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.

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Ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films possess specific characteristics as follows: (a) the appearance of additional energy levels in diamond bandgap [1]; and (b) large absorption coefficients ranging from visible to ultraviolet [2], both of which might be due to large number of grain boundaries between UNCD grains and those between UNCD grains and a-C:H [1,3]. Owing to the above-mentioned specifics, UNCD/a-C:H films are expected to be applied to photovoltaics such as UV sensors. Actually, we have fabricated pn heterojunction diodes comprising UNCD/a- C:H films and Si substrates, and confirmed their photocurrents [4,5]. Although the minority-carrier lifetime is an important factor for photovoltaics, it has never been studied for UNCD/a-C:H. In this work, we experimentally measured the minority carrier lifetimes for typical samples.