Plasma deposition of carbon films at room temperature from C₂H₂-Ar mixtures: Anodic vs. Cathodic films

Amorphous carbon films were deposited at room temperature on the anode and cathode of a radio-frequency glow discharge reactor by the dissociation of acetylene-argon mixtures. The film's properties ranged from low density, soft polymer-like to higher density a-C:H. The changes in the atomic structure, optical and electrical properties were studied as functions of the pressure. Anodic films are highly resistive, have a low refractive index (n≈1.76-1.94) and a wide band gap (E_g≈1.7-2.5 eV), whereas cathodic films are more conductive (σ≈10^-12-10^-9 S cm^-1), have a higher refractive index (n≈2.0-2.35) and a smaller band gap (E_g≈1-1.3 eV). The conductivity and activation energy of cathodic films can be controlled by the addition of phosphine or diborane to the gas mixture, suggesting that doping is possible in a-C:H as in hydrogenated amorphous silicon.