Generation- Recombination Noise in Amorphous Semiconductors
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Generation- Recombination Noise in Amorphous Semiconductors Charlie Main1, Steve Reynolds1, Rashad I. Badran2 1. School of Science and Engineering, University of Abertay Dundee, Dundee DD1 1HG, UK, 2. Department of Physics, The Hashemite University, Zaqar Jordan, ABSTRACT We examine different approaches to the analysis of noise in amorphous hydrogenated silicon associated with trapping and generation – recombination processes, which appear to predict very different noise spectra. In one approach the broad noise spectrum observed is assumed to be composed of a distribution of Lorentzian noise spectra, each associated with traps at a given energy depth, with appropriate weighting according to the energy distribution of characteristic time constants. This latter weighting is taken to mirror the energy distribution of states in the gap. This represents a linear superposition of the (weighted) contribution from individual trapping levels, each with its own characteristic time constant. This approach thus assumes that each trap level is an independent source of fluctuation in free carrier number, unaffected by the presence of other traps in the material. At first sight this assertion seems plausible, since in the multi-trapping situation envisaged, cross-correlation effects must be very small. However, the presence of several groups of traps, or, in the limit, a continuum, results in a distribution of characteristic time constants, which is not a simple linear superposition of the time constants for each level. Thus the assertion that a flat density of states, or a region which is flat, such as the top of a broadened level, results in a region of 1/f slope in the noise spectrum, may not be valid. We present an alternative model in which the distribution of time constants is appropriately incorporated, and compare the predictions of this model with the ‘superposition’ approach, using computed noise spectra.
INTRODUCTION Conductance fluctuations are manifested as noise in the dc current flowing through a material or device when a voltage is imposed. There is considerable present interest in conductance fluctuations in amorphous semiconductors, and particularly in amorphous silicon. Studies [1-5] have reported on current noise in both undoped and n-type material, and appear to reveal two qualitatively different noise régimes. At low current densities, the fluctuations in current have a Gaussian amplitude distribution, the power spectrum SI (ω ) is roughly of 1/f form, but with significant variation between different reports, and is proportional to the square of the dc current. At higher current densities, the character of the fluctuations changes dramatically, exhibiting a 'random telegraphic' switching behaviour. In this régime, the amplitude distribution is non-Gaussian, and the power spectrum increases less rapidly with dc current, while the frequency dependence remains close to 1/f. It has been proposed [4] that this latter form of noise is an indicator of filamentary conduction paths arising from electronic and structu
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