Thermal induced structural changes of a-C and a-C:Ti films analyzed by NEXAFS and XPS

Carbon K-edge NEXAFS and XPS measurements were performed on a-C and 7.5 at% a-C:Ti films annealed in-situ up to 1300 K. During annealing of a-C, the sp2 content increased from ~80 to ~95 %, associated with the development of a peak at 285.4 eV in the NEXAFS spectrum, which is assigned to sp2 carbon atoms in a 'graphite-like' local order. After annealing of the a-C:Ti film to 470 K, an additional peak shows up, indicating the creation of carbonyl functionalities. This is explained by the increased reactivity of the carbon phase by Ti-doping. Oxygen contents of ~30 at% are determined by XPS up to 900 K. For a-C at temperatures >420 K, the C 1s peak width (FWHM) decreased continuously with annealing temperature, which is not the case for a-C:Ti. This is explained by oxidised carbon and by Ti diffusion during carbide crystallite formation, partly disturbing the ordering of the carbon phase.