Analogue Memory Effects in Metal/ a-Si:H /Metal Thin Film Structures
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ANALOGUE MEMORY EFFECTS IN METAL/ a-Si:H /METAL THIN FILM STRUCTURES
A.J. Snellt, J. Hajtot, M.J. Rose*, I.S. Osborne*, A. Holmest, A.E. Owent and R.A.G. Gibson* t Department of Electrical Engineering, University of Edinburgh, Edinburgh EH9 3JL U.K. * Department of A.P.E.M.E., University of Dundee, Dundee DDl 4HN U.K. ABSTRACT
The ac conductivities of non-volatile analogue memory states are measured in electro-formed Cr/p+/V amorphous silicon structures for a broad frequency range (from 0.1 Hz to 32 MHz). The results suggest that the memory action is associated with electronic processes. INTRODUCTION
We have previously shown that a-Si:H metal/p+/metal amorphous silicon structures exhibit non-volatile, polarity dependent analogue memory phenomena after an initial forming process by means of a moderately high applied potential",2 . An essential feature of this 'forming' process is the creation of a filamentary region of highly conducting material. The filament is associated with local diffusion of the of the top metal contact into the a-Si:H, resulting in a region of mixed metal and silicon of unknown composition. The role of the top metal contact has been thoroughly investigated by fabricating devices with a range of different top contacts but with otherwise identical parameters. The analogue switching voltage window AV was found to be significantly dependent on the top metallization contact 3 . Due to the lack of any precise information about the geometry and composition of the filament it is not easy to determine the physical mechanism uniquely responsible for the analogue switching phenomenon3 . In this paper we summarize new experimental results aimed at giving a better understanding of the electronic process associated with the analogue memory action. We have concentrated on the Cr-p+-V devices i.e. devices with vanadium top contact, because these show typical non-volatile analogue memory switching between any two memory states within the range from -1 k(2 to -1 MQ by selecting the correct polarity and magnitude (- 2-6 Volt) of single pulses of 100 nsec duration 3 . EXPERIMENTAL METHOD
The samples used for this work were a-Si:H Cr-p+-V sandwich structures having an active area defined by a pore 10 rm in diameter, the preparation and geometry of which have been described elsewhere 3. The structures have an initial low voltage resistance of - 109 fQ which can be lowered to - 103 Q by applying a sequence of 300 nsec voltage pulses of increasing magnitude up to - 14 V. The positive polarity is applied to the vanadium top contact. After this forming step the devices exhibit fast analogue switching i.e. they show a continuum of non-volatile states between RON = 103 Q and RoF = 106 j under the influence of single voltage pulses of 100 nsec duration, with magnitudes from 2 to 7 Volt. The analogue memory samples were then switched into different analogue memory states and their AC-behaviour were measured by a Schlumberger 1260 impedance analyzer over a frequency range from 0.1 Hz to 3.2x10 7 Hz. The amplitude of the
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