Portland, Ore. - Carbon nanotubes recently set a record for carrier mobility in a semiconductor at room temperature, surpassing the previous record, set in 1955 by indium antimonide (InSb), by about 23 percent. The new record, measured by professor Michael Fuhrer and colleagues at the University of Maryland's Center for Superconductivity Research (College Park), indicates that silicon combined with nanotubes could outpace even the most exotic alternatives available today.
Mobility-the figure typically used to compare the performance capabilities of semiconductors-is defined as the conductivity of a material divided by the number of charges present to carry current. Mobility "determines a number of important properties, such as how fast a transistor can operate," said Fuhrer. "It determines how fast the charge carriers will move.
"We measured a factor of 10 higher mobility [than today's highest-mobility silicon transistors] for carbon nanotubes. It remains to be seen if we can turn this into a useful device, but it certainly sounds good so far."
Fuhrer's group measured 100,000-cm2/volt-second mobility at room temperature for the nanotube structures. That's about 70 times the 1,500-cm2/V-s mobility of standard silicon chips and 10 times the 10,000 cm2/V-s achievable by silicon's mobility leaders, discrete high-electron-mobility transistors (HEMTs). The mobility record set by InSb in 1955 was 77,000.
"There is a lot of progress on manufacturing and sorting carbon nanotubes. What we are doing is jumping ahead of that by isolating one device and studying it very carefully so we can really find out what the basic properties of the nanotube are," said Fuhrer. "We are getting at the fundamental materials properties of the carbon nanotube."
In the lab, Fuhrer fabricated nanotubes atop silicon wafers on which electrodes had already been fashioned. High-aspect-ratio nanotubes-with channels measuring in hundreds of microns, compared with the 2- to 4-nanometer diameter of conventional nanotubes-were later located using a scanning electron microscope. The SEM was needed to identify which electrodes were properly bridged by a nanotube acting as a transistor channel. The architecture used a back gate connected to all devices.
"We build our nanotubes on top of a silicon wafer, on top of silicon dioxide, and then the nanotube lies on top of that. And the silicon chip is highly doped so that it can act as a back gate," said Fuhrer.
Nanotubes were discovered in 1991 by Sumio Iijima (NEC, Japan) as a fourth form of carbon, after graphite, diamond and fullerenes. Measuring only a few nanometers in diameter, they are essentially atomically thin layers of graphite rolled into tubes.
Because carbon nanotubes have such a high measured mobility, they should yield not only faster transistors but also more sensitive ones. For chemical sensors, for instance, nanotubes are treated with a molecule that bonds with whatever the sensor is supposed to sense, thereby changing the nanotube's response.
"Our carbon nanotubes are perched atop the silicon dioxide, exposed to the environment, and they still operate very well-so the idea is to put some kind of chemical on the silicon dioxide that changes its charge when exposed to what you are trying to sense," said Fuhrer. "From our results, it looks like pushing this to the single-molecule sensitivity level is really possible."
Last year, Fuhrer's research group demonstrated a carbon-nanotube transistor that could detect single electrons in a semiconductor memory. For the future, he wants to characterize carbon-nanotube transistors further, starting with their high-frequency limits.
"I'd like to see a high-frequency transistor with a nanotube that would operate faster than HEMTs-something on the order of a terahertz, "Fuhrer said. "And our results indicate it might be possible to have a terahertz carbon-nanotube transistor. That would be quite a milestone."
Fuhrer predicts at least another year's hard laboratory work before announcing the results of his group's attempts to create a terahertz carbon-nanotube transistor.
For the recently reported project, Fuhrer was assisted in the lab by doctoral candidate Tobias Durkop and undergraduate student Enrique Cobas. The group's work at the university (www.physics.umd.edu/condmat/mfuhrer) is supported by the National Science Foundation.