Comparative study of the structure of a-CN x and a-CN x:H films using NEXAFS, XPS and FT-IR analysis

Abstract

Amorphous carbon nitride thin films have become a matter of great attention due to their remarkable electronic and mechanical properties. It has been shown that hydrogen and nitrogen incorporation deeply modifies the properties of carbon films. Therefore, the optimization of their properties requires a deep knowledge of various kind of chemical bonds composing in the film matrix. The topic of this presentation is to get more insight into the different local environment of the C and N atoms for hydrogenated and hydrogen-free amorphous carbon nitrides films. H-incorporation has been varied using different deposition technique from plasma-enhanced chemical vapor deposition (PECVD) leading to highly hydrogenated films up to 40 at.% to radio-frequency (RF) magnetron sputtering providing nearly hydrogen-free films. The study of the local structure is done using the combination of Fourier transform infrared (FT-IR), X-ray photo-emission spectroscopy (XPS) and high resolution near edge X-ray absorption fine structure (NEXAFS) analysis. FT-IR spectroscopy is widely used to probe the bonding configurations in the carbonaceous materials, especially the C N, C H and N H bonds in a-CN x :H. In addition, XPS and NEXAFS provide surface information on the environment around C and N atoms and on the chemical composition. NEXAFS gives a better description of the π* states inside the films, due to its remarkable energy resolution. The combination of both characterizations FT-IR and NEXAFS may leave the controversy about the interpretation of the XPS spectra, and allows a fine analysis of the evolution of the local structure as a function of nitrogen incorporation, according to the hydrogen concentration into the films. Fundamental differences can be revealed between hydrogenated and hydrogen-free carbon nitride: in the former, hydrogen promotes double bonds C N H, whereas in the latter nitrogen atoms prefer to substitute to carbon or interconnect aromatic rings through single bonds (>C N). A strong conjugation of imines (N C N H) for PECVD film and nitrile (N C N) groups for hydrogen-free films is evidenced.