Memory Switching in Ion Bombarded Amorphous Silicon Carbide Thin Film Devices
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Memory Switching in Ion Bombarded Amorphous Silicon Carbide Thin Film Devices R.G. Gateru1, J.M. Shannon and S. R. P. Silva Advanced Technology Institute, School of Electronics and Physical Sciences, University of Surrey, Guildford, Surrey, GU2 7XH, United Kingdom
ABSTRACT
Metal-semiconductor-metal (MSM) memory devices have been made in hydrogenated amorphous silicon carbide (a-SiCx:H). Switching in silicon-rich amorphous silicon alloys is believed to be related to the concentration of defects in the semiconductor material. We have used ion bombardment to introduce a controlled number of damage centres into amorphous silicon carbide. Both analog and digital switching behaviours are reported.
INTRODUCTION
Electrical switching has been observed in a wide range of materials ranging from chalcogenide glasses [1] to nickel oxides [2], amorphous silicon [3], and hydrogenated amorphous silicon (aSi:H) [4]. Memory switching devices of a-Si:H and its alloys are of great interest due to their potential use in compact high density non-volatile memory arrays. The use of large area technology also leads to low production costs. However, various difficulties such as lack of reproducibility and a low switching ratio have been reported in a-Si:H devices. Memory switching is said to occur when a solid-state device, usually a metalsemiconductor-metal (MSM) structure, changes from a non-conducting (OFF) state to a highly conducting (ON) state by the application of an appropriate electric signal. The change is permanent (unchanged over a long period of time) making these devices non-volatile memory devices. It is believed that formation of the memory element is related to the presence and density of defects in the semiconductor material [5]. Current before pulse stressing is controlled by the Schottky barriers at both junctions of the MSM device [6]. However, during current stressing, defects are generated in the bulk of the semiconductor material driven by the energy released during band-to-band recombination of carriers [7]. After prolonged stress, the defect density is high enough to create a defect band and current flows through these defect states via hopping in a Poole-Frenkel type conduction mechanism. Conduction then changes from contact controlled – characterised by asymmetric J-V characteristics – to one where transport through the bulk of the semiconductor is the dominant process. Here the J-V characteristics are symmetric
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with the polarity of the bias voltage. The current has been reported to increase with increase in the defect density introduced in the material [5]. Hydrogenated amorphous silicon carbide has been shown to possess several material properties that make it quite interesting for switching investigations as compared to other a-Si:H alloys, especially a-SiNx:H. Firstly, it has been shown that in the unstressed form, a-SiCx:H has a much lower density of charged defects than
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