Spectroscopy aids optoelectronics

A new generation of ultra-fast optoelectronics based on semiconducting nanowires could be possible with a spectroscopy technique developed by UK and Australian researchers.

The team, from Oxford University and the Australian National University, are aiming to understand the way electrons move in nanowires with radii as small as 30nm.

Principal investigator Michael Johnston, a physicist at Oxford University, said it is extremely challenging to attain this information because it is difficult to separate the properties of the nano-material from those of the contact material.

Johnston and his research team are exploring the limits of a contactless method called optical pump terahertz probe spectroscopy. The technique involves an ultra-fast laser that emits pulses of light, covering a wide frequency range that peaks in the terahertz region.

The pulses are emitted into a spectroscopy device every millisecond. Inside the device, each pulse is split into two beams and sent through a maze of mirrors and channels.

One beam passes through a nanowire sample before continuing through the maze and exiting. The other beam is used as a time reference.

The device uses detectors to determine what the electric field of the beam is before and after it hits the nanowire. The researchers then chart the electrical properties of the material after the pulse hits it.

With this, the team is able to reveal information on the movement of the electrons in the nanowire.

Johnston said his team can also unveil what happens after charged particles emitted from the laser light pulses are injected into the material. The researchers compared the lifetime of the charged particles in nanowires made of gallium arsenide and aluminium to gallium arsenide semiconductor wafers.

Johnston said the results show that the carriers had a dramatically reduced lifetime in nanowires. This characteristic is useful, he said, for making ultra-fast switching components for electronic devices.

Another benefit of their technique, Johnston said, is it can determine whether a nanowire is properly doped. ‘We can do this on an ensemble of nanowires so we can get a fast turn around from growing the structures to making doped devices,’ he said.

Johnston said he hoped by the end of their project in 2013 to have a new generation of nanowires ready for electronic devices.

Siobhan Wagner