Tuesday, August 26, 2008

IBM demonstrates light-emitting nanotube

EE Times

PORTLAND, Ore. — Electric control of the spectrum, direction and efficiency of light-emitting nanotubes (LENs) has been demonstrated by researchers at IBM Corp.'s Thomas J. Watson Research Center, bringing silicon photonics one step closer to reality.

IBM Research (Yorktown Heights, N.Y.) previously demonstrated record-breaking silicon optical waveguides and higher electroluminescent efficiency for LENs compared to LEDs. Now, it has put a LEN inside an optical waveguide to achieve directional surface emission, wavelength selectivity and the potential for ultrahigh efficiency.

"Like most light-emission sources, nanotubes emit light in all directions. Their spectrum was relatively broad and their efficiency was not very high," said Phaedon Avouris, IBM Fellow and manager of Nanometer Scale Science and Technology at IBM Research. "We attacked all these problems, making its light directional so it can be coupled to optical filters or to a device to transport it. We controlled its spectrum with an optical cavity and we have proposed a theory to help us achieve higher efficiency."

By fabricating an optical cavity around light-emitting nanotube mirrors at the bottom and top, wavelengths were confined to the desired 1.55-micron communications frequency.

IBM achieved surface emission by combining a single nanotube-based field-effect-transistor with a pair of metallic mirrors, one above and below the nanotube which lies flat on the silicon chip. The bottom mirror was made from silver, with a top half-mirror made from gold. Light was emitted from the nanotube in the cavity, which was filled with transparent dielectric.

The distance between the top and bottom mirrors was calculated to be half of the desired emission wavelength, which was set to be near a communications wavelength of 1.55 microns. Light was reflected upward off the bottom of the cavity, where half was passed as a surface emission from the LEN while the other half was reflected back down to the bottom mirror to reinforce the desired emission wavelength.

"We confined the emission in an optical cavity with two mirrors, so that light forms a standing wave between the mirrors which enhanced the frequencies, whose wavelength were equal to half the size of the cavity," said Avouris. "We used lithography to form the cavities, which achieved a dramatic enhancement--confining the spectrum to about 10 percent of what it was without the cavity, and giving us an overall enhancement [in the efficiency] of the emission of 400 percent."

Nanotubes have slightly different diameters (in this case, about 2 nanometers). As a result, they have slightly different bandgaps, and thus emit light at slightly different frequencies. However, by integrating the nanotube inside a cavity, physical confinement in the structure "eliminates unwanted frequencies thus [solving] the problem of nanotubes having slightly different diameters," according to Avouris.

IBM has demonstrated two methods of light emission in nanotubes: one that injects hot carriers into each end and another in which one end gets electrons while the other end gets holes. Another method injects excitons into one end. By characterizing these two methods, IBM claims to have finally answered the question of how electroluminescence compares to photoluminescence.

"There has always been a controversy over whether electroluminescence and photoluminescence involve the same states, so through comparisons using Raman scattering we have now proven that they both use the same states," said Avouris.

IBM has also proposed a theory for how heat diverts energy from luminescence, thus reducing the efficiency of LENs. While further experimentation will be required to prove the theory, IBM claims it is now only a matter of time until virtually all wasted energy that formerly generated heat can be eliminated by changing the electronic structure of a device.

"There are two types of emission from an object, radiative and nonradiative, with the latter being the energies lost by heat," said Avouris. Radiative emission "was always thought to be a fixed property of the material, but what we realized was that it is not only the material that is quantized--that has discrete states--but the photons also are part of a field that has quantized states.

"Emission comes by coupling these two fields. We now feel that by using an electric field we can change the electronic structure of nanotubes so that heat cannot be generated," he added.

Besides improving the efficiency of future devices by eliminating heat generation, IBM researchers also plan to experiment with methods of aligning nanotubes to a superlattice. This would allow an array of LENs to be fabricated on future silicon photonic chips.