Optical absorption parameters of amorphous carbon films from Forouhi–Bloomer andTauc–Lorentz models: a comparative study

Abstract

Parametrization models of optical constants, namely Tauc–Lorentz (TL), Forouhi–Bloomer(FB) and modified FB models, were applied to the interband absorption of amorphouscarbon films. The optical constants were determined by means of transmittance andreflectance measurements in the visible range. The studied films were prepared by rfsputtering and characterized for their chemical properties. The analytical models were alsoapplied to other optical data published in the literature pertaining to films producedby various deposition techniques. The different approaches used to determineimportant physical parameters of the interband transition yielded different results. Afigure-of-merit was introduced to check the applicability of the models and the resultsshowed that FB modified for an energy dependence of the dipole matrix elementadequately represents the interband transition in the amorphous carbons. Further, themodified FB model shows a relative superiority over the TL ones for concerning thedetermination of the band gap energy, as it is the only one to be validated by anindependent, though indirect, gap measurement by x-ray photoelectron spectroscopy.Finally, the application of the modified FB model allowed us to establish someimportant correlations between film structure and optical absorption properties.