FOR
IMMEDIATE RELEASE
December
3, 2003
|
UM
Physicists Show Nanotubes are Best Semiconductors
|
COLLEGE
PARK, Md. - University of Maryland physicists have found that semiconducting
carbon nanotubes have the highest mobility of any known material at
room temperature. Mobility is a measure of how well a semiconductor
conducts electricity.
A team of researchers led by Michael Fuhrer, assistant professor of
physics in the university's Center for Superconductivity Reearch, have
fabricated a semiconducting nanotube transistor that shows a mobility
almost 25 percent higher than any previous semiconducting material and
more than 70 times higher than the mobility of the silicon used in today's
computer chips. The results, published online in the journal Nano Letters,
provide new evidence that semiconducting carbon nanotubes hold great
promise for replacing conventional semiconductor materials in applications
ranging from computer chips to biochemical sensors.
"This is the first measurement of the intrinsic conduction properties
of semiconducting nanotubes," said Fuhrer, who heads the university's
Nanoelectronics Research Group. "It is an important step forward
in efforts to develop nanotubes into the building blocks of a new generation
of smaller, more powerful electronics."
Mobility is the conductivity of a material divided by the number of
charges, which carry the current, and is the number typically used to
compare the conduction properties of one semiconductor to another. Fuhrer's
group, in research supported by the National Science Foundation, found
that the mobility of their carbon nanotube exceeds 100,000 square-centimeters
per volt-second at room temperature. The previous record for room temperature
mobility was 77,000 square-centimeters per volt-second in indium antimonide
and was first measured in 1955. The mobility in the silicon used to
make today's computer chips is only about 1,500 square-centimeters per
volt-second. To perform their measurements, the team had to prepare
extremely long nanotubes, and be able to place metal wires precisely
on one single nanotube. They synthesized nanotubes with lengths up to
0.3 millimeters, or about 100,000 times the nanotubes' diameter. This
is some 100 times longer than nanotubes previously studied in electronic
measurements. The nanotubes were grown directly on flat silicon chips.
A special technique using a scanning electron microscope had to be developed
in order to locate the nanotubes on the chip so that wires could be
connected to them.
Carbon nanotubes can be thought of as a single atom-thick sheet of graphite,
rolled into a seamless cylinder. Nanotubes were discovered in 1991 by
Sumio Iiijima (NEC, Japan), and since then have been the subject of
research around the world. Today nearly every major research university
has at least one group studying carbon nanotubes. Nanotubes are being
considered for many applications in electronic devices including field-effect
transistors, memory cells, and chemical and biochemical sensors. In
each of these applications mobility is the key to how well the device
can perform. Mobility dictates how fast the charges move through a device,
so it determines the ultimate speed of a transistor.
Mobility
also determines the change in conductivity that is caused by a nearby
electrical charge. Thus, mobility also is a measure of the sensitivity
of a transistor for detecting charge (as in a memory cell) or detecting
a nearby molecule (as in a chemical or biochemical sensor).
Fuhrer's group demonstrated last year that high-mobility semiconducting
nanotube transistors could detect single electrons in a memory cell.
This suggests that chemical sensors made from nanotubes could detect
a single molecule of a target compound. The International Technology
Roadmap for Semiconductors, an assessment of the semiconductor industry's
technology requirements, says that a replacement material for silicon
with higher mobility will be necessary by the year 2010. According to
Fuhrer, the new findings by he and his colleagues indicate nanotubes
could fill that role. "Many challenges remain before nanotubes
can be used instead of silicon in computer chips," notes Fuhrer.
"The contact resistance between nanotube and metal electrodes must
be controlled. Nanotube batches must be prepared that contain only semiconducting
nanotubes. And nanotubes must be placed with precision on substrates."
However, significant progress is taking place in all these areas, and
the challenges do not seem insurmountable," he said.
___________________________# # #
"Extraordinary Mobility in Semiconducting Carbon Nanotubes,"
Nano Letters, published online December 03, 2003, T. Dürkop, S.
A. Getty, Enrique Cobas, M. S. Fuhrer, Department of Physics and Center
for Superconductivity Research, University of Maryland.
Related earlier paper
"High-Mobility Nanotube Transistor Memory" Nano Letters, published
online May 30, 2002, M. S. Fuhrer, B. M. Kim, T. Dürkop, and T.
Brintlinger, Department of Physics and Center for Superconductivity
Research, University of Maryland.
CONTACTS:
Lee
Tune
301-405-4679
ltune@umd.edu or
Karrie Hawbaker
301-405-5945
karrie@physics.umd.edu