Optical characterisation of high-quality homoepitaxial diamond

Recently, great effort has been devoted to the deposition of homoepitaxial diamond for electronic applications. Its single-crystal nature avoids the problems due to grain boundaries in polycrystalline diamond films, although the quality of such materials may still be very high. However, the optimisation of the deposition process of single-crystal diamond has not yet been achieved. In fact, the surface morphology of homoepitaxial diamond seems to be very sensitive to the quality of the diamond substrate and to even slight variations in the deposition parameters (composition of the gas mixture, microwave power, substrate temperature). Moreover, the growth of flat samples needs very low growth rates, which are not practical for technological exploitation of the material. Thus, a deposition process at relatively high growth rates needs to be developed. In this view, characterisation studies of the material deposited play a key role, because they may give precious hints for the optimisation of a fast growth process of electronic-grade homoepitaxial diamond. In this work, we report on the characterisation of single-crystal diamond grown on diamond substrates of different origin. The samples have been deposited by means of microwave plasma-enhanced chemical vapour deposition (MWPECVD) with a CH 4-H2 gas mixture at different methane concentrations and at approximately 560° C substrate temperature. The growth rates range between approximately 1 μ m/h and 4.5 μ m/h. Optical and scanning electron microscopy have been used to study the surface morphology of the samples. A Raman investigation has been carried out to study the crystalline quality and the spatial homogeneity of the material, by means of accurate measurements of position and width of the diamond Raman peak at different points in the samples. Photoluminescence (PL) has been used to investigate both the phase purity and the distribution of possible impurities in the deposited material. The results indicate that homoepitaxial CVD diamond samples having very high crystalline quality (full width of the diamond Raman peak ~ 1.75 cm-1) and no impurity-related defect centers can be obtained in the adopted deposition conditions.