Thursday, November 26, 2009
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.
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.
SOURCE