Photodarkening and photobleaching in a-C:H films

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Abstract
Irreversible photodarkening and photobleaching effects in a‐C:H are discussed on the bases of measured excitation spectra and infrared absorptions. From infrared absorption studies, it was found that three absorptions due to CHn, C=0 and OH bonds changed with illumination. The CHn absorption decreased for photodarkening as well as photobleaching, while C=0 and OH absorptions increased only for photobleaching. From excitation spectra, photodarkening effects were found to be strongly dependent on illuminating energy. The knee at about 3.6 eV of the excitation spectrum appears to correspond to the dissociation energy of the CH bond. These results seem to indicate that the photo‐dissociation of the CH bond plays an essential role. The temperature dependence of photodarkening seems to imply that thermal energy also participates the process. The estimated quantum efficiency of this dissociation is relatively high, for example, about 0.2 for 365 nm illumination at room temperature.

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