Light Induced Changes in the Non-Gaussian Noise Statistics in Doped Hydrogenated Amorphous Silicon
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Light Induced Changes in the Non-Gaussian Noise Statistics in Doped Hydrogenated Amorphous Silicon J. FAN and J. KAKALIOS The University of Minnesota, School of Physics and Astronomy, Minneapolis, 55455 USA
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ABSTRACT The power spectrum of coplanar current fluctuations in n-type doped hydrogenated amorphous silicon (a-Si:H) has been measured before and after metastable defect creation by light soaking. The average magnitude and spectral slope of the 1/f noise are not affected by illumination, however significant changes in the higher order statistics are observed. Associated with the decrease of conductivity upon light soaking (the Staebler-Wronski effect), there is a decrease in the correlation of the noise power which characterize the nonGaussian noise in the annealed state. These changes in the noise statistics are reversible by annealing. The light-induced changes in the non-Gaussian statistics provide experimental support for models of light induced defect creation which involve long-ranged and many body interactions. INTRODUCTION Most models to account for light-induced conductance changes (the Staebler-Wronski effect)l in hydrogenated amorphous silicon (a-Si:H) involve bond breaking of weak Si-Si bonds due to the nonradiative recombination of photo-excited charge carriers, and consider the defect creation process to involve2 only the localized region of the strained Si-Si bond, though hydrogen motion which is invoked to stabilize the newly created dangling bond can involve long ranged diffusion. Recently, alternative models 3 - 7 have been proposed that suggest that the photo-created defects also induce changes in long-ranged potential fluctuations or strain fields which alter the electronic properties of the current carrying extended states. A unique feature of these models is that the metastable conductance changes result (at least in part) from non-local or many body interactions. Experimental support for such models is difficult to come by, since most transport data are only sensitive to conductivity variations over the length scale of the inelastic scattering length 8 (which is - 5 - 7 A at room temperature). Measurements 9-11 of the 1/f noise power spectra in doped a-Si:H have been interpreted as reflecting the presence of inhomogeneous current filaments whose resistance is time and temperature dependent, presumably due to hydrogen motion. We have measured the 1/f noise spectral density of coplanar current fluctuations in n-type doped a-Si:H before and after light soaking, and find a dramatic change in the non-Gaussian statistics which characterizes the noise power in a-Si:H. 1 2 In the annealed state the noise is strongly non-Gaussian, with large correlation of the noise power between differing frequency octaves. Following light exposure the 1/f noise becomes Gaussian and the correlation coefficients of the noise decrease. These light induced changes in the nonGaussian statistics are reversible upon annealing at 450 K for 30 min. The changes in the non-Gaussian statistics are consistent with an inc
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