Laboratory and Field Performance of Megapixel QWIP Focal Plane Arrays
ARMY RESEARCH LAB ADELPHI MD
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The quantum well infrared IR photodetector QWIP was first demonstrated nearly 20 years ago. Two- dimensional staring focal plane arrays of QWIPs were developed relatively soon after this. Unlike HgCdTe, the principal competitive technology for detection in the long wavelength IR or LWIR, the main limitation to the production of large format FPAs for QWIPs has been the availability of large format readout integrated circuits ROICs. At the time that QWIPs were first being exploited, the largest ROIC format available was 128 X 128 pixels used by manufacturers of medium wavelength IR MWIR detector arrays using InSb. In contrast, LWIR HgCdTe has presented major detector material challenges in terms of compositional uniformity and defect density that have kept yields low and costs high. Since QWIPs are made from the relatively mature GaAsAlGaAs material system for which a large infrastructure exists, it is a straightforward task to design, grow, and fabricate QWIP FPAs using molecular beam epitaxy MBE and standard process tools on GaAs substrates as large as 6 inches 15.24 cm in diameter. An FPA with a format of 1024 x 1024 pixels with a pixel pitch of 20 micrometers will be approximately 20.5 cm long on each side. On a 6 in substrate, it is possible to process at least 16 FPAs of this size. However, the ZnCdTe substrates necessary for high-quality HgCdTe FPAs are limited to a diameter of approximately 1 in. It is impossible to process more than one 1024 x 1024 FPA on each substrate. As a result, the production yield for 1024 x 1024 LWIR FPAs using HgCdTe will be quite low until either ZnCdTe becomes available in significantly larger sizes or it is possible to grown HgCdTe on Si wafers. Much progress has been made in growing MWIR HgCdTe on Si but growth of LWIR layers on Si remains a significant challenge. Therefore, QWIP technology may be the only choice for affordable high-performance FPAs in large formats.
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