Electronic transport properties of nitrogen doped amorphous carbon films deposited by the filtered cathodic vacuum arc technique

Highly tetrahedral amorphous carbon thin films were deposited by the filtered cathodic vacuum arc technique at room temperature. Nitrogen was found to be a good n-type dopant of the tetrahedral amorphous carbon thin films. The Fermi level shifts from 0.91 eV above the valence band to 0.65 eV below the conduction band with increasing nitrogen flow rate from null to 16 sccm (nitrogen partial pressure from 0 to 2.4 × 10^-4 Torr). At the same time, the optical band gap drops from 2.7 to 1.8 eV. Three electronic transport mechanisms, namely, transport in extended states, in band tails by hopping and variable range hopping (VRH) near the Fermi level, were observed from the thermal activation measurements in the temperature range from 100 to 450 K. The VRH transport parameters for ta-C films are studied, and the density of states near the Fermi level extracted from the hopping transport parameters was found in the range of 6.5 × 10¹⁷-9.7 × 10¹⁹ cm^-3 eV^-1. The dominant doping configuration is the substitution in the sp³ coordination at low N concentration and adoption of sp² bonding at high N concentration.