In recent years, Type-II InAs/GaSb superlattice (T2SL) has emerged as a new material technology suitable for high performance infrared (IR) detectors operating from Near InfraRed (NIR, 2-3μm) to Very Long Wavelength InfraRed (LWIR, λ > 15μm) wavelength domains. To compare their performances with well-established IR technologies such as MCT, InSb or QWIP cooled detectors, specific electrical and radiometric characterizations are needed: dark current, spectral response, quantum efficiency, temporal and spatial noises, stability… In this paper, we first present quantum efficiency measurements performed on T2SL MWIR (3-5μm) photodiodes and on one focal plane array (320x256 pixels with 30μm pitch, realized in the scope of a french collaboration ). Different T2SL structures (InAs-rich versus GaSb-rich) with the same cutoff wavelength (λc= 5μm at 80K) were studied. Results are analysed in term of carrier diffusion length in order to define the optimum thickness and type of doping of the absorbing zone. We then focus on the stability over time of a commercial T2SL FPA (320x256 pixels with 30μm pitch), measuring the commonly used residual fixed pattern noise (RFPN) figure of merit. Results are excellent, with a very stable behaviour over more than 3 weeks, and less than 10 flickering pixels, possibly giving access to long-term stability of IR absolute calibration.
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.
A VGA format type-II superlattice focal plane array (FPA) for the mid-wave infrared (MWIR) atmospheric window has been designed, manufactured and characterized. The detector material is based on a heterojunction structure with a barrier that effectively decreases the Shockley-Read-Hall based component of the dark current. A very effective passivation method has been used which successfully inhibits all surface leakage currents. The barrier structure has a 50 % cutoff at 5 µm and 65 % quantum efficiency without antireflective coating. The dark current density is 3×10-6 A/cm2 at -0.05 V bias and 120 K. The optical cavity of the detector has been optimized for maximum capture of available light in the MWIR window. A focal plane array with 640 by 512 pixels and 15 µm pitch was processed based on this barrier structure. High-quality imagery in a system with high F-number will be presented.
A mid wave infrared type-II superlattice focal plane array with 320x256 pixels, 30 μm pitch and 90 % fill factor was
fabricated in house, using a conventional homojunction p-i-n photodiode design and the ISC9705 readout circuit. High-quality
imaging up to 110 K is demonstrated with the substrate fully removed. The absorber is 2 μm thick, and no anti-reflection
coating was used, so there is still room for significant improvement of the quantum efficiency, which is in the
40 % range.
Studies of the dark current vs. temperature behavior indicate that the device is limited by Shockley-Read-Hall generation
from the depletion region. The activation energy of this dark current component is 0.13 eV, suggesting an unidentified
recombination center positioned halfway into the 0.24 eV bandgap.
Furthermore, we report on detectors with 100 % cut-off at 13 μm. The dark current density at 60 K and -50 mV bias is
2x10-4 A/cm2. Quantum efficiency, NETD and BLIP temperature are also calculated.
Position-sensitive photocurrent measurements on mesa-etched superlattice material were made at low temperatures using
a focused laser spot. The lateral diffusion length for holes was extracted and is reported.
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
temperature.
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.
The use of arsine in metal-organic vapor phase epitaxy (MOVPE) growth is well established in the compound semiconductor industry but associated with large potential risks due to its high toxicity. Worldwide efforts are therefore being made to replace it with less hazardous source materials. Acreo, a commercial supplier of quantum well infrared photodetector (QWIP) focal plane arrays (FPA), is working towards an MOVPE process where tertiarybutylarsine (tBAs) instead of arsine is used in the growth of AlGaAs/GaAs n-type QWIP epiwafers. In this paper we investigate the performance of QWIP FPA produced from conventional arsine and alternative tBAs arsenic precursors. We also discuss the two growth processes regarding uniformity, crystalline purity and production cost. The performance of our QWIP structures grown using tBAs and arsine is comparable in terms of response and response-to-the dark current ratio.
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.
Acreo in Sweden has been invovled in Quantum Well IR Photodetector (QWIP) reserach and development since 1986. During the first years a small group led by Jan Andersson was dedicated to research on QWIP structures and means of coupling radiation into the quantum-well structure. One of the resarech results is a 2D optical grating couler to create an optimal elecgtric field pattern for highest possible absorption and responsivity. Acreo holds a patent for this grating coupler. Since 1988 FLIR Systems, later FMV, Saab Dynamics, Celsius Tech and NUTEK have sponsoered an R and D project with the goal to develop QWIP detectors and start up production. Soon it became clear that an adapted ROIC deisng and the hybridization of the focal plane array are key issues in order to achieve the highest possible performance and operability of the complete detector device for voluem production. Extended measures where taken in 1996. In 1997 the industrial interest increased further, ClesiusTech and Saab Dynamics, merged in 2000, and now a division of FLIR Systems AB started to sponsor the R and D project. Because of its success during the last years it has now been expanded and scheduled until 2003 and beyond. Volume production of QWIP FPAs started in 2000.
Multiple quantum well spatial light modulators (MQW SLMs) are promising devices for future high-speed applications. We present results obtained with a single-pixel amplitude modulator. We discuss the status of our work on a 128x128-pixel ternary SLM. This SLM will run at 10 kHz and have one low-reflectance level and two high reflectance levels with a phase difference of pi. We also present a study of the relation between the coding domain and the structural design of modulators.
As CMOS technology evolves, more and more functions can be integrated on the infrared focal plane array (FPA). This paper presents a study on the integration of analog to digital (A/D) conversion onto the FPA. A possible application for this is high-resolution (640 X 480 pixels) quantum well infrared photodetector (QWIP) FPAs operating at 70K. Operation at liquid nitrogen temperatures and below gives both advantages and disadvantages. CMOS transistors are performing better at these temperatures: increased transconductance and no leakage currents. On the other hand power dissipation needs to be limited to prevent a high load on the cooling system. The system aspects will ultimately determine the requirements for the A/D converters on the FPA. Some of the most stringent requirements are on: power dissipation, number of bits, die area and throughput. An FPA lends itself very well for the utilization of parallelism, so a trade-off can be made between sample rate per A/D converter and number of converters. With all these parameters in mind, an overview of state-of-the-art A/D converters at room temperature will be given. Trends will be identified and different architectures like delta/sigma and successive approximation will be evaluated. Also different implementation technologies such as switched current and switched capacitor will be reviewed with respect to their applicability in our application.
To obtain the optimal performance of quantum well IR photodetectors (QWIP) focal plane array, the readout circuit (ROIC) has to be tailored to the detector characteristics. Different concepts will be compared with respect to signal to noise performance, linearity and power consumption. In particular an active direct injection (DI) circuit will be compared with a passive direct injection circuit. The active and passive DI circuits give similar temporal NETD but the passive DI is shown to have superior spatial noise performance. Furthermore, the latter circuit type offers higher flexibility with respect to on-chip signal processing. The intended application is in high-resolution QWIP focal plane arrays.
A camera system has been designed using a focal plane array with 320 X 240 pixels. The detector array is based on quantum wells in the GaAs/AlGaAs material system grown onto a GaAs substrate and flipchip mounted to a readout circuit. The camera system uses f-number equals 1.5 optics to create an image of the scene on the FPA. The detector is cooled to approximately 70 K by an integrated Stirling cooler. The system also includes electronics for amplification and analog to digital conversion of the detector signal. The images are either displayed on a monitor or stored in digital format on an integrated hard disk. The short-term temporal noise was measured and the noise equivalent temperature difference was calculated to 16 mK. The spatial noise was found to be comparable to the temporal noise. The properties of the infrared images were valuated with respect to short and long term stability. The stability was found to be very good, giving a high quality image even 1 hour after a calibration. The number of dead pixels was less than 0.1% for several detectors.
Due to the well established GaAs material and processing technology QWIPs are viable candidates for high resolution (greater than 128 by 128 pixels), low cost LWIR (8 - 12 micrometer) focal plane arrays (FPAs). Usually n-doped AlGaAs/GaAs QWIPs are used since, at least to date, these have been shown to provide the highest performance. Fabrication and evaluation of 320 by 240 pixels QWIP arrays have also been done. The fabrication involves hybridizing GaAs chips consisting of detector mesas to specially designed CMOS readout chips. The hybridization is effected by indium bump flip-chip bonding. Optical coupling into the detectors is performed by using optimized, etched, two-dimensional gratings combined with GaAs substrate thinning down. The advantages of substrate removal are: (1) reduction of thermal mismatch between materials and thus permitting large array sizes, (2) enhancement of absorptance, and (3) elimination of optical cross-talk between pixels. The intended operating temperature range is 70 to 73 K, achievable by a miniature Stirling cooler. Excellent wafer uniformities resulting in responsivity uniformities of 3.3% across an array are found, and a temperature resolution NETD (noise equivalent temperature difference) equals 40 mK is achieved. Finally, the presence of fixed-pattern noise and its influence on the image performance are discussed.
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