Improvements in performance and reductions in cost of silicon-based nonvolatile memories, such as flash random access memory (flash-RAM), have rendered floppy discs and many other forms of portable storage obsolete. But for many uses that require data to be written only once, such as archiving and security applications, their cost effectiveness is limited. So-called write-onceread- many (WORM) memories made from low-cost polymer materials could provide a solution.

The devices, developed by Professors Moonhor Ree, Ohyun Kim, Su-Moon Park and their research teams, are based on hyperbranched copper phthalocyanine (HCuPc) polymer. The starting material of the HCuPC polymer is copper phthalocyanine (CuPc) which is a common organic semiconductor. The CuPc material, which is usually grown by vacuum deposition techniques, does not usually exhibit switching behaviour that could be used for data storage. But when fabricated from solution to form thin polymer films, the HCuPC reveals electrical switching characteristics.

The HCuPc polymer films initially exhibited a high-conductivity state (ON-state). The conduction processes within the device were found to be dominated by hole injection rather than electron injection. The active layer in the devices is short-circuited during the ON-state. Specifically, when a voltage of just over 2.5 V is applied across such an HCuPC film sandwiched between indium-tin-oxide and gold contacts, it switches from the ON-state to low conductivity state (OFF-state). This switching-OFF process of the devices is influenced by the magnitude of the injection current. This switching-OFF process is thought to be governed by the rupture of filaments which takes place when a voltage greater than the turn-off voltage is applied. The conductivity of the OFF-state was found to be more than millions of times smaller than the ON-state, making it easy to read the state of a device. And once switched to the OFF-state the devices remained permanently in that state, even when tested again a full year later.

The simplicity of the devices and the fact that the films of which they are based can be fabricated from solution, not only makes them potentially much cheaper than silicon-based memories, it could enable them to store much larger amounts of data more efficiently.

The density of the data that can be stored in silicon-based memories can only be improved by making the size of individual memory cells smaller. This is because they can only be fabricated in two-dimensions on the surface of a silicon chip. In contrast, because the HCuPC films are fabricated from solution, it should be possible to build 3-dimensional (3D) arrays of devices by spincoating or dip-coating multiple layers, to achieve very high storage densities.

Overall, the HCuPc devices exhibited excellent WORM memory characteristics. These properties open up the possibility of a low-cost mass production of highly dense, and very stable digital non-volatile WORM memory devices.

These research results were published in the journal Advanced Materials (volume 20, 1766-1771, 2008), and further highlighted by Nature Asia Materials (June 4, 2008).

Professor Moonhor Ree
Department of Chemistry
Tel: +82-54-279-2120
Fax: +82-54-279-3399
E-mail: ree@postech.edu

Professor Su-Moon Park
Department of Chemistry
Tel: +82-54-279-2102
Fax: +82-54-279-3399
E-mail: smpark@postech.edu

Professor Ohyun Kim
Department of Electronic and Electrical Engineering
Tel: +82-54-279-2215
Fax: +82-54-279-2903
E-mail: ohkim@postech.edu