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issue: May 2004 APPLIANCE Magazine

Technology Report
Semiconducting Carbon Nanotubes


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A team of researchers at the University of Maryland have found the highest mobility of any known material at room temperature, a discovery which may hold promise for replacing conventional semiconductor materials.

Physicists at the University of Maryland (UMD) have discovered that semiconducting nanotubes have the highest mobility—a measure of how well a semiconductor conducts electricity—of any known material at room temperature.

This was ascertained during a study conducted at the university. With use of the nanotubes, mobility was found to be 25 percent higher than any other previous semiconducting material and 70 times more mobile than the silicon used in today’s computers. Currently, silicon is used for computer chips not because of its high mobility, but because it is possible to grow a good insulating oxide on it, which is used for a gate dielectric, says Dr. Michael Fuhrer, an assistant professor of physics in the university’s Center for Superconductivity Research and leader of the team of researchers.

There are two reasons why high mobility was achieved through nanotubes in this study. Graphite naturally has a high mobility due to weak interaction of the phonons (lattice vibrations) in graphite with the electrons. “The mobility of graphite is about 20,000 cm2/Vs at room temperature,” Dr. Fuhrer notes. “However, graphite is not a semiconductor, so the conduction can’t be turned off completely in a transistor. The nanotube can be thought of as a way to engineer a bandgap in graphite, so we can make transistors out of it.”

Secondly, high mobility is achieved in this environment because of the one-dimensional nature of the nanotubes. Electrons traveling along the nanotube can only scatter completely backwards, and this requires a large momentum transfer, Dr. Fuhrer tells APPLIANCE. “This momentum is not present in the phonons at room temperature, so scattering is reduced,” he says. “I believe this is responsible for the higher mobility in carbon nanotubes (100,000 cm2/Vs) than in graphite.”

A carbon nanotube is essentially a sheet of graphene (singular for graphite) rolled up into a seamless tube, Dr. Fuhrer explains. Graphite is metallic in some directions and semiconducting in others, he says, but this is not identifiable by connecting wires to graphite, since the electrons are free to travel in the metallic directions. Graphite, however, can conduct currents so it looks like a semi-metal. But if graphene is rolled into a tube, a particular direction is determined—the direction along the tube axis, which is the metallic or semiconducting direction, Dr. Fuhrer says. “This depends on how sensitively you roll up the graphene to make the nanotube,” he notes.

Dr. Fuhrer also points out that the International Technology Roadmap for Semiconductors predicts that by 2010, silicon dioxide will need to be replaced with a higher K dielectric and silicon with a higher mobility material. According to the UMD study, the semiconducting nanotube transistor that was fabricated shows promise for replacing conventional semiconductor materials in applications that range from computer chips to biochemical sensors. “This is the first measurement of the intrinsic conduction properties of semiconducting nanotubes,” Dr. Fuhrer tells APPLIANCE. “It is an important step forward in efforts to develop nanotubes into the building blocks of a new generation of smaller, more powerful electronics. Although mobility isn’t everything, it is important.”

For appliance designers and engineers, this discovery means that now the groundwork has been laid to open up new areas of applications in flexible/wearable/disposable electronics, Dr. Fuhrer adds. He admits, however, that there are many technical hurdles before this happens.

“My feeling is that in the nearer term what we will see is some low- to medium-performance nanotube device technologies in which nanotubes can be used as an alternative where the processing constraints are unusual,” Dr. Fuhrer predicts.

 

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