"LEDs work like our previous ambipolar nanotube emitter, where electrons and holes were injected from the source and drain electrodes separately. But our new device generates a single type of carrier locally by accelerating it under a high local electrical field to create strongly correlated electron-hole pairs," Chen said.
Other research groups have reported light emission from carbon nanotubes stimulated to photoluminescence with a pulsed laser, but IBM claims its technique creates an exciton density that's a hundredfold higher. The mechanism, according to IBM, is radiative recombination electron-hole recombination whereby the band-to-band recombination energy of the radiation released corresponds to the bandgap of the semiconductor.
"We can adjust the wavelength of the emission by changing the diameter of the nanotubes," Chen said. "So far we have been able to adjust it to any communications wavelength between 1 and 2 microns."
IBM credits the extreme efficiency of its new technique to the confinement of the hot carriers within the nanometer-thin suspended carbon nanotubes.
Because of the nanotubes' extreme aspect ratio only nanometers in diameter but micrometers (microns) long mathematically they are the equivalent of one-dimensional conduits. That one-sional aspect ratio, according to IBM, resulted in weak electron-phonon scattering and strong electron-hole binding.
At the edge of the suspension interface, the electrons and holes were quickly accelerated by band bending, caused by reduced capacitive coupling to the backgate over the suspended portion of the nanotube. Electrons jumped to the conduction band but remained bound to a specific hole, thereby forming into excitons, which emitted light to shed energy when the electron in a pair fell back from the conduction band to the valence band and combined with its matched hole.
"Our drain voltage was just 5 volts, and yet the energy levels of our excitons were extremely high," said Chen. "Nanotubes are better conductors than copper or aluminum, and because they are also a direct bandgap material, they can make excellent optical devices too."
IBM fabricated the light-emitting device by etching 0.4- to 15-micron-wide trenches in a 200-nanometer-thick silicon dioxide film on a highly doped silicon wafer that acted as a backgate to the nanotube transistor. Carbon nanotubes with diameters of 2 or 3 nanometers were grown over trenches by chemical vapor deposition, and palladium source and drain electrodes were overlaid with lithography over each end of the suspended nanotube. The nanotubes, acting as the channel of a field-effect transistor with a backgate, measured between 4 and 80 microns in length.
The resulting device emitted light of exponential strength relative to the drive current on the backgate, IBM reported.
For the future, the researchers plan to improve the LEN transistor's efficiency and to experiment further with the confinement effects of forcing hot carriers down a nanometer-thin conduit.
"We also want to do more experiments on these low-dimensional materials," said Chen. "After all, a nanotube is practically one-dimensional, and as a result its properties are very different.
"These low-dimensional materials can enable many new device concepts and functionalities."