Bandtail States and Carrier Transport in Amorphous Silicon

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In this study, we have pursued a theory for nonequilibrium carrier kinetics in amorphous semiconductors containing localized bandtail states among which finite interactions are allowed via tunneling. Fourier domain solutions are explored intensively in order to quantitatively interpret the frequency-dependent photocarrier drift mobility determined by modulated photocurrent measurement. The theoretical analysis is in good agreement with experiments carried out for hydrogenated amorphous silicon over a wide range of frequencies and temperatures, and discloses that the inclusion of tunneling transitions considerably accelerates carrier thermalization in localized bandtail states. A generalized recombination model which considers both direct capture of band carriers and tunneling transfer of bandtail carriers into recombination centers is also examined in detail, suggesting that even for room temperature, the tunneling recombination takes place preferentially. Also performed in this study is the frequency-domain measurement of junction photocurrent under a modulated photoexcitation, called the frequency-resolved time of flight, in order to evaluate the drift mobilities of electrons and holes, separately. The electron time of flight spectra measured for a p-i-n sample exhibit an oscillatory structure due to a transit time effect in a non-dispersive transport regime. The analysis of the spectral beat determines the frequency dependent drift mobility, and yields quantitative assessments of the transport properties of this material. Under an intense light exposure, the time of flight experiments observe a sizable reduction of the hole drift mobility, which illustrates that the light induced disordering of amorphous silicon network gives a strong impact on the electronic states near the valence band edge.