Advanced electro-optical systems are designed towards a more compact, low power, and low cost solution with respect to traditional systems. Integration of several components or functionalities, such as infrared imager, laser designator, laser range finder (LRF), into one multi-function detector serves this trend. SNIR Read-Out Integrated Circuit (ROIC) incorporates this high level of signal processing and with relatively low power consumption. In this paper we present measurement results from a Focal Plane Array (FPA) where the SNIR ROIC is Flip-Chip bonded to a 15µm pitch VGA InGaAs detector array. The FPA is integrated into a metallic vacuum sealed package. We present InGaAs arrays with dark current density below 1.5 nA/cm<sup>2</sup> at 280K (typically 1fA), Quantum Efficiency higher than 80% at 1550 nm and operability better than 99.5%. The metallic package is integrated with a low power proximity electronics which delivers Camera Link output. The overall power dissipation is less than 1W, not including Thermal-Electric Cooling (TEC), which is required in some applications. The various active and passive operation modes of this detector will be reviewed. Specifically, we concentrate on the "high gain" mode with low readout noise for Low Light Level imaging application. Another promising feature is the Asynchronous Laser Pulse Detection (ALPD) with remarkably low detection thresholds.
Short wavelength Infra Red (SWIR) imaging has gained considerable interest in recent years. The main applications
among others are: active imaging and LADAR, enhanced vision systems, low light level imaging and security
In this paper we will describe SCD's considerable efforts in this spectral region, addressing several platforms:
1. Extension of the mature InSb MWIR product line operating at 80K (cut-off wavelength of 5.4μm).
2. Extension of our new XB<sub>n</sub><i>n</i> InAsSb "bariode" technology operating at 150K (cut-off of 4.1μm).
3. Development of InGaAs detectors for room temperature operation (cut-off of 1.7μm)
4. Development of a SNIR ROIC with a low noise imaging mode and unique laser-pulse detection modes.
In the first section we will present our latest achievements for the cooled detectors where the SWIR region is combined
with MWIR response. Preliminary results for the NIR-VIS region are presented where advanced substrate removal
techniques are implemented on flip-chip hybridized focal plane arrays.
In the second part we will demonstrate our VGA, 15μm pitch, InGaAs arrays with dark current density below 1.5nA/cm<sup>2</sup>
at 280K. The InGaAs array is hybridized to the SNIR ROIC, thus offering the capability of low SWaP systems with
laser-pulse detection modes.
We demonstrate the suppression of the bulk generation-recombination current in nBn devices based on an InAsSb active layer (AL) and a AlSbAs barrier layer (BL). This leads to much lower dark currents than in conventional InAsSb photodiodes operating at the same temperature. When the BL is p-type, very high doping must be used in the AL (nBpn+). This results in a significant shortening of the device cutoff wavelength due to the Moss-Burstein effect. For an n-type BL, low AL doping can be used (nBnn), yielding a cutoff wavelength of ∼4.1 μm and a dark current close to ∼3 × 10−7 A/cm2 at 150 K. Such a device with a 4-μm-thick AL will exhibit a quantum efficiency (QE) of 70% and background-limited performance operation up to 160 K at f/3. We have made nBnn focal plane array detectors (FPAs) with a 320 × 256 format and a 1.3-μm-thick AL. These FPAs have a 35% QE and a noise equivalent temperature difference of 16 mK at 150 K and f/3. The high performance of our nBnn detectors is closely related to the high quality of the molecular beam epitaxy grown InAsSb AL material. On the basis of the temperature dependence of the diffusion limited dark current, we estimate a minority carrier lifetime of ∼670 ns.
The XB<sub>n</sub><i>n</i> high operating temperature (HOT) detector project at SCD is aimed at developing a HOT (~150K) mid-wave
infrared (MWIR) detector array, based on InAsSb/AlSbAs barrier detector or "bariode" device elements. The essential
principle of the XB<sub>n</sub><i>n</i> bariode architecture is to suppress the Generation-Recombination contribution to the dark current
by ensuring that the depletion region of the device is contained inside a large bandgap <i>n</i>-type barrier layer (BL) and
excluded from the narrow bandgap <i>n</i>-type active layer (AL). The band profile of the XB<sub>n</sub><i>n</i> device leads to effective
blocking of electron transport across the BL while maintaining a free path for the holes, thus assuring a high internal
quantum efficiency (QE). Our devices exhibit a very large minority carrier lifetime (~700 ns), leading to a very low
dark current of <10<sup>-6</sup> A cm<sup>-2</sup> at 150K, which is essentially diffusion limited. We compare bariode devices with both a <i>p</i>-type
GaSb contact layer (CL) and an n-type InAsSb CL (termed C<sub>p</sub>B<sub>n</sub><i>n</i> and <i>n</i>B<sub>n</sub><i>n</i>, respectively). Apart from a ~0.3V
shift in the operating bias, the optical and electrical properties of both architectures are virtually identical,
demonstrating the generic nature of the XB<sub>n</sub><i>n</i> barrier detector family. We have fabricated FPAs from <i>n</i>B<sub>n</sub><i>n</i> bariode
arrays bonded both to a 320×256, 30 μm pitch Read-Out Integrated Circuit (ROIC) and a 640×512, 15 μm pitch ROIC.
For lattice matched FPAs the cut-off wavelength at >50% of maximum response is ~ 4.1 μm. We show an image
registered at 150K with a 640×512/15 μm Pelican FPA, using f/3.2 optics. The operability at 150K is >99.5% and the
measured NETD, limited only by shot and Read-Out noise, is 20 mK for a 22 ms integration time. At this f/number, the
detector has a background limited performance (BLIP) up to ~165K.
An XBn photovoltaic device has a band profile similar to that of a standard homojunction p-n diode, except that the
depletion region is made from a wide bandgap barrier material with a negligible valence band offset but a large
conduction band offset. In this notation, "X" stands for the n- or p-type contact layer, "B", for the n-type, wide bandgap,
barrier layer, and "n", for the n-type, narrow bandgap, active layer. In this work, we report on the fabrication of XBn
devices, which were grown by Molecular Beam Epitaxy (MBE) on GaSb substrates. Each structure has an InAsSb
active layer of thickness ~1.5μm and a 0.2-0.5μm thick AlSbAs barrier layer. Good growth uniformity was achieved
with lattice matching of better than 500ppm. Selected layers have been processed into devices which operate with a
high internal quantum efficiency at a bias of ~0.1-0.2V, and which exhibit a very low dark current due to the strong
suppression of the current component due to bulk Generation-Recombination processes. From dark current
measurements, a minority carrier lifetime of >670nS has been estimated in devices with an active layer doping of
~4×10<sup>15</sup>cm<sup>-3</sup>. In optimized, lattice matched, devices with this doping and an active layer thickness of 4μm, a cut-off
wavelength of ~ 4.0 - 4.1μm is expected at 160K, with a dark current density of ~10<sup>-6</sup> A cm<sup>-2</sup> and a quantum efficiency
of >70% (λ<4μm). These figures correspond to BLIP operation at 160K with a photocurrent to dark current ratio of ~4
Recently, a new "XB<i>n</i>" device architecture, based on heterostructures, has been proposed as an alternative to a
homojunction photodiode. The main difference is that no depletion layer exists in any narrow bandgap region of the
device. Instead, the depletion layer is confined to a wide bandgap barrier material. The Generation-Recombination (G-R)
contribution to the dark current is then almost totally suppressed and the dark current becomes diffusion limited.
This lowering of the dark current allows the device operating temperature to be raised relative to that of a standard
photodiode made from the same photon absorbing material, with essentially no loss of performance. At SCD we have
been developing XB<i>n</i> devices grown on GaSb substrates with an InAsSb photon absorbing layer and an AlSbAs barrier
layer. The results of optical and electrical measurements are presented on devices with a bandgap wavelength of about
4.1μm. Strong suppression of the G-R current is demonstrated over a range of almost two orders of magnitude in the
doping of the photon absorbing active layer (AL), while at the same time very high internal quantum efficiencies are
achieved. A model of the spectral response is developed which can reproduce the observed behaviour very well at 88K
and 150K over the whole AL doping range. In properly optimized devices, the BLIP temperature is shown to be in the
region of 160K at <i>f</i>/3.
Detectors composed of novel Antimonide Based Compound Semiconductor (ABCS) materials offer some unique
advantages. InAs/GaSb type II superlattices (T2SL) offer low dark currents and allow full bandgap tunability from the
MWIR to the VLWIR. InAs<sub>1-x</sub>Sb<sub>x</sub> alloys (x~0.1) also offer low dark currents and can be used to make MWIR devices
with a cut-off wavelength close to 4.2μm. Both can be grown on commercially available GaSb substrates and both can
be combined with lattice matched GaAlSbAs barrier layers to make a new type of High Operating Temperature (HOT)
detector, known as an XBn detector. In an XBn detector the Generation-Recombination (G-R) contribution to the dark
current can be suppressed, giving a lower net dark current, or allowing the same dark current to be reached at a higher
temperature than in a conventional photodiode. The ABCS program at SCD began several years ago with the
development of an epi-InSb detector whose dark current is about 15 times lower than in standard implanted devices.
This detector is now entering production. More recently we have begun developing infrared detectors based both on
T2SL and InAsSb alloy materials. Our conventional photodiodes made from T2SL materials with a cut-off wavelength
in the region of 4.6μm exhibit dark currents consistent with a BLIP temperature of ~ 120-130K at f/3. Characterization
results of the T2SL materials and diodes are presented. We have also initiated a program to validate the XBn concept
and to develop high operating temperature InAsSb XBn detectors. The crystallographic, electrical and optical properties
of the XBn materials and devices are discussed. We demonstrate a BLIP temperature of ~ 150K at f/3.
Accurate and reliable numerical simulation tools are necessary for the development of advanced semiconductor devices.
SCD is using the Silvaco Atlas simulation tool to simultaneously solve the Poisson, Continuity and transport equations
for 3D detector structures.
In this work we describe a set of systematic experiments performed in order to calibrate the Atlas simulation to SCD's
backside illuminated InSb focal plane arrays (FPA) realized with planar technology. From these experiments we extract
physical parameters such as diffusion length, surface recombination velocity, and SRH lifetime. The actual and predicted
performance (e.g. dark-current and MTF) of present and future detectors is presented.
We have studied arrays with pitch in the range of 15 to 30 μm. We find that the MTF width is inversely proportional to
the pitch. Thus, the spatial resolution of the detector improves with decreasing pixel size as expected. Using the Atlas
simulation we predict the performance of planar InSb arrays with smaller pixel dimensions, e.g., 12 and 10 μm.
Over the last decade, SCD has developed and manufactured high quality InSb Focal Plane Arrays (FPAs), that are currently used in different applications worldwide. SCD's production line includes InSb FPAs with mid format (320x256 elements), and large format (640x512 elements), all available in various packaging configurations, including fully integrated Detector-Dewar-Cooler Assemblies (DDCA). Many of SCD's products are fully customized for customers' needs, and are optimized for each application with respect to the weight, power, size, and performance.
In 2006, SCD has added to its broad InSb product portfolio the new "Pelican" detector family. All Pelican detectors include a large format 640×512 InSb FPA with 15&mgr;m pitch, which is based on the FLIR/Indigo ISC0403 Readout Integrated Circuit (ROIC). Due to its small size, the Pelican FPA fits in any mid format Dewar, enabling upgrading of mid format systems with higher spatial resolution due to its good MTF.
This work presents the high performance of Pelican products. As achieved in all SCD's InSb DDC's, the Pelican detectors demonstrate high uniformity and correctability (residual non uniformity less than 0.05% std/DR) and remarkable operability (typically better than 99.9%). The Pelican FPA can be integrated in various DDCA configurations as per application needs, such as light weight, low power and compact form for hand held imagers, or a rigid configuration for environmentally demanding operating and storage conditions.
The two-dimensional spatial response of a pixel in SCD's back-side illuminated InSb Focal Plane Array (FPA) is
measured directly for arrays with a small pitch, namely 30, 20 and 15&mgr;m. The characterization method uses a spot-scan
measurement and de-convolution algorithm to obtain the net spatial response of a pixel. Two independent methods are
used to measure the detector spatial response: a) direct spot-scan of a pixel with a focused beam; b) uniform illumination
upon back-side evaporated thin gold coating, in which sub-pixel apertures are distributed in precise positions across the
array. The experimental results are compared to a 3D numerical simulation with excellent agreement for all pitch
dimensions. The spatial response is used to calculate the crosstalk and the Modulation Transfer Function (MTF) of the
pixel. We find that for all three pixel dimensions, the net spatial response width (FWHM) is equal to the pitch, and the
MTF width is inversely proportional to the pitch. Thus, the spatial resolution of the detector improves with decreasing
pixel size as expected. Moreover, for a given optics and smaller array pitch, the overall system spatial resolution is
limited more by the optical diffraction than by the detector. We show actual improved spatial resolution in an imaging system with a detector of smaller array pitch.