Threshold Voltage and Field for Metal Filament Formation in Hydrogenated Amorphous Silicon
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THRESHOLD VOLTAGE AND FIELD FOR METAL FILAMENT FORMATION IN HYDROGENATED AMORPHOUS SILICON P. Stradins, H. M. Branz, W. B. Jackson1, R.S. Crandall, J. Hu, and Q. 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 Electrical switching due to metallic filament formation in hydrogenated amorphous silicon (a-Si:H) is studied in metal/a-Si:H/metal structures. We examine the effects of a-Si:H switch layer thickness, applied voltage and polarity, metal contact material, and contact interface properties. For switching, the voltage applied to the contacts must be large enough to establish: 1) a minimum threshold voltage of about 2V at the contacts and 2) a bias field of about 1 MV/cm in the bulk. Changing contact material and polarity strongly affects the switching behavior.
INTRODUCTION Switching from a high to a low resistance state in semiconductors and insulators has attracted strong attention as a potential element of an inexpensive thin-film memory technology. Working switch-diode and CMOS devices have been demonstrated [1-3] , an important step toward write-once memories. The physics of the switching is interesting and far from clear. For a-Si:H sandwiched between two metallic contacts it has been demonstrated that switching is accompanied by metallic filament formation [4-6]. For Cr/a-Si:H/Ag devices the filaments are formed by the mixture of metals from both contacts [5]. In TiN/a-Si:H/TiN system, there are strong indications that filament formation starts at the negative electrode [3]. The above studies investigated only the final state of the sample, i.e., after the switching has been detected electronically, the sample current is interrupted and the metallic filament is completely formed. The interface at which switching is initiated may not correspond to the contact from which metal first diffuses. Metallic filament formation is the final result, but not necessarily the cause of the switching. Initiation of switching is likely of electronic origin and has not been studied in detail. It has been shown that to switch, the current-supplying contact should be blocking [7]. Switching depends strongly on voltage polarity in Cr/a-Si:H/Ag [5]. To unravel the mechanism of filament formation, it is necessary to understand the electronic conditions for the initiation of switching.
EXPERIMENTAL The a-Si:H switch layers were deposited by hot wire chemical vapor deposition from SiH4 gas at a rate about 10 Å/s onto Corning 7037F glass substrates previously coated with electron beam deposited metal layers of Cr or TiW. The intrinsic a-Si:H layer had a nearly linear thickness wedge from 9 to 100nm across the 25 mm of sample length which was deposited using a moving shutter in our combinatorial HWCVD reactor [8]. The film thickness profile was
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determined by optical reflectance and transmission spectroscopy (n&k Analyzer 1280). Wedges were made at substrate temperatures of 25, 60 and 1
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