Group III-nitride is one of the best candidate for UV lasers-diodes, LEDs and UV sensors. Gallium nitride-based opto-electronic devices may possess high emission efficiency because of its electronic properties. In order to obtain high quality GaN crystalline films, the problem of finding an optimized substrate material must be solved. In this paper, we investigate one useful task of using Si as substrate with SiC buffer layer grown by carbonization of the Si surface and germanium (Ge) incorporation. The optimization of the GaN/SiC/Si structure is achieved by using an approach based upon the elasticity theory of strained interfaces. This approach provides a strain-dynamics relationship implying buffer layer composition-dependent parameters. By optimizing this composition, we can provide a theoretical basis for further investigations on the GaN/SiC/Si system based on the buffer layer technique.