array could form chips
Ted Smalley Bowen,
Technology Research News
The imperative to shrink all things electronic
has led researchers to fashion transistors from carbon nanotubes, which
are thousands of times thinner than a human hair. But before these minuscule
components can find their way into computer chips or other devices they
must be produced en masse.
A group of researcers from the Samsung Advanced Institute of Technology
and the Chonbuk National University in Korea has made nanotube field-effect
transistors in bulk by growing them in vertical bunches, then using electron
beam lithography and ion etching to make the source, gate and drain electrodes
that control the flow of electrons.
These high-density arrays of transistors can be controlled with little
energy, according to Won Bong Choi, project manager at the Samsung Advanced
Institute of Technology.
The multi-walled nanotubes, which are sheets of carbon atoms rolled up
into tubes, measure 20 nanometers wide. A nanometer is one millionth of
a millimeter. Two trillion of the transistors would fit into a square
The arrays of tiny transistors could eventually be used to make smaller,
faster computer chips and very sensitive sensors, according to Choi.
To make the arrays, the researchers deposited a layer of silicon dioxide
on an aluminum oxide plate, then used a beam of electrons to mark a grid
of microscopic dots on the surface, and applied chemicals to eat away
the marks to form pores in this template. The researchers released a hot
vapor containing carbon into the pores to form the nanotubes.
They added source, drain and gate electrodes to the nanotubes by applying
silicon dioxide lines across the grid of nanotubes, using electron beam
lithography and ion milling to etch the silicon dioxide, then adding gold
and titanium to form the electrodes. In ion milling, high-energy ions
are fired at a surface to eject individual atoms of the surface material.
The source electrodes are essentially rows across the bottom of the carbon
nanotubes, and the drain electrodes are perpendicular rows across the
top of the nanotubes. The gate electrodes are also near the top of the
nanotubes. In field-effect transistors the gate electrodes control the
flow of electrons through the transistor. When current is applied to the
gate electrode, the resulting electrical field turns the transistor on,
and electric current flows from the source to the drain electrode.
The researchers' carbon nanotube transistors worked at temperatures up
to an extremely cold -243 degrees Celsius, according to Choi. The transistors
will need to work at much warmer temperatures to be used in practical
The vertical alignment of the nanotubes allows many nanotubes to be packed
in the small spaces needed for producing small, dense electrical components.
The method also allows for close control of the the manufacturing process,
said Choi. It allows fabricators to "control the position of carbon nanotubes,
manipulate [them, and] control [their] diameter and thickness," he said.
Conventional transistors are typically made of layers of silicon or germanium
semiconductor material that have different electrical properties due to
small amounts of impurities - usually boron, arsenic, or iridium. The
electrical properties of carbon nanotubes can be altered by varying the
angle at which the carbon atoms are aligned.
Transistors are used as amplifiers, oscillators, photocells and switches,
which are the key component of the integrated circuits that make up computer
chips. Computers use the on and off states of transistors to represent
the ones and zeros of binary code.
The researchers' method is still very rough, and they did not demonstrate
that individual transistors could be accessed, according to Zhen Yao,
assistant professor of physics at the University of Texas.
"Each nanotube can in principle be addressed using a source line at the
bottom and a drain line at the top. However, the authors didn't really
demonstrate that they could address individual nanotubes. Instead, the
field-effect transistor characteristics were measured over the entire
array of nanotubes," he said.
In addition, the level of current running through the array is very low,
said Yao. The transistors' structure and materials are part of the problem,
"Because the transistor gate, which controls the flow of electrons through
the source and drain electrodes, was placed over the ends of the vertically-aligned
nanotubes, "the gate coupling would be too weak for practical device operations,"
said Yao. In addition, the nanotubes contain a lot of defects, which makes
them difficult to analyze electrically, he said.
The nanotubes rough composition limits their use, said Yue Wu, associate
professor of physics at the University of North Carolina. "The carbon
nanotubes are very defective. The device won't work at room temperature
because the tubes are not clean semiconductors," he said.
Nonetheless, carbon nanotubes sensors that use the arrays could be possible
within five years, and computer memory chips made with the process could
be practical by 2010, said Choi.
Choi's research partners were Jae Uk Chu, Kwang Seok Jeong, Eun Ju Bae,
and Jo-Won Lee of the Samsung Advanced Institute of Technology, and Ju-Jin
Kim and Jeong-O Lee of the Chonbuk National University Department of Physics.
They published the work in published in the November 26 issue of Applied
Physics Letters. The work was funded by Korean Ministry of Science and
Timeline: 5-8 years
TRN Categories: Semiconductors; Integrated circuits; Materials
Science and Engineering
Story Type: News
Related Elements: Technical paper, "Ultrahigh-density nanotransistors
by using selectively grown vertical carbon nanotubes", Applied Physics
Letters, November 26, 2001.
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