Preparation method of low stress state composite substrate for GaN growth

The invention discloses a preparation method of a low stress state composite substrate for GaN growth. The preparation method comprises that a GaN monocrystal epitaxial layer is prepared on a sapphire substrate; a stress compensation layer is deposited at the back side of a thermally and electrically conductive transfer substrate of high welding point; bonding dielectric layers of high welding point are prepared at the surfaces of a GaN epitaxial wafer and the transfer substrate respectively; the GaN epitaxial wafer is bonded to the thermally and electrically conductive substrate by the high-temperature diffusion bonding technology; and the composite substrate with high temperature stability and low stress state for GaN growth is obtained. According to the composite substrate of the invention, homoepitaxy can be realized and a vertical structural device can be directly prepared as a traditional composite substrate, the low stress state and high-temperature stability can be also realized, and the quality of subsequent GaN epitaxial growth and chip preparation can be effectively improved.