Optical properties of plasma deposited amorphous carbon nitride films on polymer substrates

Amorphous carbon nitride thin films were deposited on polymer substrates using radio frequency (rf) plasma in a mixture of nitrogen (N2) and acetylene (C2H2) gasses. The samples were prepared at different rf plasma power (350, 400, 450, 500, and 550 W), at constant plasma exposure time of 10 min, and constant N2/C2H2 ratio of 50%. The crystal structure and surface morphology of the prepared samples were examined using X-ray diffraction and atomic force microscopy analysis, respectively. The absence of the carbon nitride diffraction peaks confirms the amorphous nature of these films. The root mean square roughness of the films increased from 3.77 to 25.22 nm as the power increased from 350 to 550 W. The thickness and the deposition rate were found to increase with increasing plasma power. Over the whole studied wavelength range, from 200 to 2500 nm, the transmittance decreased with increasing plasma power. A shift in the onset of absorption towards higher wavelengths with increasing plasma power, indicating a decrease in the optical band gap, has been observed. The refractive index values were found to decrease while the extinction coefficient increased with increasing plasma power.