Optical absorption of amorphous hydrogenated carbon thin films

Amorphous hydrogenated carbon layers were deposited from ethylene in a capacitively coupled r.f.-discharge reactor under different self-bias voltages and by addition of different amounts of phosphine as an assumed ''doping'' gas. The optical constants of the thin films were determined by analysing reflection, transmission and photothermal deflection measurements. Accordingly, the absorption behaviour below an absorption coefficient alpha of 10(4) cm(-1) can be fitted very well by a simple power law. Extrapolating the absorption to lower excitation energies suggests an increase of the density of states at the Fermi level accompanying the increase of the electrical conductivity with increasing self-bias voltage or phosphine concentration. No hint of any ''doping'' effect is found.