Methods and systems for making a composite substrate is provided. The method includes depositing a silicon layer on a surface of a silicon carbide wafer. The method includes smoothing the deposited silicon layer by Chemical Mechanical Polishing (CMP) and first annealing to produce a flat silicon surface on the silicon carbide wafer. The method includes bonding the flat silicon surface of the silicon carbide wafer with a gallium oxide wafer. The method includes second annealing the bonded silicon carbide wafer and gallium oxide wafer. The method includes thinning the bonded gallium oxide wafer to a thickness of about 2 to about 25 microns.