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There is great interest in MWIR Ga-free type II strained layer superlattice (T2SLS) nBn detectors for background limited photodetectors (BLIP) operating at temperatures higher than InSb (T ≥ 150K compared to T≈ 80-90K). Recently, Ting et al. [Proc. SPIE, Vol. 10624, 1062410-1 (2018)] reported on measurements of the dark current and quantum efficiency (QE) for e-SWIR (λ>1.7 μm), MWIR, and LWIR Ga-free nBn T2SLS detectors. Of particular interest is the reported MWIR nBn T2SLS data, since the measured dark current and QE provide the opportunity to analyze the measured detector optoelectronic characteristics using optical properties obtained separately from the hole minority carrier lifetime and optical absorption coefficient data, hence no adjustable parameters. The goal is to develop and utilize robust modeling techniques to explain real, measured nBn detector data to understand the technology limitations and the improvements needed to optimize performance and device designs. A notable result is the observation that the dark current data under reverse bias (-100 mV < bias ≤ 0 mV) obeys the ideal diode equation, where the saturation dark current is in agreement with the radiative recombination rate obtained from the measured absorption coefficient and a 9 μs Shockley-Read-Hall (SRH) lifetime obtained from the measured hole lifetime data. Most important is that the expected generation-recombination (G-R) space charge current based on the 9 μs SRH lifetime is not observed as expected for an ideal nBn heterojunction detector and that increasing excess dark currents are observed with decreasing temperature. The in-band external QE measured at 100K is in the range of 30% and is observed to increase with increasing operating temperature which indicates effects influenced by hole mobility anomalies. In contrast to the above listed observations, the Type II Ga-free nBn detector data reported by D. Ting et al. [Appl. Phys. Lett., 113, 021101, 2018] exhibits a hole energy barrier, G-R dark current like characteristics, and both temperature/voltage dependent QE. |
