Researchers discover predictable behavior in promising material for
computer memory

Date:
November 1, 2021
Source:
Georgia Institute of Technology
Summary:
A team of researchers has discovered unexpectedly familiar behavior
in the antiferroelectric material known as zirconium dioxide,
or zirconia.



FULL STORY ==========================================================================
In the last few years, a class of materials called antiferroelectrics
has been increasingly studied for its potential applications in modern
computer memory devices. Research has shown that antiferroelectric-based memories might have greater energy efficiency and faster read
and write speeds than conventional memories, among other appealing
attributes. Further, the same compounds that can exhibit antiferroelectric behavior are already integrated into existing semiconductor chip
manufacturing processes.


==========================================================================
Now, a team led by Georgia Tech researchers has discovered unexpectedly familiar behavior in the antiferroelectric material known as zirconium
dioxide, or zirconia. They show that as the microstructure of the material
is reduced in size, it behaves similarly to much better understood
materials known as ferroelectrics. The findings were recently published
in the journal Advanced Electronic Materials.

Miniaturization of circuits has played a key role in improving memory performance over the last fifty years. Knowing how the properties of an antiferroelectric change with shrinking size should enable the design
of more effective memory components.

The researchers also note that the findings should have implications in
many other areas besides memory.

"Antiferroelectrics have a range of unique properties like high
reliability, high voltage endurance, and broad operating temperatures
that makes them useful in a wealth of different devices, including high-energy-density capacitors, transducers, and electro-optics
circuits." said Nazanin Bassiri-Gharb, coauthor of the paper and professor
in the Woodruff School of Mechanical Engineering and the School of
Materials Science and Engineering at Georgia Tech. "But size scaling
effects had gone largely under the radar for a long time." "You can
design your device and make it smaller knowing exactly how the material
is going to perform," said Asif Khan, coauthor of the paper and assistant professor in the School of Electrical and Computer Engineering and the
School of Materials Science and Engineering at Georgia Tech. "From our standpoint, it opens really a new field of research." Lasting Fields


==========================================================================
The defining feature of an antiferroelectric material is the peculiar
way it responds to an external electric field. This response combines
features of non- ferroelectric and ferroelectric materials, which have
been much more intensively studied in physics and materials science.

For ferroelectrics, exposure to an external electric field of sufficient strength makes the material become strongly polarized, which is a state
where the material exhibits its own internal electric field. Even when
the external electric field is removed, this polarization persists,
similar to how an iron nail can become permanently magnetized.

The behavior of a ferroelectric material also depends on its size. As a
sample of material is made thinner, a stronger electric field is required
to create a permanent polarization, in accordance with a precise and predictable law called the Janovec-Kay-Dunn (JKD) law.

By contrast, application of an external electric field to an
antiferroelectric does not cause the material to become polarized --
at first. However, as the strength of the external field is increased,
an antiferroelectric material eventually switches to a ferroelectric
phase, where polarization abruptly sets in. The electric field needed
to switch the antiferroelectric to a ferroelectric phase is called the
critical field.

Size Scaling In the new work, the researchers discovered that zirconia antiferroelectrics also obey something like a JKD law. However, unlike for ferroelectrics, the microstructure of the material plays a key role. The strength of the critical field scales in the JKD pattern specifically
with respect to the size of structures known as crystallites within the material. For a smaller crystallite size, it takes a stronger critical
field to switch an antiferroelectric material into its ferroelectric
phase, even if the thinness of the sample remains the same.



========================================================================== "There had not been a predictive law that dictates how the switching
voltage will change as one miniaturizes these antiferroelectric oxide
devices," said Khan. "We've found a new twist on an old law." Formerly,
thin antiferroelectrics had been difficult to produce in comparable sizes
as ferroelectrics, the researchers said. Nujhat Tasneem, the doctoral
student leading the research, spent "day and night" in the lab according
to Khan to process and produce leakage-free antiferroelectric zirconium
oxide films of single nanometers in size. The next step, according
to Khan, is for researchers to figure out exactly how to control the crystallite size, thereby tailoring the properties of the material for
its use in circuits.

The researcher also collaborated with researchers from the Charles
University in Czech Republic and the Universidad Andres Bello in Chile
for X-ray diffraction characterization and first-principles based
calculations, respectively.

"It was truly a collaborative effort, spanning multiple continents,"
said Tasneem.

The results should also speak to fundamental physics questions, according
to Bassiri-Gharb. In recent years, something of a mystery has arisen in
the study of antiferroelectrics, with the way that microscopic crystalline structures cause a macroscopic polarization being called into question.

"Finding two very different types of materials -- ferroelectric and antiferroelectrics with different atomic structures -- to follow similar behaviors and laws is particularly exciting," said Bassiri-Gharb. "It
opens doors for searching for more similarities and transferring more
of our knowledge across the fields." The work was supported by the
National Science Foundation, the Semiconductor Research Corporation,
the Defense Threat Reduction Agency, the European Regional Development
Fund, and ANID FONDECYT in Chile. This work was performed in part at the Georgia Tech Institute for Electronics and Nanotechnology, a member of
the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the National Science Foundation.

========================================================================== Story Source: Materials provided by Georgia_Institute_of_Technology. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Nujhat Tasneem, Yasmin Mohamed Yousry, Mengkun Tian, Milan Dopita,
Sebastian E. Reyes‐Lillo, Josh Kacher,
Nazanin Bassiri‐Gharb, Asif Islam Khan. A
Janovec‐Kay‐Dunn‐Like Behavior at Thickness
Scaling in Ultra‐Thin Antiferroelectric ZrO 2 Films.

Advanced Electronic Materials, 2021; 2100485 DOI:
10.1002/aelm.202100485 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211101141800.htm

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