Bi-stable State for WORM Application Based on Carbazole-containing Polymer

Abstract

Recently, several studies have been done by various groups to understand the memory effects behind organic materials, which include understanding in terms of conformation changes, conjugation modification and oxidation-reduction process. 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 toluene solution of PCz was spin-coated on the ITO, followed by solvent removal in a vacuum chamber. Finally, Al was thermally evaporated through shadow mask onto the PCz film.

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 linker between the carbazole groups and backbone present in PCz which play a part in the conduction mechanism.