Optical absorption parameters of amorphous carbon films from Forouhi-Bloomer and Tauc-Lorentz models: a comparative study

Parametrization models of optical constants, namely Tauc-Lorentz (TL), Forouhi-Bloomer (FB) and modified FB models, were applied to the interband absorption of amorphous carbon films. The optical constants were determined by means of transmittance and reflectance measurements in the visible range. The studied films were prepared by rf sputtering and characterized for their chemical properties. The analytical models were also applied to other optical data published in literature, pertaining to films produced by various deposition techniques. The different approaches to determine important physical parameters of the interband transition yielded different results. A figure-of-merit was introduced to check the applicability of the models and the results showed that FB modified for an energy-dependence of the dipole matrix element represents adequately the interband transition in the amorphous carbons. Further, the modified FB model shows a relative superiority over TL one’s for what concerns the determination of the band gap energy, as it is the only one to be validated by an independent, though indirect, gap measurement by XPS. Finally, the application of the modified FB model allowed to establish some important correlations between film structure and optical absorption properties.