Nanoscale light detection

The geometry of semiconducting nanowires makes them uniquely suited for light detection, according to a new

study that highlights the possibility of nanowire light detectors with single-photon sensitivity.

Nanowires are crystalline fibres and their inherent properties are expected to enable new photodetector architectures for sensing, imaging, memory storage, intrachip optical communications and other nanoscale applications, according to a new study in an issue of the journal Nano Letters. The UCSD engineers illustrated why the large surface areas, small volumes and short lengths of nanowires make them extremely sensitive photodetectors, much more sensitive than larger photodetectors made from the same materials.

‘These results are encouraging and suggest a bright future for nanowire photodetectors, including single-photon detectors, built from nanowire structures,’ said Deli Wang, an electrical and computer engineering professor from the UCSD Jacobs School of Engineering and corresponding author on the Nano Letters paper.

For a nanowire to serve as a photodetector, photons of light with sufficient energy must hit the nanowire in such a way that electrons are split from their positively charged holes. Electrons must remain free from their holes long enough to move along the nanowire and generate electric current under an applied electric field, a sign that light has been detected.

The new research demonstrates that the geometry of nanowires – with more surface area compared to volume – makes them inherently good at trapping holes. Dangling bonds on vast nanowire surfaces trap holes – and when holes are trapped, the time it takes electrons and holes to recombine increases. Delaying the reunion of an electron and its hole increases the number of times that electron travels down the nanowire, which in turn triggers an increase in current and results in ‘internal photoconductive gain.’

‘Different kinds of nanowires detect different wavelengths of light. You could make a red-green-blue photodetector on the nanoscale by combining the right three kinds of nanowires,’ said Cesare Soci, one of two primary authors on the Nano Letters paper and a postdoctoral researcher in the Deli Wang lab at the Jacobs School.

This work supports recent theoretical work from Peter Asbeck’s High Speed Device Group, also at the Jacobs School.

‘Our theoretical work showed that light-induced conductivity in nanowires can be increased by more than 10 times over similar bulk structures under the same illumination level. The work from Deli Wang’s lab has confirmed some of our calculations and provides further support for the idea that nanowires will be increasingly incorporated into photodetection and photovoltaic applications,’ said Asbeck.

In the new work, short pulses of ultraviolet light (hundreds of femtoseconds wide) were detected on time scales in the nanosecond range. Moreover, using electronic measurement of photocurrent, the engineers reported internal photoconductive gain (G) as high as 10^8, one of the highest ever reported.