yield nanotube transistors
Technology Research News
Scientists from IBM have moved a significant
step closer to realizing the dream of using carbon nanotubes as computer
chip transistors with a method that allows them to selectively destroy
metal nanotubes while leaving those that are semiconducting intact.
Carbon nanotubes, which form naturally when sheets of graphite roll up
under high heat and are a component of soot, can be smaller than one nanometer
Whether a nanotube conducts electrons freely like a metal or with some
resistance like a semiconductor
depends on the combination of the angle of the roll and the diameter of
Commercially made nanotubes are a mix of the two types of tubes, and separating
the microscopic cylinders, which naturally stick together in clumps, is
a tedious process. The researchers' method of separation is one step toward
making nanotubes viable for mass-produced applications.
While studying how much current they could pass through the nanotubes,
the researchers found that nanotubes can handle a billion or more amps
per square centimeter, or 1,000 times more than copper or aluminum. When
they increased the energy of the electrons to about 5 volts, however,
the nanotubes started breaking down. "That work... gave us the idea to
utilize this destructive event in a constructive manner to actually get
rid of the metallic nanotubes," said Phaedon Avouris, manager of nanometer-scale
science and technology at IBM's T. J. Watson Research Center.
The researchers have found two distinct uses for metal nanotube destruction.
The first is to create an array of semiconducting nanotubes, which has
significant implications for eventually using them as transistors on computer
While metallic nanotubes can be used as wires, semiconducting nanotubes
can be used as field effect transistors (FETs), which use an electric
field to affect whether the device conducts current or not, effectively
turning the flow on and off. An array of nanotubes that includes both
types, however, means current will always pass through the metallic nanotubes
acting as wires, making the semiconducting nanotubes irrelevant. "The
metallic nanotubes cannot be affected by the gate, so they're always on,"
The researchers made an array of semiconducting nanotubes by depositing
rope-like clumps of both metallic and semiconducting nanotubes on a substrate,
and covering them with electrodes, which they used to stop any current
from running through the semiconducting nanotubes. They then identified
the shorts, or places where current was passing through metallic nanotubes,
and fixed these shorts by applying current strong enough to destroy them.
The resulting array consisted of ropes of semiconducting nanotubes connecting
pairs of electrodes.
The method is essentially a new way of fabrication that doesn't require
separation or orientation of the nanotubes, said Avouris. "We just cover
them with electrodes and do the final fabrication by current rather than
by chemistry or any other technique," said Avouris.
The researchers have made arrays using several thousand nanotubes and
could easily scale that up, said Avouris. "You can make them as big as
you want," he said.
The second application for the researcher's method of destroying metal
nanotubes is to selectively shape individual multiwall nanotubes, which
are essentially nested groups of tubes, by destroying individual tube
layers from the outside in.
This allows them to choose the exact diameter of a nanotube, which determines
its electrical properties. Using the method, the researchers have fabricated
nanotube FET's with bandgaps, or propensity to channel electrons, of their
choice. "The bandgap of nanotubes, unlike silicon, is not fixed. It depends
on the diameter of the tube. If you start removing [the shells] one by
one, the diameter decreases and correspondingly the bandgap increases.
So you can stop where you want and you have a transistor with the desired
bandgap," Avouris said.
The researchers have also characterized the electrical properties of these
transistors, and according to Avouris they are close to those of p-type
silicon transistors. P-type transistors use positive holes to carry current,
while n-type transistors use electrons.
The nanotube transistor characteristics include contact resistance, meaning
how well the nanotubes connect with the electrodes bringing them current,
transconductance, which measures how fast the current changes, and mobility,
or the ease with which charge carriers move.
The researcher's next steps are to make top-gated transistors, which are
the type used in computer chips, and to optimize the transistor characteristics.
Making transistors and arrays using semiconducting nanotubes are significant
steps toward eventually making circuits from nanotubes, which have the
potential to be more than an order of magnitude smaller than the circuits
that make up today's computer chips.
Today's Pentium IV chips, for example, sport about 42 million transistors
with features as narrow as 180 nanometers. Computer chips use transistors
to form the circuits that perform basic logic functions, and in order
to continue to build faster, more powerful computers, chip manufacturers
must cram transistors into a smaller space.
Although the lithography techniques used to make these transistors for
the past few decades have improved enough to double the number of transistors
on a chip every 18 months or so, they're expected to run into the laws
of physics within the next decade.
Self-assembled carbon nanotubes are a good candidate to eventually provide
transistors small enough to go beyond the lithography size limits.
"Experimentalists already know how to make individual nano-transistors
using... single semiconducting nanotubes or other molecules," said Vincent
Crespi, assistant professor of physics at Penn State University. The IBM
work is "a first step towards practical integration of multiple nano-devices
on a single chip. Thatís why itís important. In the grand scheme of integrated
electronics, itís only a baby step, but at least the baby has started
to walk,Ē he said.
"Itís still unclear exactly which techniques of nanoelectronics will pan
out into practical devices, Crespi added. "But this work has a reasonable
chance, in 10 years, of being seen as one of the important enabling advances
for a new technology."
The IBM researchers are "the first group to show a rational approach to
making devices out of nanotubes," said Charles Lieber, a chemistry professor
at Harvard University. "It's a really nice advance [but] it's going to
be pretty hard to scale up," he said.
IBM's destructive method is "another strategy that may help to achieve
that next step," toward working nanoelectronics, said Lieber, adding that
self assembled materials are another possibility.
Although the IBM process is a step toward working nanoelectronics, it's
a long way between nanotube arrays and nanotube logic circuits that can
be used in computer chips. Circuits require both p-type and n-type transistors,
and lot of work remains to develop large-scale manufacturing processes,
making nanotube computer chips likely a decade away, Avouris said.
There's also the possibility of hybrid technologies the use both silicon
and nanotubes, he added. "Both silicon and carbon are in the same column
of the periodic table and they have many similar properties, so using,
for example, nanotubes as interconnects -- metallic wires that connect
one device to another -- that's also very promising."
At this point all the work geared toward nanotube circuits is, of course,
research, Avouris said. "There are a lot of unknowns... I think as we
get closer to 2010 and the end of silicon the pressures to have a working
successor will intensify," he added.
Avouris' research colleagues were Philip G. Collins and Michael S. Arnold
of IBM's T. J. Watson Research Center. They published the research in
the April 27, 2001 issue of the journal Science. The research was funded
Timeline: 10 years
TRN Categories: Nanotechnology; Integrated Circuits
Story Type: News
Related Elements: Technical paper, "Engineering Carbon Nanotubes
and Nanotube Circuits Using Electrical Breakdown," Science, April 27,
Jolts yield nanotube
hints at quantum computer power
Metal makes DNA more
process points to nanotech production
Plastic pins DNA
molecules in place
Research News Roundup
Research Watch blog
View from the High Ground Q&A
How It Works
News | Blog
Buy an ad link