The simultaneous detection of infrared signatures in separate spectral bands or sub-bands provides an advantage in many applications such as missile warning sensors. It allows an improved target identification and discrimination in front of structured natural or artificial background scenarios. AIM has been producing MCT-based dual color infrared SW/MW and MW/MW detectors grown by MBE on GaAs substrate for several years. These detectors allow a temporal and spatial coincident detection in two bands or sub-bands in the infrared spectral range. Since available space, allowed weight and power consumption are limited in many applications, an increase of the detector operation temperature is one way to cope with these constraints. It allows the usage of smaller and lighter coolers with less power consumption. Another way to enhance detector performance is the reduction of the pixel pitch. This allows for an increased spatial resolution or smaller detectors to improve the situational awareness and/or reduces the overall size and power consumption of the device. In this paper, we present investigations on AIM's 320 x 256 pixels and 30 mu m pitch infrared bispectral SW/MW and MW/MW detectors regarding elevated operating temperatures. The main limitation to operate this kind of infrared detectors at higher temperatures is the increasing dark current, resulting in increased temporal noise. We present measurements of the dark current and noise-related figures of merit like NETD. We investigate parameters like cooler power consumption to show the positive effects of higher operating temperatures regarding SWaP. Finally, we give an overview of the current activities regarding pixel pitch reduction and higher operation temperature of bispectral detectors at AIM.

AIM as one of the global leaders in higher operating temperatures (HOT) mercury cadmium telluride (MCT)-based infrared detectors presents its status on small pixel pitch short wavelength infrared (SWIR), mid wavelength infrared (MWIR), long wavelength infrared (LWIR) and very long wavelength infrared ( VLWIR) focal plane detector arrays (FPA) and gives a glimpse of the near future in the field. As of now, pixel operabilities exceeding 99.7% and excellent radiometric performance are attained in the measurement under typical operating conditions and at increased detector operating temperatures of 235K in the SWIR, 170K in the MWIR, 110K in the LWIR, and 70K in the VLWIR. Pixel pitches range between 7.5 mu m and 15 mu m. Thus, infrared applications with reduced size, weight, power dissipation and cost can be served. We outline the essential technological contributions of material science and device processing making this evolutionary step possible.

The dark current is a fundamental figure of merit to characterize the performance of high-sensitivity, low-noise mid- and far-infrared barrier photodetectors. In the context of HgCdTe barrier photodetectors, the trend is to use very low doping concentrations, in an attempt to minimize recombination processes. In the present work, through TCAD simulations, we delve deeper into the design of low-dark-current $p$ B $n$ detectors, showing the possible existence of an optimum doping. This occurrence is investigated and interpreted also by means of closed-form expressions for the lifetimes, emphasizing the role of the interplay between Auger and Shockley-Read-Hall generation processes.

In mid- and far-infrared HgCdTe photodetectors, the maximum operating temperature is determined by the dark current, which in turn depends on charge carriers lifetimes. The ability to reduce dark current and operate at the radiative limit allows for significantly higher operating temperature, but in this new scenario some approximations, commonly and historically applied in the literature to estimate and optimize lifetimes and dark current, in some cases should be avoided.

Pixel pitch reduction as well as high operating temperatures (HOT) of IR focal plane arrays (FPA) have attracted huge development effort in recent years. While the development journey is continuously moving towards an ultra-small pixel pitch of ~5μm for MWIR FPAs by several manufacturers, for their LWIR pendants the focus has not been on pitch size reduction and HOT so far. This comes as a surprise, since low SWaP (size, weight, and power) LWIR modules in combination with high spatial resolution are of great interest due to their superior electro-optical (e/o) performance in particular for scenarios with degraded visual environment. An SXGA-format (1280x1024 pixels) MWIR module using a 7.5μm pixel pitch is currently under development at AIM. Excellent performance data were obtained with the first prototypes up to elevated operating temperatures of ~150K thanks to the AIM HOT MCT p/n detector technology. SWaP characteristics are analyzed for both a compact configuration using the in-house ‘SX035’ cooler and for an even smaller variant referred to as ‘HOT Cube SX’. Moreover, a low SWaP VGA-format MWIR module is under development at AIM using a 10μm pixel and a HOT rotary cooler. First measurement results will be shown. Promising first e/o data of XGA-format 10μm pitch LWIR FPAs manufactured in a planar MCT p/n technology have already been presented by AIM last year. Recently, the performance has been improved which is underlined by a high thermal resolution of ~30mk and a low level of noisy pixel at HOT of ~100K. Latest results as well as key features of the possible low SWaP IR-module configurations will be discussed.

Multiphysics modeling of a planar HgCdTe-based mid-wavelength infrared (MWIR) focal plane array with $3\,\mu$ m-wide pixels enlightens the role of surface plasmon-polaritons observed in gold nanorods arranged on its illuminated face. Simulations indicate that the proposed plasmonic detector, which employs a $1\,\mu$ m-thick absorber layer, exhibits a reduction of diffusive inter-pixel crosstalk by more than one order of magnitude with respect to more conventional, non-plasmonic detectors with a $5\,\mu$ m-thick absorber layer, without penalizing responsivity and achieving increased detectivity in the whole MWIR band by taking advantage of the absorber volume reduction.

In recent years development focus for higher operating temperatures (HOT) and optimized size, weight and power (SWaP) characteristics has been on MWIR modules at AIM. However, HOT as well as small pixel pitch combined with high spatial and thermal resolution is of great interest for LWIR applications, too, e.g. for scenarios with degraded visual environment. Up to now LWIR detector arrays were manufactured in AIM’s mature planar MCT n/p technology and are typically operated at ~75K. In-depth studies of a p/n pixel structure at AIM showed significant reduction of the dark current and lower noisy pixel levels compared to their n-on-p pendants. Encouraged by these results a p/n pixel polarity is currently introduced to LWIR modules using different detector formats and pitch sizes to optimize performance and to increase operating temperature towards 100K. For VGA-format LWIR detector arrays with 15μm pitch a ROIC with p/n pixel polarity combined with a high charge handling capacity (CHC) of ~14Me- is available. With this approach we strive to increase the operating temperature to enable usage of smaller cooler types. Performance data and benefits in SWaP characteristics are discussed. An HD-format (1280x720 elements) 12µm pitch LWIR-module with NETD values of ~30mK and a high frame rate of 120Hz has already been demonstrated. Now electro-optical performance of LWIR detector arrays having a p/n diode polarity and a cutoff-wavelength of up to ~11μm were characterized. An even smaller pixel pitch of 10µm is currently introduced at AIM for LWIR detector arrays in a XGA-format. For the dedicated ROIC a CHC of ~7Me- was realized enabling high thermal resolution of ~30mK. First performance results and key features of the associated IR-module will be presented.

This work investigates the spectral quantum efficiency and inter-pixel crosstalk of a MWIR-LWIR dual band, HgCdTe-based focal plane array (FPA) photodetector (MWIR and LWIR stand for mid- and long-wavelength infrared bands). Pixels are 10 μm-wide with truncated pyramid geometry and separated by deep trenches. Three-dimensional combined full-wave electromagnetic and electrical simulations in the drift-diffusion approximation allowed to describe the complex, standing-wave-like spectral features resulting from the light interference and diffraction due to the pixels and illuminating beam aperture. The inter-pixel crosstalk for the MWIR operation demonstrated to be very sensitive to the trench depth, in contrast to the LWIR electrooptical response, left almost unchanged. The present work also investigates the causes of performance worsening in the two IR bands when pixel pitch is reduced to 5 μm, hence well below typical LWIR wavelengths and close to the diffraction limited operation.

The absorption properties of HgCdTe-based infrared detectors can be greatly increased in the mid-infrared band, by incorporating nanostructured plasmonic arrays on the illuminated detector face. The array periodicity, combined with the excitation of surface plasmon-polariton stationary modes, enhances the absorption efficiency by a substantial amount, allowing to reduce in turn the HgCdTe absorption thickness.

Majority carrier depletion has been proposed as a method to suppress the dark current originating from quasi-neutral regions in HgCdTe infrared focal plane array detectors. However, a very low doping level is usually required for the absorber layer, a task quite difficult to achieve in realizations. In order to address this point, we performed combined electromagnetic and electric simulations of a planar $ 5 \times 5 $5×5 pixel miniarray with 5 µm wide square pixels, assessing the effect of the absorber thickness, its doping level in the interval $ {N_D}{ = [10^{14}}{,10^{15}}] \;{{\rm cm}^{ - 3}} $ND=[1014,1015]cm-3, and temperature in the interval 140 K-230 K, both in the dark and under illumination. Looking for a trade-off, we found that the path towards high-temperature operation has quite stringent requirements on the residual doping, whereas a reduction of the absorber thickness helps only moderately to reduce the dark current. Under illumination, interpixel cross talk is only slightly cut down by a decrease of temperature or absorber doping in the considered intervals, whereas it gets more effectively reduced by thinning the absorber.

We present three-dimensional simulations of HgCdTe-based focal plane arrays (FPAs) with two-color and dual-band sequential infrared pixels having realistic truncated-pyramid shape, taking into account also the presence of compositionally graded transition layers. After a validation against the spectral responsivity of two-color, mid-wavelength infrared detectors from the literature, the method is employed for a simulation campaign on dual-band, mid-, and long-wavelength infrared FPAs illuminated by a Gaussian beam. Simulation results underscore the importance of a full-wave approach to the electromagnetic problem, since multiple internal reflections due to metallizations and slanted sidewalls produce non-negligible features in the quantum efficiency spectra, especially in the long-wavelength band. Evaluations of the optical and diffusive contribution to inter-pixel crosstalk indicate the effectiveness of deep trenches to prevent diffusive crosstalk in both wavebands. In its present form, the detector seems to be subject to significant optical crosstalk in the long-wavelength infrared band, which could be addressed through pixel shape optimization.

Future demanding scientific space and earth observation missions such as for exoplanet atmospheric spectral analysis or earth climate monitoring will define new benchmarks in technological feasibility and come along with increasingly stringent requirements on instrument subsystems. The requirements imposed on infrared detectors are dependent on the type of optical instrument, the chosen observation approach and the spectral range to be covered. With advanced detector architectures on hand and suitable␣technology approaches in mind, various electrical and electro-optical quantities have to be traded off against each other and possibly be prioritized, reflecting the specific instruments’ needs. From such considerations, implications on the detector material, design and processing arise. In this methodological paper, we will illustrate specific trade-off considerations for focal plane detector arrays (FPAs) based on mercury cadmium telluride (MCT) as detector material, e.g. with respect to modulation transfer function (MTF), detection efficiency (QE), noise level, defect density, etc. As an illustration, we will discuss simulation and implementation approaches and solutions found for a presently ongoing cornerstone pre-development activity in the long wavelength infrared (LWIR) and very long wavelength infrared (VLWIR). In particular, we will consider the thermal infrared (TIR) channels of the European Space Agency (ESA) Land and Surface Temperature Monitoring Mission (LSTM) observation instrument, for which we will present a photodiode array design approach balanced for key electro-optical FPA performance parameters.

In this paper, Sb-based superlattice fabrication processing is based on standard III-V technology, implying lower costs of mass production and constituting a relatively new alternative for an IR material system in LWIR and VLWIR bands.

Three-dimensional, realistic full-wave electromagnetic and electrical simulations were done considering a planar HgCdTe 5 × 5 pixel miniarray with 5 μm-wide square pixels, illuminated by narrow Gaussian beams. The results suggest that, by avoiding quasi-neutral regions in the detector's absorber through majority carriers depletion, the inter-pixel crosstalk due to the diffusion of photogenerated carriers is significantly reduced.

The ability to simulate HgCdTe infrared detectors with realistic alloy composition profiles is essential for their optimization. We give practical guidelines for the realization of combined three-dimensional, realistic full-wave electromagnetic and electrical modeling for the description of detectors with compositionally-graded layers, suitable to most available numerical solvers. Following the known procedure to sample the compositionally graded layers into a number N of sublayers, we assess the effects of different choices for N, both on calculation accuracy and computational cost. Quantum efficiency spectra calculated with the proposed approach are compared with those determined through two shortcuts: ray tracing, based on classical optics, and full-wave simulation where graded layers are replaced by constant, spatially-averaged optical properties. It is shown that the former is not generally a valid alternative, since it does not address interference effects due to back-reflections, while the latter can lead to incorrect estimates of the detector cutoff wavelength.

For improved long-range reconnaissance applications, a SWIR module for active imaging was developed at AIM. Using laser illumination together with gated viewing (GV) capability reduces noise, amount of background clutter and path radiance and helps e.g. to see through smoke obscuration. Based on AIM’s MCT technology a 2D SWIR electron avalanche photodiode (eAPD) array was developed. A 640×512 15μm pitch format was chosen to allow imaging with the desired resolution. The APD arrays have been designed and fabricated by liquid phase epitaxy (LPE) growth of MCT layers for a 2.5μm cut-off on lattice-matched CdZnTe substrates. Also, a specific ROIC was designed to enable the GV capability of the module. In GV operating mode, the gate can be controlled by an external trigger signal or by internal timing parameters. The gate-delay and integration can be programmed precisely. Focal Plane Arrays (FPAs) have been fabricated and integrated into a dewar/cooler assembly. A dedicated command and control electronics has been designed providing the system interface including power supply, triggering and digital video data. To demonstrate long-range reconnaissance capability of SWIR GV a field camera demonstrator was developed. The demonstrator incorporates the SWIR GV camera, a thermal camera with a wide field of view (WFOV) for detection and the laser illuminator. A user interface was implemented to operate the demonstrator in the field, having easy access to the gating capability. The paper will present the latest performance results of the SWIR GV module including results of field trials with the demonstrator.

In recent years enormous development progress on key subcomponents of IR-modules was achieved: the operating temperature of MWIR FPAs has successfully been increased to high operating temperature (HOT) while preserving a 5μm cutoff-wavelength and small pixel pitch and miniature cryocoolers with low power consumption were introduced at AIM. This leads to new ultra-compact IR-solutions and improvements of existing IR-modules best suited to respond to the wide range of system needs in military and civilian applications. Key enabler for the realization of a low SWaP MWIR engine in a XGA, 10μm pitch format is AIM’s HOT MCT p-on-n detector technology, able to provide high electro-optical (e/o) performance at 160K. This technology has now been optimized and transferred to various detector formats and pitch sizes in the range from 10μm to 30μm. At moderate operating temperatures around 100K it can be used to provide highest e/o-performance including a very low defective pixel level. To improve thermal resolution of 640×512, 15μm pitch LWIR modules the charge handling capacity (CHC) of the associated ROIC has now been increased to nearly 20Me- . Typical integration times remain below 1ms ensuring high frame rates of 120Hz. In addition, an interlaced mode allows for an even higher CHC value of ~40Me- , a 2×2 binning mode for 450Hz operation in a smaller format. Furthermore, an optimized ROIC design in a 1024×768 format, offering a high CHC of ~7.5Me- in a small pixel pitch of 10μm, has been completed to improve existing MWIR modules and to provide a good basis for a XGA 10μm pitch LWIR version currently under development at AIM. In this paper latest performance results of improved MCT MWIR and LWIR detector arrays will be presented, together with optimization of associated cooled IR-modules.

Bispectral infrared-sensors that provide detection in two different spectral regions, e.g. short-wavelength infrared and mid-wavelength infrared (SWIR/MWIR) or MWIR/MWIR, offer compared to single-color sensors improved performance in a wide variety of space and ground-based applications. Possible applications are target identification, signature recognition and clutter rejection. In particular the combination of the SWIR/MWIR or MWIR/MWIR spectral regions promote an enhanced target discrimination and identification by increasing the identification range, by enabling the target acquisition in front of strongly structured backgrounds or of targets with low thermal signature. We have extended our MBE growth technology, primarily developed for the cost-effective production of standard IR-detectors in the MWIR spectral range, to the growth of mercury-cadmium-telluride (MCT) multi-layers with different cutoff-wavelengths. The design of the bispectral pixel with two indium bumps per cell allows for temporal and spatial coincidence. To demonstrate the capabilities of the bispectral IR sensors, FPAs with a format of e.g. 320x256 pixels and a 30 mu m pitch have been fabricated. In this paper we present the key performance parameters of recently optimized SWIR/MWIR and MWIR/MWIR bispectral MCT detectors along with images taken with the bispectral detectors by using SWIR or MWIR optics. The detectors demonstrate improved quantum efficiency, very low color cross-talk, and an excellent NETD in conjunction with low defect densities. Processing of the bispectral images with algorithms for combining the information from both spectral ranges provide striking evidence for the potential of these bispectral detectors in various applications.