June 19, 2002 Researchers at the UCLA School of Engineering have created an organic, nonvolatile memory device that is cheaper and faster than those currently in use.
Associate Professor Yang Yang from the Materials Science and Engineering Department, who leads the research at the Henry Samueli School of Engineering and Applied Science, will present his findings to the International Conference on Science and Technology of Synthetic Metals in Shanghai on June 29.
According to Yang, such devices "have tremendous potential" for use in personal computers, personal digital assistants and digital cameras because of their lighter weight and faster response times. They are up to 3 million times faster than conventional, nonvolatile memory and can be manufactured less expensively.
The triple-layer devices are composed of an embedded metal layer between two organic films. This package is coupled to two electrodes. A positive voltage is applied to turn on the device. This transition from the off state to the on state is equivalent to the "writing" process.
The device remains in this state even when the power is turned off. In tests, the devices remained in the on state for several days to weeks. So the device can be used for applications in which it is important to retain whatever is in memory, but is not practical to keep the power turned on constantly.
By simply applying a reverse voltage pulse, the memory can be reset to the off state. This is the equivalent of the "erasing" process.
The organic memory device operates on the principle of electrical bistability, a phenomenon in which an object exhibits two states of different conductivity at the same applied voltage. Because it is made of organic material and exhibits electrical bistability, it is known as an Organic Bistable Device (OBD).
This behavior is also ideal for switching and memory applications. In addition to the rewriting capability, the retention of the on and off states and its performance under stress are important for practical applications, Yang said.
The precise control over the on-off states, multiple rewriting ability and device stability are the key issues for applications such as digital cameras.
"More than 3 million write-erase cycles were conducted on our OBD with good rewritable characteristics," Yang said.
Electrical bistability in organic materials has been studied in the past and attributed to various mechanisms. "The major difference in our OBDs is the presence of the embedded metal layer within the organic films," Yang said.
In fact, Yang said the electrical bistability of the device can only be observed when the metal layer is of a certain critical thickness. "When the layer is too thin or in the absence of the metal layer, the bistability phenomenon disappears," he said. Experiments have been conducted using aluminum, silver, gold and copper, Yang said. However, "the precise mechanism is still under investigation."
This research supported by the Office of Naval Research, the Air Force Office of Scientific Research and the National Science Foundation.
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