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1 / f NOISE IN HYDROGENATED AMORPHOUS SILICON C. Parman and J. Kakalios The University of Minnesota, School of Physics and Astronomy, 55455 USA

Minneapolis, MN

ABSTRACT Measurements of co-planar current fluctuations in n-type doped hydrogenated amorphous silicon (a-Si:H) find that the spectral density of the noise accurately obeys a 1/f frequency dependence over the frequency range of 1 Hz to 1 kHz for temperatures ranging from room temperature to 450K. The noise displays a power law dependence on the d.c. curent passing through the sample, with a In addition, the resistance of the atemperature dependent power law exponent. Si:H as a function of time displays switching phenomena; a surprising result 3 given the effective volume ( -10-6 cm ) of the sample. INTRODUCTION The performance of hydrogenated amorphous silicon (a-Si:H) based An understanding of electronic devices is ultimately limited by electrical noise. the microscopic mechanisms responsible for noise in amorphous semiconductors is therefore crucial if these materials are to fulfill their technological potential. Recent proposals 1 have suggested that the same physical process responsible for The flicker or I/f noise can also account for stretched exponential relaxation. latter phenomena has been attributed in a-Si:H to the dispersive diffusion of the 2 The initial motivation of the present study was bonded hydrogen in the material. to determine whether hydrogen motion did indeed underlie both 1/f noise and stretched expoential decay in a-Si:H. A wide variety of experimental systems display conductance fluctuations 1 A basic assumption of which vary as 1/f over a large range of frequency f. ,3-5 many models for 1/f noise, realized in most though not all experimental systems, is that the current passing though the material does not cause the noise, but is This "linear" criterion yields, for simply a probe of the conductance fluctuations. Ohmic systems, a noise spectral density which varies as the square of the In contrast, the noise in ameasurement parameter (e.g. voltage, current, etc.). Si:H is highly non-linear, that is, the noise spectral density displays a power-law dependence on the d.c. current passing through the sample, with a temperature 6 One possible explanation for this result is that dependent power-law exponent. the noise arises from inhomogeneous current paths, possibly due to the hydrogen Further support for this proposal is the microstructure in the amorphous silicon. observation of switching events in the time record of the co-planar resistance of a-Si:H. EXPERIMENTAL TECHNIQUE The flicker noise measurements were performed using standard two probe and four probe methods. In the two probe configuration a constant voltage is applied across the co-planar electrodes (length 0.3 cm, separation 0.1 cm) and the The fluctuations in the current passing through the a-Si:H sample are measured. current is amplified (Ithaco 564) and sent to a spectrum analyzer (HP 3564A) where the current spectral density SI is calculated for a frequency range of 0