Nitrogen doping of highly tetrahedral amorphous carbon

Abstract

Successful control of the conductivity of tetrahedrally bonded amorphous carbon (ta-C) by incorporation of N during film growth is reported. N is introduced into the films during growth by injecting ${\mathrm{N}}_2$ gas into a plasma stream formed by a carbon cathodic vacuum arc. X-ray-photoemission-spectroscopy studies of films prepared over a range of ${\mathrm{N}}_2$ partial pressures show that the N concentration varies from 2% to below the detection limit. Spectroscopic studies using electron-energy-loss spectroscopy confirm that the ta-C films with N contents up to 1 at. % retain their predominantly tetrahedral amorphous structure. The as-deposited ta-C, with no intentional N incorporation, is p type with a resistivity of ${10}^7$ Ω cm and an activation energy of 0.22 eV. The addition of N under varying injection levels from ${10}^{\mathrm{-}3}$ to 10 standard cubic cm causes the resistivity to initially go through a maximum at ${10}^9$ Ω cm and then decrease monotonically to 10 Ω cm. A systematic variation of activation energies with increasing ${\mathrm{N}}_2$ flow rate is observed, going through a maximum at about 1.0 eV before decreasing to 0.12 eV. The optical band pseudogap is shown to vary only marginally from the usual 2-eV benchmark. The properties of the doped film can be explained using a simple model of the electronic structure of the tetrahedral amorphous carbon.