X-Ray Diffraction Reveals Switching Mechanism In Phase-Change Materials

27/06/2019 Being able to switch between these two “0” and “1” states – a crystalline state with high electrical conductivity and a meta-stable amorphous state with low electrical conductivity – makes these materials promising for new types of non-volatile memory that could help meet the world’s ever-increasing demand for digital information, the volume of which is doubling every two years.

Researchers led by Klaus Sokolowski-Tinten of the University of Duisburg-Essen and Peter Zalden from European XFEL in Germany have now succeeded in observing the processes that occur during switching – something that has never been done before because of the short time-scale involved.

The work will be important for designing improved PCMs in the future, they say.

The researchers used hard femtosecond-long X-ray pulses from an X-ray Free Electron laser (the Linear Coherent Light Source – LCLS – at SLAC) to resolve the atomic structure of two PCMs, Ag4In3Sb67Te26 and Ge15Sb85, during the entire switching cycle. Both these materials are used as PCMs in optical (Ag4In3Sb67Te26) and electronic (Ge15Sb85) memory devices.

Two liquid states

To their surprise, they found that two liquid states of the materials are involved in the process – one that has more rigid chemical bonds, and which helps stabilize the glassy “off” state at ambient conditions, and one that is rather metallic and can therefore crystallize very quickly to produce the “on” state.

“These results provide a microscopic understanding of how PCMs work,” explains Zalden, who is lead-author of the study, “and why some materials – like Ag4In3Sb67Te26 and Ge15Sb85 – are PCMs while others (such as Ge15Te85) are not.”

Diffract-before-destroy

The researchers used the LCLS because at the time of the experiment it was the only X-ray source able to resolve the phase change process with a single X-ray pulse. “We can only use one pulse because the PCMs degrade when exposed to high intensity X-rays,” says Sokolowski-Tinten. “XFELs thus offer us the possibility of probing the atomic structure of these materials before the X-rays destroy them.

“We made use of a pump-probe scheme and combined this X-ray source with a laser-melt-quench technique in which we heat the PCM and transform it into the liquid state using an optical laser pulse (just like on optical re-writeable discs, where PCMs are also employed).”

The liquid rapidly quenches into a super-cooled state (that is, one whose temperature is lower than the melting temperature of the material). From here, the material then either crystallizes or forms a glass.

Reconstructing the entire switching cycle

By measuring the atomic structure of the material at various times after optically exciting it, the researchers say they can reconstruct the entire switching cycle. Each pump-probe event has to be performed on a fresh part of the material, however, because of the destructive impact of the X-ray pulse, explains Sokolowski-Tinten.

According to the team, the transition in PCMs comes mainly from the onset of Peierls distortions. This mechanism is a fundamental process that lowers the energy of a material by breaking its symmetry.

“In our case, the octahedral, six-fold coordinated environment of the high-temperature liquid in Ag4In3Sb67Te26 and Ge15Sb85 is distorted in the low-temperature liquid by displacing an atom along a diagonal of this configuration – if the liquid is quenched fast enough to sufficiently low temperatures, as in our experiments,” says Zalden. “This quenching shortens three chemical bonds and elongates the three opposite ones. As confirmed by the simulation results of our collaborators from RWTH Aachen, the bond shortening localizes the valence electrons, turning the materials from being metallic in the high-temperature liquid to covalent in the low-temperature liquid.”

Just as metallic materials are known to be very ductile, the atoms in the high-temperature state are very mobile, he adds. This allows the atoms in the high-temperature state to arrange themselves very quickly on a periodic lattice (and crystallize). The covalent bonds of the low-temperature liquid, on the other hand, are more like those of window glass and help stabilize the glassy state of these materials.

A new design parameter

“The results point to a new design parameter for when it comes to making future PCMs – the ratio of liquid-liquid transition temperature over the melting temperature,” says Zalden. “In a ‘good’ PCM, this value needs to be low and has to induce a strong Peierls distortion. Here, crystallization can take place at the highest rates over a wide range of temperatures and the glassy state is stable over the long-term at ambient conditions.

“More generally, the work and our time-domain approach, can also help us to understand how liquids of other classes of materials behave when they are rapidly cooled to temperatures well below the melting point,” he adds, “and why some liquids are more likely to form a glass than others.”

The researchers, reporting their work in Science 10.1126/science.aaw1773, say they now plan to perform similar measurements on other classes of materials. “We believe that a related structural phase transition mechanism could be occurring during the rapid cooling of materials like silicon,” says Sokolowski-Tinten. “In these materials, we observe different atomic structures between the glass and the liquid, but it is not possible to observe the transition between the two because of the rapid onset of crystallization upon quenching.

“Such studies should allow for a more efficient design of new technical glasses for specific applications, allowing these materials to be used wherever crystalline materials are employed today,” he tells Physics World.

The study, coordinated by the University of Duisburg–Essen and European XFEL and carried out at the Linac Coherent Light Source of SLAC National Accelerator Laboratory in the US, was part of an international collaboration that includes 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.







Source: https://bit.ly/2Y8Ujlh, via Physics World