Researchers from the University of Illinois have experimented with carbon nanotubes by pushing them close to their breaking point. They saw a huge increase in the current-carrying capacity of the nanotubes, considerably more than what was previously thought possible.
Using an avalanche process, the researchers made the semiconducting carbon nanotubes drive electrons to more paths, increasing in this way the number of electrons which could flow at a particular moment though them.
Eric Pop, a renowned professor of electrical and computer engineering from Illinois University stated that “Single-wall carbon nanotubes are already known to carry current densities up to 100 times higher than the best metals like copper. We now show that semiconducting nanotubes can carry nearly twice as much current as previously thought.”
After deep investigations the researchers found that electrons and holes can create additional electron-hole pairs at high electric fields (10 volts per micron), leading in this way to an avalanche effect. The free carriers are able to quickly multiply and the current rapidly increases until the nanotubes break down. Eric Pop said that the increase in current is due to the onset of avalanche impact ionization. This phenomenon was observed in some semiconductor diodes and transistors at high electric fields, but not before in nanotubes.
The maximum current carrying capacity for metallic nanotubes has been measured at about 25 microamps. The maximum current carrying capacity for semiconducting nanotubes is less established. Previous theoretical predictions showed a similar limit for single-band conduction in semiconducting nanotubes.
To understand current behavior, Pop and two of his students Albert Liao and Yang Zhao, grew single-wall carbon nanotubes by chemical vapor deposition from a patterned iron catalyst. For measurements were used palladium contacts and the nanotubes were pushed close to their breaking point in an oxygen-free environment. The current first plateaued near 25 microamps, and then had a sharp increase at higher electric fields. Repeated measurements revealed currents of up to 40 microamps, nearly twice those of previous reports. To simulate the avalanche process, very high electric fields were induced in the nanotubes, and some of the charge carriers were driven into nearby subbands. Instead of using just one “lane”, electrons and holes could occupy several available lanes, resulting a much greater current measured.
“Our results suggest that avalanche-driven devices with highly nonlinear turn-on characteristics can be fashioned from semiconducting single wall nanotubes” said Pop.
The avalanche process cannot be observed in metallic carbon nanotubes due to the energy gap which is required for electron-hole multiplication.