**1. Introduction**

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This Chapter covers recent advances in Short Wavelength Infrared (SWIR), Medium Wave‐ length Infrared (MWIR) and Long Wavelength Infrared (LWIR) materials and device technol‐ ogies for a variety of defense and commercial applications. Infrared technology is critical for military and security applications, as well as increasingly being used in many commercial products such as medical diagnostics, drivers' enhanced vision, machine vision and a multi‐ tude of other applications, including consumer products. The key enablers of such infrared products are the detector materials and designs used to fabricate focal plane arrays (FPAs).

Since the 1950s, there has been considerable progress towards the materials development and device design innovations. In particular, significant advances have been made during the past decade in the band-gap engineering of various compound semiconductors that has led to new and emerging detector architectures. Advances in optoelectronics related materi‐ als science, such as metamaterials and nanostructures, have opened doors for new ap‐ proaches to apply device design methodologies, which are expected to offer enhanced performance and low cost products in a wide range of applications.

This chapter reviews advancements in the mainstream detector technologies and presents different device architectures and discussions. The chapter introduces the basics of infrared detection physics and various infrared wavelength band characteristics. The subject is divid‐ ed into individual infrared atmospheric transmission windows to address related materials, detector design and device performance. Advances in pixel scaling, junction formation, ma‐ terials growth, and processing technologies are discussed.

We discuss the SWIR band (1-3 microns) and address some of the recent advances in In‐ GaAs, SiGe and HgCdTe based technologies and their applications. We also discuss MWIR band that covers 3-5 microns, and its applications. Some of the key work discussed includes InSb, HgCdTe, and III-V based Strained Layer Super Lattice (SLS) and barrier detector tech‐

© 2013 Dhar et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Dhar et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

nologies (nBn). Each of these technologies has a place in the IR applications where a variety of detector configurations can be used.

system. Emissivity is a physical property of materials that describes how efficiently it radi‐ ates heat. Because cloth has a lower emissivity than skin, the former will appear darker in a

Advances in Infrared Detector Array Technology

http://dx.doi.org/10.5772/51665

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At the MWIR and LWIR wavelengths, infrared radiation behaves differently from visible light. For example, glass is transparent to wavelengths less than 3.0 μm, so glass optics can be used and windows can be seen through at these wavelengths. However, glass is opaque in the LWIR band and blocks most energy in the MWIR band. Consequently, the optics in LWIR and MWIR imaging systems cannot use inexpensive glass lenses, but are forced to use more expensive materials, such as germanium. Because glass windows are not transparent at the longer wavebands, they can appear to be brighter or darker according to their temper‐ atures. Another difficulty with radiation in the MWIR and LWIR bands is that it is not trans‐ mitted through water. Imaging of a water (rain) coated scene with MWIR-LWIR wavelengths can wash out much of the scene's thermal contrast, resulting in a duller image. The choice of wavelength band to exploit for IR imaging depends on the type of atmospher‐ ic conditions/obscurants between the target and the imager. Generally, atmospheric obscur‐ ants, such as haze or conventional smoke, cause much less scattering in the MWIR and LWIR bands than in the VIS-NIR or SWIR bands. This is because the haze or smoke particle size (~0.5 um) is much smaller than the IR wavelength (Rayleigh scattering). Obscurants such as fog and clouds can cause more scattering, since the particle size is comparable with the IR wavelength (Mie scattering). Infrared cameras sensitive to the longer wavelengths are more tolerant to smoke, dust and fog. In addition to obscurants, atmospheric turbulence can dictate the choice of IR wave band for a given application. The effects of optical turbulence, due to the fluctuations in the refractive index of the atmosphere, can add up over very long distances to impact range performance (blurring and image motion), allowing LWIR an edge over MWIR. As a rule of thumb, longer the wavelength better is the transmission

According to Wien's Law, hotter objects emit more of their energy at shorter wavelengths. A blackbody source at 300 K has a peak exitance (power per unit area leaving a surface) at a wavelength of about 9.7 μm. For a source at 1000 K, the maximum exitance occurs at 2.9 μm. Therefore, detectors operating in the LWIR band are well suited to image room temperature

thermal imager even when both are exactly at the same temperature.

**Figure 1.** Definition of IR Spectral Band.

through the earth's atmosphere.

We also present a discussion on the LWIR band that covers the wavelength range between 8 and 14 microns. The technologies that are addressed are bolometer (Microbolometer Ar‐ rays), HgCdTe arrays, and a variety of very ingenious band-gap engineered devices using III-V compound semiconductor materials.
