Diamond composite substrate for semiconductor devices

A composite substrate or a method of manufacturing a composite substrate comprising polycrystalline diamond layer 12 bonded to single crystal silicon or silicon carbide cleaved layer 14 which is 15mm or less in thickness. Polycrystalline diamond layer 12 (50mm to 150mm thick) is grown by Chemical Vapour Deposition (CVD) at 700Â DEG C to 1200Â DEG C on single crystal silicon or silicon carbide wafer 10 0.3mm to 2.0mm thick. A strain field to enable cleavage is generated in wafer 10 by heating or cooling. Cleavage may be triggered by mechanical force (e.g. using a knife blade), thermal shock or wafer 10 may cleave automatically during heating/cooling due to mismatch in the thermal expansion coefficient between the silicon/silicon carbide and the polycrystalline diamond. Wafer 10 may be treated to promote cleavage by forming a buried oxide damage layer, implanting a weakened cleave plane or forming nanostructured surface pits. Wafer 10 may be reused to grow further diamond substrates. A semiconductor material 16 may be epitaxially grown on a buffer layer over a planarised surface of cleaved layer 14 to form a semiconductor device e.g. thin film transistor, diode or high power switching device.