One giant leap for IR technology
Researchers have successfully demonstrated that InAs/InAsSb Type-II superlattice (T2SL) materials could be a better alternative to conventional InAs/InGaSb T2SLs.
This discovery was made by scientists at Arizona State University and MBE foundry IQE.
Time-resolved photoluminescence of the gallium-free LWIR superlattice exhibited a lower-limit 412 ns minority carrier lifetime at 77 K, more than an order of magnitude increase compared to 30 ns for conventional LWIR InAs/InGaSb T2SL type structures.
Researchers at the US Army Research Laboratory say they have, for the first time, measured lifetimes approaching the reported record (~1 μs) for HgCdTe alloys. This, they say, could pose a realistic alternative to HgCdTe for high performance IR detector and focal plane array (FPA) applications.
Figure 1: Time-resolved photoluminescence measurements of an InAs/InAs0.72Sb0.28 T2SL at 77 K for excess carrier densities ranging from 4 x 1015 to 1 x 1017 cm-3
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Figure 2 : Carrier lifetimes extracted from the PL decay are shown as points as a function of 1000/T. Also plotted are the temperature dependences of the SRH lifetime (tSRH µ T1/2 , dotted line), radiative lifetime (tRad µ T3/2, dashed line), and a combination of both SRH and radiative lifetimes (solid line).
State-of-the-art infrared imagers made using HgCdTe technology suffer from high costs due to expensive substrates, device processing, a decreasing yield with longer cut-off wavelength, and the need for a low-temperature cryogenic dewar.
The breakthrough in the minority carrier lifetime demonstrated in an InAs/InAsSb T2SL could eventually enable infrared imagers to be manufactured with much lower costs using commercial III-V processes and the vast resources of MBE foundries.
What's more, T2SL materials have long been predicted to possess longer carrier lifetimes than HgCdTe due to the suppression of Auger recombination. However, Shockley-Read-Hall recombination in the previous conventional III-V T2SL materials prevented the demonstration of the theoretical high performance and limited the minority carrier lifetime to ~30 ns in the LWIR (8-12 μm) range.
With the drastic increase in the carrier lifetime observed in the gallium-free InAs/InAsSb T2SL, future T2SL photodetectors should have much lower dark currents and enable operation at much higher temperatures, allowing the use of lower-cost, smaller-sized cryogenic coolers.
With the recent advances in the size and availability of 4 inch GaSb substrates, future infrared FPA assemblies would cost much less and expand commercial applications into new areas such as the automotive, law enforcement, environmental monitoring, and safety surveillance industries.
The researchers at Arizona State University and their collaborators are very optimistic about the future of gallium-free T2SL materials and the potential to improve the material with further bandgap engineering and defect reduction.
It is worth noting that the carrier lifetime currently observed demonstrates a lower bound as the sample structures in this study have not been completely optimised. Better understanding of the recombination mechanisms, as well as improved bandgap designs are expected to further advance the existing gallium-free T2SL performance.
Further details of the work have been published in the paper, "Significantly improved minority carrier lifetime observed in a long-wavelength infrared III-V type-II superlattice comprised of InAs/InAsSb", by E. H. Steenbergen et al, Applied Physics Letters, 99, 251110 (2011); DOI:10.1063/1.3671398.
This work is partially supported by AFOSR and an ARO MURI program. Figures are reprinted with permission of E.H. Steenbergen and the co-authors of the paper. Copyright 2011, American Institute of Physics.
