Time-Resolved Switching Studies in a-Si:H and Related Films
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Time-resolved switching studies in a-Si:H and related films P. Stradins, W. B. Jackson1, H. M. Branz, J. Hu, C. L. Perkins, and Qi Wang National Renewable Energy Laboratory, 1617 Cole Blvd. Golden, Colorado 80401, USA Hewlett Packard Laboratories, 1501 Page Mill Rd., Palo Alto, CA 94304, USA
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ABSTRACT Switching in a-Si:H and a-Si:HNx layers is investigated by pulse current transient and Auger scanning microspectroscopy measurements. Switching in a-Si:H with Ag and Cr contacts exhibits 2 different regimes depending on the voltage pulse polarity. With a positive top Ag contact, switching occurs in nanoseconds after a certain latency time, which depends on voltage exponentially. For a negative Ag contact, there is no latency time provided the voltage exceeds a certain critical value. This might be related to interface effects on contact properties or fieldassisted metal diffusion. Scanning Auger element micromaps reveal metallic filaments in the switched films. They contain both Ag and Cr throughout the film thickness. Two phases of the filament formation are suggested – a precursor phase and a post-switching phase characterized by local heating and atomic diffusion. Soft and hard switching are observed in a-Si:HNx films simultaneously and their rates depend strongly on the contact material and applied voltage. Soft switching might be related to the charge trapping in this wide bandgap material. INTRODUCTION Switching in a-Si:H based devices is an interesting physical phenomenon and has a potential for electronic technology[1, 2]. Despite the recent progress, there is no satisfactory explanation of the switching mechanism, particularly in a-Si:H layers with metallic contacts. It is not clear whether the effect is caused by the avalanche-type electrical breakdown, current, thermal runaway, charge accumulation or their combination. The role of contact and interface properties also needs further understanding. There is experimental evidence that switching is accompanied by the formation of submicron diameter filaments extending from the electric contacts into the amorphous switching layer [3-5]. Metal atoms, Ag in particular, are assumed to migrate into these filaments and form permanent conductive paths[6]. Nevertheless, spatially resolved structure analysis has not yet been performed to establish the existence and composition of such metallic filaments on a microscopic level. Lastly, wide badgap materials such as a-Si:HNx are of interest as switches because of their very low conductivity and ability to accumulate charges at room temperature. The above problems are addressed in this work. EXPERIMENTAL Devices studied in this work consisted of an amorphous switching layer sandwiched between two conducting electrodes. Both intrinsic and 2000ppm p-type a-Si:H switching layers were 100nm thick while a-Si:HNx was 30nm. These samples were prepared in our standard hot filament CVD reactor at a rate of several Å/s at a typical temperature of 160 oC. A Cr film on glass served as a bottom contact for a-Si:H switches. Fo
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