The main driving force for High Operating Temperature (HOT) detectors is the strong need for low cost, compact IR imaging solution capable of supporting a wide range of military and civilian applications. In the HOT regime where imagers can be cooled with multi-stage thermoelectric coolers, the major portion of the cost is due to the die-level back-end process, from the chip hybridization to final packaging. We present here an approach to achieve significant cost reduction of MWIR imagers by monolithically integrating III-V devices directly on Silicon substrates for wafer-scale fabrication and packaging of focal plane arrays (FPAs). High quality InAs films can be grown on a blanket Silicon wafer by metal-organic chemical vapor deposition (MOCVD) in a low growth temperature regime that complies with the thermal budget of the Si-electronics. High Resolution Transmission Electron Microscopy reveals predominantly oriented, single-crystal-like InAs films, with Σ3(111) twin boundaries, which our band structure calculations predict to be electrically benign. More intriguingly, selective-area growth on SiO<sub>2</sub>-masked ROIC-like templates is demonstrated with single-crystal-like InAs film nucleation at small Si(001) openings, together with the suppression of unwanted deposition on the dielectric mask. High crystallinity lateral epitaxial overgrowth of the InAs islands and film coalescence is achieved, enabling the potential to fully cover the entire patterned substrate. MBE-grown MWIR devices (λcut-off = 4.1 μm) on blanket InAs/Si templates exhibit a dark current of 2x10<sup>-5</sup> A/cm<sup>2</sup> , a specific detectivity of 6x1011 Jones and a quantum efficiency (QE) above 60% at 100K. The QE remains constant at high temperatures (<200K) where the dark current approaches that of baseline single-crystal HOT devices grown on native substrates At 230K, it is 6x10<sup>-2</sup> A/cm<sup>2</sup>, yielding a specific detectivity of 10<sup>10</sup> Jones.
Recent advances over the last several years in III-V strained-layer superlattice-based infrared detectors have lead this material system to emerge as a solid alternative to HgCdTe for dual-band focal plane arrays (FPAs). Rapid development of superlattice-based detectors has been realized by capitalizing on mature, III-V foundry-compatible processing. Furthermore, superlattice-based epitaxial wafers exhibit a high degree of lateral uniformity with low macroscopic defect densities (< 50 cm<sup>-2</sup>) and can achieve dark current levels comparable to HgCdTe detectors. In this paper, we review our recent efforts towards producing HD-format (1280x720, 12 μm pitch) superlattice-based, dual-band MWIR/LWIR FPAs. For a representative FPA, characterization was conducted in a pour-fill dewar at 80K, f/3 and using a blackbody range of 22°C to 32°C. For the MWIR band, the noise equivalent temperature difference (NETD) was 14.9 mK with a 3x median NETD operability of 99.91%. For the LWIR band, the median NETD was 28.1 mK with a 3x median NETD operability of 99.66%. To illustrate the manufacturability of superlattice technology, we will present results on 1280x720, 12 μm pitch MWIR/LWIR FPAs built over the last year at HRL through multiple fabrication lots utilizing 4" epiwafers.
Recent advances in superlattice-based infrared detectors have rendered this material system a solid alternative to HgCdTe for dual-band sensing applications. In particular, superlattices are attractive from a manufacturing perspective as the epitaxial wafers can be grown with a high degree of lateral uniformity, low macroscopic defect densities (< 50 cm<sup>-2</sup>) and achieve dark current levels comparable to HgCdTe detectors. In this paper, we will describe our recent effort on the VISTA program towards producing HD-format (1280x720, 12 μm pitch) superlattice based, dual-band MWIR/LWIR FPAs. We will report results from several multi-wafer fabrication lots of 1280x720, 12 μm pitch FPAs processed over the last two years. To assess the FPA performance, noise equivalent temperature difference (NETD) measurements were conducted at 80K, f/4.21 and using a blackbody range of 22°C to 32°C. For the MWIR band, the NETD was 27.44 mK with a 3x median NETD operability of 99.40%. For the LWIR band, the median NETD was 27.62 mK with a 3x median operability of 99.09%. Over the course of the VISTA program, HRL fabricated over 30 FPAs with similar NETDs and operabilities in excess of 99% for both bands, demonstrating the manufacturability and high uniformity of III-V superlattices. We will also present additional characterization results including blinkers, spatial stability, modulation transfer function and thermal cycles reliability.
Recent efforts in developing InAs/GaSb strained-layer superlattices for LWIR detectors are
described. The structural properties of the devices grown by MBE at HRL were evaluated using
optical microscopy, x-ray diffraction, and atomic force microscopy. Epilayer roughness and surface
morphology are briefly described. Small format focal plane arrays were fabricated to serve as a
baseline for device study, and to determine the effects of underfill epoxy on detector performance. A
novel approach for epilayer transfer on silicon is also presented.
InAs/GaSb-based type II superlattices (T2SL) offer a manufacturable FPA technology
with FPA size, scalability and cost advantages over HgCdTe. Work at Jet Propulsion
Laboratory (JPL), Naval Research Laboratory (NRL), and Northwestern University
(NWU) has shown that the performance gap between HgCdTe and T2SL FPAs has
narrowed to within 5-10x over the last two years<sup>1,2,3</sup>. Due to the potential of T2SL
technology for fabrication of large format (> 1k x1k) and dual-band arrays, HRL has
recently resurrected efforts in this area<sup>4</sup>. We describe the progress on the FastFPA
program funded by the Army Night Vision Labs towards the development of detectors
and focal plane arrays (FPAs). Progress made in the areas of MBE growth, mesa diode
fabrication, dry etch processing, and FPA fabrication over the last one year is presented.
We have evaluated selective doping techniques for the fabrication of type II LWIR superlattice planar
detectors. Ion-implantation and diffusion of dopants were evaluated for selective doping of the electrical
junction region in planar photodiodes. Residual damage remains when superlattice structures are implanted
with Te ions with an energy of 190 keV and a dose of 5x10<sup>13</sup> cm<sup>-2</sup>, at room temperature. Controlled Zn
diffusion profiles with concentrations from 5x10<sup>16</sup> to > 5x10<sup>18</sup> cm<sup>-3</sup> in the wide bandgap cap layer was
achieved through a vapor phase diffusion technique. Planar p-on-n diodes were fabricated using selective
Zn diffusion. The I-V characteristics were leaky due to G-R and tunneling in the homojunction devices, for
which no attempts were made to optimize the n-type absorber doping level. Work is underway for the
implementation of planar diodes with the n-on-p architecture through selective Te diffusion. Due to
increased minority carrier lifetimes for p-type InAs/GaSb superlattice absorber layers, planar device with
the n-on-p architecture have the potential to provide improved performance as compared to the p-on-n
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.
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.