The conventional processing of the III-V nBn photodetectors defines mesa devices by etching the contact n-layer and stopping immediately above the barrier, i.e., a shallow etch. This processing enables great suppression of surface leakage currents without having to explore surface passivation techniques. However, devices that are made with this processing scheme are subject to lateral diffusion currents. To address the lateral diffusion current, we compare the effects of different processing approaches and epitaxial structures of nBn detectors. The conventional solution for eliminating lateral diffusion current, a deep etch through the barrier and the absorber, creates increased dark currents and an increased device failure rate. To avoid deep etch processing, a new device structure is proposed, the inverted-nBn structure. By comparing with the conventional nBn structure, the results show that the lateral diffusion current is effectively eliminated in the inverted-nBn structure without elevating the dark currents.
The e-SWIR wavelength band is a performance gap for infrared detectors. At both shorter and longer wavelengths, high
performance detector technologies exist: SWIR InGaAs detectors (1.7 micron cutoff), and MWIR (3-5 micron) detectors
such as InAs-based and GaSb-based Unipolar Barriers, MCT, and InSb. This work discusses development of high
performance e-SWIR detectors with cutoff wavelengths in the 2.7 - 2.8 micron range.
Two approaches for e-SWIR detector absorber materials were evaluated, lengthening the wavelength response of the
SWIR InGaAs technology and shortening the wavelength response of MWIR GaSb-based technology. The InGaAs e-
SWIR approach employs mismatched InGaAs absorber layers on InP substrates, using graded AlInAs buffer layers. The
GaSb-based approach uses lattice-matched InGaAsSb absorber layers on GaSb substrates. Additionally, two device
architectures were examined, pn-based photodiodes and unipolar barrier photodiodes. For both of the absorber materials,
the unipolar barrier device architecture was found to be superior.
The unipolar barrier device architecture enables both types of device to be free of effects of surface leakage currents and
generation-recombination dark currents. InGaAsSb-based devices show excellent performance, with diffusion-limited
dark current within a factor of 2-4 of the HgCdTe standard, Rule 07. They achieve background-limited (BLIP)
performance at T=210K, which is accessible by thermo-electric coolers. As expected, defects associated with latticemismatch
increase dark currents of the InP-based approach. The dark currents of the mismatched unipolar barrier
photodiodes are 30x larger than those of the lattice-matched GaSb approach, however despite the defects, the devices
still exhibit diffusion-limited operation, and achieve BLIP operation at T=190K Further improvements in the InP-based
approach are expected with refinements in the epitaxial structures. Both types of detector approaches are excellent
alternatives to conventional e-SWIR detectors.
Under elevated defect concentrations, MWIR, III-V nBn detectors exhibit diffusion limited performance with elevated dark current densities. The resulting diffusion current is limited by the generation of carriers through defect states in the neutral n-type absorber and a dark current dependence on the defect density described by one of two limits, a short absorber or long absorber limit. This characteristic contrasts that exhibited by defect limited, conventional pn junction based photodiodes which exhibit performance limited by Shockley-Read-Hall generation in the depletion layer rather than diffusion based processes.
The effect of defects on the dark current characteristics of MWIR, III-V nBn detectors has been studied. Two different types of defects are compared, those produced by lattice mismatch and by proton irradiation. It is shown that the introduction of defects always elevates dark currents; however the effect on dark current is different for nBn detectors and conventional photodiodes. The dark currents of nBn detectors are found to be more tolerant of defects compared to pn-junction based devices. Defects more weakly increase dark currents, and cooling reduces the defect-produced dark currents more rapidly in nBn detectors than in conventional photodiodes.
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