Dennis Edwall

Development of sensitive long-wave infrared detector arrays for passively cooled space missions

Abstract. The near-earth object camera (NEOCam) is a proposed infrared space mission designed to discover and characterize most of the potentially hazardous asteroids larger than 140 m in diameter that orbit near the Earth. NASA has funded technology development for NEOCam, including the development of long wavelength infrared detector arrays that will have excellent zodiacal background emission-limited performance at passively cooled focal plane temperatures. Teledyne Imaging Sensors has developed and delivered for test at the University of Rochester the first set of approximately 10 μm cutoff, 1024×1024 pixel HgCdTe detector arrays. Measurements of these arrays show the development to be extremely promising: noise, dark current, quantum efficiency, and well depth goals have been met by this technology at focal plane temperatures of 35 to 40 K, readily attainable with passive cooling. The next set of arrays to be developed will address changes suggested by the first set of deliverables.

Growth of HgCdTe for long-wavelength infrared detectors using automated control from spectroscopic ellipsometry measurements

Hg1−xCdxTe is the leading material for high-performance long-wavelength (λ∼10–20 μm) infrared detectors. At these wavelengths, highly accurate compositional control (Δx⩽0.002) is required to achieve a particular device cutoff and the detector’s performance characteristics. Spectroscopic ellipsometry has proven to be a highly accurate technique of measuring the HgCdTe composition during epitaxial growth. Here we present the growth of HgCdTe by molecular beam epitaxy using an automated control program using real-time feedback from spectroscopic ellipsometry measurements. Excellent control is demonstrated for more than 50 growth runs with a standard deviation of Δx=0.0004 observed for the error between the composition measured by ellipsometry and the target growth composition.

Performance of science grade HgCdTe H4RG-15 image sensors

We present the test results of science grade substrate-removed 4K×4K HgCdTe H4RG-15 NIR 1.7 μm and SWIR 2.5 μm sensor chip assemblies (SCAs). Teledyne’s 4K×4K, 15 μm pixel pitch infrared array, which was developed for the era of Extremely Large Telescopes, is first being used in new instrumentation on existing telescopes. We report the data on H4RG-15 arrays that have achieved science grade performance: very low dark current (<0.01 e-/pixel/sec), high quantum efficiency (70-90%), single CDS readout noise of 18 e-, operability >97%, total crosstalk <1.5%, well capacity >70 ke-, and power dissipation less than 4 mW. These SCAs are substrate-removed HgCdTe which simultaneously detect visible and infrared light, enabling spectrographs to use a single SCA for Visible-IR sensitivity. Larger focal plane arrays can be constructed by assembling mosaics of individual arrays.

Advances in large-area Hg 1-x Cd x Te photovoltaic detectors for remote sensing applications

State-of-the-art large area photovoltaic detectors fabricated in HgCdTe grown by Molecular Beam Epitaxy have been demonstrated for the Crosstrack Infrared Sounder instrument. Large area devices (1 mm in diameter) yielded excellent electrical and optical performance operating at 81K for LWIR band and at 98K for MW and SWIR bands. LWIR and MWIR detectors have near-theoretical electrical performance, and AR-coated quantum efficiency is greater than 0.70. Measured average RoA at 98K is 2.0E7 W-cm2 and near-theoretical quantum efficiencies greater than 0.90 were obtained on SWIR detectors. These state-of-the-art large area photovoltaic detector results reflect high quality HgCdTe grown by Molecular Beam Epitaxy on CdZnTe substrates in all three spectral bands of interest.

Large-format SWIR/MWIR HgCdTe infrared focal plane arrays for astronomy

We have developed 1024 X 1024 HAWAII (HgCdTe Arrays for Wide-field Astronomical Infrared Imaging) focal plane arrays (FPAs) for use in astronomical applications. These devices have been delivered to various astronomy organizations around the world and have resulted in increased sensitivities and decreased observation times for deep space imaging. The detector material is PACE-I for SWIR and Molecular Beam Epitaxy (MBE) HgCdTe on CdZnTe for MWIR. The 1024 X 1024 multiplexer has a 18.5 micrometer unit cell pitch, source follower per detector (SFD) input, and it was fabricated at or internal commercial CMOS process line with excellent yield. Mean dark currents as low as 0.02 e-/s have been measured at 77 K for 2.5 micrometer devices (1024 X 1024 format, 18.5 micrometer pitch) and 0.39 e-/s for 5.3 micrometer devices at 50 K (256 X 256 format, 40 micrometer pitch). Quantum efficiencies are greater than 50% for both SWIR and MWIR detectors; with AR coatings, these are expected to be above 75%. Noise levels of 3 e- have been measured by multiple sampling techniques for the SWIR and 75 e- for the MWIR. All of these devices are simple to operate and are readily available. We are presently developing 2048 X 2048 FPAs with 18 micrometer unit cell pitch for both SWIR and MWIR applications.

VLWIR HgCdTe photovoltaic detectors performance

Very Long Wavelength InfraRed (VLWIR; (lambda) c approximately equals 15 to 17 micrometer at 78 K) photovoltaic detector operating in the 78 K range are needed for remote sensing applications. This temperature range permits the use of passive radiators in spacecraft to cool the detectors. VLWIR ((lambda) c approximately equals 15 to 17 micrometer at 78 K) photovoltaic detectors in a range of sizes (8 micrometer diameter to 1000 micrometer diameter) have been fabricated and their performance measured as a function of temperature. Molecular Beam Epitaxy (MBE) was used to grow n-type VLWIR Hg1-xCdxTe on lattice matched CdZnTe. Arsenic was implanted and the wafer was annealed to provide the p-type regions. All the material was grown with wider bandgap cap layers and consequently the detector architecture is the Double Layer Planar Heterostructure (DLPH) architecture. Id - Vd versus temperature curves for 8 and 1000 micrometer diameter, (lambda) c equals 17 micrometer at 78 K detectors indicate that the 8 micrometer diameter detector is diffusion limited for temperatures greater than 63 K even at a -200 mV bias. There is no appreciable tunneling at T equals 50 K and at -200 mV applied bias. At T equals 40 K tunneling commences at a bias approximately equals -80 mV. Below T equals 30 K, the diode is tunneling limited. The 1000 micrometer diameter detector is diffusion limited at bias values less than -50 mV at 78 K. At zero bias, the detector impedance is comparable to the series/contact resistance. Interfacing with the low (comparable to the contact and series resistance) junction impedance detector is not feasible. Therefore a custom pre- amplifier was designed to interface with the large VLWIR detectors in reverse bias. The detector is dominated by tunneling currents at temperatures less than 78 K. The 1000 micrometer diameter, (lambda) c approximately equals 17 micrometer at 78 K detectors have dark currents approximately equals 160 (mu) A at a -100 mV bias and at 78 K. Detector non-AR coated quantum efficiency > 60% was measured at -100 mV bias in these large detectors and the response was constant across the (lambda) equals 7 micrometer to 15 micrometer spectral band. With AR- coating the quantum efficiency will be > 70%. Response was measured and non-linearity < 0.15% was calculated for the 1000 micrometer detectors. The flux values were in the 1017 ph/cm2/sec range and were changed by varying the blackbody temperature. In addition, a linear response was measured while varying the spot size incident on the 1000 micrometer detectors. This excellent response uniformity measured as a function of spot size implies that, low frequency spatial response variations are absent, for the 1000 micrometer detectors.

Growth of HgCdTe films on 7x7.5 cm2 CdZnTe substrates for science grade H4RG-15 image sensor applications (Conference Presentation)

HgCdTe films are grown by molecular beam epitaxy (MBE) on large area CdZnTe substrates to achieve low dark current, high quantum efficiency infrared image sensors with 1.7um and 2.5um cut-off respectively. We present the structural and optical characterization of our HgCdTe films with emphasis on spatial uniformity across 7x7.5cm2 wafer size. Science grade detectors are fabricated on these films and subsequently hybridized to our H4RG-15 4K x 4K readout integrated circuit (ROIC). Test results from these image sensors show low dark current ( 99%), less than 1.0% cross talk and a well capacity larger than 70,000e-. the operation temperature is between 80-110K. These image sensors are also responsive in the visible-IR region due removal of the CdZnTe substrate after hybridization. This feature enables spectrographs to use a single image sensor for both visible and IR regions. These image sensors are developed for extremely large telescopes and used in various telescopes around the world.

Update on the status of H4RG-15 SCA production and testing at Teledyne imaging sensors (Conference Presentation)

The Hawaii-4RG-15 (H4RG-15) Sensor Chip Assembly (SCA) is a 4096×4096 pixel sensor with 15 µm pixel pitch. The H4RG-15 is the newest SCA developed by Teledyne for low light level astronomical applications, providing larger format while retaining the low noise and low power of the H1RG and H2RG arrays with additional new features. The SCAs are currently being produced with mercury cadmium telluride (HgCdTe or MCT) detectors having cutoff wavelengths of 1.7 µm for near-infrared (NIR) and 2.5 µm for short-wave infrared (SWIR) applications. SCAs can also be produced with 5.3 µm cutoff wavelength for mid-wave infrared (MWIR) or optimized for visible only applications with hybrid silicon (HyViSI) detectors. Several science grade detectors have been delivered for use in new astronomical instruments. The H4RG-15 sensor has been developed to enable assembly of mosaics with high pixel fill factor, with a new package design that improves the butt-ability of the SCAs. The new package achieves a high level of flatness and is also appropriate for space flight missions, with assembly using flight qualifiable components.

HgCdTe detectors for infrared remote sensing applications

Remote sensing applications including the National Polar Orbiting Environmental Satellite System (NPOESS) require imaging in a multitude of infrared spectral bands, ranging from the 1.58 micrometer to 1.64 micrometer VSWIR band to the 11.5 micrometer to 12.5 micrometer LWIR band and beyond. These diverse spectral bands require high performance detectors, operating over a range of temperatures; room temperature for the VSWIR band 100 K for MWIR, LWIR and VLWIR, these needs can all be met using molecular beam epitaxy (MBE) to grow HgCdTe. The flexibility inherent in the MBE growth technology is its ability to vary the HgCdTe materials bandgap within a growth run and from growth run to growth run, a capability necessary for remote sensing applications that require imaging in a wide variety of spectral bands. This bandgap engineering flexibility also permits tailoring the device architecture to the various specific system requirements. This paper combines measured detector optical and electrical data, with noise model estimates of ROIC performance to calculate signal to noise ratio (SNR), D* or noise equivalent temperature difference (NE(Delta) T), for each spectral band. The SNR, D* and/or NE(Delta) T are calculated with respect to system focal plane specifications, as required for the meteorological NPOESS.