Reactive Magnetron Sputtering of CNx Thin Films at Different Substrate Bias

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The chemical binding states of C and N atoms, and optical properties of carbon nitride (CNx) thin films deposited by unbalanced magnetron sputtering, have been investigated as a function of the negative substrate bias (Vs). The film deposition rate increased slightly with increasing Vs, having a weak maximum at floating potential (~50 V), and decreased sharply to zero for Vs>150 V, while N/C ratios did not exhibit any significant variation. Raman spectroscopy was used to reveal that the structure of the film is predominantly amorphous. Fourier transform infra-red spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS) analyses showed that N atoms in the films were bound to C atoms through sp2 and sp3 configurations. Triple C–N bonds were also detected by FTIR. The ratio of sp3 to sp2 bonds increased with increasing Vs. The maximum sp3 concentration in CNx films was estimated to be ~20%. The optical band gap of CNx films was also found to increase with an increase in Vs.