Technology Research News
Wires that are structured like television
cable but are a thousand times narrower than human hair could form the
future of semiconductor technology.
Researchers at Harvard University have made microscopic wires from layers
of different materials using the semiconductor manufacturing processes
used to construct computer chips.
The layered wires are new building blocks for nanotechnology, according
to Charles Lieber, a professor of chemistry at Harvard University. The
vanishingly small nanowire components could be used to make faster computer
chips, higher-density memory and smaller lasers.
To form a 50-nanometer layered wire, the researchers started with a gold
droplet a mere 20-nanometers across. A nanometer is one millionth of a
millimeter; 20 nanometers is about the size of a line of 200 hydrogen
The researchers coaxed a semiconductor vapor to condense on one side of
the gold droplet and form a nanowire of about the same diameter as the
They then caused additional layers of material to form around the wire
by adjusting the manufacturing conditions, said Lieber. "This is actually
quite easy," he said. "The surface area of the nanowire is much, much
larger than that of the [droplet], and [layering] generally just involves...
increasing the temperature or concentration" of the vapor.
Different vapors provide different materials. The researchers made several
prototype wires, including a 50-nanometer-diameter nanowire made of a
19-nanometer core of pure silicon and a shell of silicon doped, or mixed,
with boron. Semiconductors used in electronics are often doped to make
them conduct electricity more readily.
The researchers also made several other types of nanowire: a 26-nanometer
germanium core with a 15-nanometer doped silicon shell; a 21-nanometer
doped silicon core with a 10-nanometer germanium shell; and a 20-nanometer
silicon core with a 30-nanometer middle layer of germanium and an a 4-nanometer
outer shell of doped silicon.
The researchers demonstrated the usefulness of the layered nanowire technique
by using it to make a field-effect transistor. Transistors are the basic
building blocks of computers and most other electronic devices.
A transistor consists of a channel that carries current, a source electrode
that conducts current into the channel, a drain electrode that carries
current away, and a gate electrode that turns on the transistor by increasing
the electrical conductivity of the main channel, which allows current
to flow through the device.
The researchers' built a transistor by layering 10 nanometers of germanium
over a 10-nanometer doped silicon core, then adding a 4-nanometer layer
of the insulator silicon oxide followed by a 5-nanometer outer layer of
doped germanium. The researchers attached a gate electrode to the outer
germanium layer, and source and drain electrodes to the inner germanium
The researchers' prototype transistor is several times narrower than those
used in today's computer chips.
The researchers used a similar technique last February to produce nanowire
segments composed of different materials. Together, the segmenting and
layering techniques will allow researchers to control nanowire composition
along both length and width.
The work is "a major advance," said Zhong Lin Wang, a professor of materials
science and engineering at the Georgia Institute of Technology.
Most nanoscale electronic devices made from carbon nanotubes or nanowires
are oriented lengthwise, Wang said. The Harvard work shows for the first
time that the different layers that make up field-effect transistors and
other devices can be positioned across the width of nanowires, he said.
"This research demonstrates the possibility of building much smaller devices
based on nanowires," he said
The researchers have produced a "very nice piece of work" that clearly
demonstrates the versatility and technical potential of heterostructure
semiconductor nanowires, said Peidong Yang, an assistant professor of
chemistry at the University of California at Berkeley. Yang led a research
team that recently developed a similar technique to make ribbon-shaped
nanoscale heterostructures, or structures made from two or more semiconductors.
"Heterostructures are the common components for many of the electronic
and optical devices in our daily lives, such as transistors, light-emitting
diodes and laser diodes," he said.
The Harvard researchers are working on making a high-performance field-effect
transistor that could be integrated with conventional electronic circuitry,
said Lieber. "We are pushing very hard to make [a] transistor that could
find its way into hybrid devices," he said. "This is something we're discussing
with Intel's advanced transistor group."
The researchers are also using layered nanowires to construct a new type
of nonvolatile random access memory, said Lieber. Nonvolatile memory retains
data even when the power is turned off.
They are also looking to use the nanowires to make devices that produce
light, including single-nanowire lasers, he said.
Before the nanowires can be used in practical applications, the growth
process must be controlled more finely to produce more perfect structures,
Prototype nanowire transistors could be built in two to five years, said
Lieber. The biggest technological hurdle is integrating these building
blocks into useful devices, he said. "This is the same issue facing much
of the field."
Lieber's research colleagues were Lincoln Lauhon, Mark Gudiksen and Deli
Wang. They published the research in the November 7, 2002 issue of the
journal Nature. The research was funded by the National Science Foundation
(NSF), the Office of Naval Research (ONR) and the Defense Advanced Research
Projects Agency (DARPA).
Timeline: 2-5 years
TRN Categories: Nanotechnology; Integrated Circuits; Semiconductors;
Materials Science and Engineering; Chemistry
Story Type: News
Related Elements: Technical paper, "Epitaxial core-shell
and core-multishell nanowire heterostructures," Nature, November 7, 2002
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