A method of fabricating a semiconductor device structure includes: providing a substrate comprising a layer of compound semiconductor material; forming a seed layer of nano-crystalline diamond having a layer thickness in a range 5 to 50 nm on the layer of compound semiconductor material; and growing a layer of polycrystalline CVD diamond on the seed layer using a chemical vapour deposition (CVD) technique. An effective thermal boundary resistance (TBReff) at an interface between the layer of compound semiconductor material and the layer of polycrystalline CVD diamond material is no more than 50 m2K/GW.