Anomalous Electronic Transport in Diamond-Like-Carbon Films of Different Thickness

Transport properties of amorphous diamond-like-carbon （DLC） films of largely different thicknesses （90 and 600 nm） prepared by plasma-beam ion-injection deposition （PBIID） method have been reported. The thinner film （DLCI） shows metallic behavior above 200 K and a metal-insulator transition occurs below this temperature. The metallic resistivity （p） can be fitted with the relation ρ = ρ0（sp^2）T^β （with β -1）. The much thicker film DLC2 （600 nm thick） is, however, a semiconductor over the temperature range 80-300 K showing large increase of resistivity below 150 K. The scanning electron micrographs and Raman spectroscopic studies revealed the presence of nanometer size diamond-like crystals embedded in the amorphous matrix. Presence of larger number of such nanodiamonds in DLC2 along with larger sp^3/sp^2 ratio observed from X-ray photoelectron spectroscopy （XPS） study can be attributed to the higher resistivity （-10^10 ohm cm） of DLC2 compared to that of thinner DLCI（-10^4 ohm cm）. Low temperature semiconducting behavior of the thinner film can be explained by Mott variable range hopping （VRH） model while the resistivity of the thicker film is found to follow Mott small polaron hopping （SPH） model in the high temperature range. Thickness dependence of the DLC film conductivity has also been discussed.