Properties of nitrogen-doped amorphous hydrogenated carbon films

Nitrogen-containing hydrogenated amorphous carbon (a-C:H(N)) films are grown from a dc plasma of a N2+C6H6 gas mixture. By varying the N2 fraction in this mixture films with different amounts of N are produced. The actual amount of nitrogen in the a-C:H(N) films is determined by nuclear reaction analysis and by Auger electron spectroscopy profiling. The nitrogen concentration in the films grows exponentially with nitrogen content in the gas mixture reaching concentrations as high as 10 at.% for the films grown from a N2-rich gas mixture (N2/(N2+C6H6)=0.75). The electrical and structural properties of the N2-doped films are studied by performing electrical conductivity, thermopower (TP), optical absorption, and electron-paramagnetic resonance measurements. Films with low (<1 at.%) nitrogen content exhibit fairly high resistivities, have an optical gap of 1 eV, are rich with dangling bonds (5×1020 cm−3) and their thermopower is positive and in the mV/K regime, indicating conductivity in the valence band tail. However, with increased N doping, the resistivity decreases and the optical band gap shrinks and reached zero for the highest doped film. The TPs for films containing more than 1 at.% are in the μV/K range, indicating hopping conductivity around the Fermi level. The results of all measurements are consistent with the model of Robertson for the electrical structure of amorphous hydrogenated carbon and for the proposed doping mechanism in this material.