Photoluminescence mechanism in amorphous hydrogenated 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. Thermal quenching of the luminescence occurs by hopping of carriers between tail states, which recombine non-radiatively elsewhere. The smaller temperature dependence of the luminescence efficiency of a-C:H than a-Si:H is attributed to the greater width of its tails. The luminescence efficiency is also quenched by narrow band gaps, because carriers can tunnel to defects and recombine. The dependence on optical gap arises because the tunnel barriers are narrower in narrow gap a-C:H.