X-Ray Photoelectron Study of Ion Implanted Tetrahedral Carbon

Samples of thin film (d~40nm) tetrahedral amorphous carbon (ta-C), deposited by filtered cathodic vacuum arc (FCVA), have been implanted with N+ and Ga+ at ion energy E = 20 keV and ion fluences D = 3.1014÷3.1015 cm-2. This results in optical properties modification, best manifested by a significant shift of the optical absorption edge to lower photon energies, which is accompanied by a considerable increase of the absorption coefficient (photo-darkening effect) in the measured photon energy range (0.5÷3.0 eV). These effects could be attributed both to additional defect introduction and increased graphitization, as confirmed by X-ray photo-electron spectroscopy (XPS) measurements. The nonimplanted films show the expected variety of carbon-carbon chemical bonds: three- and fourfold coordinated carbon, while the X-ray results show that ion implantation leads to the introduction of additional disorder in the films. The X-ray photoelectron spectra of the implanted films show that, in addition to the already mentioned changes, the ion bombardment results in an increase of the threefold coordinated as compared to the fourfold coordinated carbon bonds, i.e. increased graphitization of the carbon content in the films. These structural modifications, due to the ion implantation, are the reasons for the observed changes in the optical properties of the films, which could be made use of in the area of high-density optical data storage using focused Ga+ ion beams.