Atomic-Level Mechanisms of Phase-Change Memory Materials Revealed

Scientists from the University of Cambridge in the UK have uncovered the atomic-level interactions that occur when a class of “phase-change” memory materials stores information. Their work, reported in the March 23 online edition of Nature Materials, may open up new avenues for research into these fascinating materials, possibly leading to a new generation of “super” memory materials for electronic devices.

The materials studied are each composed of the elements germanium, antimony, and tellurium (Ge, Sb, and Te). They are used in devices that can retain information even when powered off, such as rewritable optical DVDs and a new type of electrical random-access memory intended to be a replacement for Flash memory, which is used in memory sticks, digital cameras, cell phones, and other portable devices.

The materials work by undergoing changes in their atomic structure. To store information, the structure rapidly switches from amorphous (disordered) to crystalline (ordered) in response to either an optical or electrical pulse. When the information is erased, the structured becomes amorphous again.

Despite many theoretical and experimental studies to understand the phase-change mechanism, the microscopic interactions have remained a mystery. This is due in part because the phase changes occurs so quickly, over only about one nanosecond, and the dimensions of the materials in the active regions on the devices are so small, only about 10 nanometers.

“For the first time, we have determined what goes on at the atomic level when one of these materials, Ge₂Sb₂Te₅, stores information,” said University of Cambridge researcher Stephen Elliott to PhysOrg.com. “We have gained deep and valuable insight into this process.”

Elliott and co-researcher Jozsef Hegedüs set out to answer some basic questions about phase-change memory materials, including why the crystallization process occurs so quickly, taking only about one nanosecond, and why the process is so readily reversible.

They carried out a series of molecular dynamics simulations using computer software and were able to reproduce the entire Ge₂Sb₂Te₅ read-write phase-change cycle. They modeled what would happen as the material was heated and then cooled rapidly or slowly, changing from a liquid to either an amorphous phase or a crystal, and then from amorphous to crystal upon reheating.

They found that, as the liquefied Ge₂Sb₂Te₅ cools, very high densities of square-shaped molecular rings form, which persist even into the amorphous phase. The rings form the backbone of the crystal phase, serving as nucleation points for its growth.

“The significance is that this is the first time that the entire phase change cycle has been simulated by accurate molecular dynamics simulations, and that the role of the square rings in the crystallization process has been established,” said Elliott.

“Our approach may lead to the design of superior phase-change materials in the future, if we can find compositions, for example, that nucleate at much shorter times or at lower temperatures, and by investigating the effects of doping, such as with nitrogen. These properties could allow better memory devices to be manufactured, and more rapidly.”

Citation: J. Hegedüs and S. R. Elliott Nature advance online publication, 23 March 2008 (DOI 10.1038/nmat2157)

Copyright 2008 PhysOrg.com.
All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

Related stories:

Improving computer memory, solar cells goal of UH chemist
A high-tech breakthrough in solar cells and flash drives may just come down to good old-fashioned pencil and paper calculations, says an award-winning young chemist at the University of Houston.
New Nanowire-Based Memory Could Beef Up Information Storage
Researchers from the University of Pennsylvania have created a type of nanowire-based information storage device that is capable of storing three bit values rather than the usual two—that is, "0," "1," and "2" instead of just "0" and "1." This ability could lead to a new generation of high-capacity information storage for electronic devices.
Nanoscale computer memory retrieves data 1,000 times faster
Scientists from the University of Pennsylvania have developed nanowires capable of storing computer data for 100,000 years and retrieving that data a thousand times faster than existing portable memory devices such as Flash memory and micro-drives, all using less power and space than current memory technologies.
Nanocrystals Are Hot
Scientists at the Department of Energy's Lawrence Berkeley National Laboratory have discovered that nanocrystals of germanium embedded in silica glass don't melt until the temperature rises almost 200 degrees Kelvin above the melting temperature of germanium in bulk. What's even more surprising, these melted nanocrystals have to be cooled more than 200 K below the bulk melting point before they resolidify. Such a large and nearly symmetrical "hysteresis" — the divergence of melting and freezing temperatures above and below the bulk melting point — has never before been observed for embedded nanoparticles.
New Promising Phase Change Memory Technology
IBM, Infineon and Macronix announced today a joint research initiative to explore the potential of a new form of computer memory technology called phase-change memory (PCM).
PCM is a novel technology that stores data by changing the state of a special material from an amorphous to a crystalline structure, rather than storing data as an electrical charge. While in its early stages, the technology shows potential for high speed, high density storage of data, while retaining data even when power is turned off. Such attributes could be beneficial in applications ranging from high performance servers to consumer electronics.
Digital memory enters a new phase
With the recent explosion in the popularity of digital music, digital photography and even digital video, the demand for faster, higher-capacity and cheaper computer memory has never been greater. Writing in the April issue of Nature Materials (DOI: 10.1038/nmat1350), Martijn Lankhorst (Philips Research Laboratories, Eindhoven, The Netherlands) and colleagues describe a new memory device that could meet this ever-growing need.
STMicroelectronics Unveils Advanced Non-Volatile Memory and Advanced CMOS Platform Developments
STMicroelectronics, one of the world's leading suppliers of semiconductor devices, will participate as presenter or co-author in fifteen papers at the IEDM 2004 (International Electron Devices Meeting) Conference, which takes place during December 13-15 in San Francisco, California. Eleven papers originating from Crolles, France, cover developments in leading-edge CMOS technology, while four are devoted to advances in Non-Volatile Memory technology developed at ST's Central R&D facility in Agrate, Italy.
STMicroelectronics Advances in Development of Future Non-Volatile Memory Technology
Leading Flash memory supplier confirms viability of 'Post-Flash' candidate

Geneva, June 16, 2004 - STMicroelectronics, one of the world's leading semiconductor suppliers, has announced significant progress in the development of a new type of electronic memory that could eventually replace the Flash memory technology. Today, Flash is a key component of many electronic applications, from mobile phones, digital cameras, and set-top boxes to automotive engine controllers. The new technology, called Phase-Change Memory (PCM), potentially offers better performance than Flash. Most importantly, PCM is better suited to continuing the rapid shrinking of features for cost and speed advantage that has long characterized the semiconductor industry than Flash and other so-called "Non-Volatile Memories" (NVM), which are able to store information even when their power is switched off.

News discussion:

Physics news