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New laser doping method heightens infrared detector sensitivity

A new laser doping method has been developed by researchers at five institutions, including the Massachusetts Institute of Technology (MIT) in the USA, which removes the limitations of developing a detector that is responsive to infrared light. The approach, published in January’s issue of journal Nature Communications, could be used to create new infrared imaging systems or form more efficient solar cells that harness a broader range of the Sun's energy.

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. ‘Silicon usually has very little interaction with infrared light,’ said Tonio Buonassisi, associate professor of mechanical engineering at MIT.

Various treatments of silicon can mitigate this behaviour, but such methods have significant negative effects on the electrical performance of silicon; for example only being functional at very low temperatures.

However, the new system is capable of working at room temperature and provides a broad infrared response. The approach works by implanting gold atoms into the top hundred nanometres of silicon, and then using a laser to melt the surface for a few nanoseconds. The end result 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,’ explained Buonassisi.

Usually, even the tiniest particle of gold can destroy the usefulness of a silicon microchip - so much so that in many chip-manufacturing facilities, the wearing of gold jewellery is prohibited. ‘It’s one of the most dangerous impurities in silicon,’ said Buonassisi. However, 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 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. ‘This is especially attractive because we can show broadband infrared response in silicon at room temperature,’ said MIT graduate student Jonathan Mailoa. 

While this is early-stage work, for some specialised purposes, such as a system for adjusting infrared laser alignment, it might be available relatively quickly. The approach could lead to the formation of new imaging arrays for security systems.

According to Buonassisi, the efficiency of the method may be too low for use in silicon solar cells, but it could be applicable to different materials that would be useful for making solar cells.

The new approach is described in a paper published in the journal Nature Communications by MIT Jonathan Mailoa, Tonio Buonassisi, and 11 others. The research was funded by the US Army Research Office, the National Science Foundation, the US Department of Energy, and the MIT-KFUPM Center for Clean Water and Energy - a joint project of MIT and the King Fahd University of Petroleum and Mining in Saudi Arabia.


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