Ultraviolet detection

By depositing thin films of silicon nanoparticles on silicon substrates, researchers at the University of Illinois at Urbana-Champaign have fabricated a photodetector sensitive to ultraviolet light. Such silicon-based ultraviolet sensors could prove very handy in military, security and commercial applications.

‘Silicon is the most common semiconductor, but it has not been useful for detecting ultraviolet light until now,’ said Munir Nayfeh, a professor of physics at Illinois and a researcher at the Beckman Institute for Advanced Science and Technology.

‘Ultraviolet light is usually absorbed by silicon and converted into heat, but we found a way to make silicon devices that absorb ultraviolet light and produce electrical current instead.’

As will be reported in the August issue of the journal Photonics Technology Letters, the technique behind silicon sensing of ultraviolet light is compatible with conventional IC technology. Conveniently, both the sensor and the computer could be incorporated on the same chip.

To create their ultraviolet-based photodetectors, Nayfeh, graduate students Satish Rao, Adam Smith and Joel Therrien, and undergraduate student Osama Nayfeh, begin with nanoparticles dispensed from silicon wafers using electrochemical etching. The nanoparticles are about 1 billionth of a metre in diametre and contain about 30 silicon atoms.

The researchers then deposit a thin film of the nanoparticles in a hole etched into the surface of another silicon wafer using standard lithographic techniques. Small conductive pads of gold complete the assembly. Electricity flows when ultraviolet light strikes the nanoparticles.

‘We start with a p-type Si wafer. An oxide layer 500nm thick is then grown on the substrate. A single pattern or an array of patterns is etched into the surface of the wafer. We then immerse the wafer, positively biased relative to a platinum electrode, in a suspension of particles in alcohol. A current is established which deposits nanoparticles in the hole, producing a film around 500nm thick with an active region of 5mm in diameter,’ said Professor Nayfeh.

Semitransparent gold of 4nm thickness is then deposited on the particle film. Next, 300nm thick gold pads are deposited on the gold layer and on the back of the substrate, providing a means to bias the device.

In use, ‘Ultraviolet light efficiently couples to the nanoparticles and produces electron-hole pairs,’ said Nayfeh, ‘with the result that the device produces nearly 750mA per Watt of incident UV at a low biasing of between 1-1.5V’.

That’s because, contrary to what occurs in bulk silicon, the electron-hole pairs in the new device do not appreciably recombine by non-radiative processes. Strong quantum confinement allows for charge separation and collection. The quantum confinement prevents the charge from recombining rapidly in terms of generation of heat. It’s the contacts and potential that collect the current instead.

‘Combining silicon nanoparticles with conventional silicon wafers could offer the best of both material systems’, Nayfeh added.

‘Placing a thin layer of nanoparticles on the front of a silicon solar cell, for example, could improve the cell’s efficiency and its lifetime.’

Other applications include ultraviolet-based detectors for missile-warning systems and airborne biological agents, industrial flame sensors and suntan monitors.

The National Science Foundation; the state of Illinois; the Grainger Foundation; and the Technology Research, Education, and Commercialization Center funded the work.

TRECC is managed by the National Center for Supercomputing Applications and funded by a grant from the Office of Naval Research. The researchers have applied for a patent.