We have used infrared (IR) absorption spectra to characterize a-C:H and its alloys (a-C:H,N,F). The samples were deposited in an rf plasma-enhanced chemical-vapor deposition system with various mixtures of ${\mathrm{CH}}_4$, ${\mathrm{N}}_2$, and ${\mathrm{NF}}_3$ gases. IR spectra are analyzed in detail and the peak position assignments are based on the data published in the literature and calculation of the normal mode vibrational frequencies by assuming a simple force field and linear-molecular model. From IR analysis, we found that as the concentration of nitrogen and fluorine increases, the ratio of ${\mathrm{sp}}^2$/${\mathrm{sp}}^3$ increases, while the hydrogen content decreases. The role of nitrogen is to break the symmetry and activate the olefinic C=C stretching mode. The role of fluorine is to replace hydrogen and to form stronger C–F bond. This explained the improvement of the thermal stability of electrical conductivity of a-C:H,N,F samples.

DOI:https://doi.org/10.1103/PhysRevB.55.13020