Filament formation in switching devices based on V 2 O 5 gel films
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P. C. Eklund Department of Physics & Astronomy and Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky 40506 (Received 3 September 1992; accepted 18 November 1992)
Filament formation in V 2 O 5 gel films leading to a two-terminal switching device has been observed. In a previous paper, we have identified the switching with a metal-insulator transition within a permanent, current-induced channel between the electrical contacts. Here, we describe the reversible formation of a filament inside the channel, and obtain a static solution of the heat transport equation for this device which indicates a large temperature variation within the filament, and further reveals the failure mechanism for the reversible switching. The principle of least entropy production has been used to analyze the filament growth, which results in a successful simulation of the "on" state I-V characteristic of the switching process, including its negative resistance region.
I. INTRODUCTION In 1981, a two-terminal switching device based on semiconducting, lamellar V 2 O 5 • 1.6 H 2 O xerogel films was patented by Bullot and Livage1 and reported2 soon thereafter. Several years later, we reported the observation that the electrical switching in this device was not intrinsic to the xerogel film, but was associated with the irreversible formation of a current-induced channel of VO2+X between the electrical contacts3 (cf, Fig. 1). The electrochemical and/or thermal mechanism by which H2O and oxygen is removed in this channel region between the contact is not known. This channel was found3 to exhibit a metal-insulator (M-I) transition at 334 K, close to the M-I transition of VO 2 (341 K2; the metallic (M) state exists above this transition temperature). Thus, we identified the electrical switching in this system with a joule-heating-induced transition from the "off state (I) to the "on" state (M) in the channel. In this paper, we provide a detailed analysis of the switching mechanism that is attributed to current conduction within a small portion, or filamentary path, within the VO2+X channel (Fig. 1). The thermal conductivity of the substrate and channel, together with the magnitude of the current, determines the cross-sectional diameter of the filament. Thus, the heat transport equation must be solved in order to determine the I-V characteristic for the switch. Current-induced filament formation has been reported in various materials. The first report of a currentcontrolled negative resistance in filamentary devices was made by Tyler in 1954 on iron-doped germanium at liquid-nitrogen temperatures,4 and Lampert5"7 developed a "double-injection" theory for the negative resistance effect in 1962. Later, Barnett and Milnes8"10 extended Lampert's theory and analytically described the fila558 http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 3, Mar 1993 Downloaded: 18 Mar 2015
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V02+x Channel FIG. 1. The schematic of V2O5 • 1.6 H2O xerogel film device showing the VO2+X channel and the filamentary metall
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