"This is the first example of a one-dimensional nanoelectronic device that uses the possibility of creating ultra-sharp heterostructures inside nanowires," Lars Samuelson of Lund University told nanotechweb.org. "If you compare it with what has been produced previously with top-down fabrication techniques, the quality of our devices - made via a bottom-up technique - is highly superior."

To assemble the devices, the researchers used a vapour-liquid-solid growth process, seeding the nanowires onto gold aerosol particles. They used indium arsenide (InAs) for the emitter, collector and dot regions of the device, and indium phosphide (InP) as the barrier material. To swap the materials, they switched off the group III source beam to stop growth, and then changed the group V source. The resulting nanowires were 40-50 nm in diameter.

In this way, the scientists created a double-barrier resonant tunnelling device consisting of two roughly 5 nm thick InP barriers on either side of a 15 nm thick InAs quantum dot. Below a bias of about 70 mV, the device showed no current. At a bias of roughly 80 mV, however, the current-voltage plot exhibited a sharp peak. According to Samuelson, the peak-to-valley ratio was about 50:1, which is more than an order of magnitude better than the values achieved by devices made using conventional techniques.

"The bottom-up method will allow the parallel creation of thousands or millions of identical advanced heterostructure devices, each resulting from the location of the catalytic gold nanoparticles," added Samuelson. "We have previously shown that we can control not only the size but also the location of individual nanowires."

Now, Samuelson says the team aims to make devices that include not just a double-barrier structure, but a triple-barrier, quadruple-barrier, or even "one-dimensional superlattices of quantum dots". "It will be extremely nice to use this technique to create photonic devices such as single quantum dot light-emitting diodes and quantum cascade lasers," he said.

The scientists reported their work in Applied Physics Letters.