Sensing in the shortwave infrared range (wavelengths from 0.9 to 1.7 microns) has been made practical by the development of Indium Gallium Arsenide (InGaAs) sensors. Sensors Unlimited, Inc., a part of Collins Aerospace, specializes in manufacturing InGaAs one-dimensional linear arrays, two-dimensional focalplane array cameras, and SWIR systems. But why even use SWIR?
First, a basic fact: light in the SWIR band is not visible to the human eye. The visible spectrum extends from wavelengths of 0.4 microns (blue, nearly ultraviolet to the eye) to 0.7 microns (deep red). Longer wavelengths can only be seen by dedicated sensors, such as InGaAs. Although light in the shortwave infrared region is not visible to the eye, this light interacts with objects in a similar manner as visible wavelengths. That is, SWIR light is reflective light; it bounces off objects much like visible light.
As a result of its reflective nature, SWIR light has shadows and contrast in its imagery. Images from an InGaAs camera are comparable to black-and-white visible images in resolution and detail. This makes objects easily recognizable and yields one of the tactical advantages of the SWIR, namely, object or individual identification. This makes InGaAs interesting, but what makes it useful?
What Makes InGaAs Useful?
InGaAs sensors can be made extremely sensitive, literally counting individual photons. When built as focal plane array – with thousands or millions of tiny point sensors, or sensor pixels – SWIR cameras will work in very dark conditions. Night vision goggles have been around for several decades and operate by sensing and amplifying reflected visible starlight, or other ambient light, in what are called image intensification (I-Squared) tubes. This technology has worked well for direct view night vision goggles. But when an image needs to be sent to a remote location (an intelligence center, for example), there is no practical method which does not introduce reliability and sensitivity limitations (e.g. I2CCD). Because all of SUI’s SWIR sensors convert light to electrical signals, they are inherently suitable for storage or transmission.
Using SWIR at night has another major advantage. An atmospheric phenomenon called night sky radiance emits five to seven times more illumination than starlight, nearly all of it in the SWIR wavelengths. With a SWIR camera and this night radiance – often called night-glow – we can “see” objects with great clarity on moonless nights and share these images across networks as no other imaging technology can do.
But aren’t there other cameras that operate in the shortwave infrared range? Yes. Sensors constructed from materials like mercury cadmium telluride (Hg-CdTe) or indium antimonide (InSb) can be very sensitive in the SWIR band. However, in order to increase their signal-to-noise ratio to usable levels, these cameras must be cryogenically cooled. In stark contrast, similar sensitivity can be achieved at room temperatures with an InGaAs-equipped camera.
Essentially, InGaAs cameras can be small and use very little power, but give big results. Sensors Unlimited InGaAs cameras offer VGA resolution in a tiny 1.25" × 1.25" × 1.10" package and just 1.5W power draw at steady state. We also offer HD (1 Megapixel) resolution in a 2" × 2" × 2.43" package with ≤ 3.0W power draw at steady state.
SWIR and Thermal Imaging
Thermal imagers are another class of camera with excellent detection abilities. These imagers complement SWIR imaging in many applications. While thermal imagers can detect the presence of a warm object against a cool background, a SWIR imager can provide recognition and identification as well as depth perception with contrast and shadows.
Image Through Glass
Finally, one major benefit of SWIR imaging that is unmatched by other technologies is the ability to image through glass. These cameras can use conventional, cost-effective visible camera lenses for all but the most demanding applications. Special expensive optics or environmentally hardened housings are mostly unnecessary, making them available for a wide variety of applications and industries. This also allows for the SWIR camera to be mounted behind a protective glass window, providing extra flexibility when positioning the camera system in a hazardous environment.
So, Why SWIR?
Nightglow (night sky radiance)
See covert lasers and beacons
No cryogenic cooling required
Conventional, low-cost visible spectrum lenses
What is InGaAs?
InGaAs, or indium gallium arsenide, is an alloy of gallium arsenide and indium arsenide. In a more general sense, it belongs to the InGaAsP quaternary system that consists of alloys of indium arsenide (InAs), gallium arsenide (GaAs), indium phosphide (InP), and gallium phosphide (GaP). As gallium and indium belong to Group III of the Periodic Table, and arsenic and phosphorous belong to Group V, these binary materials and their alloys are all III-V compound semiconductors.
Why Go Through All the Trouble?
To a large extent, the electrical and optical properties of a semiconductor depend on its energy bandgap and whether the bandgap is “direct” or “indirect.” The energy bandgaps of the four binary members of the InGaAsP quaternary system range from 0.33 eV (InAs) to 2.25 eV (GaP), with InP (1.29 eV) and GaAs (1.43 eV) falling in between. At SUI, we emphasize photodetectors, so we care most about the optical properties of semiconductors. A semiconductor will only detect light with photon energy larger than the bandgap, or to put it another way, with a wavelength shorter than the cutoff wavelength associated with the bandgap. This “long wavelength cutoff” works out to 3.75 μm for InAs and 0.55 μm for GaP with InP at 0.96 μm and GaAs at 0.87 μm.
By mixing two or more of the binary compounds, the properties of the resulting ternary and quaternary semiconductors can be tuned to intermediate values. The challenge is that not only does the energy bandgap depend on the alloy composition, so also does the resulting lattice constant. For our four binary members the lattice constants range from 5.4505 Å (GaP) to 6.0585 Å (InAs) with GaAs at 5.6534 Å and InP at 5.8688 Å. The relationship between the lattice constant and the long wavelength cutoff of the 4 ternary alloys in the InGaAsP family are shown in Figure 2.
Let’s Get Back to InGaAs
The InAs/GaAs alloy is referred to as InxGa1-xAs where x is the proportion of InAs and 1-x is the proportion of GaAs. The lattice constants and long wavelength cutoffs of these alloys are depicted as the red lines in Figure 1. The challenge is that while it’s possible to make thin films of InxGa1-xAs by a number of techniques, a substrate is required to hold up the thin film. If the thin film and the substrate do not have the same lattice constant, then the properties of the thin film will be severely degraded.
For lots of reasons, the most convenient substrate for Inx-Ga1-xAs is InP. High quality InP substrates are available with diameters as large as 100 mm. InxGa1-xAs with 53% InAs is often called “standard InGaAs” without bothering to note the values of “x” or “1-x” because it has the same lattice constant as InP and therefore the combination leads to very high-quality thin films.
Standard InGaAs has a long wavelength cutoff of 1.7 μm. This means it is sensitive to the wavelengths of light that suffer the least signal dispersion and transmit furthest down a glass fiber (1.3 μm and 1.55 μm), therefore detecting “eye-safe” lasers (wavelengths longer than 1.4 μm). It is the optimum wavelength band for detecting the natural glow of the night sky. SUI’s core product lines are based on PIN and avalanche photodiodes and photodiode arrays made from standard InGaAs.
What Is “Extended Wavelength” InGaAs?
Standard InGaAs has a long wavelength cutoff of 1.7 μm. Many applications require the detection of light with longer wavelengths. An important example is the ability to measure moisture content in agricultural products by measuring water absorption at 1.9 μm. Another example is light detection and ranging (LiDAR), used in airplanes to detect clear air turbulence. LiDAR systems often use lasers that emit light with a wavelength of 2.05 μm. InxGa1-xAs with a longer cutoff is called “extended wavelength InGaAs.”
It seems as if all one would have to do is add a little more InAs to the mix, but it’s not so easy. This increases the lattice constant of the thin film, which causes a mismatch with the substrate, and thus reduces the quality of the thin film. SUI has put a lot of work into learning to grow high-quality extended wavelength InGaAs, and this is reflected in product offerings. The results of our efforts are summarized in Figure 2, which shows the quantum efficiency of standard InGaAs in red together with the quantum efficiencies of two extended wavelength alloys, X=0.74 (blue) and X=0.82 (green). The spectral response of a short-cutoff at 1.45 μm variant is also shown. As we like to say, “InxGa1-xAs starts where silicon leaves off.”
This article was written by engineers at Sensors Unlimited - a Part of Collins Aerospace (Princeton, NJ). For more information, visit here .