AMBIGUOUS DOPING EFFECTS IN AMORPHOUS HYDROGENATED CARBON-FILMS PREPARED BY PACVD

Amorphous hydrogenated carbon (a-C:H) films prepared by plasma-assisted chemical vapor deposition were investigated with respect to their possible semiconducting properties, especially their changes upon doping. Compared with pure a-C:H films, it was found that, in films containing nitrogen, phosphorus or boron, both the electrical conductivity and the optical properties of the films were changed-the conductivity and the relative density of states at the Fermi level increased, while the optical bandgap decreased. For films prepared with various concentrations of dopants, as well as under various conditions of preparation, there was always found to be a strong correlation between the conductivity and the optical properties; only for very low concentrations of the dopants did the conductivity of the films increase slightly without a simultaneous decrease in the optical bandgap. Together with the temperature dependence of the conductivity-which for all films was in accordance with a variable range hopping mechanism in the temperature range 80-300 K-these results give no evidence for a shift of the Fermi level with respect to the bandgap of the material. Therefore, we conclude that no doping effect in a classical sense is achieved by the incorporation of nitrogen, phosphorus or boron into a-C:H.