We present results from a SWIR/MWIR infrared hyperspectral imaging polarimeter (IHIP). The sensor includes a
pair of sapphire Wollaston prisms and several high order retarders to form an imaging Fourier transform
spectropolarimeter. The Wollaston prisms serve as a birefringent interferometer with reduced sensitivity to vibration
versus an unequal path interferometer, such as a Michelson. Polarimetric data are acquired through the use of
channeled spectropolarimetry to modulate the spectrum with the Stokes parameter information. We discuss the
operation of the IHIP sensor, in addition to our calibration techniques. Lastly, spectropolarimetric results from the
laboratory and outdoor tests are presented.
Visible light photon counters (VLPCs) are solid-state devices providing high quantum efficiency (QE) photon
detection (>88%) with photon number resolving capability and low timing jitter (~250 ps). VLPC features high
QE in the 0.4-1.0μm wavelength range, as the main photon absorption mechanism is provided by electron-hole
pair generation across the silicon bandgap. In this paper, we will discuss the optical and electrical operating
principles of VLPCs, and propose a range of device optimization paths that improves various aspects of VLPC
for advanced quantum optics and quantum information processing experiments, both in the UV and the telecom
A compact SWIR/MWIR infrared hyperspectral imaging polarimeter (IHIP) is currently under development at the
Optical Detection Lab at the University of Arizona. The sensor uses a pair of sapphire Wollaston prisms and high
order retarders to form an imaging birefringent Fourier transform spectropolarimeter. Polarimetric data are acquired
through the use of channeled spectropolarimetry to modulate the spectrum with the Stokes parameter information.
The two dimensional interferogram is Fourier filtered and reconstructed to recover the complete Stokes vector data
across the image. The IHIP operates over a +/-5° field of view and will use a dual-scan false signature reduction
technique to suppress polarimetric aliasing artifacts. We present current instrument development progress, initial
laboratory results, and our plan for future work.
The photocurrent of High Density Vertically Integrated Photodiodes (HDVIP) manufactured in LPE grown SWIR
(λc ~ 2.5 μm) HgCdTe material is modeled as a function of incident spot location using a Monte Carlo diffusion
calculation in the p-type bulk. The Monte Carlo calculation assumes a 3 x 3 mini-array of detectors surrounded by
guard detectors. Carriers generated in the n-regions are always collected. The result is a responsivity map that yields
the individual detector "spot scan" profile that is then used to calculate the detector modulation transfer function
(MTF). Fourier transforms of detector "spot scan" response profile provided experimental confirmation of MTF that
corresponded to the Monte Carlo modeled MTF.
Detectors that have broadband response from the visible (~ 400 nm) to near infrared (~ 2.5 μm) have remote
sensing hyperspectral applications on a single chip. 2.2 and 2.5 μm cutoff detectors permit operation in the 200
K range. The DRS HDVIP detector technology is a front side illuminated detector technology. Consequently,
there is no substrate to absorb the visible photons as in backside-illuminated detectors and these 2.2 and 2.5-μm-cutoff
detectors should be well suited to respond to visible light. However, HDVIP detectors are passivated
using CdTe that absorbs the visible light photons. CdTe with a direct bandgap ~ 1.6 eV strongly absorbs photons
of wavelength shorter than about 800 nm. Detectors in 320 x 6 arrays with varying thickness of CdTe
passivation layers were fabricated to investigate the visible response of the 2.5-μm-cutoff detectors. The SWIR
HDVIP detectors have well known high quantum efficiency (QE) in the near infrared region. Focus here was in
acquiring array level data in the visible region of the spectrum. 320 x 6 FPA QE and NEI data was acquired
using a 642 nm narrow band filter with 50 % points at 612 nm and 698 nm. The array QE average is ~ 70 % for
the array with CdTe passivation thickness = 44.5 nm. The NEI is ~ 5 x 1010 ph/cm2/s at a flux Φ = 5.36 x 1013
ph/cm2/s. QE for an array with CdTe passivation thickness = 44.5 nm is ~ 10 % higher than an array with CdTe
passivation thickness = 79.3 nm. In addition, a model that takes into account the complex optical properties of
every layer in the HDVIP photodiode architecture was developed to predict the QE of the detectors in the near
infrared and visible wavelength regions as a function of CdTe thickness. Measured QE as a function of
wavelength is not a good match to the model QE probably due to limitations in the measured QE and knowledge
of optical constants that are input into the model.
Backside illuminated detectors are sometimes fabricated on a thick substrates. In this work, we have
calculated detector spectral response using the stack matrix approach. This matrix formulation first constructs
a 2x2 "Stack Matrix" S that describes the optical properties of the complete stack from the detailed optical
properties (as a function of wavelength) of each layer in the entire stack. The stack matrix relates the electric
field strengths of the electromagnetic wave at the left of the stack to the electric field strengths of the
electromagnetic waves at the right of the stack. The stack matrix S is constructed from the multiplication of
matrices that track the phase and amplitude of the waves propagating across interfaces and from one side of a
layer to the other side. From the detailed optical properties (as a function of wavelength) of each material
layer, spectral response was calculated.
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.
Our group has developed the first 1024×1024 high background Si:As detector array, the Megapixel Mid-Infrared array
(MegaMIR). MegaMIR is designed to meet the thermal imaging and spectroscopic needs of the ground-based and airborne
astronomical communities. MegaMIR was designed with switchable capacitance and windowing capability to
allow maximum flexibility. We report initial test results for the new array.
The interdigitation concept demonstration utilized lc[60 K] ~ 15 &mgr;m HgCdTe pixels in a 96 × 96 array format. Each pixel consisted of four interdigitated sub-pixels. Electronic circuitry on the ROIC deselects defective sub-pixels. High detector response is maintained across the pixel, even if one or two interdigitated sub-pixels are deselected, because interdigitation provides that the predominance of minority carriers photogenerated in the pixel is collected by the selected sub-pixels. An interdigitated sub-pixel is deselected where there is a short in at least one of the detectors of the sub-pixel. The configuration of the interdigitated sub-pixels for a pixel is selected such that photogenerated charge carriers generated anywhere within the pixel would be collected by any adjacent, interdigitated sub-pixels within the same pixel that are not deselected because the diffusion length for the charge carriers is long enough. Deselected interdigitated sub-pixels are disconnected so that no charge will be collected on deselected sub-pixels. Therefore, only detectors of adjacent selected interdigitated sub-pixels collect substantially all of the photogenerated charges corresponding to the impinging radiation.
Photoresponse modeling of the interdigitated subpixel approach was performed. An example is that for a 20 &mgr;m diffusion length, the calculated QE changed from 85 % with 0 sub-pixels deselected, to 78 % with 1 sub-pixel, 67 % with two sub-pixels and 48 % with three sub-pixels deselected. A good comparison has been obtained between modeled and measured performance as a function of sub-pixel deselect.
We present the design of an infrared (IR) photon counting array consisting of an array of mid IR HgCdTe APDs read out with a CMOS application specific integrated circuit (ASIC) developed for x-ray imaging called the Medipix2. The Medipix2 is an array of 256x256 pixels, each of which amplifies and counts pulse events. When combined with an APD whose gain is high enough, the Medipix2 will integrate these detected photons, and the binary readout of these counters will be fast (~ 1kHz framerate) and noiseless. Initial feasibility tests of this concept using individual APDs from DRS technologies Inc. wirebonded to Medipix input pads are discussed.
DRS Sensors & Targeting Systems, under contract to the Space Dynamics Laboratory of Utah State University, is
providing the focal plane detector system for NASA's 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 MCT and Si:As
BIB focal plane arrays (FPAs) utilized in the WISE FPMAs are both megapixel class indium-bump hybridized devices
fabricated by Teledyne Imaging Systems and DRS Sensors & Targeting Systems, respectively. This paper reports
performance of the WISE Si:As BIB FPAs that are used for the WISE 12- and 23-μm wavelength bands.
DRS LPE-grown SWIR, MWIR and LWIR HgCdTe material are fabricated in the High-Density Vertically
Integrated Photodiode (HDVIP) architecture. Instruments manufactured for certain strategic applications have
severe constraints on excess low frequency noise due to the effect the noise has on the image quality with
subsequent consequences on the period of calibration. This paper will present data and analysis of excess low
frequency noise in LWIR (&lgr;c ~ 10.5 &mgr;m @ 60 K) HDVIP HgCdTe detectors.
The vehicle for noise measurements is a multiplexed 320 x 6 array of 40 &mgr;m x 50 &mgr;m, 10.5 &mgr;m cutoff, HgCdTe
detectors. Noise has been measured on a column of 320 detectors, at 60 K, as a function of frequency at zero and 50
mV reverse bias. Integration time for the measurement was 1.76 ms. Output voltage for the detectors was sampled
every 10th or every 100th frame. 32,768 frames of time series data were collected for a total record length of 98
minutes. Since the total time for collecting the 32,768 time data series points is 98 minutes, the minimum frequency
is 170 &mgr;Hz. Time series and Fourier transform data on individual detectors at 0 mV and 50 mV reverse bias in the
dark have been studied. Examination of the detector current time series and Fourier transform curves thereof, reveal
a variety of interesting characteristics: (i) time series displaying switching between four states characteristic of
random telegraph signal (RTS) noise, the noise current power spectrum having Lorentzian type characteristics; (ii)
time series data exhibiting slight wave-like characteristics with the noise current power spectrum being 1/f-like at
low frequencies; (iii) pronounced wave-like characteristics in the time series with the noise current power spectrum
being 1/f2-like at low frequencies; and (iv) time series having a mean value independent of time with the noise
current power spectrum being white. The predominance of detectors examined had minimal excess low frequency
noise down to ~ 10 mHz. In addition some isolated diodes had characteristics that lay between the four main types
Intrinsic carriers play a dominant role especially in the long wavelength (8-12 μm cut-off) HgCdTe material near ambient temperatures due to high thermal generation of carriers. This results in low minority carrier lifetimes caused by Auger recombination processes. Consequently, this low lifetime at high temperatures results in high dark currents and subsequently high noise. Cooling is one means of reducing this type of detector noise. However, the challenge is to design photon detectors to achieve background limited performance (BLIP) at the highest possible operating temperature; with the greatest desire being close to ambient temperature operation. We have demonstrated a unique planar device architecture using a novel approach in obtaining low arsenic doping concentrations in HgCdTe. Results indicate Auger suppression in P+/π/N+ devices at 300K and have obtained saturation current densities of the order of 3 milli Amps-cm2 on these devices.
DRS uses LPE-grown SWIR, MWIR and LWIR HgCdTe material to fabricate High-Density Vertically Integrated
Photodiode (HDVIP) architecture detectors. 2.5 μm, 5.3 μm and 10.5 μm cutoff detectors have been fabricated into
linear arrays as technology demonstrations targeting remote sensing programs. This paper presents 320 x 6 array
configuration technology demonstrations' performance of HDVIP HgCdTe detectors and single detector noise data. The
single detector data are acquired from within the 320 x 6 array. Within the arrays, the detector size is 40 μm x 50 μm.
The MWIR detector array has a mean quantum efficiency of 89.2% with a standard deviation to mean ratio, σ/μ = 1.51%. The integration time for the focal plane array (FPA) measurements is 1.76 ms with a frame rate of 557.7 Hz.
Operability values exceeding 99.5% have been obtained. The LWIR arrays measured at 60 K had high operability with
only ~ 3% of the detectors having out of family response. Using the best detector select (BDS) feature in the read out
integrated circuit (ROIC), a feature that picks out the best detector in every row of six detectors, a 320 x 1 array with
100% operability is obtained. For the 320 x 1 array constituted using the BDS feature, a 100% operable LWIR array
with average NEI value of 1.94 x 1011 ph/cm 2/s at a flux of 7.0 x 1014 ph/cm2/s has been demonstrated.
Noise was measured at 60 K and 50 mV reverse bias on a column of 320 diodes from a 320 x 6 LWIR array.
Integration time for the measurement was 1.76 ms. Output voltage for the detectors was sampled every 100th frame.
32,768 frames of time series data were collected for a total record length of 98 minutes. The frame average for a
number of detectors was subtracted from each detector to correct for temperature drift and any common-mode noise.
The corrected time series data was Fourier transformed to obtain the noise spectral density as a function of frequency.
Since the total time for collecting the 32,768 time data series points is 98.0 minutes, the minimum frequency is 170 μHz.
A least squares fit of the form (A/f + B) is made to the noise spectral density data to extract coefficients A and B that
relate to the 1/f and white noise of the detector respectively. In addition noise measurements were also acquired on
columns of SWIR detectors. Measurements were made under illuminated conditions at 4 mV and 50 mV reverse bias
and under dark conditions at 50 mV reverse bias. The total collection time for the SWIR detectors was 47.7 minutes.
The detectors are white noise limited down to ~10 mHz under dark conditions and down to ~ 100 mHz under
Remote sensing programs require detectors with a variety of wavelengths. One example of remote sensing applications is the GOES-ABI program that requires linear arrays of detectors with cutoff wavelengths ranging from the visible to the VLWIR (λc ~ 15 μm). In order to target the variety of remote sensing applications, an internal task was conducted to develop detectors and linear arrays operating under nominal remote sensing applications. SWIR [λc(295 K) ~ 2.5 μm] test detectors have been measured as a function of temperature between 170 K and 295 K. At 200 K the RoA values are in the 106 ohm-cm2 range. MWIR [λc(60 K) = 5.3 μm] and LWIR [λc(60 K) = 10.5 μm] HgCdTe detectors in a 320 x 6 array format have also been measured at 60 K. Within the arrays, the detector size is 40 μm x 50 μm. The MWIR detector array has a mean quantum efficiency of 89.2 % with a standard deviation to mean ratio, σ/μ = 1.51 %. The integration time for the focal plane array (FPA) measurements is 1.76 ms with a frame rate of 557.7 Hz. Operability values exceeding 99.5 % have been obtained. In addition, test diodes at the edge of the array that did not go through a read out integrated circuit (ROIC) were also measured and had quantum efficiency ~ 86 % that agreed well with the ~ 87 % quantum efficiency measured for detectors in the array that were located near the test detectors. The LWIR arrays, measured at 60K also had high operability with only ~ 3 % of the detectors having out of family response. Using best detector select (BDS) feature in the read out integrated circuit (ROIC), a feature that picks out the best detector in every row of six detectors, a 320 x 1 array with 100 % operability is obtained. For the 320 x1 array constituted using the BDS feature, a 100 % operable LWIR array with average NEI value of 1.94x1011 ph/cm2/s at a flux of 7.0x1014 ph/cm2/s has been demonstrated.
We present a description of a new 1024×1024 Si:As array designed for ground-based use from 5 - 28 microns. With a maximum well depth of 5e6 electrons, this device brings large-format array technology to bear on ground-based mid-infrared programs, allowing entry to the megapixel realm previously only accessible to the near IR. The multiplexer design features switchable gain, a 256×256 windowing mode for extremely bright sources, and it is two-edge buttable. The device is currently in its final design phase at DRS in Cypress, CA. We anticipate completion of the foundry run in October 2005. This new array will enable wide field, high angular resolution ground-based follow up of targets found by space-based missions such as the Spitzer Space Telescope and the Widefield Infrared Survey Explorer (WISE).
The National Polar-orbiting Operational Environmental Satellite System (NPOESS), is overseen by the Integrated Program Office (IPO), a joint effort of the Department of Defense, Department of Commerce and NASA. One of the instruments on the NPOESS satellite is the Cross-track Infrared Sounder (CrIS) instrument. CrIS is a Fourier Transform interferometric infrared (FTIR) sensor used to measure earth radiance at high spectral resolution to derive pressure, temperature, and moisture profiles of the atmosphere from the ground on up. Each CrIS instrument contains three different cutoff wavelength (λc)focal plane modules (FPMs): an SWIR FPM [λc(98 K) ~ 5 mm], MWIR FPM [λc(98 K) ~ 9 mm] and a LWIR FPM [λc(81 K) ~ 15.5 mm]. There are nine large (850 mm diameter) photodiodes per FPM, the nine detectors being arranged in a 3 x 3 array. The nine detectors are placed under tight tolerances in the X, Y, and Z dimensions. The steps involved in the transfer of photodiodes as part of a newly fabricated wafer to the mounting of the photodiodes on the FPM involves many processing steps including a significant amount of dicing, cleaning, wire bonding and baking at elevated temperatures.
Quantum efficiency and 1/f noise in Hg1-xCdxTe photodiodes are critical parameters that limit the sensitivity of infrared sounders. The ratio α, defined as the noise current in unit bandwidth in(f = 1 Hz, Vd, Δf = 1 Hz) to the dark current Id(Vd), that is, α = in/Id is one of the parameters used to select photodiodes for placement in FPMs. α is equivalent to √αH/N that appears in the well-known Hooge expression. For the sixty-one, λc ~ 9 μm photodiodes measured at 60 mV reverse bias and at 98 K, the average value of αdark = 1.3 x 10-4 in the dark and αPHOTO = in/IPHOTO is ~ 2 x 10-6 under illuminated conditions. These values of α are a factor of two lower than that reported previously. The λc ~ 15.5 μm photodiodes have average αdark = 1.3 x 10-5 with the highest performance, diffusion current limited photodiodes having values of αdark in the mid 10-6 range. All of the 850 μm diameter, λc ~ 15.5 μm photodiodes measured have excess low frequency noise, with the best performers having in(f = 100 Hz, Vd =-60 mV , Δf = 1 Hz) ~ 2 x 10-11 A/Hz1/2 and the best photodiode αdark = 3.92 x 10-6.
I-V measurements, noise, and visual inspections are performed at several steps in the photodiodes manufacturing process. It was observed, following FPM fabrication, photodiode dark current and noise had increased from the initial pre-mounting leadless chip carrier (LCC) measurements for some of the nine photodiodes. The performance degradation observed led to an investigation into the cause (baking at elevated temperatures, mechanical handling, electrical stress etc.) of photodiode degradation that occurred between LCC and FPM testing. Correlations between I-V, noise and surface visual defects have been performed on some λc ~ 15.5 mm photodiodes. This paper outlines the results of the study, correlating the electrical performance observed to visual defects on the surface and to defects seen following cross sectioning of degraded photodiodes. In addition, other lessons learned and the corrective actions implemented that led to the successful manufacture of SWIR, MWIR and LWIR large photodiodes from the material growth to insertion into and successful demonstration of flight FPMs for the CrIS program are described.
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.
Detector characteristics of Au- and Cu-doped High Density Vertically Integrated Photodiode (HDVIP) detectors are presented in this paper. Individual photodiodes in test bars were examined by measuring I-V curves under dark and illuminated conditions at high bias values. Noise as a function of frequency has been measured on Au- and Cu-doped MWIR [λc(78 K) = 5 μ] HDVIP HgCdTe diodes at several temperatures under dark and illuminated conditions. No excess currents are observed above the photocurrents for reverse bias values out to 500 mV. Both Au- and Cu-doped detectors measured at 85 K, exhibit gain values between 40 and 50 at 8 V reverse bias. Gain values fell in this same range even when the flux incident on each type of detector was varied. The excess noise factor for the Cu-doped detectors ranged from 1.35 to 1.69 depending on the incident flux. Variation is probably due to measurement error. The noise at 8 V reverse bias is white for the Cu-doped detectors. The Au-doped detectors exhibited 1/f noise at 8 V reverse bias. At higher frequencies where the noise spectrum was quasi-white, the excess noise factor for the Au-doped detector was in the 1.0 to 1.5 range.
An attempt is made to connect the material parameters of Hg1-xCdxTe layer growth to the parameters measured following photovoltaic detector fabrication. We found that the Cd composition X value extracted from spectral response measurements on detectors at 78 K are lower than the X values obtained from the room temperature transmission measurements, or the X value used to fit the measured material minority carrier lifetime versus temperature data. The lateral collection length Lc that determines the thermally generated carriers that contribute to the diffusion current and Lopt extracted from the "flood-illuminated" to "focused-spot" photocurrent ratio are in excellent agreement. Devices exhibit near theoretical RoA uniformity at 77K for MWIR, LWIR and VLWIR. RoAopt was also found to be uniform throughout the range of detector dimensions measured such as 8 μm diameter circular to 250 μm x 250 μm square. Median RoAopt values are 1266, 66 and 0.75 ohm-cm2 for the 9.7, 11.3 and 15.4 μm cutoff wavelengths respectively. The uniformity in RoAopt confirms that the detector performance is limited by the bulk properties of the material, and not by surface effects.
Cu-doped HDVIP detectors with different cut-off wavelengths are routinely manufactured. The DRS HDVIP detector technology is a front-side-illuminated detector technology. There is no substrate to absorb the visible photons as in backside-illuminated detectors and these detectors should be well suited to respond to visible light. However, HDVIP detectors are passivated using CdTe that absorbs the visible light photons. CdTe strongly absorbs photons of wavelength shorter than about 800 nm. Detectors with varying thickness of CdTe passivation layers were fabricated to investigate the visible response of the 2.5-μm-cutoff detectors. A model was developed to predict the quantum efficiency of the detectors in the near infrared and visible wavelength regions as a function of CdTe thickness. Individual photodiodes (λc = 2.5 μm) in test bars were examined. Measurements of the quantum efficiency as a function of wavelength region will be presented and compared to the model predictions.
Photon detectors and focal plane arrays (FPAs) are fabricated from HgCdTe and silicon in many varieties. With appropriate choices for bandgap in HgCdTe, detector architecture, dopants, and operating temperature, HgCdTe and silicon can cover the spectral range from ultraviolet to the very-long-wavelength infrared (VLWIR), exhibit high internal gain to allow photon counting over this broad spectral range, and can be made in large array formats for imaging. DRS makes HgCdTe and silicon detectors and FPAs with unique architectures for a variety of applications. Detector characteristics of High Density Vertically Integrated Photodiode (HDVIP) HdCdTe detectors as well as Focal Plane Arrays (FPAs) are presented in this paper. MWIR[λc(78 K) = 5 μm] HDVIP detectors RoA performance was measured to within a factor or two or three of theoretical. In addition, 256 x 256 detector arrays were fabricated. Initial measurements had seven out of ten FPAs having operabilities greater than 99.45% with the best 256 x 256 array having only two inoperable pixels. LWIR [λc(78K)~10 μm] 640 X 480 arrays and a variety of single color linear arrays have also been fabricated. In addition, two-color arrays have been fabricated. DRS has explored HgCdTe avalanche photo diodes (APDs) in the λc = 2.2 μm to 5 μm range. The λc = 5 μm APDs have greater than 200 DC gain values at 8 Volts bias. Large-format to 10242 Arsenic-doped (Si:As, λc ~ 28 μm), Blocked-Impurity-Band (BIB) detectors have been developed for a variety of pixel formats and have been optimized for low, moderate, and high infrared backgrounds. Antimony-doped silicon (Si:Sb) BIB arrays having response to wavelengths > 40 μm have also been demonstrated. Avalanche processes in Si:As at low temperatures (~ 8 K) have led to two unique solid-state photon-counting detectors adapted to infrared and visible wavelengths. The infrared device is the solid-state photomultiplier (SSPM). A related device optimized for the visible spectral region is the visible-light photon counter (VLPC). The VLPC is a nearly ideal device for detection of small bunches of photons with excellent time resolution. Finally, DRS makes imaging arrays of pin-diodes utilizing the intrinsic silicon photoresponse to provide high performance over the 0.4-1.0 μm spectral range operating near room temperature. pin-diode arrays are particularly attractive as an alternative to charge-coupled devices (CCDs) for space applications where radiation hardening is needed. In addition, wire grid micropolarizers have been demonstrated and two color doped silicon detectors using diffractive microlenses are being developed. Precision alignment of sensor chips with respect to a base mounting plate has been demonstrated to be within 2 μm. A similar technique is also utilized to align single large detectors for sounder applications in focal plane arrays (FPAs). FPAs for space applications with the associated cold and warm electronics and packaging/cables have been fabricated.
Detector characteristics of Cu- and Au-doped High Density Vertically Integrated Photodiode (HDVIP) detectors as well as Cu-doped HDVIP Focal Plane Arrays (FPAs) are presented in this paper. Individual photodiodes in test bars were examined by measuring I-V curves and the associated resistance-area (RA) product as a function of temperature. The Au-doped MWIR [λc(78 K) = 5 μm] HDVIP detectors RoA performance was within a factor of two or three of theoretical. Noise as a function of frequency has been measured on Au-doped MWIR HgCdTe HDVIP diodes at several temperatures under dark and illuminated conditions. Low-frequency noise performance of the Au-doped MWIR diode in the various environments is characterized by the ratio α of the noise current spectral density at 1 Hz to the value of the diode current. For photocurrent at 140 K, αPHOTO = 1.8 x 10-5. The value of αPHOTO is the same at both zero bias and 100 mV reverse bias. At 160 K, αPHOTO is slightly lower but still in the low 10-5 range. Excess low-frequency noise measured at 140 K and 100 mV reverse bias in the dark has αDARK = 1.4 x 10-5. At 160 K and 100 mV reverse bias, αDARK is in the mid 10-5 range. At 140 K,the dark current at 8.2 V reverse bias was equal to the photocurrent at 100 mV reverse bias and close to the photocurrent at zero bias. αDARK = 1.85 x 10-3 at -8.2 V. This ratio is two orders of magnitude greater than αPHOTO. At 8.2 V reverse bias, the current was amplified by avalanche processes. Similar results were obtained on the Au-doped diode at 160 K. Diffusion current dominates dark current at 100 mV reverse bias at T = 185 K and T = 220 K. The ratio, αDARK approximately αPHOTO in the low to mid 10-5 range, i.e. dark diffusion current generates excess low frequency noise in the same manner as photocurrent. In addition, 256 x 256 Cu-doped detector arrays were fabricated. Initial measurements had seven out of ten FPAs having operabilities greater than 99.45% with the best 256 x 256 array having only two inoperable pixels.
Photon detectors and focal plane arrays (FPAs) are fabricated from silicon in many varieties. With appropriate choices for detector architecture, dopants, and operating temperature, silicon can cover the spectral range from ultraviolet to the very-long-wavelength infrared (VLWIR), exhibit high internal gain to allow photon counting over this broad spectral range, and can be made in large array formats for imaging. DRS makes silicon detectors and FPAs with unique architectures for a variety of applications. Large-format, VLWIR FPAs based on doped-silicon Blocked-Impurity-Band (BIB) detectors have been developed. These FPAs comprise an array of BIB detectors interfaced via indium column interconnects to a matching read-out integrated circuit (ROIC). Arsenic-doped silicon (Si:As) BIB detector arrays with useful photon response out to about 28 μm are the most fully developed embodiment of this technology. FPAs with Si:As BIB arrays have been made in a variety of pixel formats (to 10242) and have been optimized for low, moderate, and high infrared backgrounds. Antimony-doped silicon (Si:Sb) BIB arrays having response to wavelengths 40 μm have also been demonstrated. Avalanche processes in Si:As at low temperatures (~ 8 K) have led to two unique solid-state photon-counting detectors adapted to infrared and visible wavelengths. The infrared device is the solid-state photomultiplier (SSPM). To our knowledge, it is the only detector capable of counting VLWIR photons (formula available in paper) with high quantum efficiency. A related device optimized for the visible spectral region is the visible-light photon counter (VLPC). The VLPC is a nearly ideal device for detection of small bunches of photons with excellent time resolution. VLPCs coupled to scintillating fibers have demonstrated new capabilities for energetic charged particle tracking in high-energy physics. A fiber tracking system that utilizes VLPCs is currently in operation in the D0 detector at Fermilab's Tevatron. VLPCs may also be useful for quantum cryptography and quantum computation. Finally, DRS makes imaging arrays of pin-diodes utilizing the intrinsic silicon photoresponse to provide high performance over the 0.4 - 1.0 μm spectral range operating near room temperature. pin-diode arrays are particularly attractive as an alternative to charge-coupled devices (CCDs) for space applications where radiation hardening is needed.
The Blocked Impurity Band (BIB) detector was invented in the early 1980's and subsequently developed by our team. The original arsenic-doped silicon (Si:As) detectors addressed the need for low-noise, radiation-tolerant, mid-IR detectors for defense surveillance from space. We have since developed large-format BIB focal plane arrays to address high-background requirements of ground-based telescopes and missile interceptors, low-background requirements of the Space Infrared Telescope Facility (SIRTF), and very low background requirements of the mid-IR instruments for the Next Generation Space Telescope (NGST) and Terrestrial Planet Finder. Most of these applications employ Si:As BIB detectors, but antimony-doped silicon (Si:Sb) BIB detectors are used for some SIRTF bands. Other demonstrated types including phosphorus (Si:P) and gallium-doped (Si:Ga) BIB detectors may have application niches. We have proposed development of a BIB detector type utilizing both Si:As and Si:P layers to optimize dark current vs. wavelength performance. Wavelength response for silicon BIB detectors extend to a maximum of ~40 microns (Si:Sb), but we have also demonstrated germanium BIB detectors for wavelengths extending to several hundred microns. We are currently developing germanium BIB detector arrays for astrophysics applications, including space telescopes beyond NGST.
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.
Multicolor focal plane arrays are of interest for a variety of applications. We report on a method to create a multicolor detector array of high-performance arsenic-doped silicon Blocked-Impurity-Band (BIB) detectors by using diffractive microlenses. Advantage is taken of the strong chromatic aberration characteristic of diffractive lenses to direct light within a pixel to either a central detector or to second detector concentrically disposed around the first. A theoretical calculation of the efficacy of this approach for spectral separation is presented. Fabrication of diffractive microlenses on the backside (illuminated-side) of thinned specially-designed BIB detector arrays is described. Finally, early initial results and further development plans are discussed.
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.
The wide-field infrared explorer (WIRE) is a small spaceborne cryogenic telescope specifically designed to study the evolution of starburst galaxies. The use of advanced, large format, infrared hybrid focal plane array technology provides a large sensitivity gain over previously flown missions. The hybrid focal plane arrays (HFPAs) used in this instrument are 128 by 128-element arsenic-doped-silicon blocked impurity band infrared detector arrays connected via indium column interconnects to matching cryogenic multiplexers. The WIRE instrument includes two focal plane mount assemblies (FPMAs), each of which includes a HFPA optimized for a particular wavelength band. Details concerning design, fabrication and performance of the critical components of the WIRE FPMAs are described.
Large-format, very-long-wavelength infrared (VLWIR) hybrid focal plane arrays (HFPAs) based on doped-silicon blocked-impurity-band (BIB) detectors have been developed and demonstrated for a variety of astronomy applications. An HFPA consists of a BIB detector array interfaced via indium column interconnects to a matching cryogenic signal processor/multiplexer. Arsenic-doped silicon (Si:As) BIB detector arrays with useful photon response out to nearly 30 micrometers are the most fully developed embodiment of this technology. HFPAs with Si:As BIB arrays have been optimized for low, moderate, and high infrared backgrounds in 128 X 128-pixel formats, and a high-flux 256 X 256-pixel version is under development. For high-flux applications, both the detector array and multiplexer are optimized to handle incident flux densities > 1016 photons cm-2s-1, providing high spatial uniformity, high pixel operability, and background-limited performance down to low frequencies (< 10 Hz). Antimony-doped silicon (Si:Sb) arrays and 128 X 128-pixel Si:Sb HFPAs having response to wavelengths > 40 micrometers have also been demonstrated, primarily for use at low and moderate backgrounds. BIB technology offers producible, low-cost, high-performance focal planes for astronomy in the VLWIR.
The visible light photon counter (VLPC) is an excellent candidate for scintillating fiber applications, meeting the requirements of high quantum efficiency, high gain with low gain dispersion, and good time resolution. The mechanism of impurity band conduction, on which the device depends, is described. Device operation is outlined, and performance characteristics are presented for a recent design. These characteristics include quantum efficiency, dark count rate, dark current, gain, and their dependence on temperature and operating voltage. Pulse height distribution and excess noise factor are also given, and shown to compare favorably with conventional avalanche photodiodes.
The course presents a fundamental understanding of two-dimensional arrays applied to detecting the infrared spectrum. The physics and electronics associated with 2-D infrared detection are stressed with special emphasis on the hybrid architecture unique to two-dimensional infrared arrays.