This work presents a fundamental investigation of the surface conduction pathways occurring along etched sidewalls in devices fabricated from InAs and GaSb. Surface leakage currents are identified by their dependence on device size and thermal activation energy, and are characterized in terms of sheet conductance. InAs is found to have a temperature-independent sheet conductance of approximately 8×10-8 mho×square. The sheet conductance of GaSb is comparable to that of InAs at room temperature, and when cooled it decreases with a thermal activation energy of 75 meV, which is approximately equal to the known separation between the valence band and surface Fermi level. The temperature dependence of the surface conductance of the two materials indicates that the surface of InAs is degenerate and the surface of GaSb is non-degenerate.
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.
The unipolar barrier is a new approach for control of dark currents in infrared photodetectors. First demonstrated in the
nBn detector and then in the unipolar barrier photodiode, unipolar barriers have been shown to block surface leakage
current. Unipolar barriers can also be implemented to filter out dark current components such as Shockley-Read-Hall
current, direct band-to-band tunneling and trap-assisted tunneling, but are not useful for blocking diffusion currents.
Current density-voltage characteristics of molecular-beam-epitaxy-grown InAs based unipolar barrier photodiodes are
presented and analyzed, showing effective limiting of noise current mechanisms for different unipolar barrier photodiode
architectures. RoA data shows near Auger-limited device performance and RoA values in excess of 1x107 Ω-cm2.
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