Proposed two-color HgCdTe focal plane array

Abstract

Recently there has been considerable interest in two-color focal plane arrays (FPAs), particularly in mercury cadmium telluride (MCT,Hg 1Cd STe). Single-color FPAS provide only the signal from the target, not its emissivity and temperature separately. The schemes that have been implemented for two-color FPAS involve two bumps per pixel, and two back-to-back diodes in four different layers, which are technologically challenging. We propose a simple scheme that requires only one bump per pixel, in a material that is not a heterostructure, and can be grown by liquid phase or molecular beam epitaxy. The two different cutoff wavelengths are obtained by implanting two differentjunction depths in a nt-on-p configuration. Since the n layer does not contribute, because of the Moss-Burstein shift, the different diodes correspond to different surface composition values, and x,2, in the graded p-type epilayer. The precise cutoff wavelengths can be chosen by appropriate slope s of the composition in the epilayer. For the purpose of radiometry it is not necessary that the two cutoff wavelengths should differ by large amounts: even 0.3 im is sufficient -easily achievable, with reasonable junction depths ( 3 tm). The two colors could form a checkerboard pattern across the FPA. However, other patterns may be used: e.g. the central pixels could be of one color, and the remaining in the other color; or patches of detectors of either one or the other color could be fabricated. The advantage of keeping the difference in cutoff wavelengths small would be that if one should so desire the FPA could just as well be used as for normal imaging purposes.Recently there has been considerable interest in two-color infrared (IR) focal plane arrays (FPAs). Thermal imaging systems that are capable of detection in two bands are advantageous for military applications such as signature recognition, decoy identification, and counter-measure avoidance. They provide an added dimension of contrast as compared to single-band imaging. Further, for a single band system, either the emissivity or the temperature of the target can be determined; in a dual-band system, both can be determined, provided that the emissivity does not vary much with wavelength. As a consequence, many groups [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] have made dual band detectors and FPAS. Essentially the detectors corresponding to the two bands are either placed side-by-side [1,12] or stacked, one on top of the other [2-11, 13-151. The earliest FPAS were bias-switchable [2] between one band and the other, and later ones allowed either sequential or simultaneous viewing in the two bands [3,4], while many recent FPAS allow independent access to both bands [5][6][7][8][9], either by triple-or quadruple-layer devices. However, the penalty for simultaneous viewing in both bands is that two indium bumps must be connected per pixel. While this has been demonstrated [3-9], it certainly adds complexity to the process of fabrication.In this paper, we propose a simple two-color FPA, ...