A method of fabricating a composite semiconductor component, the method comprising: (i) providing a bowed substrate comprising a wafer of synthetic diamond material having a thickness t d, the bowed substrate being bowed by an amount B and comprising a convex face and a concave face, wherein B is measured (a) as a height of a mid-point of the bowed substrate relative to a plane defined by an edge of the substrate or (b) as a radius of curvature of the bowed substrate; (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 t sc 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 ΔT ec; 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 ΔT ec, wherein B, t d, t sc, and ΔT ec 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, characterised by the layer of compound semiconductor material having a tensile stress after cooling of less than 500 MPa, and in that the bowed substrate comprises a layer of material on the concave face of the bowed substrate which has a lower average thermal expansion coefficient than the synthetic diamond material between the growth temperature T and room temperature, said layer of material having a thermal expansion coefficient and thickness which are selected to contribute in off-setting the bowing in the bowed substrate during cooling. (i) providing a bowed substrate comprising a wafer of synthetic diamond material having a thickness t d, the bowed substrate being bowed by an amount B and comprising a convex face and a concave face, wherein B is measured (a) as a height of a mid-point of the bowed substrate relative to a plane defined by an edge of the substrate or (b) as a radius of curvature of the bowed substrate; (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 t sc 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 ΔT ec; 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 ΔT ec,