Novel Ways to Make Phase-Change Memory Material More Rapid than Existing Flash Computer Memory

A research team from Arizona State University’s School of Molecular Sciences and Germany has come up with an interpretation of ways by which a specific phase-change memory (PCM) material can function a thousand times more rapid compared to existing flash computer memory, while staying considerably more durable in relation to the number of daily read-writes.

The team has described this in an article published online in Science Advances on November 30^th, 2018.

PCMs are a type of computer random-access memory (RAM) that store data by modifying the state of the matter of the “bits” (millions of which form the device) between glass, liquid, and crystal states. PCM technology has the ability to offer high-density, inexpensive, high-volume, high-speed, nonvolatile storage on an unmatched level.

Although Stanford Ovshinsky invented the basic concept and material in 1975, applications have been delayed because of lack of clarity on how the material achieves the phase changes on such short timescales and technical challenges related to regulating the variations with the necessary precision. Currently, high-tech companies such as IBM, Samsung, and Intel are on the race toward perfecting it.

The semimetallic material presently under investigation is an alloy of germanium, antimony, and tellurium in the ratio of 1:2:4. In this study, the researchers investigated the microscopic dynamics in the liquid state of this PCM with the help of quasi-elastic neutron scattering (QENS) for hints on what might render the phase changes so sharp and reproducible.

By order, it would be possible to change the structure of each microscopic bit in this PCM material from glass to crystal or from crystal back to glass (through the liquid intermediate) on the time scale of one-thousandth of one-millionth of a second merely by controlled heat or light pulse, where the former is now being favored. In the disordered or amorphous phase, the electrical resistance of the material is high, or the “off” state; in the ordered or crystalline phase, its resistance is decreased 1000 times or more to give the “on” state.

It is feasible to arrange these elements in two-dimensional layers between activating electrodes, which can be stacked to offer a three-dimensional array specifically high active site density, thereby rendering it possible for the PCM device to work several times faster compared to traditional flash memory, while consuming less power.

“The amorphous phases of this kind of material can be regarded as ‘semimetallic glasses’,” explained Shuai Wei, who at the time was conducting postdoctoral research in Regents’ Professor Austen Angell’s lab as a Humboldt Foundation Fellowship recipient.

Contrary to the strategy in the research field of ‘metallic glasses’, where people have made efforts for decades to slow down the crystallization in order to obtain the bulk glass, here we want those semimetallic glasses to crystallize as fast as possible in the liquid, but to stay as stable as possible when in the glass state. I think now we have a promising new understanding of how this is achieved in the PCMs under study.

Shuai Wei, Post-Doc, School of Molecular Sciences, Arizona State University.

A deviation from the expected

More than a century earlier, in his PhD thesis, Einstein wrote that the diffusion of particles that undergo Brownian motion could be perceived if the frictional force that retards the motion of a particle was that derived by Stokes for a round ball that falls through a jar of honey. The simple equation

D (diffusivity) = kBT/6πηr

where T is the temperature, η is the viscosity, and r is the particle radius, indicates that the product Dη/T should be constant as T varies, and the fascinating aspect is that this appears to be true not just for Brownian motion, but even for simple molecular liquids whose molecular motion is known to be anything but that of a ball falling through honey.

We don’t have any good explanation of why it works so well, even in the highly viscous supercooled state of molecular liquids until approaching the glass transition temperature, but we do know that there are a few interesting liquids in which it fails badly even above the melting point. One of them is liquid tellurium, a key element of the PCM materials. Another is water which is famous for its anomalies, and a third is germanium, a second of the three elements of the GST type of PCM. Now we are adding a fourth, the GST liquid itself. Thanks to the neutron scattering studies proposed and executed by Shuai Wei and his German colleagues, Zach Evenson of the Technical University of Munich and Moritz Stolpe of Saarland University on samples prepared by Shuai with the help of Pierre Lucas from University of Arizona.

C. Austen Angell, Regents’ Professor, School of Molecular Sciences, Arizona State University.

Another property common to this small group of liquids is the occurrence of a maximum in-liquid density, which is well known for the case of water. A density maximum closely followed at the time of cooling by a metal-to-semiconductor transition is also observed in the stable liquid state of arsenic telluride, (As₂Te₃), the first cousin to the antimony telluride (Sb₂Te₃) component of the PCMs. All these fall on the “Ovshinsky” line that connects antimony telluride (Sb₂Te₃) to germanium telluride (GeTe) in the three-component phase diagram.

Wei and colleagues have suggested that when antimony, germanium, and tellurium are combined together in the ratio of 1:2:4—or others along Ovshinsky’s “magic” line—both the density maxima and the related metal-to-nonmetal transitions are pushed below the melting point and, at the same time, the transition turns out to be considerably sharper compared to other chalcogenide mixtures.

Then, similar to the much-studied case of supercooled water, the fluctuations related to the response function extrema should lead to exceptionally rapid crystallization kinetics. In all cases, the high-temperature state, which is currently the metallic state, is the denser.

This would explain a lot. Above the transition the liquid is very fluid and crystallization is extremely rapid, while below the transition the liquid stiffens up quickly and retains the amorphous, low-conductivity state down to room temperature.

C. Austen Angell, Regents’ Professor, School of Molecular Sciences, Arizona State University.

In nanoscopic “bits”, it subsequently stays indefinitely stable until directed by a computer-programmed heat pulse to instantly surge to a temperature at which, on a nanosecond time scale, it flash-crystallizes to the conducting state, that is, the “on” state.

Lindsay Greer from Cambridge University has made the same argument, couched in terms of a “fragile-to-strong” liquid transition.

Another, slightly larger heat pulse can take the “bit” instantaneously above its melting point and subsequently, with no further heat input and close contact with a cold substrate, it quenches at a rate adequate to prevent crystallization and is trapped in the semiconducting state, that is, the “off” state.

“The high resolution of the neutron time-of-flight spectrometer from the Technical University of Munich was necessary to see the details of the atomic movements. Neutron scattering at the Heinz Maier-Leibnitz Zentrum in Garching is the ideal method to make these movements visible,” stated Evenson.

Regents’ Professor C. Austen Angell breaks down the new findings. (Video credit: Arizona State University)