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Ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films possess the following specific characteristics: (a) the appearance of additional energy levels in diamond bandgap and (b) large absorption coefficients ranging from visible to ultraviolet, both of which might be due to large number of grain boundaries between UNCD grains and those between UNCD grains and a-C:H. Owing to them, UNCD/a-C:H films are expected to be applied to photovoltaics such as UV sensors. Actually thus far, we have fabricated pn heterojunction diodes comprising p-type UNCD/a-C:H films and n-type Si substrates, and confirmed their photovoltaic action. In this study, the minority carrier lifetime, which is an important factor for photovoltaics, was experimentally measured by microwave reflected photoconductivity decay, and it was estimated to be 0.21 and 0.43 µs for UNCD/a-C and UNCD/ a-C:H, respectively. In addition, on the basis of the previous work on the heterojunctions, the effects of hydrogenation on the photovoltaic action of the heterojunctions were studied. The photocurrent apparently increases with an enhancement in the hydrogenation of UNCD/a-C:H films, which might be because dangling bonds in the UNCD/a-C:H films, which act as photogenerated-carrier trap centers, are terminated by hydrogen atoms.

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 π* C ≡ C and π* C = C to σ* 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 hydrogenation can be an effective tool of an enhancement of photovoltaic performance in the above sensing applications.

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