As technology continues to get smaller, and as memory needs become more demanding, the microelectronics industry requires devices that are more cost-efficient and lightweight. And, while organic memory materials have shown some promise on improvement in performance and reductions in cost, they still lack some of the essential qualities, such as durable performance even in a harsh condition, needed for application in a wide variety of fields. So-called Non-Volatile Memories (NVM) from thermally and dimensionally stable polymer materials could provide a solution.

The devices, developed by Professor Moonhor Ree, Professor Ohyun Kim, Dr. Suk Gyu Hahm, and their research teams, are based on a thermally and dimensionally stable polyimide containing carbazole moieties. The polyimide material, 6F-HAB-CBZ PI, is thermally stable from -200 up to 400oC, and the device performance is also robust even in a space temperature range from -120 to 150oC. For a common organic semiconductor, organic active materials are usually grown by delicate vacuum deposition techniques and do not usually exhibit switching behaviour used for data storage when they are exposed to an ambient condition because of its weak sustainability. But when fabricated from solution to form thin polymer films, the 6F-HAB-CBZ PI reveals perennial electrical switching characteristics.

The new polyimide, 6F-HAB-CBZ PI, exhibits excellent unipolar ON and OFF switching behavior. The PI film is initially present in the OFF state. The PI film can be electrically switched on by applying a positive or negative bias with a current compliance set at a low level, and then switched off by applying a positive or negative bias with a current compliance set at a higher level than the turn-on current compliance. The ON-switching voltages are in the ranges of +1.7V to +2.7V during the positive voltage sweep and -1.7V to -2.7V during the negative voltage sweep, and the OFF-switching voltages are in the ranges of +0.3V to +0.7V during the positive voltage sweep and -0.3V to -0.7V during the negative voltage sweep. Overall, these ON- and OFF-switching voltages are very low. These low switching voltages have the advantage for memory device applications of low operation voltages and hence of low power consumption. With this very low power consumption, the device can be repeatedly written, read and erased in air. The ON/OFF current ratio of the devices is in the range 103-1011, depending on the level of the turn-on compliance current and the reading voltage. A higher turn-on compliance current and a lower reading voltage result in a higher ON/OFF current ratio. In particular, the ON/OFF current ratio of 1011 is the highest value reported so far.

The ease of the devices and the fact that the films on which they are based can be fabricated through solution not only enable them to be fabricated with potentially much lower cost than silicon-based memories, but also make it possible to build 3-dimensional (3D) arrays of devices by spin-coating or dip-coating multiple layers to achieve very high storage densities, considering that silicon-based memories can only be improved by making the size of individual memory cells smaller because of its dimensional limitation where it can only be fabricated in two-dimensions on the surface of a silicon chip.

Overall, the high ON/OFF switching ratio and stability of these devices, as well as their repeatable writing, reading and erasing capability with low power consumption, open up the possibility of mass production of high performance non-volatile memory devices at low cost.

These research results were published in the journal Advanced Functional Materials (volume 20, 1766-1771, 2008).

Professor Moonhor Ree
Department of Chemistry
Tel: +82-54-279-2120
Fax: +82-54-279-3399
Email: ree@postech.ac.kr

Professor Ohyun Kim
Department of Electronic and Electrical Engineering
Tel: +82-54-279-2215
Fax: +82-54-279-2903
Email: ohkim@postech.ac.kr

Dr. Suk Gyu Hahm
Department of Chemistry
Tel: +82-54-279- 2922
Fax: +82-54-279- 3399
Email: hamsg@postech.ac.kr