In this presentation, we propose an effective scattering-potential approach for treating interface-roughness scattering of moving electrons in a superlattice structure. Based on obtained effective scattering potentials, we further derive a generalized Boltzmann transport equation by including a self-consistent internal scattering force. In addition, we solve this equation exactly beyond the relaxation-time approximation, and meanwhile, analyze the dependence of conduction current on interface-roughness parameters at various temperatures and DC electric fiield strengths. Finally, we reveal a microscopic mechanism associated with non-ohmic transport behavior by analyzing features in steady-state non-equilibrium electron occupation function and its dependence on interface roughness parameters.
Event-based camera (EBC) technology provides high-dynamic range operation and shows promise for efficient capture of spatio-temporal information, producing a sparse data stream and enabling consideration of nontraditional data processing solutions (e.g., new algorithms, neuromorphic processors, etc.). Given the fundamental difference in camera architecture, the EBC response and noise behavior differ considerably compared to standard CCD/CMOS framing sensors. These differences necessitate the development of new characterization techniques and sensor models to evaluate hardware performance and elucidate the trade-space between the two camera architectures. Laboratory characterization techniques reported previously include noise level as a function of static scene light level (background activity) and contrast responses referred to as S-curves. Here we present further progress on development of basic characterization methods and test capabilities for commercial-off-the-shelf (COTS) visible EBCs, with a focus on measurement of pixel deadtime (refractory period) including results for the 4th-generation sensor from Prophesee and Sony. Refractory period is empirically determined from analysis of the interspike intervals (ISIs), and results visualized using log-histograms of the minimum per-pixel ISI values for a subset of pixels activated by a controlled dynamic scene. Our tests of the Prophesee gen4 EVKv2 yield refractory period estimates ranging from 6.1 msec to 6.8 μsec going from the slowest (20) to fastest (100) settings of the relevant bias parameter, bias_refr. We also introduce and demonstrate the concept of pixel bandwidth measurement from data captured while viewing a static scene – based on recording data at a range of refractory period setting and then analyzing noise-event statistics. Finally, we present initial results for estimating and correcting EBC clock drift using a GPS PPS signal to generate special timing events in the event-list data streams generated by the DAVIS346 and DVXplorer EBCs from iniVation.
Event-based camera (EBC) technology shows promise for efficient capture of spatio-temporal information, producing a sparse data stream and enabling consideration of nontraditional data processing solutions (e.g., new algorithms, neuromorphic processors). Given the fundamental difference in camera architecture, the EBC response and noise behavior differ considerably compared to standard CCD/CMOS framing sensors. These differences necessitate development of new characterization techniques to quantify performance and assess if the EBC technology produces benefits relative to traditional imaging sensors. Here we present progress on development of basic sensor performance modeling and test capabilities for commercial-off-the-shelf visible EBCs. Laboratory characterization techniques include noise level as a function of static scene light level (termed background activity) and EBC temporal contrast response to dynamic signals. Initial environmental tests of the Prophesee PPS3MVCD event-based sensor found several addressable areas of concern but identified no showstoppers that would prevent use of this device in a high-reliability aerospace application. Two independent radiation tolerance test efforts, one for the PPS3MVCD and another for the iniVation DAVIS346 EBC (both based on 180 nm CMOS technology), indicate functional issues for total ionizing dose (TID) of greater than 30 krad(Si), and show background activity increasing with TID. However, no significant change in contrast response was observed. One DAVIS346 exhibited functional failure following final gamma radiation dose from 20 krad(Si) to 50 krad(Si), and the readout saturated during doses dominated by negative-polarity events (by a factor of 10 or greater). A second DAVIS346 locked-up during proton dose but recovered normal operation following a brief rest period and power cycling. DAVIS346 pixels include both change detection (DVS) and standard grayscale frames (APS) functionalities – driven by a single photodiode; results show a 70% increase in dark current and 23% increase in dark event noise after proton exposure to 20 krad(Si). As new versions of EBC technology are developed for infrared wavelengths, we anticipate these characterization techniques will be largely translatable to IR EBCs.
HgCdTe has been called the ideal infrared detector material for good reason: high absorption coefficients and very long Shockley-Read-Hall (SRH) recombination lifetimes lead to the highest performance infrared detectors today for space applications. III-V materials, such as InAsSb, are currently limited by short SRH recombination lifetimes due to defects, and their performance is still relatively lacking for space applications where sensitivity requirements are extremely high. However, the performance of III-V superlattice infrared detectors has improved such that it is sufficient for tactical applications, which can now take advantage of the manufacturing benefits of III-V (greater uniformity and yield). With the growing NewSpace movement, there is a need for higher-volume, lower-cost infrared detectors capable of operating in space for applications such as environmental monitoring, space-based weather, and planetary science. One way to increase volume and lower cost is to grow the detectors on large-format substrates, such as 6-inch silicon or GaAs, but lattice-matched large substrates are not available for HgCdTe or InAsSb. Here a comparison between mid-wavelength infrared HgCdTe and InAsSb infrared detectors grown on non-lattice-mismatched substrates and designed for increased proton radiation tolerance, as compared to previous designs on mismatched substrates, is given. The comparison of these recent HgCdTe photodiode and InAsSb bariode designs for space applications shows that the InAsSb bariode has an order of magnitude better dark current density proton radiation tolerance while the HgCdTe photodiode has an order of magnitude better quantum efficiency proton radiation tolerance operating at 130 K. Therefore, the choice of detector material and architecture is not clear and will depend on the required performance for a specific space application.
To improve the performance of photodiodes based on narrow-bandgap InAs/GaSb type-II strained layer superlattices (T2SLs), knowledge of the vertical minority carrier transport is necessary. For this purpose, the key parameters influencing vertical minority-carrier electron transport in an nBp MWIR detector were studied: diffusion length, lifetime, mobility. The detectors were designed with p-type, 10/10 ML, InAs/GaSb T2SL absorbers, targeting a 50% cut-off wavelength of 5.0 µm at 80 K. The nBp structure is attractive because the junction field predominately drops across a relatively wide-gap InAs/AlSb SL barrier, which reduces the expected generation-recombination dark current. Measurements of the electron beam-induced current (EBIC), combined with minority carrier lifetime results from microwave reflectance measurements, enabled the determination of the minority carrier diffusion length (Le) and mobility in the growth direction as a function of temperature. The Le was extracted at each temperature by fitting the EBIC data to analytical expressions for carrier collection efficiency. The EBIC measurements were also repeated at different electron-beam energies to vary the distribution of minority carriers near the surface to gauge the surface recombination velocity. Microwave reflectance allowed for accurate measurement of the minority carrier lifetime over a large dynamic range of excess carrier concentrations, enabling a separation of recombination mechanisms. The lifetime and extracted diffusion length data were then used to estimate the diffusion coefficient and mobility versus temperature by applying the Einstein diffusion relationship.
Accurate p-type doping of the active region in III-V infrared detectors is essential for optimizing the detector design and overall performance. While most III-V detector absorbers are n-type (e.g., nBn), the minority carrier devices with p-type absorbers would be expected to have relatively higher quantum efficiencies due to the higher mobility of their constituent minority carrier electrons. However, correctly determining the hole carrier concentration in narrow bandgap InAsSb may be challenging due to the potential for electron accumulation at the surface of the material and at its interface with the layer grown directly below it. Electron accumulation layers form high conductance electron channels that can dominate both resistivity and Hall-effect transport measurements. Therefore, to correctly determine the bulk hole concentration and mobility, temperature- and magnetic-field-dependent transport measurements in conjunction with Multi-Carrier Fit analysis were utilized on a series of p-doped InAs0.91Sb0.09 samples on GaSb substrates. The resulting hole concentrations and mobilities at 77 K (300 K) were 1.6 x 1018 cm-3 (2.3 x 1018 cm-3) and 125 cm2 V-1 s-1 (60 cm2 V-1 s-1), respectively, compared with the intended Be-doping of ~2 x 1018 cm-3.
Type-II strained-layer superlattices (T2SLs) are receiving increased interest as mid-wave infrared (MWIR) and long-wave infrared detector absorbers due to their potential Auger suppression and ability to be integrated into complex device structures. Although T2SLs show promise for use as infrared detectors, further investigation into the effects of high energy particle radiation is necessary for space-based applications. In this presentation, the effects of both 4.5 MeV and 63 MeV proton radiation on the carrier lifetime of MWIR InAs/InAsSb T2SLs will be shown. The 63 MeV proton radiation study will focus on the carrier lifetime of MWIR InAs/InAsSb T2SL samples of varying donor density. These results reveal a Shockley-Read-Hall (SRH) lifetime associated with a radiation induced defect level, which is not dependent on the donor density of the T2SL. Using 4.5 MeV proton radiation, the dependence of carrier lifetime on relative trap density in MWIR T2SLs samples is studied by varying the particle fluence. A comparison of these two radiation studies shows similar lifetime effects that will be discussed in detail. These results give insight into the viability of Ga-free T2SLs for space applications.
We present a model for the spectral external quantum efficiency (EQE) to extract the minority carrier diffusion length (Ln) of a unipolar nBp InAs/GaSb Type-II superlattice (T2SL) mid-wave infrared (MWIR) detector. The detector consists of a 4 μm thick p-doped 10ML InAs/10ML GaSb SL absorber with a 50% cut-off wavelength of 5 μm at 80 K and zero bias. The n-type doped InAs/AlSb SL barrier in the structure was included to reduce the GR dark current. By fitting the experimentally measured EQE data to the theoretically calculated QE based on the solution of the drift-diffusion equation, the p-type absorber was found the have Ln = 10 ± 0.5 μm at 80K, and Ln = 12 ± 0.5 μm at 120K and 150K. We performed the absorption coefficient measurement at different temperatures of interest. Also, we estimated the reduced background concentration and the built-in potential by utilizing a capacitance-voltage measurement technique. We used time-resolved-photoluminescence (TRPL) to determine the lifetime at 80K. With the result of the model and the lifetime measurement, we calculated the diffusion coefficient and the mobility in the T2SL detector at various temperatures. Also, we studied the behavior of different dark current mechanisms by fitting the experimentally measured and simulated dark current density under different operating temperatures and biases.
We report high quantum efficiency (QE) MWIR barrier photodetectors based on the InAs/GaSb/AlSb type II superlattice (T2SL) material system. The nBp design consists of a single unipolar barrier (InAs/AlSb SL) placed between a 4 μm thick p-doped absorber (InAs/GaSb SL) and an n-type contact layer (InAs/GaSb SL). At 80K, the device exhibited a 50% cut-off wavelength of 5 μm, was fully turned-ON at zero bias and the measured QE was 62% (front side illumination with no AR coating) at 4.5 μm with a dark current density of 8.5×10-9 A/cm2 . At 150 K and Vb=50 mV, the 50% cut-off wavelength increased to 5.3 μm and the quantum efficiency (QE) was measured to be 64% at 4.5 μm with a dark current of 1.07×10-4 A/cm2 . The measurements were verified at multiple AFRL laboratories. The results from this device along with the analysis will be presented in this paper.
The last two decades have seen tremendous progress in the design and performance of mid-wavelength infrared (MWIR) type-II superlattices (T2SL) for detectors. The materials of focus have evolved from the InAs/(In)GaSb T2SL to include InAs/InAsSb T2SLs and most recently InGaAs/InAsSb SLs, with each materials system offering particular advantages and challenges. InAs/InAsSb SLs have the longest minority carrier lifetimes, and their best nBn dark current densities are <5X Rule ’07 at high temperatures, while those of InAs/GaSb SLs and InGaAs/InAsSb SLs are <10X Rule ’07. The quantum efficiency of all three SL detectors can still be improved, especially by increasing the diffusion length beyond the absorber length at low temperatures. Evidence of low temperature carrier localization is greatest for the two SLs containing ternary layers; however, the interface intermixing causing the localization is present in all three SLs. Localization likely does not affect the high temperature detector performance (>120 K) where these SL unipolar barrier detectors are diffusion-limited and Auger-limited. The SL barrier detectors remain diffusion-limited post proton irradiation, but the dark current density increases due to the minority carrier lifetime decreasing with increased displacement damage causing an increase in the trap density. For these SL detectors to operate in space, the continued understanding and mitigation of point defects is necessary.
In this work, we compare the performance of three MWIR unipolar barrier structures based on the InAs/GaSb Type-2 strained layer superlattice material system. We have designed, fabricated, and characterized pBiBn, pBn, and pBp detector structures. All the structures have been designed so that the cut off wavelength is around 5 microns at 100 K. We fabricated single-pixel devices and characterize their radiometric performance. In addition, we have characterized the degradation of the performance of the devices after exposing the devices to 63 MeV proton radiation to total ionizing dose of 100 kRad (Si). In this report, we compare the performance of the different structures with the objective of determining the advantages and disadvantages of the different designs. This work was supported by the Small Business Innovation Research (SBIR) program under the contract FA9453-14-C-0032, sponsored by the Air Force Research Laboratory (AFRL).
A microscopic-level model is proposed for exploring degraded performance in electron transport and photodetection devices, based on pre-calculated results as initial conditions for meso-scale approaches, including ultra-fast displacement cascade, intermediate defect stabilization and cluster formation, and slow defect reaction and migration. The steady-state spatial distribution of point defects in a mesoscopic-scale layered system will be studied by taking into account the planar dislocation loops and spherical neutral voids as well. These theoretical efforts are expected to be crucial in fully understanding the physical mechanism for identifying defect species, performance degradations, and the development of mitigation strategies. Additionally, verification of the current model by device characterization is discussed.
High performance infrared sensors are vulnerable to slight changes in defect densities and locations. For example in a space application where such sensors are exposed to proton irradiation capable of generating point defects the sensors are known to suffer performance degradation. The degradation can generally be observed in terms of dark current density and responsivity degradations. Here we report results of MWIR HgCdTe/CdZnTe single element diodes dark current densities before and after exposure to 63MeV protons at room temperature to a total ionizing dose of 100 kRad(Si). We find the irradiated diodes as a group show some signs of proton-induced damage in dark current.
The continuous effort to improve space-based infrared (IR) detectors has led to a search for greater fundamental understanding of radiation damage phenomena effects on key material properties. The material parameter of interest in this paper is the minority carrier recombination lifetime (MCRL), which is directly related to detector performance and can be empirically determined. As radiation damage is incurred upon a detector structure, the MCRL can be significantly affected, and tracking this in a step-wise, in-situ fashion at a radiation source can reveal rates of defect introduction. This has been accomplished by the development of a portable MCRL measurement system employing time resolved photoluminescence (TRPL) while maintaining operational temperatures. Using this methodology is more insightful than the so-called ‘bag tests’ (i.e. characterization before and after a single 100krad dosage) due to complex parameter changes witnessed with annealing as temperatures change. In addition to the system description, MCRL data on IR detectors from its inaugural deployments at a proton radiation source are analyzed and reveal a linear relationship between inverse MCRL and proton fluence.
In this work, we report on the design, fabrication, and characterization of MWIR unipolar barrier photodetectors based on InAs/GaSb Type-II superlattice. We have designed, fabricated, and characterized band-structure engineered MWIR photodetectors based on the pBiBn architecture. The devices have been characterized using the most relevant radiometric figures of merits. At 200 K, the peak value of detectivity is 1.2 x 1011 Jones at an applied bias voltage of -0.5 V.
KEYWORDS: Resistors, Sensors, Temperature metrology, Field effect transistors, Mid-IR, Superlattices, Infrared sensors, Signal to noise ratio, Interference (communication), Infrared radiation
Type-II Strained Layer Superlattice (T2SLS) infrared photodetectors have been in ongoing development over the last decade with the goal of achieving lower dark currents and higher operating temperatures when com- pared to mercury cadmium telluride (MCT) detectors. The theoretically longer Auger recombination lifetime of T2SLS has potential to lower dark current but the presence of Shockley-Read-Hall (SRH) defects limits the recombination lifetime far below the Auger-limit. In order to reduce SRH-recombination, unipolar barriers have been incorporated into the energy bands of T2SLS materials in different forms, such as nBn, to improve performance. Here, noise spectra are presented for varyingly sized, near 90% quantum efficiency, nBn mid-wave infrared (MWIR) detectors with superlattice absorbing layers grown by MBE. Noise spectrum measurements are used to evaluate device performance and reveal mechanisms contributing to low frequency noise that often exceeds predictions based on ideal shot noise. Voltage and temperature dependent noise spectra were taken using an external trans-impedance amplifier with an internal, cooled impedance converter and feedback resistor.
InAs/GaSb type-II strained-layer superlattice (T2SLS) materials are being considered for space-based infrared detector applications. However, an inadequate understanding of the role of carrier transport, specifically the vertical mobility, in the radiation tolerance of T2SLS detectors remains. Here, progress towards a vertical transport study of proton-irradiated, p-type InAs/GaSb T2SLS materials using magnetoresistance measurements is reported. Measurements in the growth direction of square mesas formed from InAs/GaSb superlattice material were performed using two distinct contact geometries in a Kelvin mode setup at variable magnetic fields, ranging from -9 T to 9 T, and temperatures, ranging from 5 K and 300 K. The results here suggested multi-carrier conduction and a field-dependent series resistance from the contact layer were present. The implications of these results and the plans for future magnetoresistance measurements on proton-irradiated T2SLS materials are discussed.
The realization of high operating temperature (HOT) midwave infrared (MWIR) photodetectors would significantly relax the requirements imposed on the cooling system, which would lead to a reduction in the size, weight, and cost of the detection system. One of the most attractive material systems to develop HOT photodetectors is InAs/GaSb Type II Superlattice (T2SL). This is due the ability of T2SL materials to engineer the band structure of the device, which can be exploited to make devices with unipolar barriers. It has been shown that the use of unipolar barriers can dramatically reduce the dark current levels of the device, which is essential to realize HOT photodetectors. In this work, we report on the performance of a unipolar barrier mid wave infrared detector based on type-II InAs/GaSb strained layer superlattice for high operating temperatures. The device architecture is the double-barrier heterostructure, pBiBn design. Under an applied bias of -10 mV and an operating temperature of 200 K, the best performing devices show a dark current density of 4.9×10-4 A/cm2. At 200 K, the measured zero-bias specific detectivity was 4.4×1010 Jones.
Exposure to proton radiation degrades the performance of wavelength infrared (MWIR) and long wavelength infrared (LWIR) HgCdTe photodetectors to varying degrees depending on the dose and energy of the incident particles. We report an experimental characterization of test devices of multiple sizes and configurations designed to investigate the effect proton radiation has on detector performance. Photodetector devices, from test element devices to fully functional focal plane arrays, are processed into MWIR and LWIR HgCdTe material grown by molecular beam epitaxy (MBE), in both single and two-color architectures, on CdZnTe and CdTe-buffered Si substrates. The devices receive doses of 30 krad(Si) and 100 krad(Si) from an incident beam of 63 MeV protons. The lower dose induces negligible degradation. At the higher dose, MWIR detectors begin to show reduced activation energy for higher temperatures, while LWIR detectors are more strongly affected with the activation energy being halved following proton irradiation.
Minority carrier recombination lifetime (MCRL) is a key material parameter for space-based infrared (IR) detector performance affecting both dark current and responsivity. Displacement damage due to energetic massive particles in space environments, such as protons, can significantly degrade the recombination lifetime, thereby reducing detector performance. Therefore, characterizing the change in MCRL with proton dose is of general interest from a radiation-hardness perspective. So-called “bag tests,” or measurements taken prior to and following room temperature proton irradiation of the device, are often of limited value to MCRL characterization since thermal annealing effects may be present. Here, progress toward a portable MCRL measurement system employing time resolved photoluminescence (TRPL) is presented. This system can be taken to remote radiation sources where irradiation can be performed on samples followed by TRPL measurements while maintaining temperature throughout. Ideally, this system permits measurement of a lifetime radiation damage factor constant, or the change in lifetime with step-wise changes in proton dose, which is a measure of the defect introduction rate. The pulsed-laser driven TRPL measurement system is able to interrogate IR materials of interest mounted in an optical cryostat held indefinitely at a desired temperature. A system description is given and results of verification measurements are discussed for several IR detector materials.
Midwave infrared (MWIR) photodetectors that do not require cryogenic cooling would significantly reduce the complexity of the cooling system, which would lead to a reduction in the size, weight, and cost of the detection system. The key aspect to realize high operating temperature (HOT) photodetectors is to design device structures that exhibit significantly lower levels of dark current compared to the existing technologies. One of the most attractive material systems to develop HOT photodetectors is InAs/GaSb Type II Strained layer Superlattice (SLS). This is due the ability of Type II SLS materials to engineer the band structure of the device, which can be exploited to make devices with unipolar barriers. It has been shown that, compared to the traditional homojunction SLS devices, band-gap engineered unipolar barrier SLS devices can obtain significantly lower levels of dark current. In this work, we report on the design, growth, and fabrication of mid wave infrared detectors based on type-II InAs/GaSb strained layer superlattice for high operating temperatures. The device architecture is the double-barrier heterostructure, pBiBn design. Under an applied bias of -10 mV and an operating temperature of 200 K, the tested devices show a dark current density of 4 x 10-3 A/cm2 and a quantum efficiency of 27%. At 4.5 μm and 200 K, the devices show a zero-bias specific detectivity of 4.4 x 1010 Jones.
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.
For space-based imaging systems radiation tolerance to both displacement damage and total ionizing dose (TID) radiation effects continues to be a major performance concern. Here, the TID and proton irradiance tolerance of mid wave infrared interband cascade infrared photodetectors (ICIPs) based on InAs/GaSb type II strained-layer superlattice (T2SLS) absorbers is presented. Protons of energy of 63 MeV were used to irradiate the unbiased ICIP detectors at room temperature to a proton fluence of 7.5 x 1011 protons/cm2, corresponding to a TID of 100 kRads(Si). A comparison of the detector performance of a variety of ICIPs with different numbers of electron barrier sizes cascade stages is presented. Performance of detectors of varying size was characterized by dark current and quantum efficiency measurements at different temperatures. Results show changes, increase in dark current and a reduction in the quantum efficiency, consistent with an increase in the trap density.
Infrared detector arrays operating in space must be able to withstand defect-inducing proton radiation without performance degradation. Therefore, it is imperative that the proton-radiation hardness of infrared detector materials be investigated. Photoluminescence (PL) is sensitive to defects in materials, and thus can be used to quantify the effects of proton-radiation-induced defects. The excitation intensity-dependent PL was used to examine of a set of InAs/InAsSb superlattices before and after 63-MeV-proton irradiation. A proton dose of 100 kRad(Si) was applied to a different piece of each superlattice sample. The low-temperature excitation intensity dependent PL results reveal minimal increases in the carrier concentration, non-radiative recombination, and the PL full-width half-maximum. These results suggest that InAs/InAsSb superlattices are quite tolerant of proton irradiation and may be suitable for space infrared detector arrays.
The development of a broadband IR focal plane array poses several challenges in the area of detector design, material, device physics, fabrication process, hybridization, integration and testing. The purpose of our research is to address these challenges and demonstrate a high-performance IR system that incorporates a HgCdTe-based detector array with high uniformity and operability. Our detector architecture, grown using molecular beam epitaxy (MBE), is vertically integrated, leading to a stacked detector structure with the capability to simultaneously detect in two spectral bands. MBE is the method of choice for multiplelayer HgCdTe growth because it produces material of excellent quality and allows composition and doping control at the atomic level. Such quality and control is necessary for the fabrication of multicolor detectors since they require advanced bandgap engineering techniques. The proposed technology, based on the bandgap-tunable HgCdTe alloy, has the potential to extend the broadband detector operation towards room temperature. We present here our modeling, MBE growth and device characterization results, demonstrating Auger suppression in the LWIR band and diffusion limited behavior in the MWIR band.
Interband cascade infrared photodetectors (ICIPs) potentially offer mid-wave infrared detection at very high operating temperatures due to their nearly ideal photovoltaic operation. An ICIP typically makes use of several cascade stages grown in series, each of which consists of an active absorption region with a mid-wave cutoff wavelength, an intra-band relaxation region for electron transport and an inter-band tunneling region to enable electron transport to the next stage. The latter two also effectively act as a hole-barrier (hB) and an electron-barrier (eB), respectively, forming a preferential path for each carrier. Here, an ICIP with a relatively large eB was investigated. One of the key parameters to measure for detector performance is the noise spectrum, particularly to observe the behavior at low frequencies where the noise is often much larger than estimates based on the ideal shot noise expression would predict. This paper presents the results of noise spectrum measurements of differently sized ICIP devices, taken using an external trans-impedance amplifier with a cooled, internal impedance converter and a cooled feedback resistor. Measurements were taken at different operating temperatures and voltage biases in order to determine the noise-dependence on each.
Infrared (IR) detector technologies with the ability to operate near room temperature are important for many
applications including chemical identification, surveillance, defense and medical diagnostics. Reducing the need for
cryogenics in a detector system can reduce cost, weight and power consumption; simplify the detection system
design and allow for widespread usage. In recent years, infrared (IR) detectors based on uni-polar barrier designs
have gained interest for their ability to lower dark current and increase a detector's operating temperature.
Our group is currently investigating detectors based on the InAs/GaSb strain layer superlattice (SLS) material
system that utilize barrier heterostructure engineering. Examples of such engineering designs include pBp, nBn,
PbIbN, CBIRD, etc. For this paper I will focus on LW (long wave) pBp structures. Like the built-in barrier in a p-n
junction, the heterojunction barrier blocks the majority carriers allowing free movement of photogenerated minority
carriers. However, the barrier in a pBp detector, in contrast with a p-n junction depletion layer, does not significantly
contribute to generation-recombination (G-R) current due to the lack of a depletion region across the narrow band
gap absorber material. Thus such detectors potentially work like a regular photodiode but with significantly reduced
dark current from G-R mechanisms.
The mechanism of photoconductive (PC) gain has not been fully characterized in such device architectures and
in many recent studies has been assumed to be unity. However, studies conducted with similar device structures
have shown the presence of PC gain. In this report we will measure and analyze the impact of PC gain in detectors
utilizing single unipolar barriers such as the case of pBp detectors.
Infrared (IR) detectors operated in the space environment are required to have high performance while being subjected to
a variety of radiation effects. Sources of radiation in space include the trapped particles in the Van Allen belts and
transient events such as solar events and galactic cosmic rays. Mercury cadmium telluride (MCT)-based IR detectors
are often used in space applications because they have high performance and are generally relatively tolerant of the space
environment when passivated with CdTe; often, the readout-integrated circuit is far more susceptible to radiation effects
than the detector materials themselves. However, inherent manufacturing issues with the growth of MCT have led to
interest in alternative detector technologies including type-II strained-layer superlattice (T2SLS) infrared detectors with
unipolar barriers. Much less is known about the radiation tolerance properties of these SLS-based detectors compared to
MCT. Here, the effects of 63 MeV protons on variable area, single element, dual-band InAs/GaSb SLS detectors in the
pBp architecture are considered. When semiconductors devices are irradiated with protons with energies of 63 MeV the
protons are capable of displacing atoms within their crystalline lattice. The SLS detectors tested here utilize a pBp
architecture, which takes advantage of the higher mobility electrons as the minority photocarrier. These detectors are
also dual-band, implying two absorbing regions are present and separated by the unipolar barrier. The absorbers have
cutoff wavelengths of roughly 5 and 9 μm allowing for mid-wave (MWIR) and long-wave (LWIR) infrared detection,
respectively. The radiation effects on these detectors are characterized by dark current and quantum efficiency as a
function of total ionizing dose (TID) or, equivalently, the incident proton fluence.
Long-wave infrared (LWIR) detector technologies with the ability to operate at or near room temperature are very
important for many civil and military applications including chemical identification, surveillance, defense and medical
diagnostics. Eliminating the need for cryogenics in a detector system can reduce cost, weight and power consumption;
simplify the detection system design and allow for widespread usage. In recent years, infrared (IR) detectors based on
uni-polar barrier designs have gained interest for their ability to lower dark current and increase a detector's operating
temperature.
Our group is currently investigating nBn and pBp detectors with InAs/GaSb strain layer superlattice (SLS)
absorbers (n) and contacts (n), and AlGaSb and InAs/AlSb superlattice electron and hole barriers (B) respectively. For
the case of the nBn structure, the wide-band-gap barrier material (AlGaSb) exhibits a large conduction band offset and a
small valence band offset with the narrow-band-gap absorber material. For the pBp structure (InAs/AlSb superlattice
barrier), the converse is true with a large valence band offset between the barrier and absorber and a small or zero
conduction band offset. Like the built-in barrier in a p-n junction, the heterojunction barrier blocks the majority carriers
allowing free movement of photogenerated minority carriers. However, the barrier in an nBn or pBp detector, in contrast
with a p-n junction depletion layer, does not contribute to generation-recombination (G-R) current.
In this report we aim to investigate and contrast the performance characteristics of an SLS nBn detector with that of
and SLS pBp detector.
KEYWORDS: Sensors, Laser sintering, Resistors, Infrared sensors, Infrared detectors, Field effect transistors, Amplifiers, Etching, Superlattices, Signal to noise ratio
Recent experiments on conventional p-on-n and n-on-p Type II superlattices (SLS) infrared detectors still indicate larger than
theoretically predicted dark current densities, despite the well known suppression of the Auger recombination mechanism.
Rather, dark current in SLS is thought to still be limited by trap-assisted tunneling in the depletion region and surface leakage
currents resulting from lack of fully passivated mesa sidewalls. An emerging infrared detector technology utilizing a
unipolar, single-band barrier design, the so-called nBn architecture, potentially suppresses these remaining noise current
mechanisms. In this report, measurements of the noise current spectral density of a mid-wave infrared nBn detector,
composed of a type-II InAs/GaSb strain layer superlattice (SLS) absorber (n) and contact (n) layers with an AlGaSb barrier
(B), under low-temperature, low-background conditions are presented. Here, noise was measured using a transimpedance
amplifier incorporating a dewar-mounted feedback resistor RF and source-follower MOSFET, both held at 77 K. This
configuration confines high detector impedance issues to the dewar, minimizes Johnson noise due to the electronics, and
enhances bandwidth by reducing stray capacitance. Features of the detector's noise spectrums at different bias are examined.
At the Air Force Research Laboratory's Space Vehicles Directorate, we are investigating how nanostructured metal
surfaces can produce plasmon-enhanced fields to improve detectivity of a detector material placed directly below the
metal surface. We are also investigating a wavelength-tunable detector scheme that involves a coupled double quantum
well structure with a thin middle barrier between the two wells. The photocurrent from this structure will be swept out
with a lateral bias. Another form of wavelength tunability is to have a tunable filter in front of a broadband detector.
There are many avenues of research that lead to such a device. The way we are approaching this is via the new field of
metamaterials. Not only might these new materials present us a way to tune the light that is incident upon a detector, but
such research might also lead to ways to obtain sub-diffraction-limit resolution and the concentration of light using flat
lenses for increased signal-to-noise ratios. In this talk we will discuss the research efforts being pursued in the above
areas.
IR detectors operated in a space environment are subjected to a variety of radiation effects while required to have very
low noise performance. When properly passivated, conventional mercury cadmium telluride (MCT)-based infrared
detectors have been shown to perform well in space environments. However, the inherent manufacturing difficulties
associated with the growth of MCT has resulted in a research thrust into alternative detector technologies, specifically
type-II Strained Layer Superlattice (SLS) infrared detectors. Theory predicts that SLS-based detector technologies have
the potential of offering several advantages over MCT detectors including lower dark currents and higher operating
temperatures. Experimentally, however, it has been found that both p-on-n and n-on-p SLS detectors have larger dark
current densities than MCT-based detectors. An emerging detector architecture, complementary to SLS-technology and
hence forth referred to here as nBn, mitigates this issue via a uni-polar barrier design which effectively blocks majority
carrier conduction thereby reducing dark current to more acceptable levels.
Little work has been done to characterize nBn IR detectors tolerance to radiation effects. Here, the effects of gamma-ray
radiation on an nBn SLS detector are considered. The nBn IR detector under test was grown by solid source molecular
beam epitaxy and is composed of an InAs/GaSb SLS absorber (n) and contact (n) and an AlxGa1-xSb barrier (B). The
radiation effects on the detector are characterized by dark current density measurements as a function of bias, device
perimeter-to-area ratio and total ionizing dose (TID).
Over the last several years the development of type-II Strained Layer Superlattice (SLS) infrared photodetectors has
yielded devices that may offer plausible alternative technology to conventional mercury cadmium telluride (MCT)-based
photodetectors. Prevailing theory predicts that SLS-based detector technologies will have several potential advantages
over MCT technologies, including lower dark currents and higher operating temperatures. However, experimentally it
has been found that conventional p-on-n and n-on-p SLS detectors have high dark current and thus, do not reach
theoretically predicted performance benchmarks. The two prevailing contributors to this high dark current are the
generation-recombination (GR) current and surface leakage currents, the latter resulting from the mesa sidewall
exposure. A recently emerging technology that utilizes a uni-polar barrier design nBn has been shown to reduce dark
current, while keeping the inherent advantages of SLS. Specific advantages of SLS over MCT include wavelength
tunability, improved uniformity, and operability potentially at a reduced manufacturing cost. This report presents some
recent experimental findings for the electrical and optical response of an nBn detector composed of an InAs/GaSb SLS
absorber (n) and contacts (n) with an AlGaSb barrier (B). Results include the intrinsic determination of the diffusion
current, and the GR current for the nBn device. Also presented is the optical response of the InAs/GaSb nBn detector at
77K over a broad range of operating biases. Dark current measurements over the 10K-300K temperature range were
undertaken to extract the activation energies in the heterostructure.
We propose a design of diffractive and refractive optical corrective elements with zooming capability for linearizing the angular scan of a resonant mirror scanner. Considering the symmetry requirements of the refractive element a graded index of refraction and its binary amplitude version are designed based on phase lag (beam retardation due to propagation through an inhomogeneous media). The design takes the beam diameter into consideration making it robust against beam fanning.
III-V based single photon avalanche diodes (SPADs), avalanche
photodiodes (APDs) operated in Geiger-mode, are ideally suited for
ultra-weak signal detection in the near infrared for photon
counting and photon timing applications. Spaceborne SPADs would
provide a rugged, compact alternative to photomultiplier tubes
with lower operating voltage requirements, stronger near-IR
response, and the possibility for array implementation. Results
from a performance characterization of an in-house fabricated
In0.53Ga0.47As/InP SPAD are presented. Sensitivity (NEP) and timing resolution (δt) were investigated as a function of bias from T = 135 K to 165 K; an NEP ≈ 5 x 1015W/Hz1/2 at T = 150 K and δt ≈ 230 ps at T = 165 K were measured for λ = 1.55 μm light.
KEYWORDS: Quantum well infrared photodetectors, Resistors, Resistance, Sensors, Field effect transistors, Signal to noise ratio, Capacitance, Interference (communication), Amplifiers, Rutherfordium
Signal to noise ratio (SNR) is the most important figure of merit for comparing the performance of infrared detectors. When measuring SNR, it is vitally important that all of the noise sources be characterized, including those of the test apparatus. At low temperatures and/or high current, generation-recombination (G-R) noise is expected to be the main noise contributor. At high temperatures and low current, Johnson noise is expected to become comparable to the G-R noise. We present noise measurements under dark conditions on a Quantum Well Infrared Photodetector (QWIP) with careful attention to noise sources present in the measurement apparatus. The noise was dominated by Johnson noise at temperatures <40K, and by G-R noise at 77K and finite bias. A new method to measure noise in QWIPs is described.
Normal incidence InAs/In0.15Ga0.85As dots-in-a-well detectors operating at T=78K with λcut-off ~8.2 μm and a spectral width (Δλ/λ) of 35% are reported. The peak at 7.2 μm is attributed to the bound-to-bound transitions between the ground state of the dot and the states within the InGaAs well. A broad shoulder around 5 μm, which is attributed to the bound to continuum transition, is also observed. Calibrated blackbody measurements at a device temperature of 78K yield a peak responsivity of 3.58 A/W (Vb=-1V), peak detectivity= 2.7x109cmHz1/2/W (Vb=-0.3V), conversion efficiency of 57% and a gain ~25.
Self-assembled polymer photo-detectors (PPDs) composed of ruthenium complex N3 and PPDs based on thin films of poly(p-phenylene vinlyene) with sulfonated polystyrene are examined for their ability to function in a simulated space radiation environment. Examination of the PPD pre- and post- response data following gamma-ray irradiation ranging in total dose from 10 krad(Si) to 100 krad(Si) are examined. The output photovoltage was observed to decrease for all irradiated devices. The brief study was performed at room temperature and a discussion of the preliminary data and results are presented.
In the presence of a time-dependent external source such as a bias electric field or an incident optical flux, electrons in quantum well and quantum dot devices experience non-adiabatic transport through the barrier layer between two adjacent quantum wells or quantum dots. This non-adiabatic transport process induces charge density fluctuations, resulting in several transient phenomena. When a time-dependent electric field is applied to the system, a dynamical breakdown (i.e., the dark current is dominated by a dielectric displacement current) of the quantum well or quantum dot photodetector is observed. If a chopped time-dependent optical flux is incident on either system, a dynamical drop in the photo-responsivity with increasing chopping frequency is also observed.
The peak responsivity of quantum-well infrared photodetectors (QWIPs) is known to decrease or "roll-off" to a lower plateau value as the frequency of an incoming time-varying photon irradiance increases. The time constant associated with the roll-off frequency depends on the amplitude of the applied DC bias, the incoming irradiance, and the device temperature. In this paper we demonstrate the scaling law for the responsivity roll-off and use it to estimate the quantum-well capacitance by first measuring the roll-off frequency as a function of bias and optical flux and then measuring the device dynamic resistance under similar conditions. The slope of the scatter plot of the roll-off angular frequency versus the inverse dynamic resistance is related to the quantum-well capacitance. Using this approach, we estimate the quantum-well capacitance in a fifty-well, Al0.3Ga0.7As/GaAs QWIP pixel of area 2.44 x 10-4 cm-2 to be ~ 1.22pF at 50 K.
In the presence of a time-dependent external source such as a bias electric field, an incident optical flux, or the temperature, electrons in quantum well devices experience non-adiabatic transport through the barrier layer between two adjacent quantum wells. This non-adiabatic transport process induces charge density fluctuations within each quantum well, resulting in several seemingly unrelated transient phenomena. When a time-dependent electric field is applied to the system, a dynamical breakdown and a zero-bias residual dark current in the quantum-well photodetectors are predicted theoretically. If a chopped time-dependent optical flux is incident on the system, a dynamical drop in the photo-responsivity with increasing chopping frequency and an emission-current spike as the optical shutter is opened are predicted. Finally, as the device temperature is varied with time, a counter-clockwise thermal hysteresis is found theoretically in the dark current curve as a function of the changing temperature. Experimental confirmation of the above theoretical predictions is presented.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.