Thursday, September 27, 2007

Novel Nanowires for Faster Memory

Tiny phase-change wires could serve as a key element in next-generation memory.

By B. Christine Hoekenga

Memory stick: Nanowires similar to the one shown here, which is made of germanium, antimony, and tellurium, could one day make ultrafast, nonvolatile memory possible. Credit: Ritesh Agarwal, University of Pennsylvania

Researchers are now one step closer to realizing the full potential of next-generation memory devices based on phase-changing material.

Ritesh Agarwal, an assistant professor of materials science and engineering at the University of Pennsylvania, and his colleagues have pioneered a new technique for producing phase-change nanowires. The technique could make it possible to build memory devices thousands of times faster and eight to ten times more energy efficient than memory currently on the market, such as flash.

The concept of using materials that change phase (from crystalline to amorphous and back) in order to read and write data is not new. Over the past few years, however, researchers have been limited by the lithographic process used to fabricate storage materials. When working with bits of material smaller than 100 nanometers, researchers found that the process damaged the material's surface, interfering with its capacity to efficiently store and retrieve data. The ability to pack together tiny bits of the material and run them in parallel is critical to creating efficient memory devices, and many researchers began asking if nanowires could be made small enough for this purpose and still reliably retain data, according to Agarwal.

In a paper published recently in Nature Nanotechnology, Agarwal and his team offer a new strategy for producing tiny strands of the phase-changing material needed for these devices. The study describes a nonlithographic method of growing and harvesting phase-changing nanowires made from the elements germanium, antimony, and tellurium. The researchers begin with the materials in powder form, heat them up until they vaporize, and then run the vapors over a piece of silicon studded with tiny particles of gold. As the vapors cool, the gold nanoparticles act as catalysts, "seeding" the wires, which grow into threads between 30 and 50 nanometers in diameter and 10 micrometers long.

After demonstrating that the technique was capable of producing these tiny strands, the researchers tested the speed at which the materials changed phases and assessed their ability to retain data over time. In current memory devices, these properties are usually at odds with each other. Normally, a mechanism for storing data that is fast is also volatile, meaning that it will lose data over time or if the power source is removed. A nonvolatile memory device usually takes longer to write and retrieve data. (Think of current digital cameras that zone out for a few seconds while they record each snapshot.)

By running experiments at high temperatures and extrapolating their results down to room temperature, the researchers found that the undamaged wires lived up to their expectations on both accounts. They estimated that the wires can hold data for roughly 100,000 years and clocked the phase changes required to store and retrieve data at 50 nanoseconds--roughly 1,000 times faster than flash memory. They also found that the data could be encoded using a very small amount of power, indicating that devices built with this technology will be energy efficient as well.