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.
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| 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.
Labels: IBM Research - Zurich, Nature, PCM, phase change memory