Nitrogen-doped ultrananocrystalline carbon: Response to small amplitude AC signals

The response to small amplitude AC signals of nanometer-sized carbon films with nitrogen added in the gas is analyzed in a wide frequency range. Measurements have also been developed at temperatures between 300 and 600 K. Domains of sp3-coordinated carbon embedded into a connective tissue have been evidenced in SEM micrograph and confirmed by photocurrent yield measurements. Samples with nitrogen added in gas showed a sharp peak at 3.9 eV associated to a defective band close to the conduction band. Small quantities of nitrogen added in the gas appear able to relax the connective tissue, but no drastic change in surface morphology occurs upon low (5%) nitrogen addition. Admittance and impedance formalisms have been used to evidence the involvement of grain boundaries in transport by tunneling and hopping among defects at Fermi level, as well as to evidence a transport mechanism with activation energy of 0.21 eV which takes place mainly in a defective band formed below the conduction band when nitrogen is added.