Cryocoolers are needed in modern high-performance infrared cameras to establish and maintain the cryogenic temperature of infrared focal plane arrays. Attollo has a number of cryocooler development efforts ranging from low SWaP linear microcoolers for HOT MWIR sensors to high lift, high reliability coolers for 80K operation. In the case of cryocoolers designed for HOT MWIR applications, Attollo is also developing low-cost designs to address the attritable platform market. Attollo will present an overview of these design programs and data on the performance of initial units.
Unmanned airborne and dismounted soldier capability requirements continue to push for reduced size, weight, and power (SWaP) and high sensitivity infrared (IR) imaging in applications that were not previously practical. In response to these needs, Attollo Engineering has developed a 1280x1024, 5μm pixel pitch cooled mid wavelength infrared (MWIR) sensor that pushes the envelope in pixel pitch in addition to a 1280x1024, 10μm pixel sensor dual band sensor with additional sensitivity in the short wavelength infrared (SWIR) in order to exploit SWIR phenomenology including laser see spot functionality. Both of these sensors offer MWIR sensing capabilities but are also able to leverage aspects of Attollo’s detector design to enable SWIR sensing to varying degrees. This class of small pixel cooled, single and dual band IR sensor technology represents advancements in all aspects of the sensor’s design and development, and we will discuss the innovations made at Attollo to enable this capability including epitaxial detector design based on III V compound semiconductors, detector array and focal plane array fabrication, design of a low noise, dual band CTIA/DI readout integrated circuit (ROIC), vacuum dewar packaging, and electronics and firmware design. In this paper we will present on the status of high definition small pixel pitch MWIR and dual band SWIR/MWIR imaging technology at Attollo as it relates to these sensors including design and measurement data and imaging.
Attollo Engineering specializes in developing small pixel pitch infrared imaging sensors. Small pixel pitch sensors offer opportunities for imaging system SWaP reduction that open up a variety of SWaP-constrained applications that were not previously feasible. The size and weight of the optics also decrease because of the small pixel, enabling further SWaP savings. Attollo Engineering has developed high-definition high-operating-temperature SWIR and MWIR sensors with pixel pitches as small as 5 µm using III-V compound semiconductor detector materials. Additionally, we have developed a compact camera core with an integrated cooler and full featured camera electronics for these imagers.
Small pixel pitch sensors offer opportunities for imaging system SWaP reduction that open up a variety of SWaP-constrained applications that were not previously feasible. Furthermore, small pixel digital sensors provide advantages in the form of additional SWaP reduction, noise immunity, and simplified interfacing requirements. With these motivations in mind, Attollo Engineering has developed a 640x512, 5μm pixel pitch, high operating temperature MWIR sensor based on III V compound semiconductor detector materials. We have adapted our 5μm pixel pitch SWIR processes for MWIR detector materials and have been able to achieve 99.5+% operability MWIR FPAs with BLIP performance operating at 130K. Additionally, we have developed a compact camera core with an integrated cooler and full featured camera electronics. The global shutter camera is capable of frame rates of up to 220 Hz or smaller windows in excess of 1 kHz and integration times as low as 100 nanoseconds. Attollo will discuss characteristics of this sensor and other related technologies.
Attollo Engineering will present results of our research program developing extended SWIR sensors as well as the packaging and camera electronics surrounding it. The 640x512 sensor uses GaInAsSb for the active layer and has a cutoff wavelength of 2.5 m. The unipolar barrier structure enables a higher operating temperature by substantially reducing dark current caused by G-R mechanisms and surface leakage. The material is grown on GaSb and is made up of GaInAsSb absorber and contact layers separated by an AlGaSb barrier. We will present dark current and imaging results from the sensor fabrication at different temperatures. The detector array was hybridized to a 15 m pixel pitch ROIC that has a direct injection unit cell. The hybridized sensor was packaged into a custom 4-stage thermoelectrically cooled package. The package was particularly designed to minimize the heat load and maximize the thermal conduction. We will present the trades that went into designing the package and the internals of the package. The cooler stabilized the sensor temperature at 200K. The electronics used to drive the package have the ability to change biases and timing on the fly using software controls. Attollo designed these electronics to be a low-cost solution for demonstrating sensors in many different modes. We will show information regarding each stage of integration and show the results of the imaging using the eSWIR sensor and supporting equipment.
High resolution multi-band infrared detection of terrestrial objects is useful in applications such as long
range and high altitude surveillance. In this paper, we present a 640 by 512 type-II superlattice focal plane array
(FPA) in the long-wave infrared (LWIR) suitable for such purposes, featuring 100% cutoff wavelengths at 9.5μm
(blue channel) and 13μm (red). The dual band camera is single-bump hybridized to an Indigo 30μm pitch
ISC0905 read-out integrated circuit. Test pixels revealed background limited behavior with specific detectivities
as high as ~5x1011 Jones at 7.9μm (blue) and ~1x1011 Jones at 10.2μm (red) at 77K.
One of the biggest challenges of improving the electrical performance in Type II InAs/GaSb superlattice
photodetector is suppressing the surface leakage. Surface leakage screens important bulk dark current mechanisms,
and brings difficulty and uncertainty to the material optimization and bulk intrinsic parameters extraction such as
carrier lifetime and mobility. Most of surface treatments were attempted beyond the mid-infrared (MWIR) regime
because compared to the bulk performance, surface leakage in MWIR was generally considered to be a minor factor.
In this work, we show that below 150K, surface leakage still strongly affects the electrical performance of the very
high bulk performance p-π-M-n MWIR photon detectors. With gating technique, we can effectively eliminate the
surface leakage in a controllable manner. At 110K, the dark current density of a 4.7 μm cut-off gated photon diode is
more than 2 orders of magnitude lower than the current density in SiO2 passivated ungated diode. With a quantum
efficiency of 48%, the specific detecivity of gated diodes attains 2.5 x 1014 cmHz1/2/W, which is 3.6 times higher than
that of ungated diodes.
Recent efforts have been paid to elevate the operating temperature of Type II superlattice Mid Infrared
photon detectors. Using M-structure superlattice, novel device architectures have been developed, resulting
in significant improvement of the device performances. In this paper, we will compare different
photodetector architectures and discuss the optimization scheme which leads to almost one order of
magnitude of improvement to the electrical performance. At 150K, single element detectors exhibit a
quantum efficiency above 50%, and a specific detectivity of 1.05x1012 cm.Hz1/2/W. BLIP operation with a
300K background and 2π FOV can be reached with an operating temperature up to 180K. High quality focal
plane arrays were demonstrated with a noise equivalent temperature difference (NEDT) of 11mK up to
120K. Human body imaging is achieved at 150K with NEDT of 150mK.
Infrared detection technologies entering the third generation demand performances for higher
detectivity, higher operating temperature, higher resolution and multi-color detection, all accomplished with
better yield and lower manufacturing/operating costs. Type-II antimonide based superlattices (T2SL) are
making firm steps toward the new era of focal plane array imaging as witnessed in the unique advantages and
significant progress achieved in recent years. In this talk, we will present the four research themes towards
third generation imagers based on T2SL at the Center for Quantum Devices. High performance LWIR
megapixel focal plane arrays (FPAs) are demonstrated at 80K with an NEDT of 23.6mK using f/2 optics, an
integration time of 0.13ms and a 300K background. MWIR and LWIR FPAs on non-native GaAs substrates
are demonstrated as a proof of concept for the cost reduction and mass production of this technology. In the
MWIR regime, progress has been made to elevate the operating temperature of the device, in order to avoid
the burden of liquid nitrogen cooling. We have demonstrated a quantum efficiency above 50%, and a
specific detectivity of 1.05x1012 cm.Hz1/2/W at 150K for 4.2μm cut-off single element devices. Progress on
LWIR/LWIR dual color FPAs as well as novel approaches for FPA fabrication will also be discussed.
In recent years, the type-II superlattice (T2SL) material platform has seen incredible growth in
the understanding of its material properties which has lead to unprecedented development in the arena
of device design. Its versatility in band-structure engineering is perhaps one of the greatest hallmarks
of the T2SL that other material platforms are lacking. In this paper, we discuss advantages of the
T2SL, specifically the M-structure T2SL, which incorporates AlSb in the traditional InAs/GaSb
superlattice. Using the M-structure, we present a new unipolar minority electron detector coined as
the P-M-P, the letters which describe the composition of the device. Demonstration of this device
structure with a 14μm cutoff attained a detectivity of 4x1010 Jones (-50mV) at 77K. As device
performance improves year after year with novel design contributions from the many researchers in
this field, the natural progression in further enabling the ubiquitous use of this technology is to reduce
cost and support the fabrication of large infrared imagers. In this paper, we also discuss the use of
GaAs substrates as an enabling technology for third generation imaging on T2SLs. Despite the 7.8%
lattice mismatch between the native GaSb and alternative GaAs substrates, T2SL photodiodes grown
on GaAs at the MWIR and LWIR have been demonstrated at an operating temperature of 77K.
Recent efforts have been paid to elevate the operating temperature of Type II InAs/GaSb superlattice
Mid Infrared photon detectors. Optimized growth parameters and interface engineering technique
enable high quality material with a quantum efficiency above 50%. Intensive study on device
architecture and doping profile has resulted in almost one order of magnitude of improvement to the
electrical performance and lifted up the 300K-background BLIP operation temperature to 166K. At
77K, the ~4.2 μm cut-off devices exhibit a differential resistance area product in excess of the
measurement system limit (106 Ohm.cm2) and a detectivity of 3x1013cm.Hz1/2/W. High quality focal
plane arrays were demonstrated with a noise equivalent temperature of 10mK at 77K. Uncooled
camera is capable to capture hot objects such as soldering iron.
Type-II InAs/GaSb Superlattice (SL), a system of multi interacting quantum wells was first
introduced by Nobel Laureate L. Esaki in the 1970s. Since then, this low dimensional system has
drawn a lot of attention for its attractive quantum mechanics properties and its grand potential for the
emergence into the application world, especially in infrared detection. In recent years, Type-II
InAs/GaSb superlattice photo-detectors have experienced significant improvements in material
quality, structural designs and imaging applications which elevated the performances of Type-II
InAs/GaSb superlattice photodetectors to a comparable level to the state-of-the-art Mercury Cadmium
Telluride. We will present in this talk the current status of the state-of-the-art Type II superlattice
photodetectors and focal plane arrays, and the future outlook for this material system.
In recent years, Type II InAs/GaSb superlattices grown on GaSb substrate have achieved significant
advances in both structural design and material growth, making Type II superlattice infrared detector
a rival competitor to the state-of-the-art MCT technology. However, the limited size and strong
infrared absorption of GaSb substrates prevent large format type-II superlattice infrared imagers from
being realized. In this work, we demonstrate type-II superlattices grown on GaAs substrates, which is
a significant step toward third generation infrared imaging at low cost. The device performances of
Type II superalttice photodetectors grown on these two substrates are compared.
A challenge for type-II InAs/GaSb superlattice (T2SL) photodetectors is to achieve high fill factor, high aspect ratio
etching for third generation focal plane arrays (FPAs). Initially, we compare the morphological and electrical results of
single element T2SL photodiodes after BCl3/Ar inductively coupled plasma (ICP) and electron cyclotron resonance
(ECR) dry etching. Using a Si3N4 hard mask, ICP-etched structures exemplify greater sidewall verticality and
smoothness, which are essential toward the realization of high fill factor FPAs. ICP-etched single element devices with
SiO2 passivation that are 9.3μm in cutoff wavelength achieved vertical sidewalls of 7.7μm in depth with a resistance area product at zero bias of greater than 1,000 Ωcm2 and maximum differential resistance in excess of 10,000 Ωcm2 at 77K. By only modifying the etching technique in the fabrication steps, the ICP-etched photodiodes showed an order of
magnitude decrease in their dark current densities in comparison to the ECR-etched devices. Finally, high aspect ratio
etching is demonstrated on mutli-element arrays with 3μm-wide trenches that are 11μm deep.
Recent advances in the design and fabrication of Type-II InAs/GaSb superlattices allowed the
realization of high performance long wavelength infrared focal plane arrays. The introduction of an Mstructure
barrier between the n-type contact and the π active region reduced the tunneling component of the
dark current. The M-structure design improved the noise performance and the dynamic range of FPAs at
low temperatures. At 81K, the NEDT of the focal plane array was 23 mK. The noise of the camera was
dominated by the noise component due to the read out integrated circuit. At 8 μm, the median quantum
efficiency of the detectors was 71%, mainly limited by the reflections on the backside of the array.
The Type-II InAs/GaSb superlattice photon detector is an attractive alternative to HgCdTe photodiodes and QWIPS. The use of p+ - π - M - N+ heterodiode allows for greater flexibility in enhancing the device performance. The utilization of the Empirical Tight Binding method gives the band structure of the InAs/GaSb superlattice and the new M- structure
(InAs/GaSb/AlSb/GaSb) superlattice allowing for the band alignment between the binary superlattice and the M- superlattice to be determined and see how it affects the optical performance. Then by modifying the doping level of the M- superlattice an optimal level can be determined to achieve high detectivity, by simultaneously improving both photo-response and reducing dark current for devices with cutoffs greater than 14.5 μm.
LWIR and VLWIR type II InAs/GaSb superlattice photodetectors have for long time suffered from a
high dark current level and a low dynamic resistance which hampers the its emergence to the infrared
detection and imaging industry. However, with the use of M-structure superlattice, a new type II
binary InAs/GaSb/AlSb superlattice design, as an effective blocking barrier, the dark current in type II
superlattice diode has been significantly reduced. We have obtained comparable differential resistance
product to the MCT technology at the cut-off wavelength of 10 and 14μm. Also, this new design is
compatible with the optical optimization scheme, leading to high quantum efficiency, high special
detectivity devices for photon detectors and focal plane arrays.
Recent progress made in the structure design, growth and processing of Type-II InAs/GaSb
superlattice photo-detectors lifted both the quantum efficiency and the R0A product of the detectors.
Type-II superlattice demonstrated its ability to perform imaging in the Mid-Wave Infrared (MWIR)
and Long-Wave Infrared (LWIR) ranges, becoming a potential competitor for technologies such as
Quantum Well Infrared Photo-detectors (QWIP) and Mercury Cadmium Telluride (MCT). Using an
empirical tight-binding model, we developed superlattices designs that were nearly lattice-matched to
the GaSb substrates and presented cutoff wavelengths of 5 and 11 μm. We demonstrated high quality
material growth with X-ray FWHM below 30 arcsec and an AFM rms roughness of 1.5 Å over an
area of 20x20 μm2. The detectors with a 5 μm cutoff, capable of operating at room temperature,
showed a R0A of 1.25 106 Ω.cm2 at 77K, and a quantum efficiency of 32%. In the long wavelength
infrared, we demonstrated high quantum efficiencies above 50% with high R0A products of 12 Ω.cm2
by increasing the thickness of the active region. Using the novel M-structure superlattice design, more
than one order of magnitude improvement has been observed for electrical performance of the
devices. Focal plane arrays in the middle and long infrared range, hybridized to an Indigo read out
integrated circuit, exhibited high quality imaging.
In recent years, Type-II InAs/GaSb superlattice photo-detectors have experienced significant improvements in material
quality, structural designs, and imaging applications. They now appear to be a possible alternative to the state-of-the-art
HgCdTe (MCT) technology in the long and very long wavelength infrared regimes. At the Center for Quantum Devices,
we have successfully realized very high quantum efficiency, very high dynamic differential resistance R0A product
LWIR Type-II InAs/GaSb superlattice photodiodes with efficient surface passivation techniques. The demonstration of
high quality LWIR Focal Plane Arrays that were 100% fabricated
in-house reaffirms the pioneer position of this university-based laboratory.
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