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
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.