R&D: Phase-Change Materials From Smartphones May Lead to Higher Data Storage, Energy Efficiency

From Lawrence Livermore National Laboratory (LLNL)

Phase-change materials that are used in the latest generation of smartphones could lead to higher storage capability and more energy efficiency.

Lawrence Livermore researchers and collaborators
used the X-ray free-electron laser
at the Linac Coherent Light Source to show that phase-change materials
can lead to faster and more effective data storage technologies.

Data is recorded by switching between glassy and crystalline material states by applying a heat pulse. However, to date it has not been possible to study what happens at the atomic level during this process.

In a paper published in the June 14 edition of the journal Science, a group of scientists, led by researchers from European XFEL and the University of Duisburg-Essen in Germany and including researchers from Lawrence Livermore National Laboratory (LLNL), describe how they used the capabilities of the X-ray free-electron laser at the Linac Coherent Light Source (LCLS) to show that a transition in the chemical bonding mechanism enables the data storage in these materials. The results can be used to optimize phase-change materials for faster and more effecient data storage technologies. They also provide new insights into the process of glass formation.

“With the increasing amount of data that we are storing in our devices such as smartphones today, we need new techniques to store even more information,” said Stefan Hau-Riege, LLNL , co-author of the paper.

Phase-change materials made of the elements antimony, tellurium and germanium can be used to store increasingly large amounts of data, and do so quickly and energy efficiently. They 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. However, to date it has not been possible to resolve how exactly these changes of state occur on an atomic level.

In an experiment at the LCLS, the team used a technique called femtosecond X-ray diffraction to study atomic changes when the materials switch states. In the experiment that took place before European XFEL was operational, an optical laser was first used to trigger the material to change between crystalline and glassy states. During this extremely fast process, the X-ray laser was used to take images of the atomic structure. Only X-ray free-electron lasers such as LCLS or European XFEL produce pulses that are short and intense enough to capture snapshots of the atomic changes occurring on such short timeframes. The scientists collected more than 10,000 images that shed light on the sequence of atomic changes that occur during the process.

To store information with phase-change materials, they must be cooled quickly to enter a glassy state without crystallizing. They also must stay in this glassy state for as long as the data are stored. This means that the crystallization process must be very slow to the point of being almost absent, as is the case in ordinary glass. At high temperatures, however, the same material must be able to crystallize very quickly to erase the information. That a material can form as stable glass but at the same time becomes very unstable at elevated temperatures has puzzled researchers for decades.

In their experiment, 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. This low-temperature liquid can be observed only on very short timescales, before crystallization takes place. 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, i.e., to arrange in a well-ordered structure. However, when the liquid passes below a certain temperature below the boiling point, some chemical bonds become stronger and more rigid and can hold the disordered atomic structure of the glass in place. It is only the rigid nature of these chemical bonds that prevents the transformation and — in the case of phase-change memory devices — secures the information in place.

“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,” said Peter Zalden, scientist, European XFEL and co-lead author of the study. ”Our study explains how the switching process in a promising new technology can be fast and reliable at the same time.”

The results also help understand how other classes of materials form a glass. Similar experiments are already scheduled at European XFEL, where the femtosecond pulses are short and intense enough to capture snapshots of these fast processes.

The study was part of an international collaboration including scientists from European XFEL, Forschungszentrum Jülich, Institut Laue-Langevin, LLNL, Lund University, Paul Scherrer Institute, SLAC National Accelerator Laboratory, Stanford University, The Spanish National Research Council (CSIC), University of Aachen, University of Duisburg – Essen and the University of Potsdam.

Tommaso Pardini, scientist, LLNL, also contributed to this research.

Article: Femtosecond x-ray diffraction reveals a liquid–liquid phase transition in phase-change materials

Science Magazine has published an article written by Peter Zalden, Stanford PULSE Institute, SLAC National Accelerator Laboratory, 2575 Sand Hill Rd., Menlo Park, CA 94025, USA, Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Rd., Menlo Park, CA 94025, USA, European XFEL, Holzkoppel 4, 22869 Schenefeld, Germany, Florian Quirin, Faculty of Physics and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Lotharstrasse 1, 47048 Duisburg, Germany, Mathias Schumacher, Institut für Theoretische Festkörperphysik, JARA-FIT and JARA-HPC, RWTH Aachen University, Germany, Jan Siegel,Instituto de Optica, CSIC, C/Serrano 121, 28006 Madrid, Spain, Shuai Wei, I. Physikalisches Institut and JARA-FIT, RWTH Aachen, Sommerfeldstrasse 14, 52074 Aachen, Germany, Azize Koc, Faculty of Physics and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Lotharstrasse 1, 47048 Duisburg, Germany, and Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany, Matthieu Nicoul, Faculty of Physics and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Lotharstrasse 1, 47048 Duisburg, Germany, Mariano Trigo1,Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Rd., Menlo Park, CA 94025, USA, Pererik Andreasson, Henrik Enquist, Department of Physics, Lund University, Professorsgatan 1, 223 62 Lund, Sweden,Michael J. Shu, Department of Applied Physics, Stanford University, Stanford, CA 94305, USA, Tommaso Pardini, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA, Matthieu Chollet, Linac Coherent Light Source, SLAC National Accelerator Laboratory, 2575 Sand Hill Rd., Menlo Park, CA 94025, USA, Diling Zhu, Linac Coherent Light Source, SLAC National Accelerator Laboratory, 2575 Sand Hill Rd., Menlo Park, CA 94025, USA, Henrik Lemke, Linac Coherent Light Source, SLAC National Accelerator Laboratory, 2575 Sand Hill Rd., Menlo Park, CA 94025, USA and Paul Scherrer Institute, Forschungsstrasse 111, 5232 Villigen, Switzerland, Ider Ronneberger, Institut für Theoretische Festkörperphysik, JARA-FIT and JARA-HPC, RWTH Aachen University, Germany, Jörgen Larsson,Department of Physics, Lund University, Professorsgatan 1, 223 62 Lund, Sweden, Aaron M. Lindenberg,Stanford PULSE Institute, SLAC National Accelerator Laboratory, 2575 Sand Hill Rd., Menlo Park, CA 94025, USA, Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Rd., Menlo Park, CA 94025, USA, and Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA, Henry E. Fischer,Institut Laue-Langevin, 71 Avenue des Martyrs, CS 20156, 38042 Grenoble Cedex 9, France, Stefan Hau-Riege, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA, David A. Reis1,Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Rd., Menlo Park, CA 94025, USA, Riccardo Mazzarello,Institut für Theoretische Festkörperphysik, JARA-FIT and JARA-HPC, RWTH Aachen University, Germany, Matthias Wuttig, I. Physikalisches Institut and JARA-FIT, RWTH Aachen, Sommerfeldstrasse 14, 52074 Aachen, Germany, and PGI 10 (Green IT), Forschungszentrum Jülich, 52428 Jülich, Germany, and Klaus Sokolowski-Tinten, Faculty of Physics and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Lotharstrasse 1, 47048 Duisburg, Germany.

Abstract: ”In phase-change memory devices, a material is cycled between glassy and crystalline states. The highly temperature-dependent kinetics of its crystallization process enables application in memory technology, but the transition has not been resolved on an atomic scale. Using femtosecond x-ray diffraction and ab initio computer simulations, we determined the time-dependent pair-correlation function of phase-change materials throughout the melt-quenching and crystallization process. We found a liquid–liquid phase transition in the phase-change materials Ag₄In₃Sb₆₇Te₂₆ and Ge₁₅Sb₈₅ at 660 and 610 kelvin, respectively. The transition is predominantly caused by the onset of Peierls distortions, the amplitude of which correlates with an increase of the apparent activation energy of diffusivity. This reveals a relationship between atomic structure and kinetics, enabling a systematic optimization of the memory-switching kinetics.”