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Phase-change materials are used in the latest generation of smartphones enabling higher storage densities and energy efficiency. However, to date it has not been possible to study what happens at the atomic level during this process. In a paper published today in Science, a group of scientists led by researchers from the University of Duisburg-Essen and the European XFEL in Germany, show that changes of the chemical bonding mechanism in the liquid phase enables data storage in these materials.
Phase-change materials made of compositions of the elements antimony, tellurium and germanium are used, for example, in replacements for flash drives in the latest generation of smartphones. When an electrical or optical pulse is applied to heat these materials locally, they change from a glassy to a crystalline state, and vice versa. These two different states represent the ‘0’ and ‘1’ of the binary code needed to store information.
To store information with phase-change materials, their liquid phase must be cooled quickly to enter a glassy state without crystallizing. They must also stay in this glassy state for as long as the data is to be stored. At high temperatures, however, the same material must crystallize very quickly to erase the information.
The scientists used a technique called femtosecond X-ray diffraction to study atomic changes when the materials switch state. Only these short and intense pulses are able to capture snapshots of the atomic changes. This way, the researchers studied the fast cooling process, by which a glass is formed. They found that when the liquid is cooled sufficiently far below the melting temperature, it undergoes a structural change to form another, low-temperature liquid.
The two different liquids had not only very different atomic structures, but also different behaviors: The liquid at high temperature has a high atomic mobility that enables the atoms to crystallize. However, when the liquid passes below a certain temperature below the melting point, some chemical bonds become stronger and more rigid, thereby reducing the atomic mobility dramatically. These observations were supported by computer simulations performed at RWTH Aachen University. The simulations also showed that the low-temperature liquid is less metallic and more rigidly bound than the high-temperature liquid. This mechanism suppresses crystallization and stabilizes the disordered atomic structure of the glass, enabling long-term data storage.
Peter Zalden, scientist at European XFEL and lead author of the study, explains “current data storage technology has reached a scaling limit so that new concepts are required to store the amounts of data that we will produce in the future. Our study explains how the switching process in a promising new technology can be fast and reliable at the same time.”
Klaus Sokolowski-Tinten from the University of Duisburg-Essen, who initiated the project, adds: “The results and our time-domain approach can also help to understand how liquids of other classes of materials behave when they are rapidly cooled to temperatures well below the melting point and form a glass.”
The study was part of an international collaboration including scientists from Forschungszentrum Jülich, Institut Laue-Langevin, Lawrence Livermore National Laboratory, Lund University, Paul Scherrer Institute, SLAC National Accelerator Laboratory, Stanford University, The Spanish National Research Council (CSIC), University of Aachen, and the University of Potsdam.
Editors:
Dr. Rosemary Wilson, European XFEL, +49 40 8998-6729, rosemary.wilson@xfel.eu
Birte Vierjahn, Center for Nanointegration Duisburg-Essen, +49 203 37 9-8176, birte.vierjahn@uni-due.de
Dr. Klaus Sokolowski-Tinten, +49 203 37 9-4561, klaus.sokolowski-tinten@uni-due.de
Dr. Peter Zalden, +49 40 8998-6828, peter.zalden@xfel.eu
P. Zalden, F. Quirin, M. Schumacher, J. Siegel, S. Wei, A. Koc, M. Nicoul, M. Trigo, P. Andreasson, H. Enquist, M. J. Shu, T. Pardini, M. Chollet, D. Zhu, H. Lemke, I. Ronneberger, J. Larsson, A. M. Lindenberg, H. E. Fischer, S. Hau-Riege, D. A. Reis, R. Mazzarello, M. Wuttig, K. Sokolowski-Tinten.
Femtosecond X-ray diffraction reveals a liquid-liquid phase transition in phase-change materials.
Science 14 Jun 2019 : 1062-1067
DOI: 10.1126/science.aaw1773
https://science.sciencemag.org/content/364/6445/1062.editor-summary
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