“Tunable” semiconductors allow better detectors, solar cells

One of the great problems in physics is the detection of electromagnetic radiation – that is, light – which lies outside the small range of wavelengths that the human eye can see. Think X-rays, for example, or radio waves.

Now, researchers have discovered a way to use existing semiconductors to detect a far wider range of light than is now possible, well into the infrared range.

The team hopes to use the technology in detectors, but also in improved solar cells that could absorb infrared light as well as the sun's visible rays.

''This technology will also allow dual or multiband detectors to be developed, which could be used to reduce false positives in identifying, for example, toxic gases,'' said Unil Perera, a Regents' Professor of Physics at Georgia State University. Perera leads the Optoelectronics Research Laboratory, where fellow author and postdoctoral fellow Yan-Feng Lao is also a member. The research team also included scientists from the University of Leeds in England and Shanghai Jiao Tong University in China.

To understand the team's breakthrough, it's important to understand how semiconductors work. Basically, a semiconductor is exactly what its name implies – a material that will conduct an electromagnetic current, but not always. An external energy source must be used to get those electrons moving.

But infrared light doesn't carry a lot of energy, and won't cause many semiconductors to react. And without a reaction, there's nothing to detect.

Until now, the only solution would have been to find a semiconductor material that would respond to long-wavelength, low-energy light like the infrared spectrum.

But instead, the researchers worked around the problem by adding another light source to their device.

The extra light source primes the semiconductor with energy, like running hot water over a jar lid to loosen it. When a low-energy, long-wavelength beam comes along, it pushes the material over the top, causing a detectable reaction.

The new and improved device can detect wavelengths up to at least the 55 micrometer range, whereas before the same detector could only see wavelengths of about 4 micrometers. The team has run simulations showing that a refined version of the device could detect wavelengths up to 100 micrometers long.

Edmund Linfield, professor of terahertz electronics at the University of Leeds, whose team built the patterned semiconductors used in the new technique, said, ''This is a really exciting breakthrough and opens up the opportunity to explore a wide range of new device concepts including more efficient photovoltaics and photodetectors.''

Perera and Lao have filed a U.S. patent application for their detector design.

''Tunable hot-carrier photodetection beyond the band-gap spectral limit'' by Yan-Feng Lao, A.G. Unil Perera, L.H. Li, S.P. Khanna, E.H. Linfield and H.C. Liu is in the May issue of Nature Photonics.

The work was supported by the U.S. Army Research Office, the US National Science Foundation, the UK Engineering and Physical Sciences Research Council, and the European Research Council Advanced Grant ''TOSCA.''