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the demand for smaller, lighter and more compact memory devices increases, the need for alternative memory devices besides conventional semiconductor technology becomes increasingly important. Among the various emerging trends in the area of
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organic electronics, the use of polymer devices has been widely explored in the applications of switches, wires, lasers, sensors, photovoltaic cells and light emitting diodes. Polymer memories have also become an active research topic in recent years. A number of physical and economic factors, which threaten the further scaling down of silicon semiconductor devices, has motivated research into organic electronics. The advantages of polymer materials lie in their unique properties, such as good mechanical strength, flexibility, ease of processing and most importantly low cost. Instead of using sophisticated and expensive equipment and processes, polymer memory devices use solution processes such as spin coating for the deposition of polymers on a variety of substrates, such as wafers, plastic, glass and metal foils.
In many fabricated devices, a metal-insulator-metal sandwich structure is used. An electrical bistability phenomenon is observed for such devices based on a conjugated copolymer containing fluorine and chelated europium complex. These devices exhibit a write-once read-many times (WORM) memory behavior, owing to the deep trap center caused by the europium electron acceptor group (Figure 1). Before the transition to the ON-state, the current is controlled by Schottky emission from the electrode to the polymer layer, due to the energy barrier between the electrode and the polymer layer in the form of a Schottky diode. When the applied voltage is high enough, the trapped charges increase and the device shows a high current, an indication of an ON-state. The ON/ OFF current ratio is of six orders of magnitude. The rectifying effect of another WORM device based on the conjugated copolymer of fluorine and europium complex was also studied. The polymer
is now sandwiched between the top Al and bottom n-type silicon. The memory device shows an ON/OFF current ratio of four orders of magnitude and a rectification of two orders of magnitude in the ON-state. The non-linear rectifying property of memory devices is of great importance to address a given memory cell in two dimensional memory arrays, by reducing crosstalk.
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Figure
1: (a) Schematic diagram of a memory device consisting of PF6Eu (~ 40nm) sandwiched between an Al top electrode and n-type silicon substrate with Ohmic contact. (b) I-V characteristics of the device in the ON and OFF states, in forward and reverse bias, respectively. |
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Another area of the research evaluates the bistable WORM memory behavior of an acrylate polymer using the conformational change that happens within the polymer under an electric field. The memory properties of the acrylate polymers are attributed to the presence of the spacer between the pendant carbazole group and the polymer backbone. In the acrylate polymer with a suitable length molecular spacer, the molecular spacer allows the transition of the pendant carbazole groups from the disoriented to the ordered face to face configuration. The face-toface conformation change is the requisite for charge hopping and bistable memory behavior.
We also studied an electrical-bistability device based on MIM-sandwiched structure using gold nanoparticles (GNPs). Poly(N-vinylcarbazole) (PVK) mixed with GNPs serve as an active layer between 2 electrodes. After applying a voltage, the fabricated device can transit from the low-conductivity to high conductivity state (Figure 2). By simply using a reverse bias, the high conductivity state can return to the low state. An ON/OFF current ratio of five orders of magnitude was achieved. The memory effect is attributed to an electrical-field-induced charge transfer complex formed between the PVK and the GNPs.
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Figure
2: SEM image of an exchange biased PHE sensor with on-chip tapered current lines. |
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This work was done in collaboration with Prof ET Kang from Dept of ChBE and graduate students Y Song, YP Tan and research fellow EYH Teo.
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CX
Zhu is an Associate Professor in the Department of Electrical and Computer Engineering. His research is focused mainly on electronic devices made with different kinds of semiconductor materials.
Email: elezhucx@nus.edu.sg |
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