Portland, Ore. -- IBM Corp. researchers say they have characterized four types of carbon nanotube field-effect transistsor defects that can stimulate nanotube FETs to emit light.
High-bias transport in carbon nanotubes leaves them teetering on the edge of glowing, with hot carriers very close to the threshold of light emission. Electrons accelerate at defect-induced voltage drops, and the impact of collisions bumps other electrons from the valence band to the conduction band. When they fall back to the valence band, the electrons emit a photon to shed the excess energy--the underlying mechanism of electroluminescence.
The first light-inducing defect was discovered on the end contacts, where a natural Schottky barrier exists at the semi- conducting nanotube interface with its metal electrode. The second was found anywhere that charge had been inadvertently trapped in the oxide-covered silicon wafer; trapped charge locally inverted the carriers in the nanotubes atop it by forming a light-emitting intratube npn or pnp junction.
The third defect characterized was on a nanotube loop where the tube bent around and crossed over itself. At the crossover point, hot carriers could tunnel from one leg to the other, where they affected other carriers and caused light emission.
The first three defects were studied by randomly laying down nanotubes on a substrate and looking for those anomalies. But the fourth type of defect was intentionally made by partially covering a nanotube with a polymer. Where the polymer ended, a voltage drop caused carriers to collide, inducing electroluminescence.
"We are mainly interested in the electronic properties of carbon nanotubes for field-effect transistors, and optical measurements are a way of characterizing them," said IBM researcher Marcus Freitag. "But the methods we are develop- ing should also work on other candidates for light-emitting devices, such as nanowires, whose optical properties can be freely tailored."
IBM previously characterized the electroluminescence of ambipolar carbon nanotubes, which use a different mechanism to achieve electroluminescence. In an approach similar to an LED, electrons are injected into one end of the nanotube, and holes are simultaneously injected from the other end. Where they meet, they recombine, shedding energy with a photon.
"We found that they recombine in a small area inside the nanotube that can be tuned with a back gate--kind of like a pn junction, but without any chemical doping to define a junction," said Freitag. "That means that in our devices, the light-emitting spot can be moved around."
Unipolar nanotube transistors, on the other hand, have carriers traveling in one direction through the nanotube, which is the channel of the field-effect transistor. Previously, IBM had discovered that carriers would accelerate at the edge of a trench if a nanotube traversed it. At the edge of the trench, a voltage drop produced hot carriers that collided to push some electrons into the conduction band, inducing electroluminescence 1,000 times brighter than the ambipolar light emission of nanotubes.
Since then, IBM has begun characterizing less-drastic voltage drops at four different types of typical defects.
In the recent work, the nanotube transistor channels characterized were 50 microns long, though they were only about 2 nanometers in diameter. "In this research we fo- cused on unipolar emission from very long-channel devices, which we would not use for real devices, but which simplified our measurements since we could clearly see where the light was coming from," said Freitag. He said the four types of defects characterized would also be typical of the shorter transistor channels typically used in real devices.
"In all four cases you get unipolar light emission," said Freitag. "We pinpointed the nature of each of these defects and characterized their behavior."
For the future, the researchers plan to correlate the types of light emissions they get from nanotubes with different characteristics. With a catalog of light emission types and other types of measurable traits, engineers will be able to tell a nanotube type by its measurable emissions, they said.
"We want to optically characterize nanotubes of different chirality [the lattice angle at which the single-crystal graphite sheet is rolled up into a tube] so that we can provide engineers ways of determining the chirality of a nanotube from its emissions," said Freitag. "For instance, the phonon modes are very different for nanotubes of different chirality, so this is a very good tool to distinguish them."
The research team consisted of IBM researchers James C. Tsang, John Kirtley, Jia Chen and Phaedon Avouris, along with Aico Troeman and Hans Hilgenkamp of the University of Twente (Enschede, Netherlands) and Autumn Carlsen from the State University of New York at Albany.