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.
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.
Laser-based stand-off sensing of threat agents (e.g. explosives, toxic industrial chemicals or chemical warfare agents), by detection of distinct infrared spectral absorption signature of these materials, has made significant advances recently. This is due in part to the availability of infrared and terahertz laser sources with significantly improved power and tunability. However, there is a pressing need for a versatile, high performance infrared sensor that can complement and enhance the recent advances achieved in laser technology. This work presents new, high performance infrared detectors based on III-V barrier diodes. Unipolar barrier diodes, such as the nBn, have been very successful in the MWIR using InAs(Sb)-based materials, and in the MWIR and LWIR using type-II InAsSb/InAs superlattice-based materials. This work addresses the extension of the barrier diode architecture into the SWIR region, using GaSb-based and InAs-based materials. The program has resulted in detectors with unmatched performance in the 2-3 μm spectral range. Temperature dependent characterization has shown dark currents to be diffusion limited and equal to, or within a factor of 5, of the Rule 07 expression for Auger-limited HgCdTe detectors. Furthermore, D* values are superior to those of existing detectors in the 2-3 μm band. Of particular significance to spectroscopic sensing systems is the ability to have near-background limited performance at operation temperatures compatible with robust and reliable solid state thermoelectric coolers.
This paper presents the first chip-scale demonstration of an intra-chip free-space optical interconnect (FSOI) we recently
proposed. This interconnect system uses point-to-point free-space optical links to construct an all-to-all intra-chip communication
network. Unlike other electrical and waveguide-based optical interconnect systems, FSOI exhibits low latency,
high energy efficiency, and large bandwidth density with little degradation for long distance transmission, and hence can
significantly improve the performance of future many-core chips. A 1x1-cm2 chip prototype is fabricated on a germanium
substrate with integrated photodetectors. A commercial 850-nm GaAs vertical-cavity-surface-emitting-laser (VCSEL) and
fabricated fused silica micro-lenses are 3-D integrated on top of the germanium substrate. At a 1.4-cm distance, the measured
optical transmission loss is 5 dB and crosstalk is less than -20 dB. The electrical-to-electrical bandwidth is 3.3 GHz,
limited by the VCSEL.
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