This illustration by Johns Hopkins doctoral student Ming Xu depicts the shape of diamond tips used to apply pressure that uncovered important new properties in the memory medium GST. The inset represents the atomic structure of amorphous GST. |
A
team led by Johns Hopkins University engineers has discovered some previously
unknown properties of a common memory material, paving the way for
development of new forms of memory drives, movie discs and computer
systems that retain data more quickly, last longer and allow far more
capacity than current data storage media.
The work was reported April 16 in the online edition of Proceedings of the National Academy of Sciences.
The
research focused on an inexpensive phase-change memory alloy composed
of germanium, antimony and tellurium, called GST for short. The material
is already used in rewritable optical media, including CD-RW and DVD-RW
discs. But by using diamond-tipped tools to apply pressure to the
materials, the Johns Hopkins-led team uncovered new electrical
resistance characteristics that could make GST even more useful to the
computer and electronics industries.
“This
phase-change memory is more stable than the material used in the
current flash drives. It works 100 times faster and is rewritable about
100,000 times,” said the study’s lead author, Ming Xu, a doctoral
student in the Department of Materials Science and Engineering in Johns
Hopkins’ Whiting School of Engineering. “Within about five years, it
could also be used to replace hard drives in computers and give them
more memory.”
GST
is called a phase-change material because, when exposed to heat, areas
of GST can change from an amorphous state, in which the atoms lack an
ordered arrangement, to a crystalline state, in which the atoms are
neatly lined up in a long-range order. In its amorphous state, GST is
more resistant to electric current. In its crystalline state, it is less
resistant. The two phases also reflect light differently, allowing the
surface of a DVD to be read by A tiny laser. The two states correspond
to one and zero, the language of computers.
Although
this phase-change material has been used for at least two decades, the
precise mechanics of this switch from one state to another have remained
something of a mystery because it happens so quickly—in
nanoseconds—when the material is heated.
To
solve this mystery, Xu and his team used another method to trigger the
change more gradually. The researchers used two diamond tips to compress
the material. They employed a process called X-ray diffraction and a
computer simulation to document what was happening to the material at
the atomic level. The researchers found that they could “tune” the
electrical resistivity of the material during the time between its
change from amorphous to crystalline form.
“Instead
of going from black to white, it’s like finding shades or a shade of
gray in between,” said Xu’s doctoral adviser, En Ma, a professor of
materials science and engineering, and a co-author of the PNAS paper.
“By having a wide range of resistance, you can have a lot more control.
If you have multiple states, you can store a lot more data.”
Other
co-authors of the paper were Y. Q. Cheng of Johns Hopkins and the Oak
Ridge National Laboratory in Tennessee; L. Wang of the Carnegie
Institution of Washington in Argonne, Ill., and Jilin University in
China; H. W. Sheng of George Mason University in Fairfax, Va.; Y. Meng
and W. G. Wang of the Carnegie Institution of Washington; and X. D. Han
of Beijing University of Technology in China.
Funding
for the research was provided by the U.S. Department of Energy, the
Office of Naval Research, the Chinese National Basic Research Program,
the National Science Foundation, the W. M. Keck Foundation and Argonne
National Laboratory.
Source: Johns Hopkins University
