Nov 10, 2008
Core-shell improves light-emitting properties of nanowires
Nanowires could be used to fabricate a variety of optoelectronics devices, including light-emitting diodes and lasers. However, the optical properties of these materials are seriously affected by surface states and an optical "dead layer" – because of the wires' large surface-to-volume ratio. Researchers in Italy now propose a way to overcome this problem for InGaAs nanowires at least – by growing core-shell GaAs structures around them. Indeed, the team led by Faustino Martelli of the Laboratorio TASC in Trieste, has improved the infrared luminescence of InGaAs/GaAs nanowires at low temperatures by two to three orders of magnitude using this technique. The wires also luminesce at room temperature.
"We have made InGaAs/GaAs core-shell heterostructures that could have important applications in active devices emitting in the infrared region, important for telecommunications," Martelli told nanotechweb.org. "The material shows very good optical properties, which makes it worth investigating further."
The researchers grow the InGaAs/GaAs core-shell nanowires by molecular beam epitaxy using the vapour-liquid-solid mechanism induced by a gold nanoparticle that acts as a growth catalyst. This allows for directional, 3D growth of freestanding nanowires. When the nanowires reach a certain length, significant lateral growth occurs on the nanowire sidewalls. At this point, Martelli and colleagues close the indium effusion cell and obtain the growth of GaAs on the InGaAs nanowire sidewalls. They verify the core-shell structures by TEM.
GaAs has a larger bandgap than InGaAs, which affects the InGaAs in two ways. First, it passivates the surface states and second it acts as a reservoir of carriers, which are captured by the energetically favoured electronic states of the InGaAs core when the GaAs is excited by green light.
"These two mechanisms, the first of which is more important, increase the luminescent efficiency of the core-shell nanowires by two to three orders of magnitude at low temperatures compared with the pure InGaAs nanowires grown without a GaAs shell," explained Martelli. "This increase also allows us to observe room-temperature light emission."
The InGaAs/GaAs core-shell nanowires could be used to make active optical devices with extremely low operation currents. They could also find use in active/passive transmission lines once transferred onto specifically designed supports. Lasers, waveguides and modulators based on the core-shell heterostructures might also be envisaged. "Indeed, using wires transferred onto patterned substrates for large-area devices (or multi-device boards) based on nanostructures is becoming a very important field of research," added Martelli.
The team is continuing to investigate different InGaAs alloy composites for use in these types of structures. Ideally, the scientists would like to see emission at both 1.31 and 1.55 nm and improvement in crystal quality should help to further enhance the material's performance. "We are also studying the optical properties of these novel heterostructures in more detail," said Martelli.
The work was published in Applied Physics Letters.
About the author
Belle Dumé is contributing editor at nanotechweb.org