Surface dark current often limits the performance of infrared photodetectors, especially as detectors become smaller. Bulk dark current mechanisms are well understood, but surface dark current mechanisms are not. Here, surface dark current mechanisms are identified, and examples of detector designs that can block these currents are given.

In the pursuit of lower cost and higher performance infrared detectors, antimonide-based materials have been increasingly investigated over the past decade. One of the greatest advantages of the III-V materials system is the potential for improved uniformity stemming from the incorporation of lattice-matched wide bandgap, unipolar barrier devices (i.e. "nBn"). However, with the increasing demand for larger-format, smaller-pitch focal plane arrays (FPA) new processing and passivation techniques have become necessary. This is especially true for FPAs that require fully reticulated detector pixel structures to meet modulation transfer function (MTF) requirements. This paper will discuss our investigation of several mesa delineation strategies to improve consistency, reduce surface current, and maximize optical fill factor in antimonide based dual-band infrared photodetectors. The resulting etch profiles, surface chemistry and photoresist etch selectivity for new inductively coupled plasma etch chemistries and conditions will be discussed.

The extended-shortwave infrared wavelength range, encompassing wavelengths from 2.2 to 3 μm, is significantly underdeveloped when compared to the shortwave and midwave infrared bands. Achieving high performance detectors in the extended-shortwave range is desirable; however, it is unclear whether to approach the wavelength range via the detector structures and materials common to the shortwave regime or those common to the midwave regime. Both approaches are studied here. Electrical and optical characteristics of conventional photodiodes and nBn architecture detectors with 2.8 μm cutoff wavelengths are analyzed for detectors with both lattice-mismatched InGaAs and lattice-matched InGaAsSb absorbing regions. Regardless of the absorber material, the nBn detectors show nearly 3 orders of magnitude improvements in performance over the conventional photodiode architecture, and the lattice-matched InGaAsSb nBn exhibits a further reduction in the dark current by more than an order of magnitude when compared to the lattice-mismatched InGaAs nBn. The InGaAsSb nBn exhibits high quality optical detection resulting in a high performance detector in the extended-shortwave infrared band.

This manuscript describes an investigation of the effects of growth temperature on InAs epitaxial layers and InAs-based nBn detectors grown by molecular beam epitaxy (MBE). The motivation for this work is to improve the overall performance of InAs-based nBn detectors, which depends both on the bulk material quality of the individual device layers, particularly the infrared absorbing layer, as well as on the quality of the layer interfaces, particularly the interface between the absorber and barrier layers. Absorber layer bulk quality and absorber/barrier interface quality are presumably optimized by performing InAs growth at different temperatures, thus the preferred MBE growth strategy is not immediately apparent. InAs epitaxial layers of 2 μm thick are grown at several temperatures ranging from 420 to 490 °C, and are examined by differential interference contrast microscopy, atomic force microscopy, steady-state photoluminescence, and time-resolved photoluminescence measurements. Absorber layers of 2 μm thick in nBn detectors are also grown at the same temperatures as the InAs single layers, and the resulting devices are evaluated on the basis of dark current density. Competitively high InAs material quality and low nBn dark current densities have been achieved across the range of investigated growth temperatures. The material quality of the InAs single epitaxial layers is found to improve monotonically with growth temperature over the investigated range, and likewise, the reverse saturation dark current density of the nBn detectors is found to decrease monotonically with growth temperature. nBn detectors with dark current density within a factor of 5 of Rule 07 are reported. Finally, it is noted that this work uses an InAs growth rate of 0.9 μm/h, whereas many other studies have chosen to use InAs growth rates in the range of 0.2–0.5 μm/h. The results of this study demonstrate that high performance InAs-based detectors can be grown at this more convenient rate.

Resonant cavity detectors based on III–V materials have been designed, grown entirely by molecular beam epitaxy, fabricated, and tested. They offer a low noise (dark current densities of 0.4 mA/cm2 were measured at 298 K, close to the predicted value of 0.31 mA/cm2), narrow response detector (full width at half maximum of 57 nm in GaSb and 45 nm in InAs) in the mid-infrared region, with future applications in spectroscopy, gas sensing, and optical communications.

Resonant cavity detectors have been grown by MBE. They offer a low noise, narrow response detector in the mid-infrared region, with possible applications in spectroscopy, gas sensing, and optical communications.

Infrared detector epitaxial structures employing unipolar barriers exhibit greatly reduced dark currents compared to simple pn-based structures. When correctly positioned within the structure, unipolar barriers are highly effective at blocking bulk dark current mechanisms. Unipolar barriers are also effective at suppressing surface leakage current in infrared detector structures employing absorbing layers that possess the same conductivity type in their bulk and at their surface. When an absorbing layer possesses opposite conductivity types in its bulk and at its surface, unipolar barriers are not solutions to surface leakage. This work reviews empirically determined surface band alignments of III–V semiconductor compounds and modeled surface band alignments of both gallium-free and gallium-containing type-II strained layer superlattice material systems. Surface band alignments are used to predict surface conductivity types in several detector structures, and the relationship between surface and bulk conductivity types in the absorbing layers of these structures is used as the basis for explaining observed surface leakage characteristics.

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.

This work presents a pn-based unipolar barrier detector architecture that exhibits no band bending under zero bias. A zero-bias flat band structure is created by utilizing materials with pre-aligned Fermi levels, which prevents charge transfer across the junction and depletion layer formation. The ideal structure shows no detectable g–r, tunneling, or surface leakage currents, and is more tolerant to variations in layer composition than other barrier detector architectures.

Mid-wave infrared, nBn detectors remain limited by diffusion current generated in the absorber region even when defect concentrations are elevated. In contrast, defect-limited conventional pn-junction based photodiodes are subject to Shockley-Read-Hall generation in the depletion region and subsequent carrier drift. Ideal nBn-architecture devices would be limited by Auger 1 generation; however, typical nBn detectors exhibit defect-dominated performance associated with Shockley-Read-Hall generation in the quasi-neutral absorbing region. Reverse saturation current density characteristics for defect-limited devices depend on the minority carrier diffusion length, absorbing layer thickness, and the dominant minority carrier generation mechanism. Unlike pn-based photodiodes, changes in nBn dark current due to elevated defect concentrations do not manifest at small biases, thus, the zero bias resistance area product, RoA, is not a useful parameter for characterizing nBn-architecture photodetector performance.

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 eSWIR 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 lattice-mismatch 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.

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.

When properly employed, unipolar barriers can significantly improve the performance of infrared photodetectors; however, the barriers must be correctly engineered and properly located in the epitaxial structure in order for detectors to function optimally. Unipolar barrier concepts, design, and implementation in several device architectures are discussed. nBn and unipolar barrier photodiodes are demonstrated in the InAs materials system, and the limitations of unipolar barriers are considered.

Control of dark current mechanisms is essential to improving the performance of infrared photodetectors and many other electronic devices. Unipolar barriers can readily be applied to practically and efficiently filter out multiple dark current components exhibited by infrared photodetectors. Via careful placement of unipolar barriers in a standard photodetector architecture, effective suppression of dark currents due to surface leakage, direct band-to-band tunneling, trap-assisted tunneling, and Shockley-Read-Hall generation is demonstrated. We present unipolar barrier photodiodes exhibiting six orders of magnitude improvement in RoA and near Auger-limited device performance.

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.

Searching for hidden illicit sources of gamma radiation in an urban environment is difficult. Background radiation profiles are variable and cluttered with transient acquisitions from naturally occurring radioactive materials and medical isotopes. Potentially threatening sources likely will be nearly hidden in this noise and encountered at high standoff distances and low threat count rates. We discuss an anomaly detection algorithm that characterizes low count sources as threatening or non-threatening and operates well in the presence of high benign source variability. We discuss the algorithm parameters needed to reliably find sources both close to the detector and far away from it. These parameters include the cutoff frequencies of background tracking filters and the integration time of the spectrometer. This work is part of the development of the Standoff Radiation Imaging System (SORIS) as part of DNDO's Standoff Radiation Detection System Advanced Technology Demonstration (SORDS-ATD) program.

We describe an undergraduate laboratory that combines an accurate measurement of the speed of light, a fundamental investigation of a basic laser system, and a nontrivial use of statistical analysis. Students grapple with the existence of longitudinal modes in a laser cavity as they change the cavity length of an adjustable-cavity HeNe laser and tune the cavity to produce lasing in the TEM00 mode. For appropriate laser cavity lengths, the laser gain curve of a HeNe laser allows the simultaneous operation of multiple longitudinal modes. The difference frequency between the modes is measured using a self-heterodyne detection with a diode photodetector and a radio frequency spectrum analyzer. Asymmetric effects due to frequency pushing and frequency pulling, as well as transverse modes, are minimized by simultaneously monitoring and adjusting the mode structure as viewed with a Fabry–Pérot interferometer. The frequency spacing of longitudinal modes is proportional to the inverse of the cavity length with a proportionality constant equal to half the speed of light. By changing the length of the cavity, without changing the path length within the HeNe gas, the speed of light in air can be measured to be (2.9972±0.0002)×108 m/s, which is to high enough precision to distinguish between the speed of light in air and in vacuum.

This manuscript profiles an unattended and fully autonomous radiation detection system sensitive to gamma-ray and neutron emissions. The Lynx design is intended for locations that require radiation detection capabilities for detection of special nuclear materials but lack supporting infrastructure. Signal-starved data is common in these environments since little or no control may be exerted over measurement conditions. The fundamental sensing elements of the Lynx system are traditional NaI(Tl) and 3He detectors. The new developments reported here center on two themes: low-power electronics and computationally simple analysis algorithms capable of discriminating gamma-ray signatures indicative of special nuclear materials from those of naturally occurring radioactive material. Incorporating tripwire-detection algorithms based on gamma-ray spectral signatures into a low-power electronics package significantly improves performance in environments where sensors encounter nuisance sources.

Abstract