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

Absorption coefficient of a-C:H film prepared by saddle-field glow-discharg e chemical vapor deposition was measured by means of photothermal deflectio n spectroscopy and transmission measurements. The absorption spectra of the se films are divided into a larger absorption region and a smaller absorpti on 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 a bsorption region can be analyzed in terms of the Urbach tail and yields inf ormation about the band tails of the a-C:H film. The effect of doping on th e 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 concen tration of the impurities, phosphorous or hydrogen. Also there is a correla tion between the band tail spread and the optical gap. The breadth of the b and tail increases with the optical gap. (C) 1999 Published by Elsevier Sci ence B,V. All rights reserved.