1024 × 1024 Format pixel co-located simultaneously readable dual-band QWIP focal plane

This paper reports the first demonstration of the megapixel-simultaneously-readable and pixel-co-registered dual-band quantum well infrared photodetector (QWIP) focal plane array (FPA). The dual-band QWIP device was developed by stacking two multi-quantum-well stacks tuned to absorb two different infrared wavelengths. The full width at half maximum (FWHM) of the mid-wave infrared (MWIR) band extends from 4.4 to 5.1 μm and the FWHM of a long-wave infrared (LWIR) band extends from 7.8 to 8.8 μm. Dual-band QWIP detector arrays were hybridized with custom fabricated direct injection read out integrated circuits (ROICs) using the indium bump hybridization technique. The initial dual-band megapixel QWIP FPAs were cooled to 70 K operating temperature. The preliminary data taken from the first megapixel QWIP FPA has shown system NEΔT of 27 and 40 mK for MWIR and LWIR bands, respectively.     

QWIP focal plane arrays on InP substrates for single and dual band thermal imagers

Alternative material systems on InP substrate provide certain advantages for mid-wavelength infrared (MWIR), long-wavelength infrared (LWIR) and dual band MWIR/LWIR quantum well infrared photodetector (QWIP) focal plane arrays (FPAs). While InP/InGaAs and InP/InGaAsP LWIR QWIPs provide much higher responsivity when compared to the AlGaAs/GaAs QWIPs, AlInAs/InGaAs system facilitates completely lattice matched single band MWIR and dual band MWIR/LWIR FPAs.We present an extensive review of the studies on InP based single and dual band QWIPs. While reviewing the characteristics of InP/InGaAs and InP/InGaAsP LWIR QWIPs at large format FPA level, we experimentally demonstrate that the cut-off wavelength of AlInAs/InGaAs QWIPs can be tuned in a sufficiently large range in the MWIR atmospheric window by only changing the quantum well (QW) width at the lattice matched composition. The cut-off wavelength can be shifted up to ～5.0 μm with a QW width of 22 Å in which case very broad spectral response (Δλ/λ_p = ～30%) and a reasonably high peak detectivity are achievable leading to a noise equivalent temperature difference as low as 14 mK (f/2) with 25 μm pitch in a 640 × 512 FPA. We also present the characteristics of InP based two-stack QWIPs with wavelengths properly tuned in the MWIR and LWIR bands for dual color detection. The results clearly demonstrate that InP based material systems display high potential for dual band MWIR/LWIR QWIP FPAs needed by third generation thermal imagers.     

Large format voltage tunable dual-band QWIP FPAs

Third generation thermal imagers with dual/multi-band operation capability are the prominent focus of the current research in the field of infrared detection. Dual band quantum-well infrared photodetector (QWIP) focal plane arrays (FPAs) based on various detection and fabrication approaches have been reported. One of these approaches is the three-contact design allowing simultaneous integration of the signals in both bands. However, this approach requires three In bumps on each pixel leading to a complicated fabrication process and lower fill factor.If the spectral response of a two-stack QWIP structure can effectively be shifted between two spectral bands with the applied bias, dual band sensors can be implemented with the conventional FPA fabrication process requiring only one In bump on each pixel making it possible to fabricate large format dual band FPAs at the cost and yield of single band detectors. While some disadvantages of this technique have been discussed in the literature, the detailed assessment of this approach has not been performed at the FPA level yet. We report the characteristics of a large format (640 × 512) voltage tunable dual-band QWIP FPA constructed through series connection of MWIR AlGaAs–InGaAs and LWIR AlGaAs–GaAs multi-quantum well stacks, and provide a detailed assessment of the potential of this approach at both pixel and FPA levels. The dual band FPA having MWIR and LWIR cut-off wavelengths of 5.1 and 8.9 μm provided noise equivalent temperature differences as low as 14 and 31 mK (f/1.5) with switching voltages within the limits applicable by commercial read-out integrated circuits. The results demonstrate the promise of the approach for achieving large format low cost dual band FPAs.     

High performance two-color one megapixel CMOS ROIC for QWIP detectors

FLIR Systems, Inc. has designed and fabricated the ISC0501 CMOS readout integrated circuit (ROIC) for quantum well infrared photodetectors (QWIPs). The ISC0501 is a two-color 1024 × 1024 format array with a 30 μm pixel pitch. The ROIC contains a separate analog signal path for each wavelength band. Separate signal paths allow the two-colors to have optimized detector biases, integration times, offsets and gains. This architecture also allows both colors to simultaneously sample a scene and readout the pixel data. This paper will describe the interface, design and features of the ROIC as well as a summary of the characterization test results. A sample image is included from a focal plane array (FPA) built by the Jet Propulsion Laboratory (JPL) using the ISC0501 ROIC with QWIP detectors designed by JPL.     

Evolution of array format of longwave infrared Type-II SLS FPAs with high quantum efficiency

In the remarkably short span of 2 years, longwave infrared focal plane arrays (FPAs) of Type-II InAs/GaSb strained layer superlattice (SLS) photodiodes have advanced from 320 × 256 format to 1024 × 1024 format while simultaneously shrinking the pitch from 30 μm to 18 μm. Despite a dark current that is presently higher than state-of-the-art mercury cadmium telluride photodiodes with the same ∼10 μm cutoff wavelength, the high pixel operability and high (∼50%) quantum efficiency of SLS FPAs enable excellent imagery with temporal noise equivalent temperature difference better than 30 mK with F/4 optics, integration time less than 1 ms, and operating temperature of 77 K or colder. We present current FPA performance of this promising sensor technology.     

Bias-selectable mid-/long-wave dual band infrared focal plane array based on Type-II InAs/GaSb superlattice

In this paper, a mid-/long-wave dual-band detector which combined PπMN structure and unipolar barrier was developed based on type-II InAs/GaSb superlattice. A relevant 320 × 256 focal plane array (FPA) was fabricated. Unipolar barrier and PπMN structure in our dual band detector structure were used to suppress cross-talk and dark current, respectively. The two channels, with respective 50% cut-off wavelength at 4.5 μm and 10 μm were obtained. The peak quantum efficiency (QE) of mid wavelength infrared (MWIR) band and long wavelength infrared (LWIR) band are 53% at 3.2 μm under no bias voltage and 40% at 6.4 μm under bias voltage of −170 mV, respectively. And the dark current density under 0 and −170 mV of applied bias are 1.076 × 10⁻⁵ A/cm² and 2.16 × 10⁻⁴ A/cm². The specific detectivity of MWIR band and LWIR band are 2.15 × 10¹² cm·Hz^1/2/W at 3.2 μm and 2.31 × 10¹⁰ cm·Hz^1/2/W at 6.4 μm, respectively, at 77 K. The specific detectivity of LWIR band maintains above 10¹⁰ cm·Hz^1/2/W at the wavelength range from 4.3 μm to 10.2 μm under −170 mV. The cross-talk, selectivity parameter at 3.0 μm, about 0.14 was achieved under bias of −170 mV. Finally, the thermal images were taken by the fabricated FPA at 77 K.     

Quantum well earth science testbed

A thermal hyperspectral imager is underdevelopment which utilizes the compact Dyson optical configuration and the broadband (8–12 μm) quantum well infrared photodetector (QWIP) focal plane array technology. The Dyson configuration uses a single monolithic prism-like grating design which allows for a high throughput instrument (F/1.6) with minimal ghosting, stray light and large swath width. The configuration has the potential to be the optimal high resolution imaging spectroscopy solution for aerial and space remote sensing applications due to its small form factor and relatively low power requirements. The planned instrument specifications are discussed as well as thermal design trade-offs. The current design uses a single high power cryocooler which allows operation of the QWIP at 40 K with adequate temperature stability.Calibration testing results (noise equivalent temperature difference, spectral linearity and spectral bandwidth) and laboratory emissivity plots from samples are shown using an operational testbed unit which has similar specifications as the final airborne system. Field testing of the testbed unit was performed to acquire plots of emissivity for various known standard minerals (quartz, opal, alunite). A comparison is made using data from the ASTER spectral library. The current single band (8–9 μm) testbed utilizes the high uniformity and operability of the QWIP array and shows excellent laboratory and field spectroscopic results.     

C-QWIP focal plane arrays

We have been developing corrugated quantum well infrared photodetector (C-QWIP) technology for long wavelength applications. A number of large format 1024 × 1024 C-QWIP focal plane arrays (FPAs) have been demonstrated. In this paper, we will provide a detailed analysis on the FPA performance in terms of quantum efficiency η and compare it with a detector model. We found excellent agreement between theory and experiment when both the material parameters and the pixel geometry were taken into account. By changing the number of quantum wells, doping density, spectral bandwidth and pixel size, a range of η from 13% to 37% was obtained. This range of η, combined with the wide spectral width, enables C-QWIPs to be operated at a high speed. For example, model analysis shows that a C-QWIP FPA with 10.7 μm cutoff and 25 μm pitch will have a thermal sensitivity of 16 mK at 2 ms integration time with f/2 optics in the presence of 900 readout noise electrons.     

High-performance modulation-doped AlGaAs/InGaAs thermopiles for uncooled infrared FPA application

Novel thermopile based on modulation doped AlGaAs/InGaAs heterostructures is proposed and developed for the first time, for uncooled infrared FPA (Focal Plane Array) image sensor application. The high responsivity with the high speed response time are designed to be 4900 V/W with 110 μs under the 2 μm design rule. Based on integrated HEMT–MEMS technology, the 32 × 32 matrix FPA is fabricated to demonstrate its enhanced performances by black body measurement. The technology presented here demonstrates the potential of this approach for low-cost uncooled infrared FPA image sensor application.     

Mid-wavelength type II InAs/GaSb superlattice infrared focal plane arrays

We have demonstrated 384 × 288 pixels mid-wavelength infrared focal plane arrays (FPA) using type II InAs/GaSb superlattice (T2SL) photodetectors with pitch of 25 μm. Two p-i-n T2SL samples were grown by molecular beam epitaxy with both GaAs-like and InSb-like interface. The diode chips were realized by pixel isolation with both dry etching and wet etching method, and passivation with SiN_x layer. The device one with 50% cutoff wavelength of 4.1 μm shows NETD ∼ 18 mK from 77 K to 100 K. The NETD of the other device with 50% cutoff wavelength at 5.6 μm is 10 mK at 77 K. Finally, the T2SL FPA shows high quality imaging capability at the temperature ranging from 80 K to 100 K which demonstrates the devices’ good temperature performance.     

Fabrication of type-II InAs/GaSb superlattice long-wavelength infrared focal plane arrays

In this paper, we present an InAs/GaSb type-II superlattice (SL) with the M-structure for the fabrication of a long-wavelength (10 μm range) infrared (LWIR) focal plane arrays (FPA), which are grown by molecular beam epitaxy (MBE). The M-structure is named for the shape of the band alignment while the AlSb layer is inserted into the GaSb layer of InAs/GaSb SL. A 320 × 256 LWIR FPA has been fabricated with low surface leakage and high R₀A product of FPA pixels by using anodic sulfide and SiO₂ physical passivation. Experiment results show that the devices passivated with anodic sulfide obviously have higher R₀A than the un-sulphurized one. The 50% cutoff wavelength of the LWIR FPA is 9.1 μm, and the R₀A is 224 Ω cm² with the average detectivity of 2.3 × 10¹⁰ cm Hz^1/2 W⁻¹.     

15 μm Quantum Well Infrared Photodetector for thermometric imagery in cryogenic windtunnel

Quantum Well Infrared Photodetector (QWIP) usually suffer from a too moderate quantum efficiency and too large dark current which is often announced as crippling for low flux applications. Despite this reputation we demonstrate the ability of QWIP for the low infrared photon flux detection. We present the characterization of a state of the art 14.5 μm QWIP from Alcatel-Thales III–V Lab. We developed a predictive model of the performance of an infrared instrument for a given application. The considered scene is a cryogenic wind tunnel (ETW), where a specific Si:Ga camera is currently used. Using this simulation tool we demonstrate the QWIP ability to image a low temperature scene in this scenario. QWIP detector is able to operate at 30 K with a NETD as low as 130 mK. In comparison to the current detector, the operating temperature is 20 K higher. The use of a QWIP based camera would allow a huge simplification of the optical part.     

Optimization of light polarization sensitivity in QWIP detectors

The current development of QWIPs (Quantum Well Infrared Photodetectors) at III–V Lab led to the production of 20 μm pitch, mid-format and full TV-format LWIR starring arrays with excellent performances, uniformity and stability. At the present time III–V Lab, together with TOL (Thales Optronics Ltd.) and SOFRADIR (Société Française de Détecteurs Infrarouges), work on the demonstration of a 20 μm pitch, 640 × 512 LWIR focal plane array (FPA) which detects the incident IR light polarization. Manufactured objects present a strong linear polarization signature in thermal emission. It is of high interest to achieve a detector able to measure precisely the degree of linear polarization, in order to distinguish artificial and natural objects in the observed scene.In this paper, we present a theoretical investigation of the optical coupling in polarization sensitive pixels. The QWIP modeling is performed by the Finite Difference Time Domain (FDTD) method. The aim is to optimize the sensitivity to light polarization as well as the performance of the detector.     

Complementary barrier infrared detector (CBIRD) with double tunnel junction contact and quantum dot barrier infrared detector (QD-BIRD)

The InAs/GaSb type-II superlattice based complementary barrier infrared detector (CBIRD) has already demonstrated very good performance in long-wavelength infrared (LWIR) detection. In this work, we describe results on a modified CBIRD device that incorporates a double tunnel junction contact designed for robust device and focal plane array processing. The new device also exhibited reduced turn-on voltage. We also report results on the quantum dot barrier infrared detector (QD-BIRD). By incorporating self-assembled InSb quantum dots into the InAsSb absorber of the standard nBn detector structure, the QD-BIRD extend the detector cutoff wavelength from ∼4.2 μm to 6 μm, allowing the coverage of the mid-wavelength infrared (MWIR) transmission window. The device has been observed to show infrared response at 225 K.     

Quantum transport in quantum well infrared photodetectors in the tunneling regime

Quantum well infrared photodetectors (QWIP) are good candidates for low photon flux detection in the 12–20 μm range. For particularly low incident power applications, it can be interesting to reduce the operating temperature to reach the ultimate performance of the QWIP (low dark current, low noise, high detectivity). Nevertheless, once the QWIP operates in the tunneling regime, the dark current is no longer improved by reducing the temperature. Thus, further improvement of the performance needs a microscopic understanding of the physical phenomena involved in QWIP operation in the tunneling regime. In this paper we focus on the dark current of QWIP operated at very low temperature (4–20 K). Experimental results obtained on a 14.5 μm peaking device revealed a plateau regime in the IV curves. We first modeled the dark current using the WKB approximation, but it failed to reproduce the shape and order of magnitude of the phenomenon. As an improvement, we developed a scattering formalism. Our model includes all the most common interactions observed in GaAs: optical phonon, acoustical phonon, alloy disorder, interface roughness, interaction with ionized impurities and between carriers. We demonstrate that, as far as the tunneling regime is concerned, the dominant interaction is the one between electron and ionized impurities, which allows us to conclude on the influence of the doping profile on the dark current.     

Manufacturability of type-II InAs/GaSb superlattice detectors for infrared imaging

Type-II InAs/GaSb superlattice detectors and focal plane arrays (FPAs) with cut-off wavelength at 5.1 μm have been studied. For single pixel devices, dark current densities of 1 × 10⁻⁶ A/cm² and quantum efficiencies of 53% were measured at 120 K. From statistics of manufactured FPAs, an average FPA operability of 99.87% was observed. Furthermore, average temporal and spatial noise equivalent temperature difference (NETD) values of 12 mK and 4 mK, respectively, were deduced. Excellent stability of FPAs after non-uniformity correction was observed with no deterioration of the ratio between spatial and temporal noise during a two hour long measurement. Also after several cooldowns the ratio between spatial and temporal NETD stayed below 0.6.     

Realization of compatible stealth material for infrared,laser and radar based on one-dimensional doping-structure photonic crystals

To inhibit the radiant infrared energy between 8 and 14 μm, which is the infrared atmospheric window, and decrease the echo power of detecting laser and radar, to achieve compatible stealth, a doping structural one-dimensional photonic crystal (1-D PC) with Ge, ZnSe and Si was fabricated; and then combine it with radar absorbing material (RAM) to make a compound. After that, the reflection spectra of this compound was tested, and the result shows a high average reflectance (89.5%) in 8–14 μm waveband, and a reflective valley (39.8%) in the wavelength of 10.6 μm, which is the wavelength of CO₂ laser; and the reflectance in radar band shows that at high frequency, especially between 7.8 and 18 GHz, the radar power is strongly absorbed by this material and the reflected energy attenuate over 10 dB within the range from 11.1 GHz to 18.3 GHz, even 24.5 dB to the most in the frequency of 14.6 GHz.     

QWIP Status at THALES

Since 2002, the THALES Group has been manufacturing sensitive arrays using QWIP technology based on GaAs and related III–V compounds, at the Alcatel-Thales-III-V Lab (formerly part of THALES Research and Technology Laboratory).In the past researchers claimed many advantages of QWIPs. Uniformity was one of these and has been the key parameter for the production to start. Another widely claimed advantage for QWIPs was the so-called band-gap engineering and versatility of the III–V processing allowing the custom design of quantum structures to fulfil the requirements of specific applications such as very long wavelength (VLWIR) or multi-spectral detection. In this presentation, we give the status of our LWIR QWIP production line, and also the current status of QWIPs for MWIR (<5 μm) and VLWIR (>15 μm) arrays.As the QWIP technology cannot cover the full electromagnetic spectrum, we develop other semiconductor compounds for SWIR and UV applications. We present here the status of our 320 × 256 SWIR module with InGaAs photodiodes.     

Device-level vacuum packaged uncooled microbolometer on a polyimide substrate

Uncooled infrared detectors (IR) on a polyimide substrate have been demonstrated where amorphous silicon (a-Si) was used as the thermometer material. New concepts in uncooled microbolometers were implemented during the design and fabrication, such as the integration of a germanium long-pass optical filter with the device-level vacuum package and a double layer absorber structure. Polyimide was used for this preliminary work towards vacuum-packaged flexible microbolometers. The detectors were fabricated utilizing a carrier wafer and low adhesion strength release layer to hold the flexible polyimide substrate during fabrication in order to increase the release yield. The IR detectors showed a maximum detectivity of 4.54 × 10⁶ cm Hz^1/2/W at a 4 Hz chopper frequency and a minimum noise equivalent power (NEP) of 7.72 × 10⁻¹⁰ W/Hz^1/2 at a biasing power of 5.71 pW measured over the infrared wavelength range of 8–14 μm for a 35 μm × 35 μm detector. These values are comparable to other flexible microbolometers with device-level vacuum packaging which are found in literature.