Development of HgCdTe for LWIR Imagers
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DEVE•OPMEIT OF HgCdTe FOR LWIR IMAGERS
JOSEPH L. SCHMIT Honeywell Physical Sciences Center, 10701 Lyndale Ave. So. Bloomington, MN 55420 ABSTRACT This paper provides a historical perspective on the emergency of HgCdTe as the material of choice for long wavelength infrared (LWIR) imagers. The need for devices which see room temperature objects through the atmospheric window actually drove the development of this material. The lack of elemental or compound semiconductors having the desired wavelength response forced the choice of the alloy semiconductor, HgCdTe. The development of this material in several The countries and companies beginning in the late 1950's is traced. crystal growth methods used to grow HgCdTe have included melt growth techniques such as Bridgman, zone-melting, quench-anneal and slushgrowth. The solution growth techniques include growth from HgTe-rich, Te-rich and Hg-rich solutions. Vapor phase growth has included evaporation, sputtering, molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). No perfect method has yet been developed, but several have provided material for the large area arrays needed for modern Imagers. NEED FOR 0.1 eV BAND GAP Paul Kruse has given an excellent historical overview for the development of HgCdTe [1]. The military need for a thermal imaging capability at longer wavelengths drove the development of HgCdTe. In the late 1950's infrared detectors were either lead salts or InSb responding to 3-5um radiation or doped Ge which requires cooling to 30K. Figure 1 indicates one reason for interest in the longer wavelengths. It is a plot of radiant emission from a black body at three temperatures as a function of wavelength. Note that a 3000K black body (such as the filament of a light bulb) emits radiation in the visible range. Since our eye can detect radiation in the visible, we can see a hot filament and also things reflecting its radiation. However, the output from a 300K black body is 10,000 times lower and its peak intensity occurs at 10um, where our eye cannot see. Clearly, we need detectors operating near 10um if we expect to "see" room temperature objects such as people, trees and trucks without the aid of reflected light. Usually the desire to see in the dark also includes the desire to see at long distances through air, therefore an additional constraint is added by the water and CO2 absorption present in air. Figure 2 is a plot of the transmission through one mile of air as a function of wavelength [2]. Specific absorption bands of water and carbon dioxide are indicated. For longer distances, the transmission outside of the 3 to 5 and the 8 to 14 us "windows" drops nearly to zero. More detailed plots are available but this is sufficient to indicate one reason why people want detector response to be at specific wavelengths. The desired detector should thus have a band gap near 0.1eV and operate at 77K or above. Elemental and compound semiconductors do not exist with the right band gaps to operate in the desired ranges. Figure 3 is a plot o
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