Ultrananocrystalline diamond (UNCD) films with up to 0.2% total nitrogen content were synthesized by a microwave plasma-enhanced chemical-vapor-deposition method using a CH4(1%)/Ar gas mixture and 1%–20% nitrogen gas added. The electrical conductivity of the nitrogen-doped UNCD films increases by five orders of magnitude (up to 143 Ω−1 cm−1) with increasing nitrogen content. Conductivity and Hall measurements made as a function of film temperature down to 4.2 K indicate that these films have the highest n-type conductivity and carrier concentration demonstrated for phase-pure diamond thin films. Grain-boundary conduction is proposed to explain the remarkable transport properties of these films.

Studies on structure and electronic properties of amorphous nitrogenated carbon films prepared in dual electron cyclotron resonance–radio frequency plasma from a mixture of methane and nitrogen are presently reported. These films are characterized by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), electrical conductivity measurement, and optical absorption spectroscopy. Symmetry breaking of aromatic rings are at a very small amount of nitrogen incorporation is understood from FTIR spectra. The relative contribution of C=N and C–N bonds is found to change with the variation of the nitrogen content in the samples, which shows a similar trend with the shift of the $G$ peak to a higher wave number and the increase of the $I_D{\mathrm{/I}}_G$ ratio. From decomposition of XPS $\mathrm{C}\text{\hspace{0.17em}1s}$ and $\mathrm{N}\text{\hspace{0.17em}1s}$ peaks a three-phase model of CN bonds is proposed. UPS valence band spectra obtained by using a Helium II source, are decomposed into $\mathrm{p-\pi ,}$ $p$ -σ, $\text{2s}$ bands and a mixture of $\mathrm{s-p}$ band. The intensity of $\mathrm{p-\pi }$ band increases as a function of nitrogen concentration, confirming the increase of ${\mathrm{sp}}^{\mathrm{2}}$ bonds in the samples. An enhancement of the room temperature electrical conductivity and a decrease of the optical gap are observed with the addition of nitrogen in the films. The effect of nitrogen doping in carbon films is also emphasized. Our analyses establish an interrelationship between the microstructure and electronic structure of nitrogenated carbon films, which helps to understand the change in electronic properties of the carbon films due to a low amount of nitrogen incorporation.

Amorphous nitrogenated carbon $\mathrm{(a-}\mathrm{CN:H})$ films are prepared from a mixture of methane and nitrogen in an electron cyclotron resonance plasma at a pressure of 2 mTorr and applying a substrate bias of $\text{−300\hspace{0.17em}}\mathrm{V}.$ Based on the characterization done by x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy on the films containing different amounts of nitrogen, a similarity in variation between the binding energy of $\mathrm{C}\text{\hspace{0.17em}1s}$ peak and position of Raman active $G$ peak is noticed. XPS $\mathrm{C}\text{\hspace{0.17em}1s}$ and $\mathrm{N}\text{\hspace{0.17em}1s}$ are deconvoluted into four components to separate the contribution of CC, $\mathrm{C–N}$ , $\mathrm{C=N}$ , and $\mathrm{C\equiv N}$ bonds. The change of intensity ratio of Raman active $D$ and $G$ peaks shows a marked correlation with the intensity ratio of single and double bonds between carbon and nitrogen. Combined XPS and Raman spectroscopy throw some light on the variation in structure of carbon films as a function of nitrogen concentration.

Dramatic enhancement of the $n$-type conductivity of nanocrystalline diamond films by introducing up to $0.2\mathrm{at.}\%$ of nitrogen into the film has been reported. Previously, there were some theoretical predictions about the change of electronic structure of the films by nitrogen incorporation, but the origin of the enhanced conductivity is still not clearly understood. In this article the mechanism of high conductivity and the change of the electronic structure by nitrogen incorporation is investigated by low-temperature conductivity and other supporting measurements. It is shown that nitrogen induces percolative paths in the grain boundary regions and there is an increase in the density of states at the Fermi level that helps increase the conductivity. Low-temperature conductivity has been explained from a change over from Arrhenious behavior to Efros-Shklovokii–Pollak–Mott variable range hopping conductivity. Using a model combining band and hopping conduction electrical conductivity of highly doped samples over wide range of temperature has been explained. This approach also helps us to improve the understanding of the electronic structure and transport of conducting amorphous carbon by resolving some typical problems in the analysis of temperature dependent conductivity.