High Quality CVD Diamond for Detection Applications: Structural Characterization

High quality synthetic diamonds have been grown on single-crystal silicon by microwave plasma enhanced chemical vapour deposition, using a CH4-H2 gas mixture at variable methane concentrations in the range 0.6-2.2%. Film surface morphology and preferential orientation have been investigated by scanning electron microscopy and X-ray diffraction. Raman spectroscopy and photoluminescence (PL) have been utilized to monitor the crystalline quality and to study the spatial distribution of defects in the deposited films, in view of their application as active materials for particle detectors. In particular, a detailed micro-Raman study shows that the diamond crystals on the growth surface have intrinsic quality comparable with that of the best natural diamonds, as evidenced by Raman peaks at 1332 cm-1 narrower than 2.4 cm-1, by the complete absence of any non-diamond carbon feature at approximately 1500 cm-1 and by a very weak luminescence background in the 100-6500 cm-1 spectral range. A broad PL band at approximately 2.05 eV, almost completely absent at the growth surface, is detected moving towards the film-substrate interface. A worsening of the global quality of the films is found with increasing methane concentration in the gas mixture, as witnessed by the increased broadband PL in the films grown at higher methane concentrations. The excellent quality of the films was independently assessed through their high detection-performance. The alpha-particle spectra, measured by using a 5.5 MeV 241Am source, showed a decrease in the detection efficiency with increasing methane concentration. This worsening in the detection performance is correlated with the lower global quality of the films grown at higher CH4 concentrations, clearly evidenced by the structural characterization.