Paper explains ferroelectrics memory losses
hile
the memory inside electronic devices may often be more reliable than
that of humans, it, too, can worsen over time.
Now a team of scientists from UW-Madison and Argonne National Laboratory
may understand why. The results are published in the online edition
of the journal Nature Materials.
Smart cards, buzzers inside watches and even ultrasound machines all
take advantage of ferroelectrics, a family of materials that can retain
information, as well as transform electrical pulses into auditory or
optical signals, or vice versa.
"The neat thing about these materials is that they have built-in
electronic memory that doesn't require any power," explains Assistant
Professor Paul
Evans , a co-author of the recent paper.
But there's a problem preventing many of these materials from being
used more widely in other technologies, including computers. As Evans
says, "Eventually they quit working."
The ability of ferroelectrics to store information resides in their
arrangement of atoms, with each structure holding a bit of information.
This information changes every time the material receives a pulse of
electricity, basically switching the arrangement of atoms.
However, each electric pulse - and corresponding change in structure
- gradually diminishes the capability of these materials to store and
retrieve information until they either forget the information or quit
switching altogether.
"It could switch 10,000 or even millions of times and then stop
working," says Evans.
Engineers call this problem fatigue. With little evidence for what
happens to the structure of ferroelectrics as the material's memory
fatigues, Evans and his colleagues decided to look inside this material
as its arrangement of atoms, controlled by electrical pulses, switched
inside an operating device.
"We'd like to understand how it switches so we could build something
that switches faster and lasts longer before it wears out," says
Evans.
To create a detailed picture of how the atoms rearrange themselves
inside an operating device during each electrical pulse, the researchers
used the Advanced Photon Source — the country's most brilliant source
of X-rays for research, located at the Argonne National Laboratory -
to measure changes in the location of atoms. By seeing how the atoms
changed their positions, the researchers could determine how well the
material switched, or remembered information.
"One advantage to working with X-rays is their ability to penetrate
deep into materials, which is why they are so extensively used today
in medical imaging," says Eric Isaacs, director of Argonne's Center
for Nanoscale Materials, and one of the paper's co-authors. "Utilizing
this property of X-rays, [we] were able to peer through layers of metal
electrodes in order to study ferroelectric fatigue in a realistic operating
device."
He adds that the very high brightness of the Advanced Photon Source
allowed the researchers to focus X-rays to unprecedented small dimensions.
The X-rays showed that, as the researchers repeatedly pulsed the device,
progressively larger areas of the device ceased working, suggesting
that the atoms were switching structures less and less.
"After 50,000 switches, the atoms were stuck - they couldn't switch
anymore," says Evans, adding that a stronger electrical charge
did put the atoms back in motion.
When the researchers used a higher voltage of electricity from the
beginning, switching stopped 100 times later, as reported in the paper.
And, in this instance, applying an even stronger pulse made no difference.
"With higher voltages, the material can't switch because something
has changed about the material itself," says Evans. "When
you use bigger voltages, it's not just the switching that stops working,
but something even more fundamental."
Because previous researchers have not peeked inside working ferroelectric
materials to understand their arrangement of atoms - key to the ability
to recall information — the reasons why switching eventually stops had
not been clearly identified.
"The electronic memory is stored in the structure of atoms, and
that's why it's so important to see what the structure looks like,"
explains Evans. By looking inside these devices, he says engineers can
begin to understand why the atoms stop switching and then manufacturers
can start to design better devices.
With this promise, Evans asks, "Wouldn't it be nice to have a
computer that doesn't forget what it's doing when you turn it off?"
Other researchers involved in the work include Chang Beom Eom, Dong
Min Kim and the paper's first author, Dal-Hyun Do, from UW-Madison;
and Eric Dufres, from the University of Michigan.
