PHOTOLUMINESCENCE MECHANISM IN HYDROGENATED AMORPHOUS-SILICON STUDIEDBY FREQUENCY-RESOLVED SPECTROSCOPY

In order to reconsider the photoluminescence mechanism in hydrogenated amorphous silicon (a-Si:H), the lifetime distribution of photolumines cence, G(tau), was evaluated over a wide lifetime range between 5.3 X 10(-7) s and 8.0 X 10(-2) s using frequency-resolved spectroscopy. The most remarkable finding is that G(tau) is dominated by two kinds of l ifetime components characterized by specific peak lifetimes at low tem peratures. At 12 K, only the lifetime component peaked at about 1 ms i s dominant, whereas another component appears distinctively at around 10 mu s and grows gradually with increasing temperature. Such a discon tinuous change of the lifetime from 1 ms to 10 mu s takes place at tem peratures lower than 60 K where the photoluminescence intensity is alm ost constant. Interestingly, the peak lifetimes for both lifetime comp onents are quite insensitive to the Urbach energy or the emission ener gy as long as the excitation intensity is held low enough to satisfy t he condition for geminate-pair recombination. These characteristic fea tures observed in G(tau) do not reconcile with the generally accepted model of tunneling recombination between carriers trapped at the tail states after thermalization. In particular, dominance of the 1-ms life time component at 12 K is interpreted following the model as the elect ron-hole separation enlarges up to a value corresponding to the 1-ms r ecombination after photogeneration. However, it is hard to understand the absence of the 10-mu s lifetime component at 12 K, since the 10-mu s recombination is expected to take place at much shorter electron-ho le separation and is actually observed at elevated temperatures. Becau se of the several arguments against the generally accepted model and o f the coexistence of two lifetime components having specific peak life times, it is more appropriate to consider that the photoluminescence i n a-Si:H comes from special localized luminescent centers correspondin g to the respective lifetime components. Since the lifetime changes di scontinuously from 1 ms to 10 mu s with increasing temperature while t he luminescence intensity remains constant at the low temperatures, so me correlation is expected to exist between the two kinds of luminesce nt centers.