In this article, the optical density (Dopt) and the skin depth (d) of films were estimated and discussed. The skin depth of NiCu-a-C: H films contenting 0% Cu in the optical band gap range was higher than other films and the maximum value is about 190 nm. Also, optical energy gap, Urbach energy, or the band tail width and Urbach parameters are calculated for thin films. By increasing the Cu percentage of films up to 5% the optical density of films was increased and then from 5% to 75% the optical density of films was decreased. The energy band gap and the width of the Urbach tail can be presented by linear relation Eg ¼ 2.6 1.48 Eu. To observe the broadening of the optical absorption edge due to carrier interactions, the steepness parameter (r) was evaluated; films deposited with 5% Cu have maximum value of the steepness parameter in about of 4.08. Electron-phonon interaction in films deposited with 5% Cu has minimum value in about of 16.4. Films deposited with 75% Cu have minimum values of the dispersion energy (Ed) and the single-oscillator energy ( E0) in about of 17.83 and 3.46 eV, respectively. With considering Nc ¼ 8, Za ¼ 1, Ne ¼ 8 for these films, the coordination number constants b were calculated and for films deposited with Ni pure (0% Cu) it has maximum value in about of 0.36
