Stability and Mobility: Two Liquids Are the Key

17/06/2019

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Phase-change random access memory, PC-RAM for short, is one of the latest, yet most mature memory technologies introduced in the pursuit for higher, non-volatile storage densities, lower energy consumption, and improved scaling.

 

In PC-RAMs, data is written by switching between glassy and crystalline material states when heat pulses are applied. However, the processes on the atomic level, underlying the fast switching capability of the phase-change material inside the PC-RAM memory cells, have been left unclear so far. A possible solution to this puzzle is offered by a study from an international collaboration, including solid state physicists from RWTH Aachen University, published in the Science journal. With a combination of state-of-the-art, ultrafast X-ray scattering methods and advanced computer simulation techniques, the researchers uncovered and characterized a transition between a high- and a low-temperature supercooled liquid phase in phase-change materials. Their results provide both new insights into the process of glass formation and possibilities to improve related memory technology.

Phase-change materials, PCMs for short, made of compositions 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 advanced DRAM with non-volatile back-up functionality, in storage-class memory and in embedded applications, for example in the automotive sector. When an electrical or optical pulse is applied to heat these materials locally, they switch rapidly from a glassy to a crystalline state, and vice versa. The two states are differentiated electrically between logic 0 and 1, by high resistance in the amorphous and low resistance in the crystalline state, respectively. In contrast to the various applications and enormous potential of PCMs, to date it has not been possible to resolve how exactly the unique properties of PCMs, namely the fast crystallization at moderately high temperature, yet the stability at room temperature for decades of the disordered state is realized on the atomic level.

The experiments and simulations conducted by the international research group behind this publication provide new insights on the atomic level. In an experiment at the Linac Coherent Light Source, LCLS for short, in California, scientists from the European XFEL, the University of Duisburg-Essen, and international co-workers collected over 10,000 scattering patterns from femtosecond X-ray diffraction, performed while the materials switch state, in order to study atomic changes. Triggering the change between the crystalline and glassy states by an optical laser pulse, the X-ray laser was used to take images of the atomic structure during this extremely fast process. The experiments showed that when the high-temperature liquid, generated by the laser heat pulse, is cooled sufficiently below the melting temperature, it undergoes a structural transition to a different, low-temperature liquid. Both liquids differed significantly in their structural and kinetic properties.

Carrying out Simulations

Ab initio Molecular Dynamics, AIMD for short, simulations were performed at the Institute for Theoretical Solid State Physics at RWTH to investigate the structural changes during the fast melt-quench process. The special aim of their analysis was to obtain detailed information about local structural motifs and thereby resolve the mechanism responsible for the liquid-liquid transition. Investigating the distribution of the six nearest-neighbors of selected atoms by means of a statistical analysis of the AIMD trajectories, the computational physicists found a clear separation of inter-atomic distances into distinct groups of three short and three long average distances. More advanced statistical measures, indicative of correlations between almost aligned triplets of atoms, confirmed the separation to originate from a so-called Peierls-like distortion, which turned out to be the dominant mechanism of the structural liquid-liquid transition and directly connected to the change in kinetic properties. Furthermore, the calculations showed changes in the electronic properties. The high-temperature liquid is significantly more metallic than the low-temperature one. These findings are consistent with the increased bond stability preventing spontaneous crystallization from the glassy state: As observed in the experiments, the liquid at high temperature shows high atomic mobility that enables the atoms to crystallize, or arrange itself in a well-ordered structure, whereas in the liquid at low temperature, some chemical bonds proved to be stronger, consistent with a reduced mobility. The disordered atomic structure is therefore preserved in the resulting glassy state. Corresponding to the ascribed physical processes, the information storage cycle in PCMs is realized by fast quenching of the high-temperature liquid to enter a glassy state via the low-temperature or -mobility liquid without crystallization, retention of the glassy state for the desired data life time, and crystallization upon a thermal stimulus through the high-temperature or -mobility liquid. However, the decisive properties of PCMs are not merely the high mobility of the liquid at high temperature, which makes fast crystallization possible, and the low mobility of the liquid at low temperature and in the glass, too, to guarantee long retention times up to the point where crystallization is almost absent. In fact, it is the unique splitting into two liquid phases, both of them with very different dependencies of the kinetics on temperature that separates well the realms of glass stability and fast crystallization ability. The observation that materials can form a stable glass, but also become very unstable at elevated temperatures has puzzled researchers for decades.

The results also help to understand how other classes of materials form a glass, or vitrification. In order to extend the knowledge of the physical processes which occur during the fast cooling and vitrification from the high-temperature liquid of materials of different types, similar experiments and computer simulations are already scheduled at European XFEL and RWTH.

The published study was part of an international collaboration including researchers from RWTH, European XFEL, 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 for short, University of Duisburg-Essen, and the University of Potsdam. The contributing researchers from RWTH are members of the SFB 917 “Nanoswitches”, which, thanks to funding from the German Research Foundation, DFG for short, has been promoting knowledge in the field of microscopic resistive switching processes since 2011. The project will continue to run until 2023.

Original Publication:

Femtosecond X-ray Diffraction Reveals a Liquid-Liquid Phase Transition in Phase-Change Materials.
Zalden et al., DOI: 10.1126/science.aaw1773