Search

For mid-wave infrared detectors, we designed a meta-surface to enhance quantum efficiency (QE) across 2 to 6 microns. The relative enhancement depends on the intrinsic absorption coefficient $\alpha $ of the material. If $\alpha $ is large at $2 \times 10^{4}$ /cm, a 0.1 micron-thick meta-detector can yield a peak QE of 90%, which is 2.6 times higher than the conventional detector. The improvement is larger with a smaller $\alpha $ . When it is 2000/cm, the improvement is about 10 times with a peak QE of 49%. For energy harvesting devices, we designed several nanostructures etched on top of silicon solar cells to enhance their absorption. Employing an array of nano-columns increases absorption from 55% to 97% at 0.7 microns and from 5.0% to 37% at 1.0 microns. An array of nano-cones further increases the average absorption to 95% between 0.4 to 0.8 microns. The overall integrated absorption is increased by 74% for nano-columns and 92% for nano-cones. For GaAs solar cells, a metasurface can improve photocurrent by 26% from a planar solar cell with a 100 nm-thick absorber.

For MWIR IR detectors, we designed a meta-surface to enhance QE across 2 to 6 microns. The relative enhancement depends on the intrinsic absorption coefficient, α, of the material. If α is large at 2x104/cm, a 0.1 micron-thick metadetector can have a peak QE of 90%, which is 2.6 times higher than the conventional detector. The improvement is larger at a smaller α. When it is 2000/cm, the improvement is about 10 times with a peak QE of 49%. For energy harvesting devices, we designed several nanostructures etched on top of silicon solar cells to enhance their absorption. Employing an array of nano-columns increases absorption from 55% to 93% at 0.7 microns and from 5.0% to 20% at 1.0 microns. An array of nano-cones further increases the average absorption to 95% between 0.4 to 0.8 microns. The overall integrated absorption is increased by 71% for nano-columns and 91% for nano-cones. For GaAs solar cells, a metasurface can improve photocurrent by 26% from a planar solar cell with a 100 nm-thick absorber.

In an infrared photodetector, noise current (dark current) is generated throughout the volume of the detector. Reducing the volume will reduce dark current, but the corresponding smaller area will also reduce the received signal. By using a separate antenna to receive light, one can reduce the detector area without reducing the signal, thereby increasing its signal-to-noise ratio (SNR). Here, we present a dielectric resonator antenna (DRA)-coupled infrared photodetector. Its performance is compared to a conventional resonant cavity-enhanced slab detector. The noise equivalent power (NEP) is used as a figure of merit for the comparison. Formulas for the NEP are derived for both cases. A pBp photodiode detector is assumed in the comparison. The active region of the photodiode is an InAs/GaSb Type II Superlattice (T2SL). A Genetic Algorithm is used to optimize the dimensions of the detector and the DRA to achieve the smallest NEP. The result is a photodetector that converts over 85% of the incident light into carriers with a volume reduced by 95%. This optimal geometry leads to an NEP reduced by 6.02 dB over that of the conventional resonant cavity-enhanced slab detector.

A Dielectric Resonator Antenna Coupled Infrared Antimonide Photodetector (DRACAD) is presented. The large capture area of the DRA feeds incident radiation to the subwavelength dimension detector. The reduced dimension of the detector reduces the Noise Equivalent Power (NEP). The large capture area of the DRA increases the captured signal. The combination leads to high Signal-to-Noise Ratios (SNR). The DRACAD is designed using a Genetic Algorithm Optimization to obtain dimensions which maximize the absorbed infrared radiation with the smallest possible detector. The plasmonic metals are modeled as effective lossy sheet impedances. Absorption spectrum simulations between $8-12\mu m$ wavelengths show the DRACAD absorbs nearly 73% of the incident light at $9\mu m$ after all metal losses are accounted for.

With the increasing use of resonant structures in optical devices, broadband optical characterization of the refractive index and extinction coefficient is necessary for accurate simulation and device design. For resonance-enhanced photodetectors, the complex refractive index is necessary to impedance match not only the resonator to air, minimizing the reflection, but also the resonator to the detector element, ensuring absorption occurs in the photodiode. To work towards better resonator-detector coupling, we present the complex refractive index for GaSb and an InAs/GaSb strained layer superlattice designed to be the absorber layer for a long-wave infrared photodetector. The optical properties were extracted using spectroscopic ellipsometry. Several modeling methods will be discussed for both the superlattice and the single-side polished bulk GaSb. Comparison to transmission and reflection values as well as absorption coefficients from literature provide additional confidence in the extraction process. Future work will incorporate these values into a resonance-enhanced photodetector.

In 1988 DARPA provided funding to NASA’s Goddard Space Flight Center to support the development of GaAs Quantum Well Infrared Photodetectors (QWIP). The goal was to make a single element photodetector that might be expandable to a two-dimensional array format. Ultimately, this led to the development of a 128 x 128 element array in collaboration with ATT 2) broad spectral response and; 3) higher sensitivity. We have collaborated extensively with QmagiQ, LLC and Northwestern University to further pursue and advance the SLS technology ever since we started back in 2012. In December of 2018 we launched the first SLS-based IR camera system to the International Space Station on board the Robotic Refueling Mission #3 (RRM3). This paper will describe the evolution of QWIP technology leading to the current development of SLS-based imaging systems at the Goddard Space Flight Center over the past 30 years.

Long-wavelength infrared (LWIR) focal plane arrays (FPAs) needed for Earth Science imaging, spectral imaging, and sounding applications have always been among the most challenging in infrared photodetector technology due to the rigorous material growth, device design and fabrication demands. Future small satellite missions will present even more challenges for infrared FPAs, as operating temperature must be increased so that cooler (and radiator) volume, mass, and power can be reduced. To address this critical need, we are working on following three technologies. 1) Type-II superlattice (T2SL) nBn type barrier infrared detector, which combines the high operability, spatial uniformity, temporal stability, scalability, producibility, and affordability advantages of III-V detectors. 2) The resonator pixel technology, which uses metasurface light trapping techniques to achieve strong absorption in a small detector absorber volume, thereby enabling enhanced QE and/or reduced dark current. 3) High dynamic range 3D Readout IC (3D-ROIC), which integrates a digital reset counter with a conventional analog ROIC to provide a much higher effective well capacity than previously achievable. The resulting longer integration times are especially beneficial for high flux/dark current LWIR applications as they can improve signal-to-noise ratio and/or increase the operating temperature. By combining these three technologies, this project seeks to demonstrate a cost-effective, high performance LWIR FPA technology with significantly higher operating temperature and sensitivity than previously attainable, and with the flexibility to meet a variety of Earth Science TIR measurement needs, particularly the special requirements of small satellite missions.

Long-wavelength infrared (LWIR) focal plane arrays (FPAs) needed for Earth Science imaging, spectral imaging, and sounding applications have always been among the most challenging in infrared photodetector technology due to the rigorous material growth, device design and fabrication demands. Future small satellite missions will present even more challenges for LWIR FPAs, as operating temperature must be increased so that cooler (and radiator) volume, mass, and power can be reduced. To address this critical need, we are working on following three technologies. 1) Type-II superlattice (T2SL) barrier infrared detector (BIRD), which combines the high operability, spatial uniformity, temporal stability, scalability, producibility, and affordability advantages of the quantum well infrared photodetector (QWIP) FPA with the better quantum efficiency and dark current characteristics. A mid-wavelength infrared (MWIR) T2SL BIRD FPA is a key demonstration technology in the (6U) CubeSat Infrared Atmospheric Sounder (CIRAS) funded under the ESTO InVEST Program. A LWIR T2SL BIRD FPA is also being developed under the ESTO SLI-T Program for future thermal infrared (TIR) land imaging needs. 2) The resonator pixel technology, which uses nanophotonics light trapping techniques to achieve strong absorption in a small detector absorber volume, thereby enabling enhanced QE and/or reduced dark current. 3) High dynamic range 3D Readout IC (3DROIC), which integrates a digital reset counter with a conventional analog ROIC to provide a much higher effective well capacity than previously achievable. The resulting longer integration times are especially beneficial for high flux/dark current LWIR applications as they can improve signal-to-noise ratio and/or increase the operating temperature. By combining the aforementioned technologies, this project seeks to demonstrate a cost-effective, high-performance LWIR FPA technology with significantly higher operating temperature and sensitivity than previously attainable, and with the flexibility to meet a variety of Earth Science TIR measurement needs, particularly the special requirements of small satellite missions.

In this paper, we report our recent efforts in achieving high performance in Antimonides type-II superlattice (T2SL) based infrared photodetectors using the barrier infrared detector (BIRD) architecture, resonator pixel light coupling mechanism, and digital read out integrated circuits (DROICs).

Long-wavelength infrared (LWIR) focal plane arrays (FPAs) needed for Earth Science imaging, spectral imaging, and sounding applications have always been among the most challenging in infrared photodetector technology due to the rigorous material growth, device design and fabrication demands. Future small satellite missions will present even more challenges for LWIR FPAs, as operating temperature must be increased so that cooler (and radiator) volume, mass, and power can be reduced. To address this critical need, we are working on following three technologies. 1) Type-II superlattice (T2SL) barrier infrared detector (BIRD), which combines the high operability, spatial uniformity, temporal stability, scalability, producibility, and affordability advantages of the quantum well infrared photodetector (QWIP) FPA with the better quantum efficiency and dark current characteristics. A mid-wavelength infrared (MWIR) T2SL BIRD FPA is a key demonstration technology in the (6U) CubeSat Infrared Atmospheric Sounder (CIRAS) funded under the ESTO InVEST Program. A LWIR T2SL BIRD FPA is also being developed under the ESTO SLI-T Program for future thermal infrared (TIR) land imaging needs. 2) The resonator pixel technology, which uses nanophotonics light trapping techniques to achieve strong absorption in a small detector absorber volume, thereby enabling enhanced QE and/or reduced dark current. 3) High dynamic range 3D Readout IC (3DROIC), which integrates a digital reset counter with a conventional analog ROIC to provide a much higher effective well capacity than previously achievable. The resulting longer integration times are especially beneficial for high flux/dark current LWIR applications as they can improve signal-to-noise ratio and/or increase the operating temperature. By combining the aforementioned technologies, this project seeks to demonstrate a cost-effective, high-performance LWIR FPA technology with significantly higher operating temperature and sensitivity than previously attainable, and with the flexibility to meet a variety of Earth Science TIR measurement needs, particularly the special requirements of small satellite missions.

In this presentation, we will report our recent efforts in achieving high performance in Antimonides type-II superlattice (T2SL) based infrared photodetectors using the barrier infrared detector (BIRD) architecture. The recent emergence of barrier infrared detectors such as the nBn [1] and the XBn [2] have resulted in mid-wave infrared (MWIR) and long-wave infrared (LWIR) detectors with substantially higher operating temperatures than previously available in III-V semiconductor based MWIR and LWIR detectors. The initial nBn devices used either InAs absorber grown on InAs substrate, or lattice-matched InAsSb alloy grown on GaSb substrate, with cutoff wavelengths of $\sim 3.2 \ \mu \mathrm{m}$ and $\sim 4\ \mu \mathrm{m}$ , respectively. While these detectors could operate at much higher temperatures than existing MWIR detectors based on InSb, their spectral responses do not cover the full ( $3 - 5.5 \mu \mathrm{m})$ Mwir atmospheric transmission window. There also have been nBn detectors based on the InAs/GaSb T2SL absorber [3], [4].

Resonator-quantum well infrared photodetectors (R-QWIPs) are the next generation of QWIP detectors that use resonances to increase the quantum efficiency (QE). To achieve the expected performance, the detector geometry must be produced in precise specification. In particular, the height of the diffractive elements (DE) and the thickness of the active resonator must be uniformly and accurately realized to within 0.05 lm accuracy and the substrates of the detectors have to be removed totally. To achieve these specifications, two optimized inductively coupled plasma (ICP) etching processes are developed. Using these etching techniques, we have fabricated a number of R-QWIP test detectors and FPAs with the required dimensions and completely removed the substrates of the test detectors and FPAs. Their QE spectra were tested to be in close agreement with the theoretical predictions. The operability and spectral non-uniformity of the FPA is about 99.57% and 3% respectively.

Rigorous electromagnetic field modeling is applied to calculate the quantum efficiency of various quantum well infrared photodetector (QWIP) geometries. We found quantitative agreement between theory and experiment for corrugated-QWIPs, grating-coupled QWIPs, and enhanced-QWIPs, and the model explains adequately the spectral lineshapes of the quantum grid infrared photodetectors. After establishing our theoretical approach, we used the model to optimize the detector structures for 12-micron pixel pitch focal plane arrays.

Corrugated quantum-well infrared photodetectors (C-QWIPs) have been proposed for long-wavelength infrared detection. In this work, we optimize the detector structure and produce a number of large format focal plane arrays (FPAs). Specifically, we adopt one-corrugation-per-pixel geometry to increase the active detector volume and incorporate a composite cover layer to preserve the large sidewall reflectivity, which results in a large detector quantum efficiency. We also optimize the detector material structure such as the final state energy, the doping density, and the number of quantum well periods to improve the FPA operation under the existing readout electronics. As a result, high FPA sensitivity has been achieved, and their characteristics are in agreement with the detector model. Based on this model, we perform a systematic analysis on the FPA performance with a wide range of detector and system parameters. We find that C-QWIP FPAs are capable of high-speed imaging especially for those with longer cutoff wavelengths.

Corrugated quantum-well infrared photodetectors (C-QWIPs) offer simple detector architectures for large-format infrared focal plane arrays (FPAs). The detector relies on inclined sidewalls to couple normal incident light into the absorbing material. Based on a simplified geometrical-optics (GO) model, this light coupling scheme is expected to be effective with little wavelength dependence. In this work, we apply the modal transmission-line (MTL) modeling technique to study in detail its light coupling characteristics and compare the results with the GO model and experimental data. We find that the results of the GO model agree reasonably well with those of the rigorous MTL model for corrugations with metal cover, and both modeling procedures are consistent with experimental data. In particular, both models predict similar increase in the quantum efficiency /spl eta/ with the size of the corrugations, and both indicate similar limiting /spl eta/ when the corrugation becomes very large. For linear corrugations with thick substrates, the maximum /spl eta/ is about 30%. On the other hand, there are also significant differences between the two models when the effects of phase coherence are important. Since the phase of the radiation is taken into account in the MTL formalism but not in the GO formalism, the MTL model is more generally applicable and is more capable of explaining different detector characteristics. For example, it predicts a smaller /spl eta/ for air or epoxy-covered C-QWIPs because of finite optical transmission through the sharp corners in the corrugations, and it indicates an oscillatory function of /spl eta/ because of the existence of optical fringes. It also reveals the wavelength dependence of the coupling, which becomes more pronounced as the thickness of the substrate layer is reduced.

In this work, we will present a detailed study on the corrugated coupling scheme, which uses optical reflection at the angled sidewalls. In particular, we have investigated, both theoretically and experimentally, corrugated quantum well infrared photodetectors (QWIPs) with different cover materials over a wide range of spectral regime. We found that the coupling is insensitive to the wavelength but depends strongly on the optical properties of the cover material. We will also present the results of C-QWIP focal plane arrays (FPA) characterized so far. An important advantage of the corrugated coupling is its scalability to small pixel size. Based on one corrugation per pixel geometry, we have fabricated a 1024 × 1024 C-QWIP FPA with 99.5% connectivity. With the corrugated coupling geometry, many application restrictions on QWIPs can be eliminated. We will further show that the lack of normal incident absorption in QWIPs can actually be an advantage. It allows one to manipulate the absorption characteristics of QWIPs through different optical coupling structures and creates new detector functionality. The present example is an infrared spectrometer. It consists of a linear array of quantum grid infrared photodetector (QGIP) elements. Each QGIP element shares the same QWIP material but has different grid geometry. The QWIP is made of broadband materials such as the binary superlattices to give an extremely broadband (∼10 μm) absorption. The structure of the grid, on the other hand, determines the specific wavelength to detect at each detector element in the array. In the QGIP spectrometer, detectors with different line widths will detect different wavelengths simultaneously, thus yielding the emission spectrum of an object. A collection of QGIP spectrometers can form an adaptive FPA.

A numerical method for global optimization of quantum-well infrared photodetector (QWIP) performance parameters is presented and experimentally verified. The single-band effective-mass Schroedinger equation is solved by employing the argument principle method (APM) to extract both the bound and quasibound eigen-energies of the quantum heterostructure. APM is combined with a simulated annealing algorithm to determine a set of device design parameters such as potential barrier height V/sub i/, layer thickness d/sub i/, number of material layers N, total device length, applied bias V/sub Bias/ etc., for which the QWIP performance is within a predetermined convergence criterion. The method presented incorporates the effect of energy-dependent effective mass of electrons in nonparabolic conduction bands. The present model can handle many optimization parameters and can incorporate fabrication constraints to achieve physically realizable devices. In addition, the method is not limited to the optimization of absorption structures, and can be used for other intersubband devices such as electron-wave Fabry-Perot filters and quantum-cascade lasers. The strength and versatility of the present method are demonstrated by the design of a bicolor equal-absorption-peak QWIP structure, and experimental verification of the zero-bias absorption spectrum is presented.

Quantum well infrared photodetectors (QWIPs) have many advantages in infrared detection, mainly due to the mature III-V material technology. The employment of the corrugated light-coupling scheme further improves the technology for its simplicity and efficiency. A C-QWIP enjoys the same flexibility as a detector with intrinsic normal incident absorption. In this chapter, we will discuss the sensitivity of C-QWIPs and their utilities in infrared detection, material characterization and electromagnetic modeling. Besides the standard corrugated structures, other exploratory detector architectures will also be described.

The infrared absorption of SF6 gas is narrowband and peaks at 10.6 μm. This narrowband absorption posts a stringent requirement on the corresponding sensors as they need to collect enough signal from this limited spectral bandwidth to maintain a high sensitivity. Resonator-quantum well infrared photodetectors (R-QWIPs) are the next generation of QWIP detectors that use resonances to increase the quantum efficiency for more efficient signal collection. Since the resonant approach is applicable to narrowband as well as broadband, it is particularly suitable for this application. We designed and fabricated R-QWIPs for SF6 gas detection. To achieve the expected performance, the detector geometry must be produced according to precise specifications. In particular, the height of the diffractive elements and the thickness of the active resonator must be uniform, and accurately realized to within 0.05 μm. Additionally, the substrates of the detectors must be completely removed to prevent the escape of unabsorbed light in the detectors. To achieve these specifications, two optimized inductively coupled plasma etching processes were developed. Due to submicron detector feature sizes and overlay tolerance, we used an advanced semiconductor material lithography stepper instead of a contact mask aligner to pattern wafers. Using these etching techniques and tool, we have fabricated focal plane arrays with 30-μm pixel pitch and 320×256 format. The initial test revealed promising results.

Resonator-Quantum Well Infrared Photodetectors (R-QWIPs) are the next generation of QWIP detectors that use resonances to increase the quantum efficiency (QE). To achieve the expected performance, the detector geometry must be produced in precise specification. In particular, the height of the diffractive elements (DE) and the thickness of the active resonator must be uniformly and accurately realized to within 0.05 μm accuracy and the substrates of the detectors have to be removed totally. To achieve these specifications, two optimized inductively coupled plasma (ICP) etching processes are developed. Using these etching techniques, we have fabricated a number of R-QWIP test detectors and FPAs with the required dimensions and completely removed their substrates. The QE spectra were tested to be in close agreement with the theoretical predictions. The operability and spectral uniformity of the focal plane array (FPA) is about 99.1% and 3% respectively.

In this paper, we report a 10 /spl mu/m-cutoff infrared hot-electron transistor (IHET) with extremely high detectivity, D*>7.5/spl times/10/sup 12/ cm/spl radic/Hz/W at 4.2 K. This large D* is accomplished by using InGaAs material in the quantum wells and by using a low filter barrier at the collector for the large photocurrent transfer ratio. We have verified the detector performance by explicitly performing noise characterization at low temperatures. We found that the noise of the quantum well infrared photodetector, which forms the emitter of the IHET, is dominated by 1/f noise and bias-independent noise. The 1/f current noise is due to the conductance fluctuation in impurity-assisted tunneling via DX centers in the quantum well barriers. The filter barrier of the IHET blocks the impurity-assisted tunneling and hence its noise at the collector and thus improves the detector sensitivity.

An analytical approach for modeling optical fields in quantum-well infrared photodetectors was developed by using a rigorous solution of the corresponding electromagnetic problem. Its application includes structures having a large number of dielectric layers, which may contain gratings having arbitrary profiles and metal-strip arrays acting as electrodes. By representing the fields inside complex photodetector structures in terms of interconnected transmission-line units, this approach helps considerably to clarify the role of each constituent of the photodetector. Examples involving realistic situations reveal that the presence of metal electrodes may affect the photodetection operation in a large class of grating structures. In particular, we show that the sensitivity of specific photodetector configurations can be enhanced by choosing grating parameters that optimize the overall photodetecting performance.

We demonstrated a 256 times 256 voltage-tunable two- color corrugated quantum-well infrared-photodetector focal plane array. The detector operation is based on photocurrent asymmetry in a double-superlattice quantum well structure. By using a broadband corrugated light coupling scheme, we obtained a quantum efficiency of 24% in the mid-wave (MW) band and 26% in the long-wave (LW) band without AR coating. Operating at 50 K, the measured noise equivalent temperature difference is 27 mK at 33-ms integration time for the MW and 90 mK at 2 ms for the LW with f/2.44 optics. The NETD operability values are 98.4% and 95.9%, respectively.

We have designed and characterized an infrared spectrometer, which uses a linear array of quantum grid infrared photodetectors (QGIPs) as its spectral sensing elements. Each QGIP element shares the same detector material but has a different grid geometry. The detector material, which is based on a binary superlattice design, provides an 8–14 μm broadband absorption medium for the spectrometer. The geometry of the grid, which is the light coupling structure under normal incidence, selects individual absorption wavelength for each element. Using a linear array of QGIPs of different geometries, multiple wavelengths can be detected simultaneously, and the array thus forms a spectrometer. Multicolor infrared imaging can then be achieved by integrating such QGIPs in unit cells of a two-dimensional array.

Inductively coupled plasma (ICP) etching has distinct advantages over reactive ion etching in that the etching rates are considerably higher, the uniformity is much better, and the sidewalls of the etched material are highly anisotropic due to the higher plasma density and lower operating pressure. Therefore, ICP etching is a promising process for pattern transfer required during microelectronic and optoelectronic fabrication. Resonator-quantum well infrared photodetectors (R-QWIPs) are the next generation of QWIP detectors that use resonances to increase the quantum efficiency (QE). To fabricate R-QWIP focal plane arrays (FPAs), two optimized ICP etching processes are developed. Using these etching techniques, we have fabricated R-QWIP FPAs of several different formats and pixel sizes with the required dimensions and completely removed the substrates of the FPAs. Their QE spectra were tested to be 30 to 40%. The operability and spectral non- uniformity of the FPA is ∼99.5 and 3%, respectively. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. (DOI: 10

We report on the temperature dependence of electron transfer between coupled quantum wells in a voltage tunable two-color quantum-well infrared photodetector (QWIP). The detection peak of this QWIP switches from 7.1 μm under positive bias to 8.6 μm under negative bias for temperatures T⩽40 K. For T⩾40 K, the 7.1 μm peak is present under both bias polarities and increases significantly with T while the 8.6 μm peak decreases correspondingly. We determine the temperature dependence of electron densities in the two QWs from the detector absorption spectra that are deduced using corrugated QWIPs and find that electron transfer is efficient only when thermionic emission is not significant.

Rigorous electromagnetic modeling based on a modal solution of the pertinent boundary-value problem was performed to study the light coupling mechanism in quantum grid infrared photodetectors with lamellar patterns. Our theory shows that vertical field component and absorption quantum efficiency η can be strongly enhanced by judiciously adjusting the width w of individual grid lines. This enhancement is further increased if the top of the grid lines is covered by metal, which behave as a collection of dipole scatterers. We have experimentally verified the dipole scattering characteristics with different w, and found that the variation of η agrees very well with the theory. We also found that, as expected, the conductivity of the metal strips affects η significantly due to internal dissipation.

Because of the high cost of HgCdTe focal plane arrays (FPAs), the quantum well infrared photodetector (QWIP), which is based on the mature GaAs material system, poses as a possible lower cost alternative. Since this layered detector material is sensitive to radiation only if the optical electric field is vertical to the layers, an optical coupler is needed to redirect the normal incident light into oblique propagation. An early theoretical analysis showed a 2-dimensional cross grating can achieve a high quantum efficiency (QE) close to 100%. This optimistic prediction prompted a wide-spread use of gratings, and tens of thousands of these QWIP cameras have been produced. Despite the commercial success, the observed QE remains modest and is in the order of 5%. While this QE is adequate for many applications, the lower performance does expose the technology to the challenges from other material systems such as InAs/GaSb type-II superlattices and exclude its use in the more demanding applications such as flight navigation and fast or cold object detection and tracking.

The Thermal Infrared Sensor (TIRS) is a QWIP based instrument intended to supplement the Operational Land Imager (OLI) for the Landsat Data Continuity Mission (LDCM). The TIRS instrument is a dual channel far infrared imager with the two bands centered at 10.8[mu]m and 12.0[mu]m. The focal plane assembly (FPA) consists of three 640x512 GaAs Quantum Well Infrared Photodetector (QWIP) arrays precisely mounted to a silicon carrier substrate that is mounted on an invar baseplate. The two spectral bands are defined by bandpass filters mounted in close proximity to the detector surfaces. The focal plane operating temperature is 43K. The QWIP arrays are hybridized to Indigo ISC9803 readout integrated circuits (ROICs). Two varieties of QWIP detector arrays are being developed for this project, a corrugated surface structure QWIP and a grating surface structure QWIP. This paper will describe the TIRS system noise equivalent temperature difference sensitivity as it affects the QWIP focal plane performance requirements: spectral response, dark current, conversion efficiency, read noise, temperature stability, pixel uniformity, optical crosstalk and pixel yield. Additional mechanical constraints as well as qualification through Technology Readiness Level 6 (TRL 6) will also be discussed.

In a quantum grid infrared photodetector (QGIP), the active multiple quantum well material is patterned into a grid structure. The purposes of the grid are on the one hand to create additional lateral electron confinement and on the other to convert part of the incident light into parallel propagation. With these two unique functions, a QGIP allows intersubband transition to occur in all directions. In this work, we focused on improving the effectiveness of a QGIP in redirecting the propagation of light using a blazed structure. The optimization of the grid parameters in terms of the blaze angle and the periodicity was performed by numerical simulation using the modal transmission-line theory and verified by experiment. With a blazed structure, the sensitivity of a QGIP can be improved by a factor of 1.8 compared with a regular QGIP with rectangular profiles.

The corrugated quantum well infrared detector (C-QWIP) offers improvements to quantum efficiency and spectral bandwidth compared to current commercial QWIPs. In addition to improved performance, the C-QWIP also uses manufacturing processes that are mature and low cost. Thus, very large format focal plane arrays (FPAs) can be fabricated with high yield. There are two applications where the C-QWIP can provide cost effective solutions. The first is very large format long-wave infrared (LWIR) sensors. Most very large format FPAs operate in the mid-wave infrared (MWIR). The MWIR band has significantly lower flux than LWIR, therefore in situations where the backgrounds are cold or there is potential motion blur, the LWIR C-QWIP offers better performance. The second application is two-color registered high-resolution wide area imagery. ARL and CE have been developing both C-QWIP detectors and read-out integrated circuits to support these needs. This paper describes the progress we've made in developing high conversion efficiency LWIR C-QWIP FPAs and MWIR/LWIR two-color FPAs and our path forward to multimegapixel C-QWIP FPAs and sensors.

Recently, we have developed a detector structure known as the resonator quantum-well infrared photodetector or R-QWIP. With this structure, we demonstrated quantum efficiency as high as 70% in single detectors and 30% to 40% in focal plane arrays (FPAs) with a 9-μm cutoff. We designed a broadband, 10-μm cutoff R-QWIP FPA using a more accurate refractive index. To achieve the theoretical prediction, the substrates of the detectors have to be removed completely to prevent the escape of unabsorbed light out of the detectors. The height of the diffractive elements (DE) and the thickness of the active resonator must also be uniformly produced within a 0.05-μm accuracy. To achieve these specifications, two optimized inductively coupled plasma (ICP) etching processes are developed. Using these etching techniques, a number of single detectors were fabricated to verify the analysis before FPA production. In general, test data support the theoretical predictions.

The sensitivity of a focal plane array in terms of the noise equivalent temperature difference ultimately depends on the detector photocurrent to dark current ratio rI. In this work, we have integrated two approaches in the quantum well technology that can increase rI into a single detector structure. The new detector is referred to as the corrugated infrared hot-electron transistor (CI-HET). In this detector structure, an energy filter is grown next to a standard quantum well infrared photodetector (QWIP) for more selective collection of the photocurrent. At the same time, corrugated structures are physically fabricated into the detector pixel to increase light absorption in the QWIP. With this combined approach, rI is able to increase by more than a factor of five compared to a standard QWIP. In addition, the new structure reduces the total current of the detector to a level that is suitable for signal integration.

A quantum-well infrared photodetector (QWIP) based on the corrugated light-coupling scheme has been fabricated and tested for remote temperature sensing. The QWIP consists of two stacks of multiple quantum wells (MQWs), each sensitive in one of the atmospheric infrared transmission windows and each with a separate readout circuit. High optical coupling efficiency is obtained in both wavelength ranges, demonstrating the use of the corrugated structure for two-color detection. By monitoring the ratio of the photocurrent generated simultaneously in each MQW stack, the temperature of the object emitting the radiation can be determined, regardless of its emissivity and the geometrical factors. This temperature sensing ability is tested by using a blackbody radiator with precision temperature control as the target. The agreement between the measured and the preset temperatures indicates that the corrugated QWIP is capable of precision thermometric measurements.

Corrugated quantum well infrared photodetectors (C-QWIPs) utilize total internal reflection to couple normal incident light into the detectors. In this work, we performed a systematic study on C-QWIPs with cutoff wavelength λc ranging from 5 to 17.3 μm and device areas ranging from 50×50 μm2 to 500×500 μm2. We found that the coupling efficiency of the C-QWIPs is, indeed, independent of both the detection wavelength and the pixel size, as expected. Compared to devices with a standard 45° edge coupling, the C-QWIP increases the background photocurrent to a dark current ratio by a factor between 2.4 and 4.4, thereby increasing the background limited temperature by 3–5 K. At the same time, the detectivity D* is also increased by a factor around 2.4. For λc=9.4 μm, the peak detectivity Dp* and the blackbody detectivity Dbb* at 78 K are measured to be 4.5×1010 cm Hz/W and 4.5×109 cmHz/W, respectively.

The versatility of quantum well infrared photodetector (QWIP) structures allows for voltage tunable detection. However, the existing designs generally suffer from limited range of tunability and substantial spectral cross-talk. In this work, we demonstrated a QWIP structure, which is capable of detecting four widely separated and narrowly peaked individual bands under different bias. The detection peaks range from mid-wavelength to long-wavelength and are centered at 4.5, 5.3, 8.3, and 10.4 μm, respectively, with Δλ/λ < 0.14. With f/1.2 optics, the detector is BLIP at 100K, 80K, 60K and 50K respectively for the increasing wavelengths. This four-color detector consists of two stacks of voltage tunable materials that are separated by a middle contact layer. Each material is designed to detect at two specific wavelengths depending on the bias polarity. In the present design, the upper stack switches between 5.3 μm and 10.4 μm, while the bottom stack switches between 4.5 μm and 8.3 μm. By applying different bias to the top and bottom contacts relative to the common middle contact, the optical signal for each color can be readout at the two contacts sequentially. The tunable material is made of repeated unit cells, and each unit cell contains two different superlattices separated by a thick barrier. Based on this design, the detection peaks can be randomly selected between 3 μm and 20 μm, and each peak can vary from narrow band to broadband. The present detector design thus improves the QWIP technology in multi-color detection.

In this letter, we report a quantum well infrared photodetector geometry for normal incidence light coupling. The new optical coupling scheme utilizes total internal reflection at the sidewalls of triangular wires to create favorable optical polarization for infrared absorption. These wires are created by chemically etching an array of V grooves through the detector active region along a specific crystallographic direction. Experimental results from the initial single color as well as two‐color detectors with linear wires and unthinned substrate show efficient light coupling comparable to the standard 45° edge coupling, without the undesirable wavelength dependence or spectral narrowing effect of a conventional grating structure. At the same time, the dark current density is substantially reduced due to the partial material removal. Further improvement is expected by creating a two‐dimensional coupling structure with substrate thinning.

We present the design and fabrication of voltage tunable two-color superlattice infrared photodetectors (SLIPs), where the detection wavelength switches from the long-wavelength infrared (LWIR) range to the mid-wavelength infrared (MWIR) range upon reversing the polarity of applied bias. The photoactive region of these detectors contains multiple periods of two distinct short-period SLs that are designed for MWIR and LWIR detection. The voltage tunable operation is achieved by using two types of thick blocking barriers between adjacent SLs - undoped barriers on one side for low energy electrons and heavily-doped layers on the other side for high energy electrons. We grew two SLIP structures by molecular beam epitaxy. The first one consists of two AlGaAs/GaAs SLs with the detection range switching from the 7-11 μm band to the 4-7 μm range on reversing the bias polarity. The background-limited temperature is 55 and 80 K for LWIR and MWIR detection, respectively. The second structure comprises of strained InGaAs/GaAs/AlGaAs SLs and AlGaAs/GaAs SLs. The detection range of this SLIP changes from the 8-12 μm band to the 3-5 μm band on switching the bias polarity. The background-limited temperature is 70 and 110 K for LWIR and MWIR detection, respectively. This SLIP is the first ever voltage tunable MWIR/LWIR detector with performance comparable to those of one-color quantum-well infrared detectors designed for the respective wavelength ranges. We also demonstrate that the corrugated light coupling scheme, which enables normal-incidence absorption, is suitable for the two-color SLIPs. Since these SLIPs are two-terminal devices, they can be used with the corrugated geometry for the production of low-cost large-area two-color focal plane arrays.

In recent years quantum well infrared (IR) photodetector (QWIP) focal plane array (FPA) technology has developed to the point where it may be considered a candidate for insertion into 3rd generation FLIR systems. Both large format (1024x1024 pixels) and multicolor (MWIR/LWIR and LWIR/LWIR) FPAs have been produced using QWIP technology. We report on the application of these new FPAs to the challenges facing today's military. These include the collection of signatures of buried land mines with a LWIR/LWIR dual-color QWIP and long-range target detection/identification using a 1024x1024 FPA. The FPAs were produced from several sources. Large format LWIR FPAs were made by an ARL/NASA/Rockwell team using ARL’s C-QWIP optical coupling scheme. Another large-format FPA was obtained from QWIP Technologies, Inc. and is commercially available. The dual-color LWIR/LWIR FPA was produced by BAE Systems. Laboratory and field imagery from both types of FPAs are presented and analyzed.

We report on the fabrication of quantum grid infrared photodetector (QGIP) arrays and demonstrate their feasibility for use as multi-channel long wavelength infrared spectrometers. The quantum well infrared photodetector (QWIP) material structure was designed to exhibit broadband absorption in the wavelength range of 7 μm to 16 μm. By fabricating QGIP devices with this QWIP material, scattering of light at an individual wavelength of interest within the material absorption range can create narrow band detection in each device. Arrays of QGIP devices with varying geometry, each tailored to respond to a discrete wavelength were fabricated. Details of the epi-growth, processing steps taken to fabricate required device features for narrow band absorption of the QGIP devices, and characterization methods will be discussed.

Gratings of different period spacings are fabricated on quantum well infrared photodetectors having two‐stack multiple quantum wells (MQW) to determine the grating coupling efficiency in multicolor detection. The peak wavelength of each MQW is 4.8 and 9.4 μm, respectively, covering the two atmospheric windows. 2D gratings of box‐shaped cavities with grating period either in 3 or 4.6 μm spacing are used to coupled normal incident light into the detector. Comparing detectors with and without gratings at either normal or 45° facet incidence, the detectors with an optimized grating show efficient light coupling for both wavelengths due to the fact that the longer wavelength in the present case is an integral multiple of the shorter wavelength. The optimized long wavelength grating couples the shorter wavelength through additional higher orders of diffraction.

A 9 μm cutoff 640×512 pixel hand-held quantum well IR photodetector (QWIP) camera has been demonstrated wiht excellent imagery. Based on the single pixel test data, a nose equivalent differential temperature (NETD) of 8 mK is expected at 65K operating temperature with f/2 optics at 300K background. This focal plane array has shown backgeround lmiited performance at 72K operating temperature with the same optics and background conditions. In this paper, we discuss the development of this very sensitive long wavelength IR (LWIR) camera based on a GaAs/AlGaAs QWIP focal plane array and its performance in quantum efficiency, NETD, uniformity, and operability.

It was demonstrated that an electron energy bandpass filter placed next to a quantum well infrared photodetector (QWIP) is not only able to suppress dark current, but also is able to select photocurrents of different energies. Since the bandpass energy of the filter changes with an applied bias across the filter, this mechanism can be used to control the detection width and the cutoff wavelength of the detector. If the photoresponse of the QWIP has more than one energy peak, the same filter can also be used to intercept a particular photoelectron peak and achieve voltage tunable multicolor infrared detection. In this work, we also show that the bandpass filter increases the background limited temperature and the detectivity of the QWIP by filtering away the lower energy dark current and the unused photocurrent.

The performance of GaAs/AlGaAs quantum well infrared photodetectors specified in terms of background limited temperature Tb and specific detectivity D* has been calculated based on realistic detector parameters. It is found that for a detector with an external quantum efficiency η of 6.9%, Tb is 76 K for a cutoff wavelength of 10 μm. This value of Tb agrees with the recent experimental result and is significantly higher than the previous estimation given by M. A. Kinch and A. Yariv [Appl. Phys. Lett. 55, 2093 (1989)]. If η is unity, the projected Tb can be as high as 88 K with a D* of 2.2×1011 cm√Hz/W. For a lower temperature operation, D* increases to 7.5×1011 cm√Hz/W at 77 K, comparable to that of a HgCdTe detector.

The noise properties of hot electrons in three different infrared hot‐electron transistors have been characterized. We observed that there is a reduction of generation‐recombination noise after the hot electrons passed through the built‐in electron energy filters. The magnitude of the reduction depends on the band structure of the filters, and can be attributed to the lack of partition noise associated with the quantum transport of the hot electrons. Based on this observation, low noise infrared hot‐electron transistors can be designed using appropriate filters.

We introduce here a novel light coupling scheme for quantum well infrared photodetectors (QWIPs). The new device structure utilizing this coupling scheme, referred to as the corrugated QWIP (C-QWIP), couples normal incident light effectively even for detector size as small as 50/spl times/50 /spl mu/m/sup 2/ and shows little spectrum narrowing effect, indicating that it can be applied to the manufacturing of high resolution focal plane arrays and is a viable candidate for multi-color or broadband infrared detection.

An optimized detector fabrication process is developed for resonator-quantum well infrared photodetectors (R-QWIPs). The R-QWIPs are the next generation of QWIP detectors that use resonances to increase the quantum efficiency (QE). To achieve the expected performance, the detector geometry must be produced in precise specification. In particular, the height of the diffractive elements and the thickness of the active resonator must be uniformly and accurately realized to within 0.05-μm accuracy. To achieve this specification, an optimized inductively coupled plasma etching process with a nearly infinite etching selectivity for the GaAs over the Al x Ga 1−x As etch-stop layer was developed. Using this etching technique, we have fabricated a number of R-QWIP test detectors with the required dimensions. Their QE spectra were tested to be in close agreement with the QE predictions.

We have conducted a detailed study on the properties of a multiple quantum well infrared photodetector (QWIP) with a single bound state. From the optical absorption experiment, we found that the peak absorption energy is determined by the resonant states associated with each individual well and not by the global miniband structure. From the transport experiment, we observed that the photoelectron distribution over the QWIP is extremely narrow and close to the top of the quantum well barriers, indicative of the diffusive nature of the hot‐electron transport in this structure. At large bias, the photoelectron distribution begins to shift up in energy, and is found strongly correlated to the observed photoconductive gain of the detector.

An infrared hot‐electron transistor with a thin (300 A)InGaAs base layer is constructed. By adopting a thin base material with a large Γ‐L valley separation, the photocurrent transfer ratio is improved by a factor of four in comparison with the GaAs base transistor. As a result, the detectivity of the transistor is increased to 1.4×1010 cm√Hz/W at 77 K with a cutoff wavelength of 9.5 μm, two times as large as the companion state‐of‐the‐art GaAs quantum well photoconductor. Combined with the lower dark current, the voltage responsivity and the noise equivalent temperature difference of a detector array can be improved by more than an order of magnitude.

Under proper external bias, we showed that photovoltage amplification can be achieved from an infrared hot‐electron transistor, and hence the transistor structure can be used to increase the voltage responsivity of a multiple quantum well photoconductor. In addition, the ouput voltage of the transistor can be very small when there is no signal light source, similar to a photovoltaic detector. As a result, the transistor is able to dc couple to a high‐gain voltage amplifier for further signal amplification without reducing its detectivity.