MWIR Detectors: A Comparison of Strained-Layer Superlattice Photodiodes with HgCdTe
William Tennant
DOI: 10.1117/3.1002245.ch16
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Photodiodes are themost sensitive photodetectors, being widely used for thermal imaging in high-performance cameras for both commercial and governmental purposes. In the thermal IR region, MWIR photodiodes (3- to 5-μm spectral range) are the most widely used andmostmature, being incorporated into arrays of hundreds of thousands to millions of elements and being widely used in applications from tactical thermal imaging to space-based astronomy. They have wavelengths sufficiently long to detect thermal infrared but short enough not to be very susceptible to tunneling (which can limit sensitivity).

Probably the most widely used high-performance MWIR photodiode material is InSb. However, InSb is a binary compound with a fixed spectral range (which increases undesirably with operating temperature) and has currents limited by defects, rather than fundamental mechanisms. Therefore it reaches its best performance at low temperatures (typically ~80-100 K). InSb is used mainly for its maturity and low fabrication cost; however, its limitations make it unsuitable to realize the low system size, weight, and power, and low life-cycle cost benefits attainable from substantially higher operating temperature.

Fortunately, two other material systems have the potential to attain high MWIR performance at high operating temperatures. Mercury cadmium telluride (HgCdTe), discovered over 50 years ago in Britain, has consistently shown the highest performance of any MWIR photodiode material at or above liquid nitrogen temperatures (80 K), and its adjustable energy gap allows tailoring of the spectral response precisely to the intended operating temperature. Moreover, it has matured to the point where its performance is limited not by defects but by fundamental Auger mechanisms that arise from the band structure itself.

Of the materials currently being investigated for high-performance detection, type-II strained-layer superlattices (T2SLSs or SLSs) are the only materials that offer a significant fundamental performance improvement over HgCdTe. These materials are artificial crystals: stacks of hundreds of groups of crystalline layers, each group composed of a few atomic layers, each of several different crystalline alloys. Typically, these SLS structures are grown by molecular beam epitaxy (MBE) from III-V alloys of AlAs, AlSb, GaAs, GaSb, InAs, and InSb. Quantum confinement and lattice strain combine to allow the material designer to tailor not just bandgap but also band structure to minimize the effects of Auger recombination.

Recent publications have indicated that both of these materials systems offer promise for high-temperature thermal imaging, as will be discussed. This chapter draws primarily on four recent (and one earlier) publications from Tennant, the Razeghi group, Bewley et al., Grein and Flatté, and Grein et al.

This review aims to provide analysis and comparison of state-of-the-art HgCdTe and SLS MWIR photodiode performance at a 150-K operating temperature. This is a highly desirable temperature for future thermal imagers, being more than 1.5 times the current InSb typical operating temperature. Because the comparison is taken from data reported, rather than data taken explicitly for comparison purposes, some conclusions will be estimates; however, an attempt will be made to show that these estimates are usefully accurate.

© 2013 Society of Photo-Optical Instrumentation Engineers (SPIE)

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