Stacy A. Masterjohn

Cross-track infrared sounder FPAA performance

The Cross-track Infrared Sounder (CrIS), an interferometric sounder, is one of the instruments within the National Polar-orbiting Operational Environmental Satellite System (NPOESS) suite. CrIS measures earth radiances at high spectral resolution providing accurate and high-resolution pressure, temperature and moisture profiles of the atmosphere. These profiles are used in weather prediction models to track storms, predict levels of precipitation etc. Each CrIS instrument contains three Focal Plane Array Assemblies (FPAAs): SWIR [λc(98 K) ~ 5 mm], MWIR [λc(98 K) ~ 9 mm], and LWIR [λc(81 K) ~ 16 mm]. Each FPAA consists of nine large (850-mm-diameter) photovoltaic detectors arranged in a 3 x 3 pattern, with each detector having an accompanying cold preamplifier. This paper describes the selection methodology of the detectors that constitute the FPAAs and the performance of the CrIS SWIR, MWIR and LWIR proto-flight FPAAs. The appropriate bandgap n-type Hg1-xCdxTe was grown on lattice-matched CdZnTe. 850-mm-diameter photodiodes were manufactured using a Lateral Collection Diode (LCD) architecture. Custom pre-amplifiers were designed and built to interface with these large photodiodes. The LWIR, MWIR and SWIR detectors are operated at 81 K, 98 K and 98 K respectively. These relatively high operating temperatures permit the use of passive radiators on the instrument to cool the detectors. Performance goals are D* = 5.0 x 1010 cm-Hz1/2/W at 14.0 mm, 9.3 x 1010 cm-Hz1/2/W at 8.0 mm and 3.0 x 1011 cm-Hz1/2/W at 4.64 mm. Measured mean values for the nine photodiodes in each of the LWIR, MWIR and SWIR FPAAs are D* = 5.3 x 1010 cm-Hz1/2/W at 14.0 mm, 1.0 x 1011 cm-Hz1/2/W at 8.0 mm and 3.1 x 1011 cm-Hz1/2/W at 4.64 mm. These compare favorably with the following BLIP D* values calculated at the nominal flux condition: D* = 8.36 x 1010 cm Hz1/2/W at 14.0 mm, 1.4 x 1011 cm-Hz1/2/W at 8.0 mm and 4.1 x 1011 cm-Hz1/2/W at 4.64 mm.

Characterization of flight detector arrays for the wide-field infrared survey explorer

The Wide-field Infrared Survey Explorer is a NASA Midex mission launching in late 2009 that will survey the entire sky at 3.3, 4.7, 12, and 23 microns (PI: Ned Wright, UCLA). Its primary scientific goals are to find the nearest stars (actually most likely to be brown dwarfs) and the most luminous galaxies in the universe. WISE uses three dichroic beamsplitters to take simultaneous images in all four bands using four 1024×1024 detector arrays. The 3.3 and 4.7 micron channels use HgCdTe arrays, and the 12 and 23 micron bands employ Si:As arrays. In order to make a 1024×1024 Si:As array, a new multiplexer had to be designed and produced. The HgCdTe arrays were developed by Teledyne Imaging Systems, and the Si:As array were made by DRS. All four flight arrays have been delivered to the WISE payload contractor, Space Dynamics Laboratory. We present initial ground-based characterization results for the WISE arrays, including measurements of read noise, dark current, flat field and latent image performance, etc. These characterization data will be useful in producing the final WISE data product, an all-sky image atlas and source catalog.

Far-infrared detector development for space-based Earth observation

DRS Sensors & Targeting Systems with silicon materials partner Lawrence Semiconductor Research Laboratory and development partner NASA Langley Research Center Earth Science Directorate are developing improved far-infrared detectors for Earth energy balance observations from orbit. Our team has succeeded in demonstrating the feasibility of extending the wavelength range of conventional arsenic-doped-silicon Blocked Impurity Band (BIB) detectors (cut-off ~28 μm) into the far infrared. The new far-IR member of the BIB detector family operates at temperatures accessible to existing space-qualified cryocoolers, while retaining the very high values of sensitivity, stability, linearity, and bandwidth typical of the broader class of silicon BIB detectors. The new detector should merit serious consideration for the Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission defined by the recent National Research Councils Decadal Survey for Earth Science. Proposed further development of this detector technology includes wavelength extension to a goal of at least 100 μm, improvements in detector design, and implementation of light-trapping packaging. These are developments that will enable increased radiometric accuracy, reduced spatial smearing, and simpler calibration approaches for CLARREO.

Noise in large-area CrlS Hg1-xCdxTe photovoltaic detectors

The National Polar-orbiting Operational Environmental Satellite System (NPOESS) Cross-track Infrared Sounder (CrIS) is a Fourier Transform interferometric sensor that measures earth radiances at high spectral resolution. Algorithms use the data to provide pressure, temperature, and moisture profiles of the atmosphere. The CrIS instrument contains photovoltaic detectors with spectral cut-offs denoted by SWIR, MWIR and LWIR. The CrIS instrument requires large-area, photovoltaic detectors with state-of-art detector performance at temperatures attainable with passive cooling. For example, detectors as large as 1 mm in diameter are required. To address these needs, Molecular Beam Epitaxy (MBE) is used to grow the appropriate bandgap n-type Hg1-xCdxTe on lattice matched CdZnTe. The p-side is obtained via arsenic implantation followed by appropriate annealing steps.

WISE focal plane module lessons learned in light of success

DRS Sensors & Targeting Systems, under contract to the Space Dynamics Laboratory of Utah State University, provided the focal plane detector system for NASAs Wide-field Infrared Survey Explorer (WISE). The focal plane detector system consists of two mercury cadmium telluride (MCT) focal plane module assemblies (FPMAs), two arsenic doped silicon (Si:As) Blocked Impurity Band (BIB) FPMAs, electronics to drive the FPMAs and report digital data from them, and the cryogenic and ambient temperature cabling that connect the FPMAs and electronics. The WISE Satellite was launched in late 2009 and has been a very rewarding success. In light of the recent success on orbit, there were many challenges and hurdles the DRS team had to overcome in order to guarantee the ultimate success of the instrument. This report highlights a few of the challenges that the team overcame in hopes that the information can be made available to the astronomy community for future use.

1/f noise in Hg1-xCdxTe detectors

This paper investigates 1/f noise performance of Hg1-xCdxTe photovoltaic detectors when detector current is varied by changing detector area, bias, temperature and incident flux. Holding detector bias and temperature constant, measured 1/f noise current is proportional to the detector current. However for all detector areas measured, non-uniformity is observed in the noise current due to the varied quality of the detectors. Even for the λc=16μm , 4-μm-radius, diffusion-limited detectors at 78K held at reverse bias, the average and standard deviation in dark current is Id=9.76+/- 1.59x10-8A while the average and standard deviation in noise current at 1 Hz in a 1 Hz bandwidth is in=1.01+/- 0.63x10-12A. For all detector areas measured at 100 mV reverse bias, the average and standard deviation in dark current to noise current ratio is α D=in/Id=1.39+/- 1.09x10-5. Defects are presumed resident in the detectors that produce greater non- uniformity in the 1/f noise as compared to the dark current at 100 mV reverse bias. Noise was also measured as a function of temperature for two λ c=16 micrometers detectors from 55 K to 100 K. The average and standard deviation in the noise current to dark current ratio is αD=in/Id=2.36+/- 0.83x10-5 for the 26-micrometers -diameter detector and (alpha) D=1.71+/- 0.69x10-5 for the 16-micrometers -diameter detector. Dark and noise current were measured while changing the bias applied to a detector. In the diffusion-limited portion of the detector I-V curve, 1/f noise is independent of bias with α D=in/Id=1.51+/- 0.12x10-5. When tunneling currents dominated, αT=in/Id=5.21+/- 0.83x10-5. The 1/f noise associated with tunneling currents is a factor of three greater than the 1/f noise associated with diffusion currents. In addition, 1/f noise was measured on detectors held at -100 mV and 78 K under dark and illuminated conditions. The average noise to current ratio αD was approximately 1.5 x 10-5 for dark and photon-induced diffusion current. However, detector-to-detector variations exist even within a single chip. The two most important points are that non-uniformities in material/fabrication need to be addressed and that each individual type of current component has an associated 1/f noise current component, the magnitude of the relationship being different depending on the source current.