COMPREHENSIVE NUMERICAL-SIMULATION OF DEFECT DENSITY AND TEMPERATURE-DEPENDENT TRANSPORT-PROPERTIES IN HYDROGENATED AMORPHOUS-SILICON

In this work the mobility-lifetime products of electrons and holes, (mu tau)(e) and (mu tau)(h), the response time of the photocurrent tau(R) and the electron drift mobility mu(de) have been numerically simulated as a function of the dangling bond density N-d and temperature T in hydrogenated amorphous silicon. We have considered all possible recombination and reemission processes occurring between extended and localized states. The simulated results are in good agreement with experimental data. (mu tau)(h) in undoped a-Si:H is insensitive to N-d in the low-N-d range in contrast to (mu tau)(e). This asymmetric N-d dependence of(mu tau)(e) and (mu tau)(h) is attributed to the inherent asymmetry of the density-of-states distribution of the conduction- and valence-band tails. The mu tau products and tau(R) decrease with the generation rate, whereas mu(de) increases. The effect of thermal broadening of the band-tail states must be taken into account in the simulation, especially the increase of the characteristic energy of the conduction band-tail states with T Apart from the defect states, the band-tail states play a very important role in the determination of the photocarrier lifetimes. In the low-N-d range, recombination via the band-tail states dominates over that via dangling bonds, while dangling bonds become the predominant recombination centers in the high-N-d range. The transition from the tail-state-dominated to the defect-state-dominated recombination process depends essentially on the defect density, the temperature, the generation rate, and the Fermi-level position.