Medium Range Order and 1/f Noise in Hydrogenated Silicon Thin Films
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Medium Range Order and 1/f Noise in Hydrogenated Silicon Thin Films T. J. Belich and J. Kakalios School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455 ABSTRACT Measurements of the second spectra and correlation coefficients of conductance fluctuations are reported for a series of n-type doped a-Si:H films for which the deposition conditions are systematically varied. The non-Gaussian character of the 1/f noise is found to decrease with decreasing deposition temperature, which is also correlated with an increase in the difference between the conductivity and thermopower activation energies (the Q-function). The decrease in non-Gaussian statistics is interpreted as a decrease in the interactions between inhomogeneous current filaments due to increasing long-range disorder in the material. INTRODUCTION Co-planar current fluctuations in hydrogenated amorphous silicon (a-Si:H) are found to have a spectral density which varies as the inverse of the frequency f, termed 1/f noise. In a-Si:H the 1/f noise has been observed to exhibit non-Gaussian statistics [1,2], indicating that the noise cannot be accounted for by an ensemble of independent Lorentzian fluctuators, but rather is due to interactions between correlated fluctuators [3,4]. The 1/f noise has been interpreted as reflecting the dynamics of inhomogeneous current filaments, believed to arise from medium range (1-10 nm) structural or electronic disorder. The presence of such medium and long-range disorder in a-Si:H, such as hydrogenated microvoids, unintended chemical impurities, charged dopants and oppositely charged dangling bonds, is well documented, as is their influence on the bulk electronic properties [5-7]. The correlations in the noise power are believed to arise from interactions between inhomogeneous current filaments, believed responsible for the observed random telegraph switching noise in a-Si:H [8], which traverse the sample through minimums in the potential energy of the conduction and valence bands. Filament interactions may arise due to changes in the silicon strain field caused by the diffusion of hydrogen which can alter the connectivity and resistance of a current microchannel [9,10]. When the motion of a hydrogen atom associated with a given current filament inserts itself into the silicon network it alters the local strain field, possibly changing the hydrogen microstructure of another filament. In this way changes in the conductivity of two filaments separated in space may become correlated. Further details regarding the structure of these current filaments have been described elsewhere [2]. The non-Gaussian statistics characterizing the 1/f noise are reflected in measurements of the second spectra, obtained by Fourier transforming the time-dependent fluctuations of the 1/f noise spectral density. The second spectra in a-Si:H films may also display a 1/f frequency dependence [11], indicating electronic correlations between differing current filaments. Previous studies on n-type films [12] and films alloyed with carbon
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