1 / f Noise Measurements of Interacting Current Filaments in Hydrogenated Amorphous Silicon
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1 / f NOISE MEASUREMENTS OF INTERACTING CURRENT FILAMENTS IN HYDROGENATED AMORPHOUS SILICON C. E. PARMAN, N. E. ISRAELOFF. J. FAN and J. KAKALIOS The University of Minnesota, School of Physics and Astronomy, Minneapolis, MN 55455 USA ABSTRACT The coplanar current in n-type doped hydrogenated amorphous silicon (a-Si:H) displays random telegraph switching noise, indicating the presence of inhomogeneous current filaments whose conductance varies with time. There are strong correlations of the 1/f noise power spectra across differing frequency octaves which are much larger than expected if the magnitudes of the fluctuators are varied in parallel. The scale invariant second spectra and the temperature dependence of the spectral slope indicate that hydrogen motion is involved in the cooperative dynamics between noise sources. A model is described wherein the properties of the current filaments are modulated by hydrogen-hydrogen interactions which are mediated by the Si strain fields. INTRODUCTION The recent observations of random telegraph switching noise in coplanar current measurements, t the non-linear current dependence of the 1/f noise spectral density 2 and of time dependent fluctuations of the I/f noise power 3 in hydrogenated amorphous silicon (a-Si:H) have all been interpreted as arising from inhomogeneous current paths in the amorphous silicon. The conductance of these filaments is time dependent, possibly due to hydrogen motion which would change the bonding configurations along the current microchannel. Conduction through a series of filaments whose resistance is highly sensitive to the local bonding along the pathway is not unlike a classical percolation network. As will be argued below, the magnitude of the switching noise, and the strong correlations of the
I/f noise power make it unlikely
that the conductance
of these
filaments is being modulated through independent parallel processes, but instead reflects strong interactions between the fluctuators which obey serial or hierarchical kinetics. In this paper we describe random telegraph switching noise measurements and statistical analysis of the noise power of doped a-Si:H which demontrate that the 1/f noise arises from current filaments which exhibit cooperative interactions. These results are discussed in terms of a model in which the interactions of the filaments are governed by hydrogen-hydrogen interactions which are mediated by distortions in the strain of the amorphous silicon subnetwork. EXPERIMENTAL METHODS The resistance switching and 1/f noise data are collected using a d.c. twoprobe measurement system. 4 A constant voltage, supplied using an HP 6212C power supply, is applied across the coplanar electrodes (200 Atm wide with a separation of 800 Atm) and the fluctuations in the current passing through the aSi:H sample are amplified using a commercial current pre-amplifier (Ithaco 564) and measured using a spectrum analyzer (HP 3561A). For the switching noise measurements the spectrum analyzer records the current directly as a function of time
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