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 measurements 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 correlation between the so-called Dasgupta's band-width and the spacing of the π and π* bands of amorphous carbon. The problem of a connection between the bandgap and the disorder is raised. We propose a generalization of the Dasgupta's assumptions on the electronic density of states which accounts well for our results and show that the density of states at the Fermi level can be assessed from PDS data.