Local order in CVD diamond films: Comparative Raman, x-ray-diffraction, and x-ray-absorption near-edge studies

We investigated the structural changes of carbon in diamond films deposited by microwave plasma-assisted chemical vapor deposition with various methane concentrations and substrate temperatures. They were studied by x-ray-absorption near-edge structure (XANES) at the C K edge. The results on composition and structure were compared to those given by Raman spectra, x-ray-diffraction patterns, and scanning electron microscopy: Unlike Raman spectroscopy, XANES at the C K edge is nearly equally sensitive whatever the nature of the carbon (diamond, graphite, amorphous,...) involved in these deposits. At T < 950 degrees C and [CH4] < 2%, the C K edge spectra and the diffraction measurements both show that the films mainly consist of diamond, despite strong morphological modifications of the films. They all display the diamond characteristics. However a small amount of amorphous component (congruent to 10%) is incorporated, while increasing the methane concentration and/or the substrate temperature. The presence of this amorphous carbon induces a drastic change in the Raman spectra with the appearance of new lines. At substrate temperatures higher than 900 degrees C and methane concentrations equal to 2%, a second characteristic modification of the Raman spectra then stems from the presence of a disordered graphitic phase. These amorphous and graphitic components exhibit quite characteristic Raman and XANES spectra. The nature of these amorphous and graphitic phases included into the diamond films is discussed.