The present invention is directed to a method for fabricating a thermal management substrate having a Silicon (Si) layer on a polycrystalline diamond film, or on a diamond-like-carbon (DLC) film. The method comprises acts of fabricating a separation by implantation of oxygen (SIMOX) wafer; depositing a polycrystalline diamond film onto the SIMOX wafer; and removing various layers of the SIMOX wafer to leave a Si overlay layer that is epitaxially fused with the polycrystalline diamond film. In the case of the DLC film, the method comprises acts of ion-implanting a Si wafer; depositing an amorphous DLC film onto the Si wafer; and removing various layers of the Si wafer to leave a Si overlay structure epitaxially fused with the DLC film.