Nanotube array could form chips

By 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 centimeter.

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 devices.

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, he said.

"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 Technology.

Timeline:   5-8 years
Funding:   Government
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.


January 23, 2002

Page One

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Nanotube array could form chips

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