Time-of-flight imaging techniques are utilized for 3D-scene reconstructions and non-line-of-sight imaging capable of revealing persons or objects hidden behind obstacles. Such sensor systems usually demand for high-performance photodetectors with single-photon detection capabilities due to the high reflection losses of the emitted laser pulses during multiple, usually diffuse reflections from a relay wall and the actual scene of interest. To further compensate the high signal loss, a laser wavelength in the eye-safe Short-Wave Infrared (SWIR) spectral range, typically around 1550 nm, allows for higher laser intensities. In turn, SWIR-matching Single-Photon Avalanche Diodes (SPADs) need to be fabricated from adequate semiconductor materials, e.g., the InGaAs/InP material system. We report on the progress of the InGaAs/InP-SPAD fabrication using a customized process technology. The key technology is the planar process technology via zinc diffusion to produce spatially confined p-type regions. For the zinc-diffusion process, a novel method of selective epitaxial overgrowth was developed. Following a prior recess-etch step, the single-step Zn-diffusion process results in the intended double-well-shaped doping profile. Experimental data of thus fabricated InGaAs/InP SPADs show the expected dark-current, photo-current, and multiplication-gain characteristics in linear-mode operating as well as dark count rates in Geiger-mode operation around 200 – 220 K, which corresponds to a typical operating temperature for InGaAs/InP SPADs achievable by thermoelectric cooling.
In this contribution, we will present a finite element method modelling study for the optimization of QE in LWIR InAs/GaSb type-II superlattice detectors via the employment of metastructures. Experiments to realize these devices will also be presented. In our approach, the average integrated QE for a structure with an absorber thickness of 2.1 µm is theoretically improved from 35% to 73%, corresponding to an improvement of 108%. For a detector with a thin absorber layer of 0.9 µm, the average integrated QE is improved from 21% to 59%, corresponding to an increasement of 180%.
Quantum sensing and quantum communication systems rely on high-performance single- or entangled-photon sources and single-photon detectors enabling experiments based on the quantum nature of single photons. In this contribution, we discuss the development of an entangled-photon source delivering entangled photon pairs with wavelengths of about 1550 nm alongside with single-photon avalanche detectors (SPADs) for the short-wave infrared (SWIR) and for the extended SWIR (eSWIR) spectral range. The fabrication processes of such quantum-enabling technologies is highlighted. The entangled-photon source is based on AlGaAs Bragg-reflection waveguides. Very low difference in effective refractive index of TE and TM polarized photons – important for high polarization entanglement without external compensation – as well as high single and coincidence count rates were achieved. For the fabrication of InGaAs/InP SWIR SPADs, the key technology is the planar process technology via zinc diffusion to produce spatially confined p-type regions. For the zinc-diffusion process, a novel method of selective epitaxial overgrowth was developed, achieving the intended double-well diffusion profile. Experimental data of thus fabricated InGaAs/InP SPADs show the expected dark-current, photo-current, and multiplication-gain characteristics in linear-mode operation as well as breakthrough behavior in Geiger-mode operation at 240 K, which is a typical operating temperature for InGaAs/InP SPADs achievable by thermoelectric cooling. GaSb-based SPADs for the eSWIR are fabricated in a mesa approach showing the expected dark current behavior as well. All three different devices are linked by enabling quantum technologies in the (e)SWIR as well as by using our III/V-semiconductor technology facilities.
The detection in short wavelength infrared (SWIR) band, ranging from 1–3 μm, provides a wide range of applications in earth observation, plastics recycling, biology and hyperspectral imaging, gas analysis and defense. In this paper, uncooled InGaAsSb-based detectors for the wavelength range beyond 1.7 μm, the extended SWIR (eSWIR), are investigated for the later use in a thermographic traffic monitoring system that is supposed to localize potentially dangerous overheated hot-spot regions. Up to the wavelength of 1.7 μm, InGaAs lattice-matched with InP is used for photodetection in the SWIR. To reach a longer cutoff wavelength, “extended InGaAs” can be employed. This requires strained growth that leads to more growth defects and reduced yield, though. InGaAsSb, however, provides a tunable bandgap for detection beyond 1.7 μm and still enables lattice-matched growth on GaSb, which makes it a viable alternative for photodetection in the eSWIR. We have demonstrated that the bandgap of InGaAsSb can be tuned in the eSWIR by modifying the stoichiometry for lattice-matched growth on GaSb. Furthermore, we have successfully realized InGaAsSb heterojunction photodiodes with an AlGaAsSb hole barrier. At room temperature, the diodes achieve a dark current density of 0.5 mA/cm2 and a responsivity better than 1 A/W resulting in an excellent peak detectivity of 9 x 1010 cm Hz1/2/W. Thus, the highperformance detector arrays operating at room temperature are within reach in order to meet application demands.
Imaging LiDAR (light detection and ranging) systems for sensing the surrounding environment are a key component for, e.g., reconnaissance and autonomous navigation. The physical basis of LiDAR systems is the time-of-flight measurement of the backscattered intensity of a pulsed laser beam. For a sufficiently high detection efficiency, the highest possible sensitivity of the photodetector is required as provided by the single-photon avalanche diode (SPAD). The maximum detection range can be even further extended by the usage of laser sources for the short-wavelength infrared (SWIR) spectral range with typical wavelengths around 1550 nm, allowing for increased optical intensities to be emitted. The InGaAs/InP material system is ideally suited for SPADs for the SWIR spectral range. The fabrication of InGaAs/InP SPADs into focal plane arrays as core component for future imaging lidar systems with a high spatial resolution are quite demanding. The key technology for the fabrication of InGaAs/InP SPADs is the planar process technology via zinc diffusion to produce spatially confined p-type regions. For the zinc-diffusion process, a novel method of selective epitaxial overgrowth was developed, achieving the intended double-diffusion profile. First experimental data of thus fabricated InGaAs/InP avalanche photodiodes show the expected dark-current, photo-current, and multiplication-gain characteristics in linear-mode operating, as well as breakthrough behavior in Geiger-mode operation at typical operating temperatures around 220 to 240 K, which are achievable by thermoelectric cooling.
Photodetectors for the non-visible region of the electromagnetic spectrum are vital for security, defense and space as well as industrial and scientific applications. The research activities at Fraunhofer IAF contribute to Europe’s non-dependence on critical components and support the European strategy for critical space technologies. A broad range of III-V material systems is developed to address the spectral region adjacent to the visible regime. For the ultraviolet (UV) spectral region, AlGaN is the material of choice with an adjustable bandgap between 3.4 and 6.0 eV, depending on the Al content, addressing the wavelength regime between 365 to 210 nm. The short-wavelength infrared (SWIR) region from 0.9 up to 3.0 µm is covered by two approaches: Lattice matched InGaAs absorber material on InP substrates for a cut-off wavelength at 1.7 µm and InGaAsSb lattice matched on GaSb substrates for 1.7 up to 3.0 µm. Through the choice of appropriate layer thickness, InAs/(In,Ga)(As,Sb) type-II superlattices (T2SLs) can be tailored to cover the wavelength range from mid- to long- up to very-long-wavelength infrared (MWIR, LWIR, VLWIR) in the spectrum of 3-15 µm.
The short-wavelength infrared (SWIR) regime between 1 and 3 μm is of high interest especially for surveillance, reconnaissance, and remote sensing applications. The availability of high-power, yet eye-safe SWIR laser sources is an important asset enabling scene illumination and implementation of advanced active imaging concepts like gated viewing (GV) or light detection and ranging (LIDAR). With atmospheric nightglow also a natural, but faint source for scene illumination is available for passive low-light-level imaging in the SWIR region. The most commonly employed material system for realizing SWIR photodetectors is InGaAs with an indium content of 53%. The spectral sensitivity of In0.53Ga0.47As with its cut-off wavelength of 1.7 μm covers a wide part of the nightglow spectrum as well the emission lines of available laser sources at typical telecom wavelengths around 1.55 μm. However, for low-light-level passive SWIR imaging a dark-current density around 10-9 A/cm² is considered mandatory. While the international state-of-the-art has already achieved this performance at room-temperature operation, today’s European stateof- the-art is still lagging behind. The development of InGaAs-based SWIR detectors at Fraunhofer IAF aims at pin as well as avalanche photodiodes (APDs) for imaging applications with 640×512 pixels. While InGaAs APDs play to their strength in GV applications with typically rather short integration times, planar processed InGaAs/InP pin photodiodes with lowest possible dark-current and noise characteristics are the detector devices of choice for passive low-light-level detection. Within a few planar-process batches, we approached the European state-of-the-art for the dark-current density of 15-μm-pitch InGaAs pin detectors by a remaining factor of five. The most recent process run yielded further slightly improved dark-current characteristics on test devices. Recently, we have started with in-house characterization of such focal plane detector arrays hybridized with suitable SWIR read-out integrated circuits.
Type-II superlattices (T2SLs) are currently recognized as the sole material system offering comparable performance to HgCdTe, yet providing higher operability, stability over time, spatial uniformity, scalability to larger formats, producibility and affordability. Hence, T2SL technology is very promising for space applications. Fraunhofer IAF played a vital role in the development of III-As/Sb T2SLs right from the beginning. Mono- and bi-spectral focal plane arrays up to 640×512 pixels for the mid- and long-wavelength infrared (IR) have been demonstrated. The growth of T2SL is performed by molecular beam epitaxy (MBE) in multi-wafer reactors. We report on the excellent homogeneity and reproducibility of the growth process, established in the past years at Fraunhofer IAF. After processing this material to detector arrays, the T2SL detectors have been characterized down to low temperatures (below 40K) with promising properties regarding the dark current. For MWIR and LWIR detectors the resolution limit of the measurement setup with a dark current density of 2×10-10 A/cm2 has been reached at 77 K and 36 K, respectively.
Type-II superlattices (T2SLs) are considered the III/V alternative to HgCdTe for infrared (IR) detectors and have already reached market maturity. Fraunhofer IAF has demonstrated mono- and bi-spectral T2SL focal plane arrays up to 640×512 pixels for mid- and long-wavelength IR. In order to develop an industry-compatible T2SL technology, we have established the complete chain for detector array fabrication including design and modelling, epitaxial growth, as well as front- and backside processing. The epitaxial growth of T2SLs is performed by molecular beam epitaxy (MBE) in multi-wafer reactors. In this paper, we report on the control of growth rates during epitaxy, uniformity and reproducibility of the growth process, as well as characterization techniques to monitor the quality of the epitaxial layers. For the superlattice period, an average thickness variation far below a single atomic monolayer is required and achieved routinely. The standard deviation of the photoluminescence peak for both colors of bi-spectral IR detectors is around 0.04 μm for consecutive growth runs. With this very stable and reproducible epitaxial growth process in conjunction with our mature front- and backside processing we have been able to set up a pilot line production for bi-spectral T2SL IR detector arrays.
Short-wave infrared (SWIR) detection systems are increasingly demanded for surveillance, reconnaissance, and remote sensing applications. Passive SWIR cameras can benefit from an extended spectral range, compared to standard nightvision goggles, and the exploitation of the faint night-glow emission from the night sky. Furthermore, eye-safe SWIR lasers can improve the contrast and range of night-vision systems. High-performance SWIR photodetectors can be realized in the InGaAs material system, providing a typical cutoff wavelength of 1.7 μm, which covers a wide part of the night-glow spectrum as well the emission lines of available laser sources at typical telecom wavelengths around 1.55 μm. However, the low photon flux in night-vision applications demand for high responsivities and very low dark-current characteristics of the InGaAs photodetectors. We report on the current development activities of InGaAs SWIR photodetectors at Fraunhofer IAF. We have implemented a planar process technology of InGaAs/InP pin photodiodes for the fabrication of low-light-level SWIR cameras with 640× 512 pixels with 15 μm pixel pitch. Electro-optical characterization results of early-stage process runs exhibit darkcurrent densities below 10-7 A/cm2 at room temperature for 15-μm pitch detector elements. The current activities focus on further reducing the dark current to reach the international state of the art. Moreover, InGaAs-based avalanche photodiode (APD) have been developed for active SWIR imaging. Gain values of M ≈ 10 on camera level at a reverse bias voltage around 20 V have been achieved by a sophisticated vertical detector design. FPAs of such InGaAs-APD material have been successfully integrated into SWIR cameras with 640 × 512 pixels at 15 μm pixel pitch and operated in SWIR laser gated viewing mode.
The fabrication and characterization of InAs/GaSb type-II superlattice long-wavelength infrared (LWIR) photodetectors for high operating temperature (HOT) are assessed regarding possible device yield. We investigate laterally-operated photoconductors with a detector cutoff wavelength in the LWIR at an operating temperature accessible with 3-stage thermoelectric cooling, realized by suitably tailoring the layer composition. Type-II superlattices with a layer composition of 14 monolayers InAs and 7 monolayers GaSb are grown on semi-insulating 3-inch GaAs substrates. We report on the growth of three different buffer layer variants that serve as growth templates for GaSb-based layers on GaAs substrates. The characterization of 75 nominally equal single element detectors per sample evidences the reliability of device processing. The electro-optical evaluation of a randomly chosen subset indicates a high uniformity of responsivity and noise of LWIR InAs/GaSb HOT photoconductors. At 210 K, the devices operate at a cutoff wavelength of 10.5 μm and achieve a mean peak spectral detectivity of 3.3 × 108 Jones.
High-performance short-wavelength infrared (SWIR) photodetectors can be realized in the InGaAs material system, providing a typical cutoff wavelength of 1.7 μm, which covers a wide part of the nightglow spectrum as well as the emission lines of available laser sources at typical telecom wavelengths around 1.55 μm. However, both, active and passive SWIR detection systems are mostly required to provide high responsivities and very low dark currents in order to detect extremely low photon fluxes. This can be achieved with pin photodiodes with high responsivities and very low dark current characteristics or by utilizing the internal signal gain as provided by avalanche photodiodes (APDs). We develop SWIR photodetectors based on InGaAs/InP pin diodes and InGaAs/InAlAs/InP APDs as single-element detectors as well as focal plane arrays (FPAs). The planar processed InGaAs/InP pin photodiodes for low-light-level SWIR cameras exhibit dark current densities of 10-7 A/cm2 at room temperature for 15 μm pitch detector elements. For the APDs, emphasis is put on the vertical detector design. Within three design iterations, the operating voltage for useful gain values M ~ 10 could be reduced from 27 V down to 18 V, which was crucial for the operation with the voltage limitation of the read-out circuit. FPAs of such InGaAs-APD material have been successfully integrated into SWIR cameras with 640 × 512 pixels at 15 μm pixel pitch. The avalanche operation on camera level has been demonstrated for both kinds, the standard (passive) as well as gated-viewing operation modes.
Through the choice of appropriate layer thicknesses, the bandgap of InAs/Ga(As)Sb type II superlattices (T2SLs) can be engineered in a wide range covering the mid-wavelength and long-wavelength infrared (MWIR, 3 μm - 5 μm and LWIR, 8 μm - 12 μm) spectral regions. Using this material system, Fraunhofer IAF develops bi-spectral MWIR image sensors based on homojunction photodiodes for missile warning applications and pursues modern heterojunction approaches as well as heteroepitaxial growth of T2SLs on GaAs. We discuss topics arising from efforts to improve the manufacturability of our bi-spectral arrays and report on the progress of the integration with MWIR heterojunction designs that exhibit reduced dark currents.
Photodetectors in the non-visible region of the electromagnetic spectrum are essential for security, defense and space science as well as industrial and scientific applications. The research activities at Fraunhofer IAF cover a broad range in the infrared (IR) regime. Whereas short-wavelength IR (SWIR, <1.7 μm) detectors are realized by InGaAs/InP structures, InAs/GaSb type-II superlattice (T2SL) infrared detectors are developed for the spectral bands from mid- (MWIR, 3-5 μm) to long-wavelength IR (LWIR, 8-12 μm). We report on the extension of the superlattice empirical pseudopotential method (SEPM) to 300 K for the design of LWIR heterostructures for operation near room temperature. Recently, we have also adapted heterostructure concepts to our well established bi-spectral T2SL MWIR detector resulting in a dark current density below 2 × 10-9 A/cm2 for a cut-off wavelength close to 5 μm. Finally, we present first results obtained with a gated viewing system based on our InGaAs/InAlAs/InP avalanche photodiode arrays.
This paper reports on advances in the electro-optical characterization of InAs/GaSb short-period superlattice infrared photodetectors with cut-off wavelengths in the mid-wavelength and long-wavelength infrared ranges. To facilitate in-line monitoring of the electro-optical device performance at different processing stages we have integrated a semi-automated cryogenic wafer prober in our process line. The prober is configured for measuring current-voltage characteristics of individual photodiodes at 77 K. We employ it to compile a spatial map of the dark current density of a superlattice sample with a cut-off wavelength around 5 μm patterned into a regular array of 1760 quadratic mesa diodes with a pitch of 370 μm and side lengths varying from 60 to 350 μm. The different perimeter-to-area ratios make it possible to separate bulk current from sidewall current contributions. We find a sidewall contribution to the dark current of 1.2×10-11 A/cm and a corrected bulk dark current density of 1.1×10-7 A/cm2, both at 200 mV reverse bias voltage. An automated data analysis framework can extract bulk and sidewall current contributions for various subsets of the test device grid. With a suitable periodic arrangement of test diode sizes, the spatial distribution of the individual contributions can thus be investigated. We found a relatively homogeneous distribution of both bulk dark current density and sidewall current contribution across the sample. With the help of an improved capacitance-voltage measurement setup developed to complement this technique a residual carrier concentration of 1.3×1015 cm-3 is obtained. The work is motivated by research into high performance superlattice array sensors with demanding processing requirements. A novel long-wavelength infrared imager based on a heterojunction concept is presented as an example for this work. It achieves a noise equivalent temperature difference below 30 mK for realistic operating conditions.
Active and passive short-wave infrared (SWIR) detection systems for surveillance and remote sensing applications are mostly required to detect extremely low photon fluxes. This can be achieved by utilizing the internal signal gain as provided by avalanche photodiodes (APDs). We report on our current development activities of SWIR photodetectors based on InGaAs/InAlAs/InP APDs, covering detector design, epitaxial growth, process technology, and electro-optical characterization results of single-element detectors and fanout hybrids. For the first time, the operation of an InGaAsbased SWIR camera with 640 × 512 pixels utilizing APDs for signal amplification is demonstrated for operating temperatures of 180 K and even 260 K.
For more than two decades, Antimony-based type-II superlattice photodetectors for the infrared spectral range between
3-15 μm are under development at the Fraunhofer Institute for Applied Solid State Physics (IAF). Today, Fraunhofer
IAF is Germany’s only national foundry for InAs/GaSb type-II superlattice detectors and we cover a wide range of
aspects from basic materials research to small series production in this field. We develop single-element photodetectors
for sensing systems as well as two-dimensional detector arrays for high-performance imaging and threat warning
systems in the mid-wavelength and long-wavelength region of the thermal infrared. We continuously enhance our
production capabilities by extending our in-line process control facilities. As a recent example, we present a
semiautomatic wafer probe station that has developed into an important tool for electrooptical characterization. A large
amount of the basic materials research focuses on the reduction of the dark current by the development of bandgap
engineered device designs on the basis of heterojunction concepts. Recently, we have successfully demonstrated
Europe’s first LWIR InAs/GaSb type-II superlattice imager with 640x512 pixels with 15 μm pitch. The demonstrator
camera already delivers a good image quality and achieves a thermal resolution better than 30 mK.
A detailed understanding of limiting dark current mechanisms in InAs/GaSb type-II superlattice (T2SL) infrared detectors is key to improve the electrooptical performance of these devices. We present a six-component dark current analysis which, for the first time, takes account of sidewall-related dark current contributions in mesa-etched T2SL photodiodes. In a wide temperature range from 30K to 130K, the paper compares limiting mechanisms in two homojunction T2SL photodiode wafers for the long-wavelength infrared regime. While the two epi wafers were fabricated with nominally the same frontside process they were grown on different molecular beam epitaxy systems. In the available literature a limitation by Shockley-Read-Hall processes in the space charge region giving rise to generation-recombination (GR) dark current is the prevailing verdict on the bulk dark current mechanism in T2SL homojunction photodiodes around 77K. In contrast, we find that investigated photodiode wafers are instead limited by the diffusion mechanism and the ohmic shunt component, respectively. Furthermore, our in-depth analysis of the various dark current components has led to an interesting observation on the temperature dependence of the shunt resistance in T2SL homojunction photodiodes. Our results indicate that the GR and the shunt mechanism share the same dependence on bandgap and temperature, i.e., a proportionality to exp(-Eg/2kT).
InAs/GaSb superlattices are characterized by a broken-gap type II band alignment. Their effective band gap can be engineered to match mid to long wavelength infrared (IR) photon energies. Fraunhofer IAF has developed image detectors for threat warning systems based on this material system that are capable of spatially and temporally coincident detection in two mid-IR wavelength ranges. We review the present status of the processing technology, report continuous improvements achieved in key areas of detector performance, including defect density and noise behavior, and present initial results for statistical characterization of ensembles of detector elements with respect to diode characteristics and noise.
To enable higher operating temperatures in InAs/GaSb superlattice detectors for the long-wavelength infrared atmospheric window at 8-12 μm, a reduction of the bulk dark current density is indispensable. To reduce the dark current of conventional homojunction pin-diode device designs, bandstructure-engineering of the active region is considered most promising. So far, several successful device concepts have been demonstrated, yet they all rely on the inclusion of Aluminum within the active layers. Driven by manufacturing aspects we propose an Al-free heterojunction device concept that is based on a p+-doped InAs/GaSb superlattice absorber layer combined with an adjacent N--doped high gap region, which again is realized with an InAs/GaSb superlattice. To calculate the superlattice band gap and the position of the conduction band edge at the heterojunction we employ the Superlattice Empirical Pseudopotential Method. With a series of three heterojunction p+N- InAs/GaSb superlattice devices with an absorber band gap of 124 meV (10.0 μm) we give a first proof of the advocated device concept.
Fraunhofer IAF can look back on many years of expertise in developing high-performance infrared photodetectors. Since
pioneering the InAs/GaSb type-II superlattice detector development, extensive capabilities of epitaxy, process
technology, and device characterization of single element detectors and camera arrays for the mid- and longwave
infrared (MWIR and LWIR) have been established up to the level of small-scale production. Bispectral MWIR/MWIR
and MWIR/LWIR cameras based on type-II superlattices or HgCdTe are key topics at Fraunhofer IAF. Moreover, the
development of InGaAs-based short-wave infrared (SWIR) photodetectors for low-light-level applications has recently
been initiated.
In this contribution, we report on the status of recent photodetector development activities at IAF, covering detector
design, epitaxial growth, process technology, and most recent electro-optical characterization results of focal plane
arrays as well as single element detectors especially for the SWIR based on InGaAs material system.
To examine defects in InAs/GaSb type-II superlattices we investigated GaSb substrates and epitaxial InAs/GaSb layers
by synchrotron white beam X-ray topography to characterize the distribution of threading dislocations. Those
measurements are compared with wet chemical etch pit density measurements on GaSb substrates and InAs/GaSb type-II
superlattices epitaxial layer structures. The technique uses a wet chemical etch process to decorate threading dislocations
and an automated optical analyzing system for mapping the defect distribution.
Dark current and noise measurements on processed InAs/GaSb type-II superlattice single element photo diodes reveal a
generation-recombination limited dark current behavior without contributions by surface leakage currents for midwavelength
infrared detectors. In the white noise part of the noise spectrum, the extracted diode noise closely matches
the theoretically expected shot noise behavior.
For diodes with an increased dark current in comparison to the dark current of generation-recombination limited
material, the standard shot-noise model fails to describe the noise experimentally observed in the white part of the
spectrum. Instead, we find that McIntyre’s noise model for avalanche multiplication processes fits the data quite well.
We suggest that within high electric field domains localized around crystallographic defects, electrons initiate avalanche
multiplication processes leading to increased dark current and excess noise.
The noise behavior of InAs/GaSb superlattice photodiodes for high-performance thermal imaging in the mid- and longwavelength
infrared atmospheric windows at 3-5 μm and 8-12 μm is complex and up to now not very well understood.
In order to characterize these devices we have developed a noise measurement setup with a noise current resolution in
the femtoampère range. First, we show that, when sidewall leakage is absent, InAs/GaSb superlattice photodiodes with a
low dark current very close to the generation-recombination limited dark current level of the bulk behave according to
the well-known shot noise expression. Next, we investigate a set of 18 large-area diodes with a bandgap in the midwavelength
infrared regime, which show an increased dark current depending linearly on the applied reverse bias. For
these diodes the common shot noise model generally fails to describe the noise experimentally observed in the white part
of the noise spectrum. Instead, we find that McIntyre’s excess noise model for electron-initiated avalanche multiplication
processes fits our data remarkably well for the entire set of diodes, which covers about three orders of magnitude in dark
current and a wide range of reverse bias voltage. Thus, to explain the mechanism leading to the increased reverse dark
current and observed excess noise we tentatively suggest that primary electrons originating from Shockley-Read-Hall
states within the space charge region might initiate avalanche multiplication processes within high electric field domains
localized around sites of macroscopic crystallographic defects.
3rd generation IR modules - dual-color (DC), dual-band (DB), and large format two-dimensional arrays - require
sophisticated production technologies such as molecular beam epitaxy (MBE) as well as new array processing
techniques, which can satisfy the rising demand for increasingly complex device structures and low cost detectors. AIM
will extend its future portfolio by high performance devices which make use of these techniques. The DC MW / MW
detectors are based on antimonide type-II superlattices (produced by MBE at Fraunhofer IAF, Freiburg) in the 384x288
format with a 40 μm pitch. For AIM, the technology of choice for MW / LW DB FPAs is MCT MBE on CdZnTe
substrates, which has been developed in cooperation with IAF, Freiburg. 640x512, 20 μm pitch Focal Plane Arrays
(FPAs) have been processed at AIM. The growth of MW MCT MBE layers on alternate substrates is challenging, but
essential for competitive fabrication of large two-dimensional arrays such as megapixel (MW 1280x1024, 15 μm pitch)
FPAs. This paper will present the development status and latest results of the above-mentioned 3rd Gen FPAs and
Integrated Detector Cooler Assemblies (IDCAs).
InAs/GaSb-based type-II superlattice photodiodes have considerably gained interest as high-performance infrared
detectors. Beside the excellent properties of InAs/GaSb superlattices, like the relatively high effective electron mass
suppressing tunneling currents, the low Auger recombination rate, and a high quantum efficiency, the bandgap can be
widely adjusted within the infrared spectral range from 3 - 30 μm depending on the layer thickness rather than on
composition. Superlattice growth and process technology have shown tremendous progress during the last years. Fully
integrated superlattice cameras have been demonstrated by several groups worldwide.
Within very few years, the InAs/GaSb superlattice technology has proven its suitability for high-performance infrared
imaging detector arrays. At Fraunhofer IAF and AIM, the efforts have been focused on developing a mature fabrication
technology for bispectral InAs/GaSb superlattice focal plane arrays for a simultaneous, co-located detection at 3-4 μm
and 4-5 μm in the mid-wavelength infrared atmospheric transmission window. A very low number of pixel outages and
cluster defects is mandatory for dual-color detector arrays. Sources for pixel outages are manifold and might be caused
by dislocations in the substrate, the epitaxial growth process or by imperfections during the focal plane array fabrication
process. Process refinements, intense root cause analysis and specific test methodologies employed at various stages
during the process have proven to be the key for yield enhancements.
InAs/GaSb short-period superlattices (SL) have proven their large potential for high performance focal plane array
infrared detectors. Lots of interest is focused on the development of short-period InAs/GaSb SLs for mono- and bispectral
infrared detectors between 3 - 30 μm. InAs/GaSb short-period superlattices can be fabricated with up to 1000
periods in the intrinsic region without revealing diffusion limited behavior. This enables the fabrication of InAs/GaSb SL
camera systems with very high responsivity, comparable to state of the art CdHgTe and InSb detectors. The material
system is also well suited for the fabrication of dual-color mid-wavelength infrared InAs/GaSb SL camera systems.
These systems exhibit high quantum efficiency and offer simultaneous and spatially coincident detection in both spectral
channels.
An essential point for the performance of two-dimensional focal plane infrared detectors in camera systems is the
number of defective pixel on the matrix detector. Sources for pixel outages are manifold and might be caused by the
dislocation in the substrate, the epitaxial growth process or by imperfections during the focal plane array fabrication
process. The goal is to grow defect-free epitaxial layers on a dislocation free large area GaSb substrate. Permanent
improvement of the substrate quality and the development of techniques to monitor the substrate quality are of particular
importance. To examine the crystalline quality of 3" and 4" GaSb substrates, synchrotron white beam X-ray topography
(SWBXRT) was employed. In a comparative defect study of different 3" GaSb and 4" GaSb substrates, a significant
reduction of the dislocation density caused by improvements in bulk crystal growth has been obtained. Optical
characterization techniques for defect characterization after MBE growth are employed to correlate epitaxially grown
defects with the detector performance after hybridization with the read-out integrated circuit.
In the past years, the development of the type-II InAs/GaSb superlattice technology at the Fraunhofer-Institute for
Applied Solid State Physics (IAF) has been focused on achieving series-production readiness for third generation dualcolor
superlattice detector arrays for the mid-wavelength infrared spectral range. The technology is ideally suited for
airborne missile threat warning systems, due to its ability of low false alarm remote imaging of hot carbon dioxide
signatures on a millisecond time scale. In a multi-wafer molecular beam epitaxy based process eleven 288×384 dualcolor
detector arrays are fabricated on 3" GaSb substrates. Very homogeneous detector arrays with an excellent noise
equivalent temperature difference have been realized. The current article presents the type-II superlattice dual-color
technology developed at IAF and delivers insights into a range of test methodologies employed at various stages during
the fabrication process, which ensure that the basic requirements for achieving high detector performance are met.
InAs/GaSb short-period superlattices (SL) based on GaSb, InAs and AlSb have proven their great potential for high
performance infrared detectors. Lots of interest is currently focused on the development of short-period InAs/GaSb SLs
for advanced 2nd and 3rd generation infrared detectors between 3 - 30 μm. For the fabrication of mono- and bispectral
thermal imaging systems in the mid-wavelength infrared region (MWIR) a manufacturable technology for high
responsivity thermal imaging systems has been developed. InAs/GaSb short-period superlattices can be fabricated with
up to 1000 periods in the intrinsic region without revealing diffusion limited behavior. This enables the fabrication of
InAs/GaSb SL camera systems with high responsivity comparable to state of the art CdHgTe and InSb detectors. The
material system is also ideally suited for the fabrication of dual-color MWIR/MWIR InAs/GaSb SL camera systems with
high quantum efficiency for missile approach warning systems with simultaneous and spatially coincident detection in
both spectral channels.
A mature production technology for Quantum Well Infrared Photodetector (QWIP) focal plane arrays (FPAs) and
InAs/GaSb superlattice (SL) FPAs has been developed. Dual-band and dual-color QWIP- and SL-imagers are
demonstrated for the 3-5 μm and 8-12 μm atmospheric windows in the infrared. The simultaneous, co-located detection
of both spectral channels resolves the temporal and spatial registration problems common to existing bispectral IRimagers.
The ability for a reliable remote detection of hot CO2 signatures makes tailored dual-color superlattice imagers
ideally suited for missile warning systems for airborne platforms.
InAs/GaSb type-II short-period superlattice (SL) photodiodes have been shown to be very promising for 2nd and 3rd
generation thermal imaging systems with excellent detector performance. A multi-wafer molecular beam epitaxy (MBE)
growth process on 3"-GaSb substrates, which allows simultaneous growth on five substrates with excellent homogeneity
has been developed. A reliable III/V-process technology for badge processing of single-color and dual-color FPAs has
been set up to facilitate fabrication of mono- and bi-spectral InAs/GaSb SL detector arrays for the mid-IR spectral range.
Mono- and bispectral SL camera systems with different pitch and number of pixels have been fabricated. Those imaging
systems show excellent electro-optical performance data with a noise equivalent temperature difference (NETD) around
10 mK.
In Germany, InAs/GaSb superlattice detector technology for the mid-wavelength infrared spectral range has been
intensively developed in recent years. Mid-IR InAs/GaSb superlattice photodiodes achieve a very high quantum
efficiency. The world's first high-performance infrared imagers based on InAs/GaSb superlattices were realized offering
high spatial and excellent thermal resolution at short integration times. Additionally, the technology for dual-color
superlattice detectors featuring simultaneous, pixel-registered detection of two separate spectral regimes in the mid-IR
has been developed. Due to the ability to detect signatures of hot carbon dioxide, dual-color superlattice detectors are
ideally suited for missile alerting sensors. The capability for small volume production of InAs/GaSb superlattice
detectors has been established.
3rd Generation IR detectors providing e.g. dual-color capability are of great benefit for applications like aircraft missile approach warning systems using this feature for achieving low false alarm rates by separating the hot CO2 missile plume from background and clutter. AIM and IAF selected antimonide based type II Superlattices (SL) for such kind of applications. The type II SL technology provides an accurate engineering of sensitive layers by MBE with very good homogeneity and yield. IAF and AIM already managed to realize a dual-color 384x288 IR-Module based on this technology. It combines spectral selective detection in the 3-4 &mgr;m wavelength range and 4-5 &mgr;m wavelength range in each pixel with coincident integration in a 384x288x2 format and 40 &mgr;m pitch. Excellent thermal resolution with NETD < 17 mK @ F/2, 2.8 ms for the longer wavelength range (red color) and NETD < 30 mK @ F/2, 2.8 ms for the shorter wavelength range (blue color) were already reported. In order to increase further the quantum efficiency and subsequently decrease further the spectral crosstalk between the two colors the layer thickness of the SL-layer was optimized.
This paper is intended to present the current status and trends at AIM on antimonide type II Superlattices (SL) IR module developments for ground and airborne applications in the high performance range, where rapidly changing scenes - like e.g. in case of missile warning applications for airborne platforms - require temporal signal coincidence with integration times of typically 1ms.
InAs/GaSb short-period superlattices (SL) for the fabrication of mono- and bispectral thermal imaging systems in the
mid-wavelength infrared region (MWIR) have been optimized in order to increase the spectral response of the imaging
systems. The responsivity in monospectral InAs/GaSb short-period superlattices increases with the number of periods in
the intrinsic region of the diode and does not show a diffusion limited behavior for detector structures with up to 1000
periods. This allows the fabrication of InAs/GaSb SL camera systems with high responsivity. Dual-color MWIR/MWIR
InAs/GaSb SL camera systems with high quantum efficiency for missile approach warning systems with simultaneous
and spatially coincident detection in both spectral channels have been realized.
The fabrication and optimization of InAs/GaSb type-II superlattice (SL) detectors for single-color and dual-color focal
plane arrays in the mid-wavelength infrared spectral range between 3-5 &mgr;m is reported. Single color focal plane arrays
with 288 x 384 detector elements and 24 &mgr;m pitch have been fabricated with high pixel yield. Camera systems with
InAs/GaSb SL detectors reveal NETD values of 27.9 mK at a cut-off wavelength of &lgr;c = 4.9 &mgr;m for an integration time
of only 1 msec with F#/2.4 optics. A dual-color MWIR/MWIR InAs/GaSb SL camera, developed for missile approach
warning systems, features simultaneous and spatially coincident detection for both spectral channels on each pixel. The
camera system with 288 x 384 detector elements in 40 &mgr;m pitch shows excellent NETD values and high pixel
operability. The fabrication of dual-color focal plane arrays on 3" GaSb substrates is presented.
The 3rd generation of infrared (IR) detection modules is expected to provide advanced features like higher resolution
1024x1024 or 1280x720 pixels and/or new functions like multicolor or multi band capability, higher frame rates and
better thermal resolution. This paper is intended to present the current status and trends at AIM on antimonide type II
superlattices (SL) dual color detection module developments for ground and airborne applications in the high
performance range, where rapidly changing scenes - like e.g. in case of missile warning applications for airborne
platforms or ground based sniper detection systems - require temporal signal coincidence with integration times of
typically 1ms.
AIM and IAF selected antimonide based type II superlattices (SL) for such kind of applications. The type II SL
technology provides - similar to QWIP's - an accurate engineering of sensitive layers by MBE with very good
homogeneity and yield. IAF and AIM managed already to realize a dual color 384x288 IR module based on this
technology. It combines spectral selective detection in the 3 - 4&mgr;m wavelength range and 4 - 5 &mgr;m wavelength range in
each pixel with coincident integration in a 384x288x2 format and 40x40 &mgr;m2 pitch. Excellent thermal resolution with
NETD < 12 mK @ F/2, 2.8 ms for the longer wavelength range (red band) and NETD < 22 mK @ F/2, 2.8 ms for the
shorter wavelength range (blue band) were reported.
In the meantime a square design of 256x256x2 pixel with a reduced pitch of 30x30 &mgr;m2 is in preparation. In this case
with 2 Indium bumps per pixel and a third common contact for all pixels required for temporal coincidence is connected
at the outer area of the array. The fill factor is approx. 65% for both wavelength ranges. The reduced size of the array
enables the use of a smaller dewar with reduced cooling power and significantly reduced weight and broadens the scope
of applications where weight and costs is essential. Design aspects and expected performances are discussed.
We report on the development of InAs/GaSb type-II superlattice focal plane arrays (FPAs) for missile warning systems
in airborne platforms. The FPA fabrication technology was developed on the basis of monospectral superlattices for the
MWIR (3-5 μm) spectral range. A monospectral 288×384 MWIR camera with 24 μm pixel pitch, a noise equivalent
temperature difference (NETD) better than 14 mK and a background-limited performance (BLIP) up to 92 K is
demonstrated. Based on the monospectral technology, the first bispectral superlattice camera was realized. The dual
color 288×384 superlattice camera features simultaneous, pixel-registered detection of both spectral channels between
3-4 μm and 4-5 μm with a NETD better than 30 mK and 17 mK, respectively. Hence, spatial or temporal registration
problems, which are common to most dual color and dual band infrared imagers are solved with the new bispectral
MWIR missile alerting sensor.
This paper is intended to present firstly the current status at AIM on quantum well (QWIP) and antimonide superlattices (SL) detection modules for multi spectral ground and airborne applications in the high performance range i.e. for missile approach warning systems and secondly presents possibilities with long linear arrays i.e. 576x7 MCT to measure spectral selective in the 2 - 11μm wavelength range.
QWIP and antimonide based superlattice (SL) modules are developed and produced in a work share between AIM and the Fraunhofer Institute for Applied Solid State Physics (IAF). The sensitive layers are manufactured by the IAF, hybridized and integrated to IDCA or camera level by AIM. In case of MCT based modules, all steps are done by AIM.
QWIP dual band or dual color detectors provide good resolution as long as integration times in the order of 5-10ms can be tolerated. This is acceptable for all applications where no fast motions of the platform or the targets are to be expected.
For spectral selective detection, a QWIP detector combining 3-5 μm (MWIR) and 8-10 μm (LWIR) detection in each pixel with coincident integration has been developed in a 384x288x2 format with 40 μm pitch. Excellent thermal resolution with NETD < 30 mK @ F/2, 6.8 ms for both peak wavelengths (4.8 μm and 8.0 μm) has been achieved. Thanks to the well established QWIP technology, the pixel outage rates even in these complex structures are well below 0.5% in both bands. The spectral cross talk between the two wavelength bands is equal or less than 1%. The substrate on the sensitive layer of the FPA was completely removed in this case and as a consequence the optical crosstalk in the array usually observed in QWIP arrays resulting in low MTF values was suppressed resulting in sharp image impression.
For rapidly changing scenes - like e.g. in case of missile warning applications for airborne platforms - a material system with higher quantum efficiency is required to limit integration times to typically 1ms. AIM and IAF selected antimonide based type II superlattices (SL) for such kind of applications. The type II SL technology provides - similar to QWIPs - an accurate engineering of sensitive layers by MBE with very good homogeneity and potentially good yield and resistivity against high temperature application i.e. under processing or storage. While promising results on single SL pixels have been reported since many years, so far no SL based detection module could be realized with reasonable performances. IAF and AIM last year managed to realize first most promising SL based detectors. Fully integrated IDCAs with a MWIR SL single color device with 256x256 pixels in 40 μm pitch have been integrated and tested. In the next step the pitch was reduced to 24μm in a 384x288 pixel configuration. With this design and further improved technology a very good pixel operabilities with very low cluster sizes (≤ 4 pixel) and performances with quantum efficiencies as high as known from MCT is reached in the meantime.
A dual color device based on SL technology on the existing 384x288 read-out circuit (ROIC) as used in the dual band QWIP device is available. It combines spectral selective detection in the 3-4.1 μm wavelength range and 4.1-5 μm wavelength range in each pixel with coincident integration in a 384x288x2 format and 40 μm pitch. Excellent thermal resolution with NETD < 17 mK @ F/2, 2.8 ms for the longer wavelength range (red band) and NETD < 30 mK @ F/2, 2.8 ms for the shorter wavelength range (blue band) has been achieved. The pixel outage rates remains below 1% in both colors. The spectral cross talk of the red band to the blue band is estimated below 1%o which is important to reduce significantly the false alarm rate in missile approach warning systems as the primarily intended use of the dual color detector is.
Real time analysis of gases, i.e. the detection of toxic or agent gases, by multi spectral detection in the IR used the characteristic infrared emission or absorption lines of different gas types. Spectroscopic systems consisting of a spectrometer with the need for large linear MCT array with small pixel sizes are used in this case. Possibilities are outlined to use long linear arrays, such as the 576x7 MCT detector, to perform spectral selective measurements in the 2-11μm wavelength range. For these applications a 576x7 MCT FPA is integrated in an open dewar cooler assy without window able to operate directly coupled in an evacuated and cooled spectrometer. The sensitivity of the array is consequently not limited by the transmission of a window for vacuum conservation in the full sensitive wavelength range of MCT up to the cut-off of 10.5 μm.
We report on bispectral imaging systems based on quantum-well infrared photodetectors (QWIPs) and InAs/GaSb type-II superlattices (SLs) for the mid-wavelength infrared spectral range between 3-5 μm (MW) and the longwavelength infrared regime at 8-12 μm (LW). A dual-band MW/LW QWIP imager and a dual-color MW/MW InAs/GaSb SL camera are demonstrated. The two systems offer a spatial resolution of 288×384 pixels and a simultaneous detection of both channels on each pixel. Both technologies achieve an excellent noise equivalent temperature difference below 30 mK in each channel with F#/2.0 optics.
The first fully operational mid-IR (3-5 μm) 256x256 IR-FPA camera system based on a type-II InAs/GaSb short-period superlattice showing an excellent noise equivalent temperature difference below 10 mK and a very uniform performance has been realized. We report on the development and fabrication of the detector chip, i.e., epitaxy, processing technology and electro-optical characterization of fully integrated InAs/GaSb superlattice focal plane arrays. While the superlattice design employed for the first demonstrator camera yielded a quantum efficiency around 30%, a superlattice structure grown with a thicker active layer and an optimized V/III BEP ratio during growth of the InAs layers exhibits a significant increase in quantum efficiency. Quantitative responsivity measurements reveal a quantum efficiency of about 60% for InAs/GaSb superlattice focal plane arrays after implementing this design improvement.
An infrared camera based on a 256x256 focal plane array for the Mid-IR spectral range (3-5 μm) has been realized for the first time with InAs/GaSb short-period superlattices. The detector shows a cut-off wavelength of 5.4 μm and reveals a quantum efficiency of 30%. The noise equivalent temperature difference (NETD) reaches 9.4 mK at 73 K with F/2 optics and 6.5 ms integration time. Excellent thermal images with low NETD values and a very good modulation transfer function are presented. Furthermore, a new method to passivate InAs/GaInSb superlattice photodiodes for the 8-10 μm regime is demonstrated. The approach is based on the epitaxial overgrowth of wet-etched mesa diodes using lattice matched AlGaAsSb. A complete suppression of surface leakage currents in small sized test diodes with 70 μm diameter is observed.
The 3rd generation of infrared (IR) detection modules is expected to provide advanced features like higher resolution 1024x1024 or 1280x720 pixels and/or new functions like multicolor or multi band capability, higher frame rates and better thermal resolution. This paper is intended to present the current status at AIM on quantum well (QWIP) and antimonide superlattices (SL) detection modules for ground and airborne applications in the high performance range. For spectral selective detection, a QWIP detector combining 3-5μm (MWIR) and 8-10μm (LWIR) detection in each pixel with coincident integration has been developed in a 384x288x2 format with 40 μm pitch. Excellent thermal resolution with NETD < 30mK @ F/2, 6.8 ms for both peak wavelengths (4.8 μm and 8.0 μm) has been achieved. Thanks to the well established QWIP technology, the pixel outage rates even in these complex structures are below 0.5% in both bands. QWIP dual band or dual color detectors provide good resolution as long as integration times in the order of 5-10ms can be tolerated. This is acceptable for all applications where no fast motions of the platform or the targets are to be expected. For rapidly changing scenes-like e.g. in case of missile warning applications for airborne platforms-a material system with higher quantum efficiency is required to limit integration times to typically 1ms. AIM and IAF selected antimonide based type II superlattices (SL) for such kind of applications. The SL technology provides-similar to QWIP's-an accurate engineering of sensitive layers by MBE with very good homogeneity and yield. While promising results on single SL pixels have been reported since many years, so far no SL based detection module could be realized. IAF and AIM last year managed to realize first most promising SL based detectors. Fully integrated IDCA's with a MWIR SL device with 256x256 pixels in 40µm pitch have been integrated and tested. The modules exhibit excellent thermal resolution of NETD<10mk @ F/2 and 5ms. Product improvement meanwhile allowed to reduce pixel outage rates below 1% i.e. down to a level as required for the military use of such detectors. Presently under development is therefore a dual color MWIR device based on SL technology and the existing 384x288 read out circuit (ROIC) used in the dual band QWIP device. This detector is primarily intended for the use in missile approach warning systems where the dual color capability significantly improves suppression of false alarms. Details of the modules and results of the electrooptical performance will be presented for the different items mentioned above.
The 3rd generation of infrared (IR) detection modules is expected to provide advanced features like higher resolution 1024x1024 or 1280x720 pixels and/or new functionalities like multicolor or multi band capability, higher frame rates and better thermal resolution. This paper is intended to present the current status at AIM on the Mercury Cadmium Telluride (MCT), quantum well (QWIP) and antimonide superlattices (SL) detection modules for ground and airborne applications in the high performance range. For high resolution a 1280x720 MCT device in the 3-5μm range (MWIR) is presently under development. For spectral selective detection, a QWIP detector combining MWIR and 8-10μm (LWIR) detection in each pixel has been developed in a 384x288x2 format with 40 μm pitch, NETD < 35mK @ F/2, 6,8 ms for both peak wavelengths (4.8 μm and 8.0 μm). The device provides synchronous integration of both bands for temporal and spatial coincidence of the events observed. QWIP dual band or dual color detectors provide good resolution as long as integration times in the order of 5-10ms can be tolerated. This is acceptable for all applications where no fast motions of the platform or the targets are to be expected. For rapidly changing scenes - like e.g. in case of missile warning applications for airborne platforms - a material system with higher quantum efficiency is required to limit integration times to typically 1ms. For this case, several companies work on molecular beam epitaxy (MBE) of MCT to have access to double or multi layer structures. AIM and IAF selected antimonide based type II superlattices (SL) for such kind of applications. The SL technology provides -- similar to QWIP's -- an accurate engineering of sensitive layers by MBE with very good homogeneity and yield. While promising results on single SL pixels have been reported since many years, so far no SL based detection module could be realized. Just recently, IAF and AIM managed to realize first most promising SL based detectors. Fully integrated IDCAs with a MWIR SL device with 256 x 256 pixels in 40 μm pitch have been integrated and tested. The modules exhibit excellent thermal resolution of NETD > 12 mk @ F/2 and 5 ms. The next step will now be to stabilize the technology and to start the development of a dual color MWIR device based on SL technology and the existing 384 x 288 read out circuit (ROIC) used in the dual band QWIP device.
We report on our QWIP focal plane array (FPA) developments for the
8 - 12 μm and 3 - 5 μm regimes. In the long-wavelength infrared, we have realized several types of QWIP FPAs with array sizes from 256 × 256 to 640 × 512 pixels and with different active regions, giving rise to photoconductive and photovoltaic operation, respectively. Best thermal resolution in the 8 - 12 μm regime is obtained with low-noise QWIP FPAs which are based on a photovoltaic QWIP structure. Special emphasis is given to our work on a 640 × 512 mid-wave QWIP FPA, which is based on strained InGaAs/AlGaAs quantum wells lattice matched to a GaAs substrate. By optimizing the carrier concentration and the geometry of the two-dimensional grating with 1.65 μm period, a high quantum efficiency of more than 10% in the long-wavelength part of the 3 - 5 µm regime is achieved, resulting in an excellent thermal resolution of only 14.3 mK.
We experimentally compare the peak responsivity R, gain g, quantum efficiency, and detectivity of GaAs/AlGaAs-QWIPs with devices based on the competing material system InGaAs/GaAs. For this purpose we use a typical n-type GaAs/AlGaAs-QWIP and three n-type InGaAs/GaAs-QWIPs with varying doping densities. R and g of the GaAs/AlGaAs-QWIP show a typical negative differential behavior, while both quantities grow monotonously with increasing bias voltage in the case of the InGaAs/GaAs-QWIPs. For identical nominal doping densities and similar cutoff wavelengths between 8.9 micrometers and 9 micrometers , InGaAs/GaAs-QWIPs show much higher responsivities than GaAs/AlGaAs-QWIPs. The ratio between these responsivities is 2.5 at the bias voltage where the GaAs/AlGaAs-QWIP has its maximum. By making use of the different bias dependence of the responsivity in both types of QWIPs a further enhancement of this factor is achieved. Nevertheless, both types of QWIPs show comparable detectivities. This is due to the fact that the gain has a negligible influence on the detectivity. In conclusion, InGaAs/GaAs-QWIPs are promising if high responsivities and short integration times are required.
We report on novel low-noise QWIP focal plane arrays (FPAs) which allow us to improve the thermal resolution of infrared sensors in the long-wavelength infrared (LWIR) atmospheric window. Our concept uses detector structures with a small photoconductive gain in order to achieve simultaneously a high internal quantum efficiency and a small responsivity. In comparison to conventional QWIPs where each period consists of a quantum well and a thermionic barrier, our approach involves additionally a combination of a narrow quantum well and a tunnel barrier. Due to these additional layers, a high emission probability of the photoexcited carriers and an efficient capture into the ground subband of the subsequent period are simultaneously achieved. FPA cameras using these detectors show an extremely low noise- equivalent temperature difference (NE(Delta) T) and a high dynamic range. In particular, NE(Delta) Ts of only 7.2 mK and 5.2 mK (at 20 ms and 40 ms integration time, respectively) are observed for a 256x256 FPA camera system which we have realized using low-noise QWIPs. This value is the best temperature resolution ever obtained for thermal imagers operating in the LWIR.
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