Evidence for excitonic behavior of photoluminescence in polymer-like a-C:H films

To understand the dynamics of energy transfer and randomization of photoluminescence polarization in hydrogen-rich polymer-like amorphous carbon a-C:H films, time-resolved investigations of intensity and anisotropy decays have been performed recently. The intensity decay rates increase exponentially as a function of emission energy with a behavior very similar to that observed in wide band-gap C-rich a-Si1-xCx:H. In addition, in polymer-like carbon, the observation of a plateau of PL anisotropy in the 100-1000 ps range, is taken as strong evidence for the existence of a finite density of excitonic species in radiative recombination phenomena; it does not fit the phonon-assisted depolarization models proposed earlier. Polarization anisotropy decays and steady-state values are consistently interpreted using a dipole-dipole non-radiative energy transfer mechanism (Forster mechanism) with a characteristic depolarization time of 50 ps rather independent of the emission energy. The latter value is likely to be related to the density of radiative centers distribution estimated independently in the constant exciton radius approximation, rather than the result of hopping in an exponential distribution of tail states. (C) 2001 Elsevier Science B.V. All rights reserved.