A new generation of ultrasmall transistors and more powerful computer chips
using tiny structures called semiconducting nanowires are closer to reality
after a key discovery by researchers at IBM, Purdue
University and the University of California at Los Angeles.
 | | Researchers are closer to using tiny devices called semiconducting nanowires to create a new generation of ultrasmall transistors and more powerful computer chips. The researchers have grown the nanowires with sharply defined layers of silicon and germanium, offering better transistor performance. As depicted in this illustration, tiny particles of a gold-aluminum alloy were alternately heated and cooled inside a vacuum chamber, and then silicon and germanium gases were alternately introduced. As the gold-aluminum bead absorbed the gases, it became "supersaturated" with silicon and germanium, causing them to precipitate and form wires. Credit: Purdue University, Birck Nanotechnology Center/Seyet LLC
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The researchers have learned how to create nanowires with layers of different
materials that are sharply defined at the atomic level, which is a critical
requirement for making efficient transistors out of the structures.
"Having sharply defined layers of materials enables you to improve and
control the flow of electrons and to switch this flow on and off," said
Eric Stach, an associate professor of materials engineering at Purdue.
Electronic devices are often made of "heterostructures," meaning
they contain sharply defined layers of different semiconducting materials, such
as silicon and germanium. Until now, however, researchers have been unable to
produce nanowires with sharply defined silicon and germanium layers. Instead,
this transition from one layer to the next has been too gradual for the devices
to perform optimally as transistors.
The new findings point to a method for creating nanowire transistors.
The findings are detailed in a research paper appearing Friday (Nov. 27) in
the journal Science. The paper was written by Purdue postdoctoral researcher
Cheng-Yen Wen, Stach, IBM materials scientists Frances Ross, Jerry Tersoff and
Mark Reuter at the Thomas J. Watson Research Center in Yorktown Heights, N.Y,
and Suneel Kodambaka, an assistant professor at UCLA's Department of Materials
Science and Engineering.
Whereas conventional transistors are made on flat, horizontal pieces of silicon,
the silicon nanowires are "grown" vertically. Because of this vertical
structure, they have a smaller footprint, which could make it possible to fit
more transistors on an integrated circuit, or chip, Stach said.
"But first we need to learn how to manufacture nanowires to exacting standards
before industry can start using them to produce transistors," he said.
Nanowires might enable engineers to solve a problem threatening to derail the
electronics industry. New technologies will be needed for industry to maintain
Moore's law, an unofficial rule stating that the number of transistors on a
computer chip doubles about every 18 months, resulting in rapid progress in
computers and telecommunications. Doubling the number of devices that can fit
on a computer chip translates into a similar increase in performance. However,
it is becoming increasingly difficult to continue shrinking electronic devices
made of conventional silicon-based semiconductors.
"In something like five to, at most, 10 years, silicon transistor dimensions
will have been scaled to their limit," Stach said.
Transistors made of nanowires represent one potential way to continue the tradition
of Moore's law.
The researchers used an instrument called a transmission electron microscope
to observe the nanowire formation. Tiny particles of a gold-aluminum alloy were
first heated and melted inside a vacuum chamber, and then silicon gas was introduced
into the chamber. As the melted gold-aluminum bead absorbed the silicon, it
became "supersaturated" with silicon, causing the silicon to precipitate
and form wires. Each growing wire was topped with a liquid bead of gold-aluminum
so that the structure resembled a mushroom.
Then, the researchers reduced the temperature inside the chamber enough to
cause the gold-aluminum cap to solidify, allowing germanium to be deposited
onto the silicon precisely and making it possible to create a heterostructure
of silicon and germanium.
The cycle could be repeated, switching the gases from germanium to silicon
as desired to make specific types of heterostructures, Stach said.
Having a heterostructure makes it possible to create a germanium "gate"
in each transistor, which enables devices to switch on and off.
Posted November 26th, 2009
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