A semiconductor device structure includes a layer of III-V compound semiconductor material, a layer of polycrystalline CVD diamond material, and an interface region with a diamond nucleation layer. A Raman signal of the diamond nucleation layer exhibits an sp3 carbon peak at 1332 cm−1 having a full width half maximum of no more than 5.0 cm−1, and one or both of: (i) an sp2 carbon peak at 1550 cm−1 having a height which is no more than 20% of a height of the sp3 carbon peak at 1332 cm−1 after background subtraction when using a Raman excitation source at 633 nm; and (ii) the sp3 carbon peak at 1332 cm−1 is no less than 10% of local background intensity in a Raman spectrum using a Raman excitation source at 785 nm. An average nucleation density at a nucleation surface is no less than 1×108 cm−2 and no more than 1×1012 cm−2.