Resistance fluctuations in hydrogenated amorphous silicon: Nonthermal equilibrium

Abstract

We present and discuss the first noise measurements on hydrogenated amorphous silicon (a-Si:H) under nonthermal equilibrium conditions. Under steady-state illumination noise measurements are carried out for temperatures ranging from 100 to 450 K. We conclusively identify generation-recombination noise as the prevailing noise mechanism in intrinsic a-Si:H. We examine the dynamics of holes with noise spectroscopy in n-type devices that include thermal activation from the hole quasi-Fermi level, a clear Meyer-Neldel relation pointing to a temperature dependence of the hole quasi-Fermi level, and diffusion-limited transport to recombination centers at low temperatures. The variance is found to be independent of illumination intensity, indicative of a uniform distribution of defect states throughout the gap. Noise measurements further suggest that metastable defects are created close to midgap in degraded samples. Under conditions of electron injection we find that the noise becomes space-charge suppressed, similar to what has been observed in crystalline semiconductors. ͓S0163-1829͑98͒07131-8͔

Generation–recombination noise studied in hydrogenated amorphous silicon

“…The response of the setup is frequency independent for all measurements presented. Spurious contact eects were determined to be absent in the temperature range studied and for the current densities used [6,7]. All measurements are performed in the ohmic regime and had a noise intensity that scaled quadratically with current at least to moderately high temperatures (<420 K).…”

“…All measurements are performed in the ohmic regime and had a noise intensity that scaled quadratically with current at least to moderately high temperatures (<420 K). We ®nd that the noise is Gaussian [6,7].…”

“…The circular top contact has a diameter of 3 mm and is only 15 nm thick to allow for illumination of the intrinsic layer with visible light. This geometry was found to be the best compromise for a study of intrinsic a-Si:H over a range of temperatures and illumination intensities [6,7]. The device-quality a-Si:H ®lms were deposited on Corning 7059 glass using plasma enhanced chemical vapor deposition (PECVD) at a substrate temperature of 470 K. The device is glued on a copper block using thermally conductive paste and mounted in a temperature controlled vacuum vessel with optical and electrical access.…”

“…However, Lee et al [5] predicted that g±r processes in amorphous semiconductors should produce near Lorentzian spectra, containing physical information as with crystalline semiconductors. Recently we demonstrated that resistance¯uctuations in a-Si:H are governed by g±r noise, both in thermal equilibrium and in non-thermal equilibrium [6,7]. In this paper we exploit noise spectroscopy to make the dynamics of charge carriers and the properties of dangling-bond defects in an amorphous semiconductor accessible for measurements under thermal equilibrium conditions.…”

Spin‐dependent transport in elemental and compound semiconductors and nanostructures

“…While in CDMR at a test frequency of 1 MHz, only transitions involving conduction band tail states and dangling bond defects are observed [24,25], NDMR experiments show that electronic noise in this material is dominated by the trapping of holes with a characteristic generation-recombination lifetime of 20 kHz [26,27]. Therefore, NDMR has allowed to unambiguously prove that holes are the main source for generation-recombination noise in amorphous silicon [28].…”