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Industry  Infrared detectors


just 3-5 µm. It is possible to increase this detector’s operating temperature to 150 K by switching from bulk InSb to more complex structures that include InAlSb and are grown by MBE. But this technology is in its infancy, and it is currently very challenging to manufacture detectors with this approach.


QWIPs is based on a far more mature materials technology, GaAs-based heterostructures. However, the sensitivity of a QWIPs detector is not as high as that for MCT, which can operate over a far wider spectral range. These II-VI devices can be tuned to cover the visible, or to detect radiation classified as short wave (1-2.5 µm), medium wave (3-5 µm), long wave (7-10 µm) or very long wave (around 14 µm).


One of the biggest downsides of the MCT detector is its cooling requirement. Although this is not as severe as that for conventional InSb detectors, commercial MCT detectors do require cooling to 80 -130 K, with the exact figure depending on the operating wavelength – it is higher for shorter wavelengths. It is possible to reach these temperatures with Stirling coolers, which operate just like refrigerators and use helium gas. And if cooling is required for a single, very short time, a Joule- Thompson cooler can be used, which involves the release of a high-pressure gas from a vessel.


Military demands


The biggest market for the MCT detector is the military, where it is often employed for surveillance. When these detectors are fitted on tanks, carriers, destroyers, submarines, fighter planes and helicopters, trimming the size, weight and power consumption of the detector’s cooling system is not necessarily a big deal. But it can make a major difference to the range of unmanned aerial vehicles (UAVs), pilotless planes that can have a wingspan of just a few metres.


Today, the power drawn by the cooler from a set of batteries typically exceeds that required to run the detector, but it should be possible to start re-dressing this balance by making detectors that can operate at significantly higher temperatures. One company that is trying to do just this is the French firm Sofradir, which is headquartered close to Paris and has its development and production facilities in Veurey-Voroize, a town within France’s ‘infrared valley’. Spun off from CEA-Leti in 1986, Sofradir has a clear long-term plan for increasing the operating temperature of its wide range of commercial MCT detectors.


The company’s standard technology combines an optimised passivation process with high-quality substrates and epilayers to yield detectors with operating temperatures up to 120 K. In future, the company plans to launch detectors with an inverted doping structure that can operate at up to 150 K, followed by more complex epitaxial designs that will drive down dark current. These more sophisticated structures, which will require a switch of growth


Sofradir’s standard ‘n-over-p’ mercury cadmium telluride detector


technology from liquid phase epitaxy to MBE, should lead to operating temperatures of 200 K.


Efforts to improve the performance of MCT detectors are carried out in partnership with CEA-Leti. “We have had a long, historical relationship, and in 2003 we set up a joint lab with CEA-Leti to share our R&D,” explains David Billon-Lanfrey, Vice President of R&D, technology and products at Sofradir. “[CEA-Leti] gives us a lot of new ideas and a lot of expertise, in terms of technology. We bring the needs of the market and industry constraints. It’s one thing to do one demonstrator – it’s another thing for our infrared technologies to turn production at large quantities.”


A key requirement for any high-quality detector is that it has very few defective pixels. According to Sofradir, the proportion of defective pixels should be less than 0.5 percent. Unfortunately, as operating temperature rises, the ratio of defective pixels to good ones increases, due to various forms of noise. Engineers at Sofradir and CEA-Leti have reduced the proportion of defective pixels by driving down the density of defects and dislocations in the MCT layer through improvements in substrate quality and epitaxy.


“Passivation layers are also playing a big role,” adds Billon-Lanfrey, because improvements to the passivation layer suppress the current leakage out of the device.


Sofradir is keeping the details of this improved passivation process under wraps, but it is willing to disclose the benefits brought to device performance. In 2010, more than thirty MCT detectors were made using a range of processes. Through optimisation of the passivation


Sofradir has demonstrated its superior passivation technology in its Scorpio detector, which is fitted to a camera for demonstration


January/February 2012 www.compoundsemiconductor.net 25


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