Effect of graphitic inclusions on the optical gap of tetrahedral amorphous carbon films

High sp(3) fraction tetrahedral amorphous carbon (ta-C) films can be prepared using the filtered cathodic vacuum arc (FCVA). A by-product of the deposition process are small micrometer sized graphitic particles which are also incorporated into the film. The particle coverage of FCVA films is typically <5%, and thus the effect of these graphite inclusions have been largely ignored in earlier optical gap measurements of ta-C. By incorporating a better filter design (e.g., S-bend filter), the particle coverage can be reduced to 0.1%. In this article, we show that the effect of these graphitic inclusions is to scatter or absorb light which significantly affects the optical gap measurement and hence reduces the "apparent" optical gap of the ta-C film. By comparing two ta-C films with different particle coverage but the same sp(3) content of 85%, we show that we can correct for the effect of these inclusions. Our results confirm that the E-04 gap of a 85% sp(3)ta-C matrix is 3.6 eV. The importance of considering these micro particles is emphasized as we find that for every 1% of area covered by particles, there is a 3-4 fold percentage difference between the corrected optical gap and measured gap of the film. (C) 2001 American Institute of Physics.