"Since doping in carbon nanotubes is not well understood and almost impossible to do in the traditional sense by using dopants, the obvious approach to making a p-n diode was to use a split gate to electrostatically dope the two halves of a single nanotube," Ji Ung Lee of GE Research told nanotechweb.org. "By biasing one gate with a negative voltage and the other with a positive voltage, a p-n junction along a single nanotube can be formed."
Lee and colleagues formed pairs of split gates on a silicon wafer by standard optical lithography and metal deposition techniques. Next, they laid down an oxide layer to act as a gate dielectric and created single-walled nanotubes by chemical vapour deposition using iron nanoparticles as a catalyst. The tubes had diameters of 0.5 to 30 nm. Finally, the researchers made contacts to the nanotubes.
In this way, the team made around 400 devices per sq. cm of the wafer. They chose only to examine the devices containing semiconducting nanotubes. These diodes acted as rectifiers, exhibiting forward conduction and reverse blocking characteristics. At low bias, the researchers say that the devices' current-voltage characteristics followed the ideal diode equation, with an ideality factor close to one.
"To date, very little effort has been devoted to making diodes, partly because this is harder to do with carbon nanotubes," said Lee. "Most of the effort has been centred around the transistor. Our attempt is the first to demonstrate a well behaved p-n junction diode that closely follows the ideal diode equation."
Lee says that since carbon nanotubes are direct band-gap materials, the devices should function as light-emitting diodes. And, as the doping is not fixed, the devices could also be multifunctional.
"The polarity of the p-n junction can be switched dynamically to create an n-p or a p-n junction," said Lee. "Furthermore, the same device will also function as a p-channel field-effect transistor (FET) or an n-channel FET."
The carbon nanotube p-n junction diodes could have applications in areas such as optoelectronics, sensors and power electronics.
Now Lee says the obvious next step is to characterize light emission from the diodes and continue to improve upon the device structure to make an efficient light emitter.
The researchers reported their work in Applied Physics Letters.