Unipolar Nanotube Devices Emit in the Near-IR

Daniel S. Burgess

Scientists at IBM’s Thomas J. Watson Research Center in Yorktown Heights, N.Y., and at Duke University in Durham, N.C., have fabricated field-effect transistors based on partially suspended carbon nanotubes that generate near-infrared radiation. The emission process is ~1000 times more efficient than in earlier devices that employed ambipolar injection. The IR sources have potential applications in the production of ultrasmall photonic devices.

Carbon nanotubes partially suspended over a trench produce near-infrared radiation under unipolar injection. Courtesy of Phaedon Avouris.

Phaedon Avouris, manager of nanometer-scale science and technology at the research center, explained that undoped nanotubes can be made to generate radiation by injecting electrons and holes into opposite ends of the structure, a feat that they demonstrated previously (see “Single Nanotubes Yield IR Radiation,” Photonics Spectra, June 2003, page 18).

“In this work, the operation of the light source is different,” he said. “We still use an undoped nanotube, but only one type of carrier — electrons or holes — is injected.” Employing unipolar injection enables the insertion of more carriers into the nanotube devices, and the excitation mechanism is orders of magnitude more efficient.

In the ambipolar devices, a carbon nanotube was supported by a film of SiO₂. In the new emitters, a segment of a 2- to 3-nm-diameter tube is suspended over a 0.4- to 15-µmwide trench etched through the 200-nm-thick SiO₂ film into the under-lying doped silicon substrate.

Avouris explained that this produces an abrupt variation in the potential along the nanotube. Encountering the point of transition from support to suspension, the injected carriers gain sufficient energy to excite bound electron-hole pairs. These excitons, which have binding energies on the order of 0.5 eV and which do not contribute to the electrical current, have a high recombination probability and emit 1- to 2µm radiation, depending on the diameter of the nanotube.

“Essentially, every injected electron produces an excited state,” Avouris said.

Although it is too soon to know what the ultimate applications of these devices will be, he said, they may have potential in the stimulation of single molecules or individual nanostructures. Because the emission wavelengths are in the range of interest to telecommunications, they also may be useful in that sector. He further noted that the single-nanotube devices may be employed as photodetectors as well as emitters.

Currently, the researchers are investigating the role of local forces on the performance of the nanotube devices. “It is clear that the environment of the nanotube strongly influences the yield,” Avouris said, “and we are trying to find the conditions that maximize it.”

Science, Nov. 18, 2005, pp. 1171-1174.