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Lee, Taek Joon,Chang, Cha-Wen,Hahm, Suk Gyu,Kim, Kyungtae,Park, Samdae,Kim, Dong Min,Kim, Jinchul,Kwon, Won-Sang,Liou, Guey-Sheng,Ree, Moonhor IOP Pub 2009 Nanotechnology Vol.20 No.13
<P>We have fabricated electrically programmable memory devices with thermally and dimensionally stable poly(<I>N</I>-(<I>N</I>′,<I>N</I>′-diphenyl-<I>N</I>′-1,4-phenyl)-<I>N</I>,<I>N</I>-4,4′-diphenylene hexafluoroisopropylidene-diphthalimide) (6F-2TPA PI) films and investigated their switching characteristics and reliability. 6F-2TPA PI films were found to reveal a conductivity of 1.0 × 10<SUP>−13</SUP>–1.0 × 10<SUP>−14</SUP> S cm<SUP>−1</SUP>. The 6F-2TPA PI films exhibit versatile memory characteristics that depend on the film thickness. All the PI films are initially present in the OFF state. The PI films with a thickness of >15 to <100 nm exhibit excellent write-once-read-many-times (WORM) (i.e. fuse-type) memory characteristics with and without polarity depending on the thickness. The WORM memory devices are electrically stable, even in air ambient, for a very long time. The devices’ ON/OFF current ratio is high, up to 10<SUP>10</SUP>. Therefore, these WORM memory devices can provide an efficient, low-cost means of permanent data storage. On the other hand, the 100 nm thick PI films exhibit excellent dynamic random access memory (DRAM) characteristics with polarity. The ON/OFF current ratio of the DRAM devices is as high as 10<SUP>11</SUP>. The observed electrical switching behaviors were found to be governed by trap-limited space-charge-limited conduction and local filament formation and further dependent on the differences between the highest occupied molecular orbital and the lowest unoccupied molecular orbital energy levels of the PI film and the work functions of the top and bottom electrodes as well as the PI film thickness. In summary, the excellent memory properties of 6F-2TPA PI make it a promising candidate material for the low-cost mass production of high density and very stable digital nonvolatile WORM and volatile DRAM memory devices. </P>
Ko, Yong-Gi,Kwon, Wonsang,Yen, Hung-Ju,Chang, Cha-Wen,Kim, Dong Min,Kim, Kyungtae,Hahm, Suk Gyu,Lee, Taek Joon,Liou, Guey-Sheng,Ree, Moonhor American Chemical Society 2012 Macromolecules Vol.45 No.9
<P>A series of aromatic polyimides (PIs) were synthesized via the polymerization of 3,3′,4,4′-diphenylsulfonyltetracarboxylic dianhydride with 4,4′-diaminotriphenylamine derivatives containing hydrogen, cyano, methoxy, or dimethylamine substituents. These PIs were thermally and dimensionally stable and produced high-quality thin films when applied in a conventional spin-coating process. Their structure and properties were characterized. Nanoscale thin films of the PIs demonstrated excellent electrical memory performance, with high stabilities and ON/OFF current ratios. The memory characteristics were found to be tunable by varying the substituents; nonvolatile write-once–read-many-times memory behavior, nonvolatile ON/OFF switching type memory behavior, and volatile dynamic random access memory behavior were observed. The memory characteristics were substantially influenced by the electron-accepting cyano- and electron-donating dimethylamine substituents but were apparently not affected by the electron-donating methoxy substituent. In addition, the film density was a significant factor influencing the observed memory behaviors, with larger film densities causing lower OFF-current levels. However, the critical switching-on voltage varied very little as the substituents were changed and was measured to be approximately ±2 V. All of the memory behaviors were found to be governed by a mechanism involving trap-limited space-charge-limited conduction and local filament formation. Overall, all of the PIs assessed in the present work were found to be suitable active materials for the low-cost mass production of high-performance, programmable unipolar memory devices that can be operated with very low power consumption, high ON/OFF current ratios, and high thermal and dimensional stability.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/mamobx/2012/mamobx.2012.45.issue-9/ma300311d/production/images/medium/ma-2012-00311d_0004.gif'></P>