Electronic properties and doping of hydrogenated tetrahedral amorphous carbon films

Hydrogenated tetrahedral amorphous carbon (ta-C:H) films were deposited over a range of ion energies using a plasma beam source. The optimum ion energy for maximum sp(3) content was found to be 92 eV per carbon ion, and films were grown at this energy using both acetylene and methane as the source gas. The conductivity of the acetylene films was found to be 3.9 x 10(-8) cm(-1). Upon addition of nitrogen, the conductivity increased, while the optical band gap remained constant up to a nitrogen partial pressure of 10(-5) mbar, which suggested n-type doping. At higher partial pressures, the band gap rapidly decreased. ta-C:H/Si heterojunction measurements using both p and n-type silicon showed that the films were n-type in character. However. there was found to be a large amount of nitrogen present in the acetylene gas, so a truly undoped film could not be produced. Thus, further films were made using high-purity methane as the source gas. The undoped films were then found to be p-type and could be doped n-type using nitrogen. Intrinsic behaviour was observed at a nitrogen partial pressure of 4 x 10(-6) mbar. (C) 1998 Elsevier Science S.A.