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/In<sub>0.15</sub>Ga<sub>0.85</sub>As/GaAs QD layers embedded between n-doped contact layers. The density of the QDs was about 9 x 10<sup>10</sup> cm<sup>-2</sup> 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<sup>-5</sup> A/cm<sup>2</sup>. 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.