Detectivity of mid-wave infrared (MWIR) detectors based on InAs/GaSb type II strained layer superlattices (T2SLs) can be significantly enhanced at select wavelengths by integrating the detector with a back-side illuminated plasmonic coupler. The application of a simple metal-T2SL structure directly on the GaSb substrate can result in radiation losses into the substrate due to the low refractive index of T2SL layer. However, insertion of a higher refractive index material, such as germanium (Ge), into the metal-SLS structure can confine the surface plasmon waveguide (SPW) modes to the surface. In this work, metal (Au)-Ge-T2SL structures are designed with an approximately 100 nm thick Ge layer. The T2SL layer utilized a p-i-n detector design with 8 monolayers (MLs) InAs/8 MLs GaSb. A plasmonic coupler was then realized inside the 300 μm circular apertures of these single element detectors by the formation of a corrugated metal (Au) surface. The T2SL single element detector integrated with an optimized plasmonic coupler design increased the quantum efficiency (QE) by a factor of three at an operating temperature of 77 K and 3 to 5 μm illumination wavelength, compared to a reference detector structure, and each structure exhibited the same level of dark current.
High-performance large-format detector arrays responsive to the 1-5μm wavelength range of the infrared spectrum
fabricated using large area HgCdTe layers grown on 6-inch diameter (211) silicon substrates are available for advanced
imaging applications. This paper reviews performance and capabilities of Raytheon Vision Systems (RVS) HgCdTe/Si
Focal Plane Arrays (FPA) and shows 2k x 2k format MWIR HgCdTe/Si FPA performance with NEdT operabilities
better than 99.9%. SWIR and MWIR detector performance for HgCdTe/Si is comparable to established performance of
HgCdTe/CdZnTe wafers. HgCdTe devices fabricated on both types of substrates have demonstrated very low dark
current, high quantum efficiency and full spectral band fill factor characteristic of HgCdTe. HgCdTe has the advantage
of being able to precisely tune the detector cutoff via adjustment of the Cd composition in the MBE growth. The
HgCdTe/Si detectors described in this paper are p-on-n mesa delineated architecture and fabricated using the same
mature etch, passivation, and metallization processes as our HgCdTe/CdZnTe line. Uniform device quality HgCdTe
epitaxial layers and application of detector fabrication processes across the full area of 6-inch wafers routinely produces
high performing detector pixels from edge to edge of the photolithographic limits across the wafer, offering 5 times the
printable area as costly 6×6cm CdZnTe substrates. This 6-inch HgCdTe detector wafer technology can provide
applications demanding very wide FOV high resolution coverage the capability to produce a very large single piece
infrared detector array, up to a continuous image plane 10×10 cm in size. Alternatively, significant detector cost
reduction through allowing more die of a given size to be printed on each wafer is possible, with further cost reduction
achieved through transition towards automated detector fabrication and photolithographic processes for both increased
yields and reduced touch labor costs. RVS continues to improve its FPA manufacturing line towards achieving low cost
infrared FPAs with the format, size, affordability, and performance required for current and future infrared applications.
We report on dual-band (mid-/long-wave infrared) InAs/GaSb strained layer superlattice detector
with nBn and pBp architectures. Two band response was registered with 50% cut-off wavelengths of 5μm
(both nBn and pBp detectors) and 9μm (nBn)/10μm (pBp). The maximum peak responsivity of MWIR
absorber equal to 1.6 A/W (at λ = 5 μm and V<sub>b</sub> = +1 V) and LWIR absorber equal to 1.2 A/W (at λ = 10 μm
and V<sub>b</sub> = -1 V) for nBn detector, with the corresponding values of D* were 1.2 x 10<sup>11</sup> Jones and 1.2 x 10<sup>10</sup>Jones for MWIR and LWIR absorbers, respectively (77 K). The maximum values of quantum efficiency were
estimated to 36% (MWIR) and 15% (LWIR) at V<sub>b</sub> = +1V and V<sub>b</sub> = -1V. For pBp detector, the responsivity
equal to 1.6 A/W (at λ = 5 μm and V<sub>b</sub> = +0.4 V) and 1.8 A/W (at λ = 9 μm and V<sub>b</sub> = -0.7 V) for MWIR and
LWIR absorbers was achieved with corresponding values of specific detectivity 5 x 10<sup>11</sup> Jones and 2.6 x 10<sup>10</sup>Jones, respectively. The maximum values of quantum efficiency were estimated to 41% (MWIR) and 25%
(LWIR) at V<sub>b</sub> = +0.4V and V<sub>b</sub> = -0.7V. Moreover, the diffusion-limited behavior of dark current at higher
temperatures was observed for MWIR absorber for pBp detector. The overall performance of the dual-band
InAs/GaSb SLS detectors with investigated designs showed comparable (nBn design) and superior (pBp
design) performance to the QWIP detectors both in MWIR and LWIR bands and comparable performance to
MCT detectors in MWIR band (nBn and pBp detector designs).
We report on the investigation of lateral diffusion of minority carriers in InAsSb based photodetectors with
the nBn design. Diffusion lengths (DL) were extracted from temperature dependent I-V measurements. The
behavior of DL as a function of applied bias, temperature, and composition of the barrier layer was
investigated. The obtained results suggest that lateral diffusion of minority carriers is not the limiting factor
for InAsSb based nBn MWIR detector performance at high temperatures (> 200K). The detector with an As
mole fraction of 10% in the barrier layer has demonstrated values of DL as low as 7 μm (V<sub>b</sub> = 0.05V) at 240K.
Oceanit Laboratories Inc. is collaborating with Raytheon Vision Systems (RVS) to develop a novel HgCdTe-based
position sensitive detector (PSD) that can ultimately be implemented in target detection and tracking or target
interception applications in the infrared spectral region.
The development of InAsSb detectors based on the nBn design for the mid-wave infrared (MWIR) spectral region is
discussed. Comparisons of optical and electrical properties of InAsSb photodetectors with two different barrier material,
namely, AlAs <sub>0.15S</sub>b<sub>0.75</sub> (structure A) and AlAs<sub>0.10S</sub>b<sub>0.9</sub> (structure B) are reported. The dark current density in the
AlAs0.15Sb0.85 is lower possibly due to the larger valence band offset. Clear room temperature spectral responses is
observed and a specific detectivity (D*) of 1.4x10<sup>12</sup> and 1.01x10<sup>12</sup> cmHz<sup>1/2</sup>/W at 0.2 V, and a responsivity of 0.87 and
1.66 A/W under 0.2 V biasing at 77 K and 3.5 μm, assuming unity gain, was obtained for structures A and B,
This paper reviews the historical progress of HgCdTe material and device development at Raytheon Vision Systems
starting with the initial work in 1965 at what was then the Santa Barbara Research Center, a subsidiary of the Hughes
Aircraft Company and progressing up to the present time. Because of the long history, all the details cannot be presented
in a single paper; instead, we focus only on a few major accomplishments. In HgCdTe material preparation these
include: the early bulk single crystal growth methods; the advent of liquid phase epitaxial growth from Hg melts; and,
the most recent molecular beam epitaxial methods. For IR photodetector devices, we started with just single element
detectors operating either in photoconductive or photovoltaic mode, then progressed to multi-element linear arrays, then
to 2-D arrays on Si read-out circuits and, finally to the very large focal plane (>2k × 2k), dual-band, and APD arrays of
today. Some applications of these devices in IR systems will be presented. Technical issues will be discussed only to
the extent necessary to support the historical narrative. Some interesting anecdotes will be included.
Raytheon Vision Systems (RVS) has developed and demonstrated the first-ever 1280 x 720 pixel dual-band MW/LWIR
focal plane arrays (FPA) to support 3rd-Generation tactical IR systems under the U.S. Army's Dual-Band FPA
Manufacturing (DBFM) program. The MW/LWIR detector arrays are fabricated from MBE-grown HgCdTe triple-layer
heterojunction (TLHJ) wafers. The RVS dual-band FPA architecture provides highly simultaneous temporal detection in
the MWIR and LWIR bands using time-division multiplexed integration (TDMI) incorporated into the readout integrated
circuit (ROIC). The TDMI ROIC incorporates a high degree of integration and output flexibility, and supports both
dual-band and single-band full-frame operating modes, as well as high-speed LWIR "window" operation at 480 Hz
frame rate. The ROIC is hybridized to a two-color detector array using a single indium interconnect per pixel, which
makes it highly producible for 20 μm unit cells and exploits mature fabrication processes currently used to produce
single-color FPAs. High-quality 1280 x 720 MW/LWIR FPAs have been fabricated and excellent dual-band imagery
produced at 60 Hz frame rate. The 1280 x 720 detector arrays for these FPAs have LWIR cutoff wavelengths ≥10.5 μm
at 78K. These FPAs have demonstrated high-sensitivity at 78K with MW NETD values < 20 mK and LW NETD values
<30 mK with f/3.5 apertures. Pixel operability greater than 99.9% has been achieved in the MW band and greater than
98% in the LW band.
Raytheon Vision Systems (RVS) is developing two-color and large format single color FPAs fabricated from molecular beam epitaxy (MBE) grown HgCdTe triple layer heterojunction (TLHJ) wafers on CdZnTe substrates and double layer heterojunction (DLHJ) wafers on Si substrates, respectively. MBE material growth development has resulted in scaling TLHJ growth on CdZnTe substrates from 10cm<sup>2</sup> to 50cm<sup>2</sup>, long-wavelength infrared (LWIR) DLHJ growth on 4-inch Si substrates and the first demonstration of mid-wavelength infrared (MWIR) DLHJ growth on 6-inch Si substrates with low defect density (<1000cm<sup>-2</sup>) and excellent uniformity (composition<0.1%, cut-off wavelength Δcenter-edge<0.1μm). Advanced FPA fabrication techniques such as inductively coupled plasma (ICP) etching are being used to achieve high aspect ratio mesa delineation of individual detector elements with benefits to detector performance. Recent two-color detectors with MWIR and LWIR cut-off wavelengths of 5.5μm and 10.5μm, respectively, exhibit significant improvement in 78K LW performance with >70% quantum efficiency, diffusion limited reverse bias dark currents below 300pA and RA products (zero field-of-view, +150mV bias) in excess of 1×103 Ωcm<sup>2</sup>. Two-color 20μm unit-cell 1280×720 MWIR/LWIR FPAs with pixel response operability approaching 99% have been produced and high quality simultaneous imaging of the spectral bands has been achieved by mating the FPA to a readout integrated circuit (ROIC) with Time Division Multiplexed Integration (TDMI). Large format mega pixel 20μm unit-cell 2048×2048 and 25μm unit-cell 2560×512 FPAs have been demonstrated using DLHJ HgCdTe growth on Si substrates in the short wavelength infrared (SWIR) and MWIR spectral range. Recent imaging of 30μm unit-cell 256×256 LWIR FPAs with 10.0-10.7μm 78K cut-off wavelength and pixel response operability as high as 99.7% show the potential for extending HgCdTe/Si technology to LWIR wavelengths.
Raytheon Vision Systems (RVS) is developing two-color, large-format infrared FPAs to support the US Army's Third Generation FLIR systems. RVS has produced 640 x 480 two-color FPAs with a 20 micron pixel pitch. Work is also underway to demonstrate a 1280 x 720 two-color FPA in 2005. The FPA architecture has been designed to achieve nearly simultaneous temporal detection of the spectral bands while being producible for pixel dimensions as small as 20 microns. Raytheon's approach employs a readout integrated circuit (ROIC) with Time Division Multiplexed Integration (TDMI). This ROIC is coupled to bias-selectable two-color detector array with a single contact per pixel. The two-color detector arrays are fabricated from MBE-grown HgCdTe triple layer heterojunction (TLHJ) wafers. The single indium bump design is producible for 20 μm unit cells and exploits mature fabrication processes that are in production at RVS for Second Generation FPAs. This combination allows for the high temporal and spatial color registration while providing a low-cost, highly producible and robust manufacturing process. High-quality MWIR/LWIR (M/L) 640 x 480 TDMI FPAs with have been produced and imaged from multiple fabrication lots. These FPAs have LWIR cutoffs ranging to 11 micron at 78K. These 20 micron pixel FPAs have demonstrated excellent sensitivity and pixel operabilities exceeding 99%. NETDs less than 25 mK at f/5 have been demonstrated for both bands operating simultaneously.
HgCdTe offers significant advantages over other semiconductors which has made it the most widely utilized variable-gap material in infrared focal plane array (FPA) technology. However, one of the main limitations of the HgCdTe materials system has been the size of lattice-matched bulk CdZnTe substrates, used for epitaxially-grown HgCdTe, which are 30 cm<sup>2</sup> in size for production and have historically been difficult and expensive to scale in size. This limitation does not adequately support the increasing demand for larger FPA formats which now require sizes up to and beyond 2048 x 2048 and only a single die can be printed per wafer. Heteroepitaxial Si-based substrates offer a cost-effective technology that can be more readily scaled to large wafer sizes. Most of the effort in the IR community in the last 10 years has focused on growing HgCdTe directly on (112)Si substrates by MBE. At Raytheon we have scaled the MBE (112)HgCdTe/Si process originally developed at HRL for 3-in wafers, first to 4-in wafers and more recently to 6 in wafers. We have demonstrated a wide range of MWIR FPA formats up to 2560 x 512 in size and have found that their performance is comparable to arrays grown on bulk CdZnTe substrates by either MBE or LPE techniques. More recent work is focused on extending HgCdTe/Si technology to LWIR wavelengths. The goal of this paper is to review the current status of HgCdTe/Si technology both at Raytheon and the published work available from other organizations.
The Navy faces an ever evolving threat scenario, ranging from sub-sonic sea skimming cruise missiles to newer, unconventional threats such as that experienced by the USS Cole. Next generation naval technology development programs are developing “stealthy” ships by reducing a ships radar cross section and controlling electromagnetic emissions. To meet these threat challenges in an evolving platform environment, ONR has initiated the “Wide Aspect MWIR Array” program. In support of this program, Raytheon Vision Systems (RVS) is developing a 2560 X 512 element focal plane array, utilizing Molecular Beam Epitaxially grown HgCdTe on silicon detector technology. RVS will package this array in a sealed Dewar with a long-life cryogenic cooler, electronics, on-gimbal power conditioning and a thermal reference source. The resulting sub system will be a component in a multi camera distributed aperture situation awareness sensor, which will provide continuous surveillance of the horizon. We will report on the utilization of MWIR Molecular Beam Epitaxial HgCdTe on Silicon material for fabrication of the detector arrays. Detector arrays fabricated on HgCdTe/Si have no thermal expansion mismatch relative to the readout integrated circuits. Therefore large-area focal plane arrays (FPAs) can be developed without concern for thermal cycle reliability. In addition these devices do not require thinning or reticulation like InSb FPAs to yield the high levels of Modulation Transfer Function (MTF) required by a missile warning sensor. HgCdTe/Si wafers can be scaled up to much larger sizes than the HgCdTe/CdZnTe wafers. Four-inch-diameter HgCdTe/Si wafers are currently being produced and are significantly larger than the standard 1.7 inch x 2.6 inch HgCdTe/CdTe wafers. The use of Si substrates also enables the use of automated semiconductor fabrication equipment.
Raytheon Vision Systems (RVS) in collaboration with HRL Laboratories is contributing to the maturation and manufacturing readiness of third-generation two-color HgCdTe infrared staring focal plane arrays (FPAs). This paper will highlight data from the routine growth and fabrication of 256x256 30μm unit-cell staring FPAs that provide dual-color detection in the mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) spectral regions. FPAs configured for MWIR/MWIR, MWIR/LWIR and LWIR/LWIR detection are used for target identification, signature recognition and clutter rejection in a wide variety of space and ground-based applications. Optimized triple-layer-heterojunction (TLHJ) device designs and molecular beam epitaxy (MBE) growth using in-situ controls has contributed to individual bands in all two-color FPA configurations exhibiting high operability (>99%) and both performance and FPA functionality comparable to state-of-the-art single-color technology. The measured spectral cross talk from out-of-band radiation for either band is also typically less than 10%. An FPA architecture based on a single mesa, single indium bump, and sequential mode operation leverages current single-color processes in production while also providing compatibility with existing second-generation technologies.
Since its initial synthesis and investigation more than 40 years ago, the HgCdTe alloy semiconductor system has evolved into one of the primary infrared detector materials for high-performance infrared focal-plane arrays (FPA) designed to operate in the 3-5 mm and 8-12 mm spectral ranges of importance for thermal imaging systems. Over the course of the past decade, significant advances have been made in the development of thin-film epitaxial growth techniques, such as molecular-beam epitaxy (MBE), which have enabled the synthesis of IR detector device structures with complex doping and composition profiles. The central role played by in situ sensors for monitoring and control of the MBE growth process are reviewed. The development of MBE HgCdTe growth technology is discussed in three particular device applications: avalanche photodiodes for 1.55 +m photodetection, megapixel FPAs on Si substrates, and multispectral IR detectors.
Significant progress has been made in the technology for MBE growth of HgCdTe infrared focal-plane arrays on Si substrates since the initial demonstration of MBE HgCdTe-on- Si heteroepitaxy in 1989. In 1995, the first all-MBE-grown detector arrays on Si were produced through direct MBE growth of (112)B-oriented II-VI films on Si without III-V initiation layers, culminating in detector performance comparable to LPE-grown detectors on bulk CdZnTe substrates. This achievement was enabled by the development of two key contributing technologies: CdTe on Si buffer layer growth and HgCdTe p-on-n double-layer heterojunction growth using p-type chemical doping with As. The MBE process for deposition of high crystalline quality CdTe buffer layers has been developed so that x-ray rocking curve FWHM less than 75 arc-sec and near-surface etch pit densities (EPD) of 2 multiplied by 10<SUP>6</SUP> cm<SUP>-2</SUP> are routinely achievable for 9-micrometer-thick CdTe buffer layers. The dependence of CdTe EPD on ZnTe initiation layer thickness, insertion of CdTe/CdZnTe strained layer superlattices, and thermal cycling to cryogenic temperatures has been investigated and is reviewed. HgCdTe baselayers deposited by MBE on these CdTe/Si composite substrates exhibit x-ray FWHM as low as 72 arc-sec and EPD of 3 - 20 multiplied by 10<SUP>6</SUP> cm<SUP>-2</SUP>. To demonstrate the potential for MBE growth of large-area HgCdTe FPAs on Si, detectors with 78 K cutoff wavelength of 7.8 micrometer have been fabricated in this HgCdTe/Si epitaxial material with array-average R<SUB>0</SUB>A product of 1.64 multiplied by 10<SUP>4</SUP> (Omega) -cm<SUP>2</SUP> (0 FOV).
To achieve the DoD objective of low cost high performance infrared focal plane arrays a manufacturing technique is required which is intrinsically flexible with respect to device configuration and cutoff wavelength and easily scaleable with respect to volume requirements. The approach adopted is to fully develop the technology of molecular beam epitaxy (MBE) to a level where detector array wafers with a variety of configurations can be fabricated with first pass success at a reduced cost. As a vapor phase process, MBE lends itself directly to: (1) the inclusion of real-time monitoring and process control, (2) a single or multiple wafer growth mode, (3) nearly instantaneous changes in growth parameters. A team has been assembled to carry out the program. It is composed of four industrial organizations -- Rockwell International, Hughes Aircraft Company, Texas Instruments, and Lockheed-Martin, and a university -- Georgia Tech Research Institute. Since team members are committed suppliers and users of IRFPAs, technology transfer among team members is accomplished in real-time. The technical approach has been focused on optimizing the processes necessary to fabricate p-on-n HgCdTe double layer heterostructure focal plane arrays, reducing process variance, and on documenting flexibility with respect to cutoff wavelength. Two device structures have been investigated and fabricated -- a 480 by 4 and a 128 by 128.
HgCdTe MBE technology offers many advantages for the growth of multi-layer heterojunction structures for high performance IRFPAs. This paper reports data on major advances towards the fabrication of advanced detector structures, which have been made in MBE technology at Hughes Research Laboratories during the last couple of years. Currently device quality materials with desired structural and electrical characteristics are grown with the alloy compositions required for short-wavelength infrared (SWIR, 1 - 3 micron) to very long- wavelength infrared (VLWIR, 14 - 18 micron) detector applications. In-situ In (n-type) and As (p-type) doping developed at HRL have facilitated the growth of advanced multi-layer heterojunction devices. Thus, high performance IR focal plane arrays (128 X 128) with state-of-the-art performance have been fabricated with MBE-grown double-layer heterojunction structures for MWIR and LWIR detector applications. In addition, the growth of n-p-p-n multi-layer heterojunction structures has been developed and two-color detectors have been demonstrated. Recently, significant preliminary results on the heteroepitaxy growth of HgCdTe double-layer heterojunction structures on silicon have been achieved.
Molecular-beam epitaxy (MBE) has been utilized to deposit single crystal epitaxial films of CdTe(112)B and HgCdTe(112)B directly onto Si(112) substrates without the use of GaAs interfacial layers. The films have been characterized with x-ray diffraction and wet chemical defect etching, and IR detectors have been fabricated and tested. CdTe(112)B films are twin- free and have x-ray rocking curves as narrow as 72 arc-seconds and near-surface etch pit density (EPD) of 2 X 10<SUP>6</SUP> cm<SUP>-2</SUP> for 8 micrometers -thick films. HgCdTe(112)B films deposited on Si substrates have x-ray rocking curve FWHM as low as 92 arc-seconds and EPD of 8 - 30 X 10<SUP>6</SUP> cm<SUP>-2</SUP>. HgCdTe/Si infrared detectors have been fabricated with R<SUB>0</SUB>A equals 4.3 X 10<SUP>3</SUP> (Omega) -cm<SUP>2</SUP> (f/2 FOV) and 7.8 micrometers cutoff wavelength at 78 K to demonstrate the capability of MBE for growth of large-area HgCdTe arrays on Si.
Molecular-beam epitaxy (MBE) has been utilized to deposit single crystal films of ZnTe and CdZnTe/ZnTe onto Si(100) and Si(112) substrates. Parallel epitaxy of ZnTe(100) and CdZnTe(100)/ZnTe(100) has been observed for growth on Si(100) substrates misoriented from 0-8 degrees towards the  direction. With ZnTe initiation layers, high quality CdZnTe(100) films have been demonstrated on both 4° and 8° misoriented Si(100) with x-ray rocking curve FWHM as narrow as 158 arc-seconds, which is comparable to that obtained with GaAs/Si composite substrates. The observed surface morphologies are superior to those obtained on GaAs/Si composite substrates. HgCdTe(100) films with x-ray FWHM as low as 55 arcseconds and average etch pit densities of 5 x 106 cm2 have been deposited by liquid phase epitaxy on these MBE CdZnTe/ZnTe/Si(100) substrates. On vicinal Si(1 12) substrates, ZnTe films are observed to nucleate in either the (1 12) or its twin (552) orientation depending on the misorientation of the Si substrate away from (1 12). For Si(1 12) misorientations of 5° or 10° towards from the [1 1-1] direction, ZnTe nucleates in a parallel (1 12) orientation, while for misorientations of 0° or 5° away from the [1 1-1] direction, ZnTe is observed to nucleate in a (552) orientation. CdTe deposited on ZnTe/Si(112) is observed to nucleate in the same orientation as the ZnTe. CdTe(552) epilayers are of substantially higher quality than (1 12)oriented films. X-ray rocking curves as narrow as 1 10 arc-seconds have been observed for the CdTe(331) reflection in the case of (552)-oriented epitaxy.