Structural characterization of sputtered hydrogenated amorphous carbon films by solid state nuclear magnetic resonance

• Published in: Journal of applied physics 2008-07, Vol.104 (1), p.013531-013531-8
• Main Authors: Cho, Gyunggoo, Yen, Bing K., Klug, Christopher A.
• Format: Article
• Language: English
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/1.2952515

Solid state nuclear magnetic resonance (NMR) methods have been used to investigate the effect of H content on the structure of sputtered hydrogenated amorphous carbon a -C : H films. Carbon-13 NMR spectra of a -C : H consist of two broad peaks at 135 and 40 ppm, which correspond to the s p 2 and s p 3 carbons, respectively. Surprisingly, the s p 2 / s p 3 carbon ratio ( 2.0 ± 0.3 ) is relatively independent of the H content in the range of 13%-35%. Hence, all a -C : H films contain approximately 66% graphitelike s p 2 carbon. Cross polarization and spectral editing experiments reveal that the nonprotonated (quaternary) s p 3 carbon fraction reaches as high as 30% at low H concentrations of 13%-23%. As the H content exceeds 23%, however, quaternary carbon bonds are terminated by H atoms to form protonated s p 3 carbons, such as methylene ( CH 2 ) and methine (CH). Our results show that the hardness of a -C : H correlates with the quaternary carbon fraction rather than the s p 2 / s p 3 ratio, as commonly believed. We also report the first extensive studies of both H 1 and C 13 spin-lattice relaxation for sputtered a -C : H . Relaxation data for hydrogen and carbon indicate that the unpaired electrons are probably equally distributed among s p 2 and s p 3 carbons. Moreover, the number of unpaired electrons in the a -C : H samples is relatively independent of H content, as revealed by spin counting experiments. C 13 and H 1 magnetization recovery curves for samples with H content up to 28% can be fitted with a single exponential, which is in contrast to the biexponential behavior reported for plasma-enhanced chemical vapor deposition (PECVD) a -C : H films. Hence, our results for sputtered a -C : H films do not support the model of proton-rich and proton-deficient regions proposed for PECVD carbon.