Positron annihilation spectroscopy of thermally annealed hydrogenated amorphous carbon films

Hydrogenated amorphous carbon (a-C:H) films are prepared using a bipolar-type plasma-based ion implantation and deposition system, followed by thermal annealing of the films at 200 °C-800 °C. The structural and compositional changes are examined by Raman spectroscopy, Rutherford backscattering spectrometry, and elastic recoil detection analysis (ERDA). Positron annihilation spectroscopy (PAS) is also conducted to investigate the behavior of voids in the films. The results of Raman and ERDA measurement clearly show that the sp2 ring clusters increase, and hydrogen content apparently decreases at temperatures between 400 °C and 800 °C. Uncorrelated behaviors between positron lifetimes and line-shape parameters are observed in the PAS results, which can be attributed to several competing effects, such as hydrogen release, bond reconstruction, and electronic-state change of voids. Furthermore, it is suggested that the graphitization of the films begins from the surface to the inside, accompanied by the hydrogen release that is related to the void behavior.