The optical properties of a-C:H films between 1.5 and 10 eV and the effect of thermal annealing on the film character

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Abstract
The optical properties of amorphous hydrogenated carbon films prepared with various techniques are studied with conventional and synchrotron‐radiation spectroscopic ellipsometry (SE) and the pseudodielectric function 〈ε(ω)〉 of diamondlike and graphitelike films is presented in the energy region 1.5–10 eV. Characteristic features of the measured 〈ε(ω)〉 and the calculated electron‐energy‐loss (EEL) function are found to serve as useful criteria for the classification of such materials. The results and information obtained by SE are compared to those obtained by EEL and Raman spectroscopy techniques, which are the most widely used for this purpose. Thermal annealing experiments up to 675 °C with in situ monitoring of the 〈ε(ω)〉 reveal the undergoing structural changes in the material character from diamondlike into graphitelike during the annealing. The major modifications which turn the material into sp2‐like are found to take place around and above 550 °C. The fundamental gap along with other optical parameters of the materials are compared to those of diamond and graphite and their shift with temperature is discussed and used to illustrate further the prevalence of the graphitic character during and after the annealing. Finally, the optimum growth parameters for the production of diamondlike material are discussed in the case of the glow‐discharge and ion‐beam deposited films.