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. 1996 The American Physical Society.