Unusual behavior of the magnetoresistance of boron carbonitride films at low temperature

We have performed resistivity and magnetoresistance measurements down to 0.3 K, and under fields up to 37 T of boron carbonitride and BC3 films prepared by chemical vapor deposition. The turbostratic structure of the as-deposited materials favors a 2D weak localization effect which is invoked to explain the negative magnetoresistance (MR) as well as the log T variation of the resistivity. However, at very low temperature a positive component is superimposed on the negative MR. At high fields, the total MR is positive and almost isotropic. Usual theories are unable to account for the observed phenomenon. Increasing heat treatments up to 1800 degrees C increase the 2D character of the deposits, which show an increasingly negative magnetoresistance. For still higher treatments, the change of the films to a 3D graphitic-like structure leads to a vanishing of the negative magnetoresistance.