In this paper, the performance of small, lightweight, and low power (SWaP) midwave infrared (MWIR) type-II superlattice detectors have been studied. The detectors cover the MWIR range from 3.7 μm to 5.1 μm with quantum efficiency higher than 60% in the entire range. Statistics from the focal plane array (FPA) production show excellent reproducibility with average temporal NETD of 20 mK, spatial NETD of 5 mK and operability values typically higher than 99.85%. Good uniformity across the arrays is demonstrated with narrow NETD histograms and highly uniform gain-correction maps. Temperature studies of the FPA performance show that the low NETD values, good uniformity and high operability are maintained up to 110 K. After integration of these FPAs in SWaP integrated detector dewar cooler assemblies (IDDCAs) with F/5.5, the high FPA performance is maintained with good imaging properties.
The III-V material system has proven to be a solid base for building infrared detector focal plane arrays (FPAs), enabling well-established designs such as bulk InSb-photodiodes, GaAs-based quantum well infrared photodetectors (QWIP) and GaSb-based type-II superlattices (T2SL). The remarkable advancement in the sensitivity and stability of such FPAs seen in the past decades calls for revision of the merits and the acceptance criteria used for the performance evaluation of infrared imaging arrays. Specifically, the early skepticism toward QWIP has largely been focused on their low quantum efficiency, and consequently low specific detectivity of the FPA pixels, as compared to the counterparts with bulk design. It was later demonstrated that unrivalled spatial uniformity of QWIP arrays (typically with a residual non-uniformity <0.05 %) not only compensated for this drawback, but rendered them superior in terms of the sensitivity, with typical noise equivalent temperature difference (NETD) <30 mK (when staring at 30 °C source through F/2.24 aperture, FPA on 15 μm pitch operated at 70 K and 60 Hz frame rate, peak absorption at 8.6 μm). This introduced QWIPs to the demanding applications, such as tactical or gas sensing, in spite of the design’s inherently low detectivity. In this paper, we propose the tightened criteria for marking the defective pixels and demonstrate how IRnova’s III-V based long-wave QWIP and mid-wave T2SL FPAs with 15 μm pitch live up to them. High spatial uniformity is reflected in symmetric and almost tail-free NETD histograms for both detector types. Only 0.13 % of the pixels exhibit NETD above ×1.5 of the median value for QWIP and 0.14 % - for T2SL detectors. At the same time, there are virtually no pixels falling outside less stringent limit of ×3 the median value. For QWIPs, this observation is consistent with flat noise spectra, which are completely free from the low-frequency “1/f”-component, as measured on the test cells. Finally, our attesting procedure allows tracing even solitary pixels exhibiting deviating types of noisy behavior, including random telegraph signal (RTS)-like, whenever they are present in the array. Low number of RTS-like pixels is a key advantage of III-V based FPAs that enables them to be used for high-operating-temperature (HOT) applications. Another characteristic feature of III-V based detectors is the outstanding stability of their performance. For IRnova’s mid-wave T2SL FPA on 30 μm pitch, using a 3-year-old non-uniformity correction map even without refreshing the offset is penalized by a mere 100 % increase in the residual fixed pattern noise relative to its temporal NETD. In the paper, we complement this with a study on the stability of the non-uniformity correction of IRnova’s QWIP and T2SL detectors on 15 μm pitch.
In this paper, results from the development of InAs/GaSb superlattice focal plane arrays (FPAs) at IRnova will be presented. A versatile and robust detector design is used that allows for adjustment of the detection cut-off wavelength from 2.5 μm up to 14.5 μm with only minor changes in the detector design. Performance of the fabricated detectors has been reviewed in terms of external quantum efficiency (EQE), dark current and noise for three designs with cut-off wavelengths of 4, 5.5 and 11 μm at 80 K (referred to as DEEP BLUE, RED HOT and VLWIR, respectively). Measurements on the 15 μm sized photodiodes demonstrated 70% EQE for the MWIR designs, and almost 40% - for VLWIR. At the same time, the dark current stayed close to the Rule07 benchmark for all studied samples. Noise mechanisms have been discussed and their relation to the passivation was examined. Mature in-house processing and passivation technique of resulted in very high spatial uniformity of VGA focal plane arrays (FPAs), i.e. low relative deviations of EQE (< 6%) and of dark current density (< 12%) and narrow noise distributions for both RED HOT and DEEP BLUE FPAs. We show also that <99.5% of these arrays operate close to the fundamental noise limit.
In this paper we demonstrate how the QWIPs are keeping all their promises in terms of performance, versatility and cost, partly thanks to excellent uniformity, stability and manufacturability, as well as the flexibility of tailoring both the detection wavelength and the polarization sensitivity.
Thanks to the intrinsic uniformity and stability of the III-V technology combined with the process maturity at IRnova, high resolution (VGA format, 15 µm pitch) QWIPs are now integrated in compact Integrated Detector Dewar Cooler Assemblies (IDDCAs). For regular high-end thermography applications, these IDDCAs operate with moderate 500 mW cryocoolers, which opens the field for handheld cameras.
Thanks to the band gap engineering, the detector’s spectral response can be tailored and optimized for different wavelength ranges. In addition to the 8.5 µm QWIP, which is optimized for thermal imaging, a QWIP with 10.5 µm peak wavelength has been tailored for detection of harmful gases, such as SF6. The performance of high resolution QWIPs with peak detection at 10.5 µm integrated in IDDCAs is demonstrated showing the impact of high sensitivity and high resolution on the image quality.
Thanks to the intrinsic sensitivity of QWIP to the polarization of the light it is possible to complement the thermal information with polarimetric information. This opens new possibilities for detection of certain features such as oil spill and de-camouflage. In this paper we demonstrate the high polarimetric contrast obtained with our 320×256 polarimetric IDDCA.
In this paper, the performance of high operating temperature (HOT) type-II superlattice FPAs (640 × 512 pixels @ 15 μm pitch), are demonstrated. The type-II superlattice design used for these FPAs has a cut-off wavelength of 5.3 μm and the quantum efficiency is extracted to 80% at FPA level. The HOT FPAs are integrated in IDDCAs with small size, weight and power (SWaP) with F#4 configuration. Excellent imaging performance is demonstrated at 110 K with temporal NETD of 21 mK, spatial NETD of 7 mK, 10 ms integration time and typical operability > 99.8 %. From modelling and studies of the temperature dependence of the FPA performance, further increase of the operating temperature up 130 K is predicted for the 5.3μm design.
Continuing with its legacy of producing high performance infrared detectors, IRnova introduces its high resolution LWIR IDDCA (Integrated Detector Dewar Cooler assembly) based on QWIP (quantum well infrared photodetector) technology. The Focal Plane Array (FPA) has 640×512 pixels, with small (15μm) pixel pitch, and is based on the FLIRIndigo ISC0403 Readout Integrated Circuit (ROIC). The QWIP epitaxial structures are grown by metal-organic vapor phase epitaxy (MOVPE) at IRnova. Detector stability and response uniformity inherent to III/V based material will be demonstrated in terms of high performing detectors. Results showing low NETD at high frame rate will be presented. This makes it one of the first 15μm pitch QWIP based LWIR IDDCA commercially available on the market. High operability and stability of our other QWIP based products will also be shared.
IRnova has been manufacturing mid wave infrared (MWIR) detectors based on InAs/GaSb type-II superlattices (T2SL)
since 2014. Results from the first years of production of MWIR focal plane arrays (FPAs) with 320 x 256 pixels on 30
μm pitch using the ISC9705 readout integrated circuit (ROIC) is presented in terms of operability, temporal and spatial
noise equivalent temperature difference (NETD) and other key production parameters. Results on image stability of
T2SL detectors show that no deterioration of image quality over time can be observed. Furthermore it is shown that the
non-uniformity correction remains stable even after repeated detector temperature cycles. Spatial and temporal NETD
for fabricated mid wave arrays show a temporal NETD of 12 mK and a spatial NETD of 4 mK with f/2 optics and 8 ms
integration time. When studied over a large scene temperature, the spatial noise is still less than 60 % of the temporal
noise. Furthermore, 640 x 512 mid wave FPAs with 15 μm pitch using the ISC0403 ROIC are entering an
industrialization phase. Temporal and spatial NETD values of 25 mK and 10 mK, respectively, are obtained with f/4
optics and 22 ms integration time and the operability is 99.85 %. A status update on the development of T2SL detectors
for short wave, mid wave and long wave infrared wavelength regions for existing and new applications is given and
recent development towards higher operating temperature, smaller pitch and larger FPA formats is presented.
IRnova has a long history of producing QWIPs for the LWIR band. In this paper we give an overview of the current
products (FPAs with 640x480 and 384x288 pixels respectively, and 25 μm pitch) and their performance. Their superior
stability and uniformity inherent to detectors based on III/V material system will be demonstrated. Furthermore, an
IDCA specifically designed for hand-held systems used for the detection of SF6 gas using a 0.5 W cooler will be
presented. The detector format is 320x256 pixels with 30 μm pitch using the ISC9705 read out circuit. The peak
wavelength is at 10.55 μm and the NETD is 22 mK.
The extension of supercontinuum (SC) sources into the mid-infrared, via the use of uoride and chalcogenide optical fibers, potentially offers the high radiance of a laser combined with spectral coverage far exceeding that of typical tunable lasers and comparable to traditional black-body emitters. Together with advances in mid-IR imaging detectors and novel tunable filter designs, such supercontinua hold considerable potential as sources of illumination for spectrally-resolved microscopy targeting applications such as rapid histological screening. The ability to rapidly and arbitrarily select particular wavelengths of interest from a broad emission spectrum, covering a wide range of biologically relevant targets, lends itself to image acquisition only at key relevant wavelengths leading to more manageable datasets. However, in addition to offering new imaging modalities, SC sources also present a range of challenges to successful integration with typical spectral microscopy instrumentation, including appropriate utilisation of their high spatial coherence. In this paper the application of SC sources to spectrally-resolved microscopy in the mid-IR is discussed and systems-integration considerations specific to these sources highlighted. Preliminary results in the 3-5μm region, obtained within the European FP7 project MINERVA, are also presented here.
Historically IRnova has exclusively been a company, focused on manufacturing of QWIP detectors. Nowadays, besides
continuous improvements of the performance of QWIP FPAs and development of new formats IRnova is involved in
development of QWIP detectors for special applications and has started the development of the next generation infrared
detectors, as well.
In the light of the development of new formats we validate experimentally theoretical calculations of the response of
QWIPs for smaller pixel size. These results allow for the development of high performance megapixel QWIP FPA that
exhibit the high uniformity and operability QWIP detectors are known for. QWIP is also being considered for space
applications. The requirements on dark current and operating temperature are however much more stringent as compared
to the terrestrial applications. We show ways to improve the material quality with as a result a higher detector operating
IRnova is also looking at antimony-based strained superlattice material for the LWIR region together with partners at the
IMAGIC centre of excellence. One of the ways to overcome the problem with surface currents is passivating
overgrowth. We will report the status and results of overgrowing the detector mesas with AlGa(As)Sb in a MOVPE
system. At the same centre of excellence a novel material concept is being developed for LWIR detection. This new
material contains a superlattice of vertically aligned and electronically coupled InAs and GaSb quantum dots.
Simulations show that it should be possible to have LWIR detection in this material. We will present the current status
and report results in this research.
The ongoing development of QWIP focal plane arrays at IRnova (formerly Acreo) has resulted in the launch of several
new formats up to 640 by 512 pixels and the introduction of major improvements to all products. The achieved
performance and imagery will be evaluated. In the light of the development of new formats, the results of hybridization a
640 by 512 detector with 20 &mgr;m pitch will be discussed. The driving forces behind these improvements have been the
demands from both industrial applications where the requirements for the operating temperature are high due to the life
time issues, and from space applications where the requirements for the quantum efficiency and dark current are
extreme. For the latter type of applications a number of QWIPs covering the 4 to 20 &mgr;m wavelength band have been
grown and evaluated. The demands for better performance are met by ongoing increases in light coupling, improvements
of the quantum well structures, as well as fine tuning of the epitaxial growth parameters. This has led to FPAs that can
operate at 75 K and operation close to 80 K is within reach. IRnova is also looking at other material systems to fulfill the
requirements of next generation photon detectors.
The ultimate performance of QWIP implies hard requirements on the response-to-dark-current ratio for both high operating temperature and low background, e.g. space, applications. A way to improve this ratio by finding the optimal combination of band structure and material parameters is suggested. Experiments have been conducted on GaAs/AlGaAs structures optimised for 8.5 to 16 μm with similar types of band profile.
The doping concentration in the quantum well (QW) is the principal parameter in such optimisation because it affects linearly the photocurrent and exponentially the dark current. As a result of the first experiment series we found an optimal QW doping concentration corresponding to the maximum response-to-dark-current-ratio, thus verifying the validity of the widely used hydrodynamic model.
Experiments with a varying number of quantum wells for a constant total thickness were also carried out and analyzed. The resulting variation in barrier thickness changes the balance between the quantum efficiency and photoconductive gain. A critical thickness was found, where the temperature-independent component of the dark current increases drastically.
For low background applications, especially in combination with long wavelength detection, it is not enough to only reduce the thermally-assisted and sequential tunnelling components of the dark current. Other sources of the dark current usually neglected at high temperature start to play a role. Interface shape and background doping in the barriers are examples of increasingly important factors. We discuss the contribution of these factors to the dark current.
We report on a quantum dots-in-a-well infrared photodetector (DWELL QDIP) grown by metal organic vapor phase epitaxy. The DWELL QDIP consisted of ten stacked InAs/In0.15Ga0.85As/GaAs QD layers embedded between n-doped contact layers. The density of the QDs was about 9 x 1010 cm-2 per QD layer. The energy level structure of the DWELL was revealed by optical measurements of interband transitions, and from a comparison with this energy level scheme the origin of the photocurrent peaks could be identified. The main intersubband transition contributing to the photocurrent was associated with the quantum dot ground state to the quantum well excited state transition. The performance of the DWELL QDIPs was evaluated regarding responsivity and dark current for temperatures between 15 K and 77 K. The photocurrent spectrum was dominated by a LWIR peak, with a peak wavelength at 8.4 μm and a full width at half maximum (FWHM) of 1.1 μm. At an operating temperature of 65 K, the peak responsivity was 30 mA/W at an applied bias of 4 V and the dark current was 1.2×10-5 A/cm2. Wavelength tuning from 8.4 μm to 9.5 μm was demonstrated, by reversing the bias of the detector.
Acreo is one of the leading producers of QWIP FPAs in the world and is also intensively running R&D activities. The European Space Agency has awarded Acreo the contracts "Far-IR Linear Detector Array" in 6-18 μm infrared range within the Darwin mission's frameworks and "Quantum Well Infrared Photodetector Arrays" in 11-15 μm range for Earth observation (EO). The Darwin project imposes hard requirements on the dark current, while for the EO project the operating temperature is a stringent constraint. The goal of both contracts is to establish and demonstrate the ultimate performance of Acreo's QWIP-technology for these applications at the highest possible operating temperature. For this purpose Acreo designed, grew and characterised QWIP material sensitive to different wavelengths in the range of 6-18 μm. To investigate transport properties and verify the validity of the hydrodynamic model of the dark current, experiments with varying numbers of quantum wells per thickness unit and periods were conducted. A structure for long infrared region with an increased number of periods revealed a drastic reduction of the dark current at transient temperature. The dependence of the capture probabilities on the electron energy in the miniband resulting in different dependencies of the photoconductive gain for the photo- and dark currents on the number of periods is suggested as the reason for that. Such hypothesis shows possibilities for improvement of the balance between the photo- and dark current. Optimisation of the photoconductive gain changes the geometrical parameters of the detector and requires optimisation of the optical coupling.