A method of fabricating a synthetic diamond coated compound semiconductor substrate, the method comprising: loading a composite substrate into a chemical vapor deposition (CVD) reactor, the composite substrate comprising a single crystal carrier wafer, a layer of single crystal compound semiconductor epitaxially grown on the carrier wafer, and an interface layer disposed on the layer of compound semiconductor, the interface layer forming a growth surface suitable for growth of synthetic diamond material thereon via a CVD technique; and growing a layer of CVD diamond material on the growth surface of the interface layer, wherein during growth of CVD diamond material a temperature difference at the growth surface between an edge and a center point thereof is maintained to be no more than 80° C., and wherein the carrier wafer has an aspect ratio, defined by a ratio of thickness to width, of no less than 0.25/100.