Evidence of hydrogen termination at grain boundaries in ultrananocrystalline diamond/hydrogenated amorphous carbon composite thin films synthesized via coaxial arc plasma

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Ultrananocrystalline diamond/hydrogenated amorphous carbon composite thin films consist of three different components: ultrananocrystalline diamond crystallites, hydrogenated amorphous carbon, and grain boundaries between them. Since grain boundaries contain many dangling bonds and unsaturated bonds, they would be a cause of carrier trap center degrading device performance in possible applications such as UV photodetectors. We experimentally demonstrate that hydrogen atoms preferentially incorporate at grain boundaries and terminate dangling bonds by means of several spectroscopic techniques. XPS measurements cannot detect quantitative transitions of sp2- and sp3-hybridized carbons in the films, resulting in 55%–59% of sp3 content. On the other hand, Fourier transform infrared spectroscopy and near-edge x-ray absorption fine structure exhibit some variations of the amounts of certain carbon hybridization for sure. The former confirms the transformation from sp2 to sp3 hydrocarbons by ∼10% by additional hydrogenation, and the latter represents chemical configuration changes from π* C≡C and π* C=C to σ* C—H as well as more σ* C—C. These results can be evidence of localized hydrogen at grain boundaries, which plays a part in terminating dangling bonds and unsaturated bonds, and they are correlated with the optical and electrical properties of the films investigated in some previous research studies. Our spectroscopic studies on the hydrogenation effects combined with the discussion on the optical and electrical characteristics confirm that the hydrogenation can be an effective tool for the enhancement of photovoltaic performance in the above sensing applications.