Compositional and structural variations of nitrogen doped amorphous carbon films grown by surface-wave mode microwave plasma CVD

In this paper, we report the effect of acetylene (C2H2) flow rates (5 to 20 sccm) on the compositional and structural variations of nitrogen doped amorphous carbon (a-C:N) thin films grown by surface-wave mode microwave plasma chemical vapor deposition at low temperature (≪100 °C). Argon was used as the main plasma source gas. The films were characterized by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, UV/VIS/NIR spectroscopy and Fourier transform infrared spectroscopy (FTIR) measurements. The deposition rate of the films was typically 10 nm/min. The Tauc optical band gap of the films was in the range 1.6–1.8 eV. The XPS results show successful doping of nitrogen in the films, whereas N atomic concentration (at.%) into the films varied in the range of 23 to 35%. The N at.% in the films did not correlate with the increase of C2H2 flow rate (i.e. decrease of nitrogen gas concentration). The maximum percentage of nitrogen take up was observed in the film grown at C2H2 flow rate of 10 sccm. The FTIR results show enhanced contribution of C=C sp² and C-N sp³ bonds with increasing C2H2 flow rate. The increase of C-H vibration mode around 3300 cm⁻¹ indicates the increase of sp³-bonded carbon in the films. The amorphous nature of the films was qualitatively understood from the Raman results.