Substrates for semiconductor devices

A method of fabricating a composite semiconductor component comprising: (i) providing a bowed substrate comprising a wafer of synthetic diamond material having a thickness td, the bowed substrate being bowed by an amount B and comprising a convex face and a concave face; (ii) growing a layer of compound semiconductor material on the convex face of the bowed substrate via a chemical vapour deposition technique at a growth temperature T to form a bowed composite semiconductor component comprising the layer of compound semiconductor material of thickness tsc on the convex face of the bowed substrate, the compound semiconductor material having a higher average thermal expansion coefficient than the synthetic diamond material between the growth temperature T and room temperature providing a thermal expansion mismatch ΔTec; and (iii) cooling the bowed composite semiconductor component, wherein the layer of compound semiconductor material contracts more than the wafer of synthetic diamond material during cooling due to the thermal expansion mismatch ΔTec, wherein B, td, tsc, and ΔTec are selected such that the layer of compound semiconductor material contracts on cooling by an amount which off-sets bowing in the bowed substrate thus pulling the bowed composite semiconductor component into a flat configuration, the layer of compound semiconductor material having a tensile stress after cooling of less than 500 MPa.