Researchers at Georgia State University, the University of Leeds and China's Shanghai Jiao Tong University have developed a way to use standard semiconductors to detect light over a much broader range of wavelengths, potentially opening up new possibilities in solar power generation and low-energy light detection ('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, Nature Photonics, May 2014 issue).
"Generating electric current from the lower energy ranges of the electromagnetic spectrum, such as infrared, is very challenging using semiconductor materials because the wavelengths involved provide little energy," notes Edmund Linfield, professor of Terahertz Electronics in the University of Leeds' School of Electronic and Electrical Engineering, whose team fabricated the patterned semiconductors. "The pay-offs are potentially very significant – from more efficient use of solar energy by utilizing a larger portion of the spectrum to developing new types of detector for use at long wavelengths," he adds.
Until now, one of the solutions to the challenge of detecting low-energy light has been to find semiconductor materials with an energy bandgap that allows a response to such long-wavelength, low-energy light in the infrared part of the spectrum. The spectral response of common optoelectronic photodetectors is restricted by a cutoff wavelength limit λc related to the activation energy (bandgap) of the semiconductor structure (or material) (Δ) through the relationship λc =hc/Δ. This rule dominates device design and intrinsically limits the long-wavelength response of a photodetector.
Instead, the new approach uses a photodetection principle based on a hot–cold hole energy transfer mechanism. Hot carriers injected into a semiconductor structure interact with cold carriers and excite them to higher energy states. So, that when the low-energy wavelengths arrive they can generate a current, overcoming the spectral limit and enabling a very long-wavelength infrared response.
The improved device can detect wavelengths up to at least the 55μm range, tunable by varying the degree of hot-hole injection, for a GaAs/AlGaAs sample with Δ=0.32eV (equivalent to 3.9μm in wavelength, the previous limit of the detector). The team has run simulations showing that a refined version of the device could detect wavelengths up to 100μm long.
"This technology will also allow dual- or multi-band detectors to be developed, which could be used to reduce false positives in identifying – for example – toxic gases," says professor Unil Perera, a Regents' Professor of Physics at Georgia State University and head of its Optoelectronics Research Laboratory, which led the study.
Because the technique extends the range of existing semiconductors rather than relying on novel materials, it also offers the potential for wafer-scale integration with electronic devices.
The study's lead researchers Perera and postdoctoral fellow Yan-Feng Lao have filed a US patent application for the detector design.
The work was supported by the US Army Research Office, the US National Science Foundation (NSF), the UK's Engineering and Physical Sciences Research Council (EPSRC) and the European Research Council grant TOSCA (TO Simulate and CAlibrate stochastic models).
