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Boost IR response in detectors with laser doping

07 Jan 2014

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A study published in the journal Nature Communications offers a simpler design approach to incite a broad infrared light response from silicon detectors. Formulated by Massachusetts Institute of Technology (MIT) researchers, the technique involves supersaturating gold atoms into the crystal structure of silicon without altering its original state, and using laser doping to test its responsiveness. The research received support from various U.S. government agencies as well as the MIT-KFUPM Centre for Clean Water and Energy, a joint project of MIT and the King Fahd University of Petroleum and Mining.

Silicon, which forms the basis of most semiconductor and solar-cell technology, normally lets most infrared light pass right through. This is because the material's bandgap—a fundamental electronic property—requires an energy level greater than that carried by photons of infrared light. Various treatments of silicon can mitigate this behaviour, usually by creating a waveguide with structural defects or doping it with certain other elements. The problem is that most such methods have significant negative effects on silicon's electrical performance, only work at very low temperatures, or only make silicon responsive to a very narrow band of infrared wavelengths.

The new approach works by implanting gold into the top hundred nanometers of silicon and then using a laser to melt the surface for a few nanoseconds. The silicon atoms recrystallise into a near-perfect lattice, and the gold atoms don't have time to escape before getting trapped in the lattice. In fact, the material contains about 1 per cent gold, an amount more than 100 times greater than silicon's solubility limit. But under certain conditions, materials can exceed their normal solubility limits, creating what's called a supersaturated solution. In this case, the new processing method produces a layer of silicon supersaturated with gold atoms.

"It's still a silicon crystal, but it has an enormous amount of gold near the surface," lead investigator and MIT professor Tonio Buonassisi said. While others have tried similar methods with materials other than gold, the MIT team's work is the first clear demonstration that the technique can work with gold as the added material, he says.

"This is especially attractive because we can show broadband infrared response in silicon at room temperature." said researcher Jonathan Mailoa. While this is early-stage work, for some specialised purposes—such as a system for adjusting infrared laser alignment—it might be useful relatively quickly.

This use of gold was a surprise: Usually gold is incompatible with anything involving silicon, according to Buonassisi. Even the tiniest particle of it can destroy the usefulness of a silicon microchip—so much so that in many chip-manufacturing facilities, the wearing of gold jewellery is strictly prohibited. "It's one of the most dangerous impurities in silicon," he says. But at the very high concentrations achieved by laser doping, gold can have a net positive optoelectronic impact when infrared light shines on the device.

While this approach might lead to infrared imaging systems, its efficiency is probably too low for use in silicon solar cells. However, this laser processing method might be applicable to different materials that would be useful for making solar cells, Buonassisi said.

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