  Ultranonocrystalline 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 a lot of dangling bonds and unsaturated bonds,
they would be a cause of carrier trap center degrading device performance in
possible applications such as UV photo-detectors. We experimentally demonstrate
hydrogen atoms preferentially incorporate at grain boundaries and terminate
dangling bonds by means of several spectroscopic techniques. XPS measurements
cannot detect quantitative transitions of sp^2- and sp^3-hybridized carbons in
the films, resulting in 55-59 % of sp^3 contents. On the other hand, FT-IR and
NEXAFS exhibit some variations of the amounts of certain carbon hybridization
for sure. The former confirms the transformation from sp^2 to sp^3 hydrocarbons
by ~10 % by additional hydrogenation, and the latter represents chemical
configuration changes from {\pi}* C{\equiv}C and {\pi}* C=C to {\sigma}* C-H.
These results can be an 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. 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 of an enhancement of photovoltaic performance in the above sensing
applications.
