Bi-stable State for WORM Application Based on Carbazole-containing Polymer
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Bi-stable State for WORM Application Based on Carbazole-containing Polymer Eric Yeow Hwee Teo1, Qidan Ling2, Yan Song1, Yoke Ping Tan1, Wen Wang2, En-Tang Kang2, DANIEL SIU HUNG CHAN1, and Chunxiang Zhu1 1 Department of Electrical and Computer Engineering, National University of Singapore, 10 Kent Ridge, Singapore, 119260, Singapore 2 Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge, Singapore, 119260, Singapore ABSTRACT In this paper, a WORM (write-once read-many times) memory device using a new polymer material 2-(9H-carbazol-9-yl)ethyl methacrylate (PCz) containing carbazole donor group has been demonstrated. The device uses a MIM (metal-insulator-metal) structure with ITO coated glass as bottom electrode, the synthesized polymer material PCz as the active layer and Al as the top electrode. The memory effect of PCz was observed in the I-V characteristic of the MIM structure. The as-fabricated device is found to be in its OFF state, and can be programmed to ON state which is not reversible. The WORM device exhibits a high ON/OFF current ratio of up to 106, and shows a good retention time for both the ON and OFF states which can be sustained within a 24 h timeframe, and extrapolated to sustain for another 10 years. The effect of continuous read pulse on the ON and OFF states was evaluated and no resistance degradation is observed for read cycles up to 107 times. By comparing the electrical characteristics of PCz and PVK as well as their optimized geometry simulation corresponding to their minimized energy states, the memory effect or bi-stable states of PCz can be attributed to the long molecular spacer between the carbazole groups and backbone present in PCz which play a part in the conduction mechanism. INTRODUCTION Recently, several excellent studies on functional memory devices based on organic materials have been reported [1-6]. For an organic material to function viably as a memory device, it must possess a high ON/OFF current ratio between the conducting and non-conducting states in order to minimize any error during the read cycle, and the retention time in both ON and OFF states must be sufficiently long. Several studies devoted to the understanding of memory effects in organic materials include the understanding of conformation change [7], modification of conjugation [2,8] and oxidation-reduction process [4-6]. Among the reported non-volatile memory devices fabricated from polymers, the WORM type memory devices show the best retention time, which is critical to real applications. In this study, we show that an acrylate polymer containing the carbazole donor moiety in the pendant groups (PCz) also exhibits bistable states for WORM memory applications. It is well known that carbazole-containing polymers are positive charge (hole) transporting materials [9] because (1) carbazole groups form relatively stable radical cations (holes), (2) some carbazole-containing compounds exhibit relatively high charge carrier mobility, and (3) different substi
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