Conductance fluctuations in n-type doped hydrogenated amorphous silicon (a-Si:H) films are described. The spectral density of the coplanar current fluctuations has a 1/f frequency dependence for frequency f from 1<f<104 Hz over the temperature range 300<T<450 K. The noise power density displays a power-law dependence on the dc current passing through the film, with a temperature-dependent power-law exponent. Random telegraph switching noise is observed in coplanar current measurements in samples with effective volumes of 10-6 to 10-7 cm3 with fluctuations as large as ΔR/R∼1%. Statistical analysis of these fluctuations indicates that the 1/f noise is strongly non-Gaussian, suggestive of cooperative interactions between fluctuators. A model is proposed in which the noise is dominated by inhomogeneous current paths whose local conductivity is modulated by bonding rearrangements enabled by hydrogen motion.
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bottom electrodes. Dynamics are introduced into this system by allowing a small fraction of the conducting bonds and adjacent broken bonds to switch places, keeping the total number of broken bonds fixed. The conductivity of the network is then recalculated and the procedure is repeated a large number of times, yielding a time record of the fluctuating resistance of the network. These dynamical percolation calculations simulate, in a crude manner, the influence of diffusing hydrogen on the current filaments in a-Si:H. The time trace of the conductance of the resistor network is shown in fig. 4 for a flip rate of 2.5 x 10 -6, defined as the fractional number of bonds switched per time step. The flip rate in fig. 4 corresponds to only (on average) four bonds exchanging places in a given time step. While we have not observed two-state telegraph noise, the time trace in fig. 4 does resemble the data in fig. 1, with discrete jumps of the conductance, along with sharp spikes in the current, which correspond to individual conducting filaments being turned on and off. That such conductance jumps can be observed for such a small flip rate is consistent with the suggestion that a small number of diffusing hydrogen atoms can influence ~ of a-Si:H over macroscopic length scales. Simulations involving more realistic models of hydrogen motion, having a distribution of special trap sites which modulate the bond flipping rate are presently underway and will be described in detail elsewhere \[13\]. This research was supported in part by the NSF DMR-9057722, E.P.R.I., and the Minnesota Supercomputer Institute; one of us (LML) was supported by a Dept. of Education fellowship. We gratefully thank C.C. Tsai of Xerox PARC for supplying the a-Si:H films.