The Staebler-Wronski Effect and 1/f Noise in Amorphous Silicon
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The Staebler-Wronski Effect and 1/f Noise in Amorphous Silicon T. J. Belich and J. Kakalios School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455 ABSTRACT Experimental measurements of the Q-function (a comparison of the conductivity and thermopower activation energies) and the non-Gaussian statistical character of the 1/f noise in ntype doped a-Si:H as a function of light soaking (the Staebler-Wronski effect) are reported. There is a significant decrease in the non-Gaussian statistical character of the 1/f noise following light soaking of a device quality film, consistent with a slight increase in the long-range disorder. However, there is no change in the Q-function following light exposure, indicating that there is no significant increase in the long-range disorder. INTRODUCTION The conventional model for the Staebler-Wronski effect in hydrogenated amorphous silicon (a-Si:H) involves the breaking of strained Si-Si bonds due to the nonradiative recombination of photoexcited charge carriers, which create new dangling bond defects [1-4]. The local motion of hydrogen is typically invoked to stabilize these newly created defect states, upon annealing above 420 °K sufficient hydrogen diffusion occurs to remove these excess defects [4,5]. Recently, models for light induced conductivity degradation have been proposed which are intrinsically nonlocal in nature, involving changes in long-range potential fluctuations or strain fields associated with compositional morphology [6-9]. A large number of recent experimental results indicate that in addition to creating additional dangling bonds, extended illumination yields significant changes in the disorder at the mobility edge, affecting the ability of the material to carry an electrical current. The nonlocal models for the Staebler-Wronski effect presume that light soaking increases or modifies the long-range disorder already present in the film, believed to arise from nonuniform distributions of bonded hydrogen as well as potential fluctuations arising from randomly located charged defects. Consequently, even though the Fermi energy is uniform throughout the a-Si:H film, the conductivity will vary spatially, suggesting that electronic transport might best be described by a classical percolation process [10]. However, since the inelastic (phase breaking) scattering length in a-Si:H is believed to be on the order of ~ 5-10 Å at room temperature [11], conventional transport measurements typically do not directly reveal information concerning disorder on much longer length scales. Nevertheless, there are several measurement techniques that are sensitive to long range inhomogeneities in amorphous silicon. These techniques include measurements of the nonGaussian conductance fluctuations [12] and comparisons of the temperature dependence of the dark conductivity and thermoelectric power (the Q-function) [13-15]. The value of the energy separation between the Fermi energy and the mobility edge in the dark conductivity Eσ is larger than that of the thermopowe
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