High-performance, low-cost IR detector technology

Mercury cadmium telluride (HgCdTe) is a semiconductor material commonly used for the detection of IR radiation. Its spectral range is controlled by the Hg:Cd ratio, which makes it suitable for a wide range of thermal imaging applications. The strong optical absorption and relatively low dark currents of HgCdTe make it superior to alternative technologies, such as III-V quantum devices. Although HgCdTe is often perceived as a difficult material to control, 30 years of development have resulted in SELEX technology, which achieves high electro-optic performance in a fundamentally low-cost manufacturing process.

In this novel procedure, HgCdTe is grown into a complex stack of layers on a gallium arsenide (GaAs) substrate, by a technique known as metal organic vapor phase epitaxy (MOVPE).¹ We selected GaAs as the substrate because it is 50 times cheaper than cadmium zinc telluride (CdZnTe), which is used in other HgCdTe processes. GaAs is also five times cheaper than gallium antimonide (GaSb), used for quantum detectors. An etch creates separate optical cavities that define the pixels and provide very sharp images. MOVPE on GaAs is routinely used for long-wavelength (LW), medium wavelength, and even dual-band devices that have few pixel defects and low dark current. An image taken with our Harrier LW camera (see Figure 1) illustrates the capability of one such product.

Figure 1. Illustration of the metal organic vapor phase epitaxy (MOVPE) technology and image quality from the long-wavelength (LW) Harrier camera. GaAs: Gallium arsenide.

Our MOVPE technology can be used to manufacture detector arrays with dimensions up to 40×40mm. We use this intrinsically low-cost, large-area capability in several airborne surveillance and low-background-flux astronomical applications in the near-IR band. An example is the Falcon 1920×1280 detector, on a 12μm pitch, that can be butted on three sides so that even larger mosaic arrays can be constructed. We are currently developing 2000×2000 pixel arrays for Earth observation applications in the short-wavelength IR band.

The MOVPE growth system is very flexible, and the devices can be designed and engineered to best match the requirements. For instance, we have recently improved the operating temperature of our sensors, yet have maintained image quality up to 210K.² More complex device structures can also be designed. Our dual-band device uses stacked diode arrays that are now routinely produced for imaging in mid-wavelength IR and long-wavelength IR on alternate frames.

We have developed device technology that achieves the best optical efficiency, with high absorption and photon containment. Combined with low junction capacitance, this provides unprecedented sensitivity. Our cameras are now used in many applications that require high speeds and short integration times. An example image from the Hot Spot system used in cricket is shown in Figure 2.

Figure 2. Image from the SLX Hawk camera used in the Hot Spot system. (Courtesy of BBG Sports.)

The unique properties of HgCdTe enable almost noise-free avalanche gain, which is ideal for very low photon flux applications.³ Our Saphira array is capable of sensing individual photons at rates up to 100,000 frames per second. This detector has been selected for use in the wavefront sensors and fringe trackers of the European Southern Observatory's Gravity program. Avalanche gain is also used in laser-gated imaging for long-range identification. Our multifunctional Swallow detector can provide either thermal or laser-gated 3D imaging. Its applications include retrofitting small turret or pod structures with enhanced electro-optic capabilities. Figure 3 illustrates a typical airborne detection and identification scenario.

Figure 3. Airborne laser-gated thermal images taken with the Swallow multifunctional detector.

Although MOVPE technology is now mature, there is still much potential to extend its performance. Future arrays may have smaller pixels, with 5μm having already been achieved in development. We are currently in the process of designing MOVPE structures for higher operating temperatures, which will have important consequences for the size, weight, power, and cost of future thermal imaging systems. Our avalanche gain devices are starting to be used in major science programs and are providing a new generation of photon-sensitive instruments.