Materials Requirements for IR Detectors and Imagers
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Materials Requirements for IR Detectors and Imagers M. A. Kinch Texas Instruments Incorporated Dallas, Texas 75265 ABSTRACT The requirements of infrared systems have increased significantly over the years, from the simple linear, low resolution, photoconductive array to the present-day, large area, high-density, photodiode (MIS and metalurgical) arrays, with on-focal-plane signal processing of considerable complexity. The success that has been achieved in meeting the performance goals appropriate for these systems has been due, to a large degree, to significant advances in the relevant materials technologies. The technologies of importance over the last twenty years are briefly reviewed, and the current state of the art, with its dominance by intrinsic alloy materials, is addressed in detail. The limitations of current bulk and epitaxial intrinsic materials technologies are considerable, both from a performance and a producibility point of view, when compared to the quality and quantity of material required by future infrared systems. These limitations are considered together with possible ways to overcome them. I.
INTRODUCTION Passive thermal imaging systems operating in the earth's atmosphere are designed to sense and display differences in emitted thermal radiation of a scene associated with variations in temperature and emissivity. The spectral density of the photon flux levels emitted by black bodies of various temperatures is shown in Figure 1(a). It is apparent that room temperature emission peaks at a wavelength - 12 pm at a flux density level 2 of -2xO1D7 photons/cm -s-pm. The infrared scene consists of minute changes of emitted flux superimposed on this large background flux level and thus the imager is designed to be sensitive to this scene contrast. The variation of scene contrast across the 3001K blackbody spectrum is shown in Figure 1(b), and is seen to increase with decreasing wavelength. The wavelength of operation for terrestrial infrared systems is determined by the spectral output shown in Figure 1(a) and the transmission of the earth's atmosphere, and is essentially limited to two spectral windows located in the 3 to 5 pm, and the 8 to 14 pm spectral ranges. For a temperature difference of 0.1 0 K Figure 1 indicates that the contrast of a 3 to 5 pm scene is Ap/4 = 1/300 superimposed upon a total emitted flux density (24y 1.3x101b photons/cm2 -s, whereas in the 8 to 12 pm window A&p/4p 1/600 with a total flux density 428xi0 17 photons/cm2 -s. E 10191 0.10 10 18
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Figure 1 (a) Spectral Radiant Photon Density vs Wavelength for Various T, and (b) Scene Contrast vs Wavelength at T = 300*K
Mat. Res. Soc. Symp. Proc. Vol. 90. ? 1987 Materials Research Society
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The heart of the infrared imaging system is the detector focal plane, and these detectors fall into two broad categories, namely thermal and photon. Both ha
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