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 assu med ''doping'' gas. The optical constants of the thin films were deter mined by analysing reflection, transmission and photothermal deflectio n measurements. Accordingly, the absorption behaviour below an absorpt ion coefficient alpha of 10(4) cm(-1) can be fitted very well by a sim ple power law. Extrapolating the absorption to lower excitation energi es suggests an increase of the density of states at the Fermi level ac companying the increase of the electrical conductivity with increasing self-bias voltage or phosphine concentration. No hint of any ''doping '' effect is found.