Negative magnetoresistance in undoped semiconducting amorphous carbon films

Amorphous carbon (a-C) films were fabricated by chemical vapor deposition on SiO2 substrate. The a-C films have nano-crystalline sp2 structure with the grain size of ∼5 nm and an optical band gap of ∼1.8 eV. The a-C films show negative magnetoresistance (MR) from 300 to 2 K and an anomalous shape change of MR-magnetic field curves at 10 K. Grain boundary scattering theory and weak localization theory were used to explain the MR mechanism and shape change of MR-magnetic field curves.