Effect of graphitic inclusions on the optical gap of tetrahedral amorphouscarbon films

High sp(3) fraction tetrahedral amorphous carbon (ta-C) films can be prepar ed using the filtered cathodic vacuum arc (FCVA). A by-product of the depos ition process are small micrometer sized graphitic particles which are also incorporated into the film. The particle coverage of FCVA films is typical ly <5%, and thus the effect of these graphite inclusions have been largely ignored in earlier optical gap measurements of ta-C. By incorporating a bet ter filter design (e.g., S-bend filter), the particle coverage can be reduc ed to 0.1%. In this article, we show that the effect of these graphitic inc lusions is to scatter or absorb light which significantly affects the optic al 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 t hese inclusions. Our results confirm that the E-04 gap of a 85% sp(3)ta-C m atrix is 3.6 eV. The importance of considering these micro particles is emp hasized 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 mea sured gap of the film. (C) 2001 American Institute of Physics.