Correlation between the sp2-phase nanostructure and the physical properties of unhydrogenated carbon nitride

Abstract

This paper consists of an investigation of the structural arrangement of the sp2 phase in amorphous unhydrogenated carbon nitride (a-CNx) films and its effect on their physical properties. The a-CNx films (0.16<x<0.25) were synthesized using a hybrid deposition system combining laser ablation of graphite and a source of atomic nitrogen. The microstructure of the films was investigated by Raman spectroscopy and electron paramagnetic resonance (EPR), while their optical and mechanical properties were determined by spectroscopic ellipsometry and nanoindentation, respectively. It was found that deposition at high laser intensities leads to an increase in the spin density (>1020∕cm3) and the EPR linewidth (of a few gausses) along with a decrease in nitrogen content. Visible Raman measurements indicate that these effects are accompanied by an increase in the degree of disorder of the sp2 phase, as inferred from the broadening and downshift of the G Raman band, and a reduction of the CN triple bond signal. The analysis of these results in terms of the structural configuration and bonding in the films, show that an enhancement of the connectivity of the sp2 phase in the layers, takes place when deposition is performed at high laser intensities. These structural modifications are strongly correlated to a decrease in the optical gap from 0.61 to 0.21 eV as well as to an increase of the hardness value of the films from 12 to 24 GPa. The transition from a reduced to an enhanced connectivity of the sp2 phase occurs when the nitrogen content decreases below 22 at. %, as a result of the detected reduction of the triply bonded CN species in the layers.