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  • Detector Sensitive from 800 nm to Far-IR

Photonics Spectra
Jun 2005
Richard Gaughan

Infrared detection is used in industrial, surveillance, medical and defense applications. Infrared emissions provide a direct insight into an object's temperature, but the shape, size and surface characteristics of an object influence the signal, so that the first-order relation between IR emission and temperature is not exact.

If an object's IR emission is measured at several wavelengths, it is possible to eliminate surface and shape effects and to reconstruct its temperature. Doing so, however, requires multiple detectors, cooling assemblies and sets of electronics, and the imaging system must establish and maintain pixel-to-pixel alignment among the detectors. Several groups thus are developing single detectors that are sensitive to multiple wavebands in the IR.

Detector Sensitive from 800 nm to Far-IR
A proof-of-principle homojunction interfacial work function internal photoemission detector measures infrared radiation from 800 nm to 70 µm. Multiband detection makes it possible to determine an object's temperature independent of its other features. Courtesy of A.G. Unil Perera.

The research group of A.G. Unil Perera at Georgia State University in Atlanta, in collaboration with scientists at the National Research Council in Ottawa and at the Russian Academy of Sciences' Institute for Physics of Microstructures in Nizhniy Novgorod, has constructed a dual-band near- and far-IR detector. The detector consists of two heavily doped GaAs layers separated by an undoped GaAs barrier layer.

Infrared radiation is incident on an 80-nm-thick P+ emitter layer that rests on a 1-µm-thick barrier layer that is atop a 1-µm-thick P++ bottom contact layer. The device, called a homojunction interfacial work function internal photoemission detector, absorbs near-IR wavelengths (below around 800 nm) by direct photoabsorption across the bandgap in the barrier layer.

Far-IR photons excite carriers at the interface of the emitter that undergo internal photoemission over the work function at the emitter/barrier interface. The value of the work function is defined by the difference between the fermi level in the emitter and the valence band in the barrier. An applied electric field sweeps out the photoexcited carriers. Because the current collection mechanism is the same for both near- and far-IR wavelengths, external filtering is required to separate response to radiation in the different bands.

The unique feature of the design is just how much it pushes the sensitivity out to the far-IR. The homojunction interfacial work function internal photoemission detector can detect radiation up to a wavelength of 70 µm, a sensitivity that can be tailored with the choice of material. Under optimum dual-band detection conditions, with 100-mV reverse bias at 20 K, the detectivities at 800 nm and at 57 µm were both a little less than 1010 cm Hz1/2/W. The same design principles can be employed to produce a GaN/AlGaN detector responding in the UV and near-IR ranges.

Perera said that the researchers are looking at designs that would separately measure the current generated in different wavebands, eliminating the need for external filtering. But the extended responsivity of the detector already makes it useful for some heavy-duty applications, such as separating the image of a missile body from that of its exhaust plume.

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