oC on the SiGeC buffer layer. By the precise control of the disilane (Si₂H₆) flux, a smooth surface without pits and Si-islands was realized successfully. The activation energy of growth rate became lower from 27.4 kcal/mol in the three-dimensional growth to 12.7 kcal/mol in the step-flow growth with the increase of growth temperature. Growth mechanisms in the 3C-SiC/Si heteroepitaxial system and the effects of Ge incorporation in the buffer layer are discussed.