Optical properties of amorphous hydrogenated carbon thin films

The optical absorption of various amorphous hydrogenated carbon thin films (a-C:H) grown by radiofrequency (RF) magnetron sputtering were investigated through photothermal deflection spectroscopy (PDS) and transmission measur ements in the visible and near IR region. We compare our data to two models : the standard Tauc model and the Gaussian bands model of Dasgupta. A good agreement with both models was found and we report a semi-empirical correla tion between the so-called Dasgupta's band-width and the spacing of the pi and pi (*) bands of amorphous carbon. The problem of a connection between t he bandgap and the disorder is raised. We propose a generalization of the D asgupta's assumptions on the electronic density of states which accounts we ll for our results and show that the density of states at the Fermi level c an be assessed from PDS data. (C) 2001 Elsevier Science B.V. All rights res erved.