Photothermal deflection spectroscopy and transmission measurements of a-C:H films

Absorption coefficient of a-C:H film prepared by saddle-field glow-discharge chemical vapor deposition was measured by means of photothermal deflection spectroscopy and transmission measurements. The absorption spectra of these films are divided into a larger absorption region and a smaller absorption region. The larger absorption region can be interpreted in terms of the Tauc model and an optical gap is obtained for the a-C:H film. The smaller absorption region can be analyzed in terms of the Urbach tail and yields information about the band tails of the a-C:H film. The effect of doping on the absorption tails and the optical gap is also reported. We found that both the optical gap and the breadth of the band tail decreased with the concentration of the impurities, phosphorous or hydrogen. Also there is a correlation between the band tail spread and the optical gap. The breadth of the band tail increases with the optical gap.