We designed R-QWIP devices for narrowband 10.6 μm detection. Despite the low doping of 0.2 and 0.3 × 10<sup>18</sup> cm<sup>-3</sup> and thin absorbing layer thickness of 1 μm, the observed QE is 29% and 26%, respectively. This level of QEs, combined with the large photoconductive gain in a thin layer, produces large conversion efficiencies of 20% and 15% for large photocurrents. The high photocurrent and low dark current brought by low doping thus allow the QWIPs to be BLIP at 65 K with an 11 μm cutoff. The projected NETD is 20 mK at 60 K, demonstrating the high performance of R-QWIPs.
We report progress in the development of long wavelength infrared (LWIR) focal plane arrays (FPAs) built on type-II
strained layer InAs/GaSb superlattice materials. Work at Raytheon Vision Systems and Jet Propulsion Laboratory has
led to successful devices with cutoff wavelengths in the 10 to 12 μm range. Pixels have been formed by wet etching
and surface passivation by plasma-deposited silicon dioxide. We present test results on arrays hybridized with indium
bump bonding to silicon readout integrated circuits, as well as analyses of current-voltage characteristics of individual
diodes. In particular, we find that, at temperatures below about 70 K the leakage current is dominated by generation-recombination
effects near zero bias and by trap-assisted tunneling in reverse bias. Although other authors have
demonstrated imaging for SWIR and MWIR type-II superlattice devices, to our knowledge no one has done so prior to
2006 in the LWIR range. We have obtained both still and video imaging with 256×256 arrays with 30-μm pixels
operating at 78 K, having high operability and a cutoff wavelength of 10.5 μm.
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.
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 1990s saw the rapid evolution of staring IR focal plane arrays (FPAs), with array formats progressing from 128 by 128 arrays at the beginning of the decade, to 1K by 1K arrays in low-rate production at the end of the decade. The maturation of large-format staring FPAs has given astronomers new capabilities for wide-field, high-resolution imaging and spectroscopy. The trends that emerged in the 1990s are continuing with larger format FPAs currently under development.
This paper will describe a 270 X 436 HgCdTe FPA/module that was developed for the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) H and M instruments. Raytheon Infrared Operations was selected by Officine Galileo and the Observatorie de Paris, Meudon to design, fabricate and deliver 4 flight modules for the VIRTIS H and M spectrometers.