Transient Decay from the Steady-State in Microcrystalline Silicon
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Transient decay from the steady-state in microcrystalline silicon
R. Brüggemann*, S. Reynolds#, C. Main# *Fachbereich Physik, Carl von Ossietzky Universität Oldenburg, 26111 Oldenburg, Germany #School of Science and Engineering, University of Abertay Dundee, Dundee DD1 1HG, Scotland, United Kingdom
ABSTRACT We measured the transient photocurrent decay from the steady state in microcrystalline silicon from plasma-enhanced chemical vapor deposition and hot-wire chemical vapour deposition. Samples exhibiting a range of photoconductive properties in terms of both the majority and minority carrier mobility-lifetime products or sub-gap absorption coefficients were studied. Measurements were made over a wide range of steady-state photogeneration rates for which we detail the variation with generation rate of the decay time. Samples with a short steady-state photocarrier lifetime show a long decay time. We relate the slow decay process to the much larger density of traps in the band gap in the poor-quality samples. Trapped carriers are released, undergo emission and trapping processes and eventually recombine in these samples, on a much longer time-scale than in the higher-quality samples so that the decay time cannot be taken as a fingerprint for photo-electronic quality. Analytical and numerical modeling indicate bimolecular recombination behavior during the decay. Results are in agreement with free-carrier interaction with exponentially distributed band-tail states.
INTRODUCTION The transient photoconductive decay from the steady state has attracted considerable interest in amorphous semiconductors in order to characterize the electronic properties [1-6]. Typically, the characteristic decay time τ, often defined as the value at which the photocurrent has dropped to 50 % of its initial value decay, is determined and the temperature and photo-generation rate dependence are analyzed. The values of τd were used to determine a drift mobility µd which can be related to the trapped carrier density and thus to the width of the conduction band tail. The time-dependence of the decay, often a power-law behavior, can also be related to the density-ofstates profile. In undoped hydrogenated amorphous silicon (a-Si:H), the excess photocarrier density in these experiments is usually much larger than the equilibrium carrier density n0. In this paper, we investigate the photocurrent decay behavior in microcrystalline silicon (µc-Si). The dark conductivity and the equilibrium carrier density n0 in µc-Si are much larger than in a-Si:H. It is thus possible to explore the regimes with the excess carrier density nph > n0, and also with nph < n0, which is usually not accessible in undoped a-Si:H. We correlate the decay time values with other material parameters like sub-gap absorption and mobility-lifetime products from steady state photoconductivity. The analysis of the experimental results is supported by numerical modeling of the photocurrent decay with a simple density-of-states model.
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