Thermally induced structural changes in amorphous carbon films observed with ultraviolet photoelectron spectroscopy

We used the characteristic changes in the electronic structure of the valence band of amorphous carbon films upon the formation of graphitic clusters to monitor the temperature induced graphitization of a variety of hydrogen-containing (a-C:H) as well as hydrogen-free (a-C) films. The valence band spectra of the films, which were prepared in situ by ion-beam deposition and electron beam evaporation, were determined using photoelectron spectroscopy with excitation energies in the ultraviolet regime (ultraviolet photoemission spectroscopy, HeI, hν=21.22 eV and HeII, hν=40.82 eV). By choosing a variety of deposition parameters we are able to illustrate that the extent of temperature induced structural changes is on one hand determined by the bulk and surface diffusion coefficients of carbon atoms, and on the other hand by the ion irradiation which inhibits the graphitization process.