Structural, mechanical and optical properties of thin films deposited from a graphitic carbon nitride target

Carbon nitride films were usually deposited from graphite targets, and a problem remains in incorporating proper nitrogen content in the film. Therefore, we deposited carbon nitride thin films with the target which had a composition close to C₃N₄. The deposited carbon nitride films were transparent, and the color of the films varied between dark brownish to light yellowish, depending on nitrogen gas pressure. The highest band gap of ~ 3.5 eV was obtained for the films with light yellowish color. Surprisingly, nano-indentation experiments showed a lower mechanical hardness (~ 1.0 to 1.5 GPa) and reduced Young's modulus (24 to 27 GPa) for the transparent film than the opaque film. Structural analysis with XRD and TEM revealed that the films were completely amorphous. XPS analysis suggests that nitrogen to carbon (N/C) ratio strongly affect the hardness and band gap. The hardness was found to increase with a decrease in nitrogen content, but the band gap has an opposite effect. Both FT-IR and XPS experiments were carried out to further understand the nature of bonding between carbon and nitrogen. The transparent film with high sp³/sp² ratio may exhibit wide bandgap, but not necessarily have high hardness.