Recombination and photoluminescence mechanism in hydrogenated amorphous carbon

The luminescence mechanism in a-C:H is described as a modification of the band tail luminescence in hydrogenated amorphous Si. The tail states of a-C:H are formed from clusters of sp(2) sites and luminescence occurs by recombination within each cluster. The paramagnetic defects are confirmed as the nonradiative recombination centers. The weaker temperature dependence of the luminescence efficiency of a-C:H than a-Si:H is attributed to its wider tails which inhibit carrier hopping. The luminescence efficiency is also quenched by narrow optical gaps, because carriers can tunnel to defects more easily in the sp(2)-rich, narrow-gap a-C:H. Defect quenching is less strong, however, because of the shorter Bohr radius of localized states in a-C:H.