40+ Year Old Challenge Solved for Phase Change Materials

Phase change materials, were first considered for storing data in the 1970s, where the two metastable states or phases of these materials, are used to store data in the form of millions of lines of binary code made of up billions of 0s and 1s. 

The concept eventually reached the consumer market, and today the most common use of these materials is in optical storage, where the phase transition is induced by heating the material with a laser beam - this is how a Blue-ray disk stores a video.

The cross-sectional tunneling
electron microscopy (TEM) image of
a mushroom-type PCM cell
is shown in this photo.
In addition to a laser, it is also possible to heat the phase change material through electrical means by placing it between two electrically conducting electrodes. This forms the basis for a novel concept called phase-change memory (PCM), a nonvolatile memory technology that promises to bridge the performance gap between the main memory and storage electronics, spaning from mobile phones to cloud data centers.

The nanometric volume of phase change material in the PCM cell can be reversible switched from the amorphous phase (logic “0”) and the crystalline phase (logic “1”) by the application of suitable voltage pulses. The resulting data can be read out by applying a much lower read voltage.

But for more than 40 years scientists have never measured the temperature dependence of crystal growth, due to the difficulties associated with the measurements which are taken at both a nanometer length and a nanosecond time scale. That was until earlier this year when, for the first time, IBM scientists in Zurich were able to take the measurements, which is today being reported in the peer-review journal Nature Communications.

On the eve of the publication of this important result, the authors answered a few questions from their lab in the Binnig and Rohrer Nanotechnology Center at IBM.

IBM scientists Abu Sebastian, Manuel Le Gallo and Daniel Krebs

Let’s start with the obvious decades old question, what is the temperature corresponding to maximum crystal growth?

Daniel Krebs: The optimum crystal growth temperature is 477 degrees Celcius (750 Kelvin), but that it really just one point on the chart (figure B) – holistically it gets much more interesting. 

What is more useful to scientists studying phase change materials is that we were able to model the entire growth velocity curve in addition to this maximum. Prior to this paper, scientists knew some of the points, but not across such a wide temperature and time scale.

It is also worth noting that we took these measurements within the cell. Typically, experiments took place outside the cell, which then had to be extrapolated. Now scientists have an excellent reference point.

Can you describe the eureka moment?

Abu Sebastian: Let me start by saying that these phase change materials are very fascinating and possess unconventional crystallization kinetics. Just by changing the temperature by a few hundred degrees, you change the crystal growth rate by 17 orders of magnitude (that is beyond a trillion). This is why it  has been so difficult to probe experimentally.

Only in the last 18-24 months have scientists begun to probe the crystallization rate within a reasonable temperature range, until this point the measurements were at very low temperatures (close to room temperature).

Our key insight was in exploiting the nanoscale dimensions and the fast thermal dynamics of the phase change memory cell to expand the temperature range all the way up to the point at which the material melts – more than 600 degrees Celsius.

Daniel: It’s called the time-temperature dilemma. At room temperature you want stability of the material to retain the data for at least 10 years, but when you want to write to the material it needs to crystallize in nanoseconds. And that is what makes this material so interesting, but it’s also what makes it challenging – particularly in how it can be accurately measured.

Manuel Le Gallo: I came to IBM to do my Masters thesis work on electrical transport in phase change materials. One of the requirements was to achieve the same amorphous volume at all temperatures. This involved a deeper understanding of melting and crystallization in the PCM cells. As we delved more into the subject, the focus of the thesis gradually shifted, culminating in the fascinating results we present in the paper.

What inherent challenges in phase change memory does this achievement address and what are the potential applications?

Daniel: If we break down the challenges of PCM into read and write operations, in this work, we are addressing the write operation. Our measurements will help devise ways to write data faster and with better retention.  

Abu: In the context of PCM, this research will help us in estimating how fast we can write, how much power is required and what the real retention time is. Going beyond memory, yet another emerging application of phase change materials is in neuromorphic engineering,  creating chips based on the biological architectures of the nervous system. So understanding the phase change mechanism is critically important for a number of applications.

Manuel: Crystal growth and subsequent change in electrical conductance has the potential to emulate the biophysics of neurons and synapses. This will also form part of my doctoral thesis work which I am currently pursuing jointly with the Institute of Neuroinformatics at ETH Zurich.

What will you study next?

Abu: It will be interesting to look at different materials and compare the temperature dependence of crystal growth. We also discovered that the crystal growth rate reduces over time, which we want to expand on further.

Daniel: The reduction in growth over time is actually very interesting for me. In the amorphous phase the materials are a glass. Like a glass window becomes thicker when it is at rest over a long period of time, like 100 years, also our amorphous material will change. In fact, it changes in such a way that it becomes more viscous. This viscosity is one of the characteristics which determines how fast the material can crystalize. Therefore it effects the write operation. It cannot crystallize as fast anymore, which is a good thing for data retention. On the other hand the glassy nature also causes the inherent problem of resistance drift in phase change memory.

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