There is great interest in MWIR Ga-free type II strained layer superlattice (T2SLS) nBn detectors for background limited photodetectors (BLIP) operating at temperatures higher than InSb (T ≥ 150K compared to T≈ 80-90K). Recently, Ting et al. [Proc. SPIE, Vol. 10624, 1062410-1 (2018)] reported on measurements of the dark current and quantum efficiency (QE) for e-SWIR (λ>1.7 μm), MWIR, and LWIR Ga-free nBn T2SLS detectors. Of particular interest is the reported MWIR nBn T2SLS data, since the measured dark current and QE provide the opportunity to analyze the measured detector optoelectronic characteristics using optical properties obtained separately from the hole minority carrier lifetime and optical absorption coefficient data, hence no adjustable parameters. The goal is to develop and utilize robust modeling techniques to explain real, measured nBn detector data to understand the technology limitations and the improvements needed to optimize performance and device designs. A notable result is the observation that the dark current data under reverse bias (-100 mV < bias ≤ 0 mV) obeys the ideal diode equation, where the saturation dark current is in agreement with the radiative recombination rate obtained from the measured absorption coefficient and a 9 μs Shockley-Read-Hall (SRH) lifetime obtained from the measured hole lifetime data. Most important is that the expected generation-recombination (G-R) space charge current based on the 9 μs SRH lifetime is not observed as expected for an ideal nBn heterojunction detector and that increasing excess dark currents are observed with decreasing temperature. The in-band external QE measured at 100K is in the range of 30% and is observed to increase with increasing operating temperature which indicates effects influenced by hole mobility anomalies. In contrast to the above listed observations, the Type II Ga-free nBn detector data reported by D. Ting et al. [Appl. Phys. Lett., 113, 021101, 2018] exhibits a hole energy barrier, G-R dark current like characteristics, and both temperature/voltage dependent QE.

In this paper, we use the theory of Evans and Landsberg, which is a generalization of the Shockley–Read–Hall recombination statistics in the space charge region (SCR), to include effects of Auger and radiative recombination processes that are also of origin in the SCR. Using analytical expressions for the current density, we calculate the total dark current density for a variety of conditions. Contributions include radiative and Auger transitions of origin in both the quasi-neutral region and the SCR. Numerical simulations are used to assess the nature of the limitations associated with the analytical calculation in the n-extrinsic region (\(N_{\rm d} \gg n_{\rm i}\), where \(N_{\rm d}\) is the doping concentration and \(n_{\rm i}\) is the intrinsic carrier concentration), and to extend the calculations to operating temperatures in the intrinsic region (\(n_{\rm i} \gg N_{\rm d}\)). Major findings include the observation that in a fully depleted \(P^+n\) double-layer planar hetero-structure, at a reverse bias voltage sufficiently high to suppress the Auger process, SRH centers are not limiting, and the dark current is due to radiative transitions of origin in the n-side SCR. From the numerical simulations, while the Auger recombination rate changes drastically with varying the carrier concentration (such as applying reverse bias), the radiative recombination rate remains nearly invariant to varying the carrier concentration, and, as such, does not appreciably change with increasing reverse bias. Using the theory of van Roosbroeck and Shockley, the radiative recombination rate is obtained by integrating the measured optical absorption coefficient over all photon energies. Hence, the theory links the measured absorption coefficient to the measured dark current density for conditions in which the dominant current component is due to radiative recombination. Finally, the numerical simulations reveal, in both the n-extrinsic and intrinsic operating regions, that, under sufficient conditions, the detector is radiatively limited.

Imaging in the extended short-wavelength infrared (eSWIR) spectral band (1.7–3.0 μm) for astronomy applications is an area of significant interest. However, these applications require infrared detectors with extremely low dark current (less than 0.01 electrons per pixel per second for certain applications). In these detectors, sources of dark current that may limit the overall system performance are fundamental and/or defect-related mechanisms. Non-optimized growth/device processing may present material point defects within the HgCdTe bandgap leading to Shockley–Read–Hall dominated dark current. While realizing contributions to the dark current from only fundamental mechanisms should be the goal for attaining optimal device performance, it may not be readily feasible with current technology and/or resources. In this regard, the U.S. Army Research Laboratory performed physics-based, two- and three-dimensional numerical modeling of HgCdTe photovoltaic infrared detectors designed for operation in the eSWIR spectral band. The underlying impetus for this capability and study originates with a desire to reach fundamental performance limits via intelligent device design.

Heterojunction device design concepts are leveraged to reduce depletion layer generation-recombination (G-R) dark current in planar P+-on-n SWIR HgCdTe infrared detectors. Shockley-Read-Hall (SRH) depletion dark current (when present) is expected to be the dominant dark current component at low temperatures, and in fact, it is beneficial for the transition from diffusion to G-R to be at such relatively low temperatures. However, it is empirically observed that even for relatively long values of the SRH lifetime (20 μs), the transition occurs at relatively high temperatures (>200 K) for material with a cut-off wavelength of 2.5 μm. A key device design parameter of P+-on-n photodiodes is the position of the electrical junction relative to the hetero-metallurgical interface. Junction formation via p-type arsenic implantation into the narrow-gap absorber layer is typically chosen for efficient collection of diffusion current, however, other configurations are possible as well. In this letter, we numerically explore the conditions that reduce depletion dark current without reducing the quantum efficiency (QE). The findings support the assertion that device design conditions exist in SWIR HgCdTe that essentially eliminate the depletion dark current without significantly reducing the QE.

A key design feature of P+-on-n HgCdTe detectors is the depth of the p-type region. Normally, homojunction architectures are utilized where the p-type region extends into the narrow-gap absorber layer. This facilitates the collection of photo-carriers from the absorber layer to the contact; however, this may result in excess generation-recombination (G-R) current if defects are present. Alternatively, properly adopting a heterojunction architecture confines the p-type region (and the majority of the electric field) solely to the wide-gap layer. Junction placement is critical since the detector performance is now dependent on the following sensitivity parameters: p-type region depth, doping, valence band offset, lifetime and detector bias. Understanding the parameter dependence near the hetero-metallurgical interface where the compositional grading occurs and the doping is varied as either a Gaussian or error function is vital to device design. Numerical modeling is now essential to properly engineer the electric field in the device to suppress G-R current while accounting for the aforementioned sensitivity parameters. The simulations reveal that through proper device design the p-type region can be confined to the wide-gap layer, reducing G-R related dark current, without significantly reducing the quantum efficiency at the operating bias V = -0.100V.

InAs/GaSb superlattices are leading candidates for next generation long-wave infrared and very-long-wave infrared photodetectors. These heterostructures are expected to hold important advantages over existing materials systems, primarily bulk HgCdTe alloys. To realize their inherent potential, however, superlattice materials with low defect density and improved device characteristics must be demonstrated. Here, we report on the molecular beam epitaxy growth and characterization of an 11 μm cutoff wavelength InAs/GaSb superlattice detector with a state-of-the-art single-pass, internal quantum efficiency of 36%. The shutter sequencing used to form the GaSb-on-InAs and InAs-on-GaSb superlattice heterojunctions is described in detail, and the latter specifically identified as a source of morphological defects in these devices.

InAs-GaSb strained layer superlattices (SLSs) form a narrow band gap material whose cut-off wavelength can be tuned from 3 um to beyond 30 um. Theory predicts that in the LWIR and VLWIR, the SLS narrow bandgap layer structures can be engineered to reduce Auger recombination, relative to other narrow bandgap materials, such as HgCdTe. This should result in the SLS diodes having better performance than currently available detectors. A key to achieving this improved performance is knowing the detailed layer structure of the superlattice, and being able to accurately model this layer structure. Having an accurate model to guide the improved performance is essential to optimizing this material system. Cross-sectional scanning tunneling microscopy data will be presented which shows that the actual layer structure differs significantly from the intended layer structure, due to the detailed dynamics of MBE growth and the very thin layers in the superlattice. Specifically, cross-sectional scanning tunneling microscopy demonstrates that the InAs contains excess antimony, and the GaSb excess indium, due to segregation from the underlying arsenide-on-antimonide, or antimonide-on-arsenide, heterojunctions respectively. These deviations from the intended structure have a significant impact on the predicted properties of the superlattice. The predicted behavior of the intended and actual superlattice structures will be compared to measured performance.

We report on Hg 1−x Cd x Te mid-wavelength infrared (MWIR) detectors grown by molecular-beam epitaxy (MBE) on CdZnTe substrates. Current-voltage (I-V) characteristics of HgCdTe-MWIR devices and temperature dependence of focal-plane array (FPA) dark current have been investigated and compared with the most recent InSb published data. These MWIR p-on-n Hg 1−x Cd x Te/CdZnTe heterostructure detectors give outstanding performance, and at 68 K, they are limited by diffusion currents. For temperatures lower than 68 K, in the near small-bias region, another current is dominant. This current has lower sensitivity to temperature and most likely is of tunneling origin. High-performance MWIR devices and arrays were fabricated with median R o A values of 3.96 × 10 10 Ω-cm 2 at 78 K and 1.27 × 10 12 Ω-cm 2 at 60 K; the quantum efficiency (QE) without an antireflection (AR) coating was 73% for a cutoff wavelength of 5.3 µm at 78 K. The QE measurement was performed with a narrow pass filter centered at 3.5 µm. Many large-format MWIR 1024 × 1024 FPAs were fabricated and tested as a function of temperature to confirm the ultra-low dark currents observed in individual devices. For these MWIR FPAs, dark current as low as 0.01 e − /pixel/sec at 58 K for 18 × 18 µm pixels was measured. The 1024 × 1024 array operability and AR-coated QE at 78 K were 99.48% and 88.3%, respectively. A comparison of these results with the state-of-the-art InSb-detector data suggests MWIR-HgCdTe devices have significantly higher performance in the 30–120 K temperature range. The InSb detectors are dominated by generation-recombination (G-R) currents in the 60–120 K temperature range because of a defect center in the energy gap, whereas MWIR-HgCdTe detectors do not exhibit G-R-type currents in this temperature range and are limited by diffusion currents.

The 1/f noise in photovoltaic (PV) molecular-beam epitaxy (MBE)-grown Hg1-xCdxTe double-layer planar heterostructure (DLPH) large-area detectors is a critical noise component with the potential to limit sensitivity of the cross-track infrared sounder (CrIS) instrument. Therefore, an understanding of the origins and mechanisms of noise currents in these PV detectors is of great importance. Excess low-frequency noise has been measured on a number of 1000-mum-diameter active-area detectors of varying "quality" (i.e., having a wide range of I-V characteristics at 78 K). The 1/f noise was measured as a function of cut-off wavelength under illuminated conditions. For short-wave infrared (SWIR) detectors at 98 K, minimal 1/f noise was measured when the total current was dominated by diffusion with white noise spectral density in the mid-10(-15) A/Hz(1/2) range. For SWIR detectors dominated by other than diffusion current, the ratio, alpha, of the noise current in unit bandwidth i(n)(f = I Hz, V-d = -60 mV, and Deltaf = I Hz) to dark current I-d(V-d = -60 mV) was alpha(SW-d) = i(n)/I-d similar to 1 X 10(-3). The SWIR detectors measured at 0 mV under illuminated conditions had median alpha(SW-P) = i(n)/I-ph similar to 7 x 10(-6) For mid-wave infrared (MWIR) detectors, alpha(MW-d) = i(n)/I-d similar to 2 x 10(-4) due to tunneling current contributions to the 1/f noise. Measurements on forty-nine 1000-mum-diameter MWIR detectors under illuminated conditions at 98 K and -60 mV bias resulted in alpha(MW-P) = i(n)/I-ph = 4.16 +/- 1.69 x 10(-6). A significant point to note is that the photo-induced noise spectra are nearly identical at 0 mV and 100 mV reverse bias, with a noise-current-to-photocurrent ratio, alpha(MW-P), in the mid 10(-6) range. For long-wave infrared (LWIR) detectors measured at 78 K, the ratio, alpha(LW-d) = i(n)/I-d similar to 6 x 10(-6), for the best performers. The majority of the LWIR detectors exhibited alpha(LW-d) on the order of 2 x 10(-5). The photo-induced 1/f noise had alpha(LW-p) = i(n)/I-ph similar to 5 x 10(-6). The value of the noise-current-to-dark-current ratio, a appears to increase with increasing bandgap. It is not clear if this is due to different current mechanisms impacting 1/f noise performance. Measurements on detectors of different bandgaps are needed at temperatures where diffusion current is the dominant current. Excess low-frequency noise measurements made as a function of detector reverse bias indicate 1/f noise may result primarily from the dominant current mechanism at each particular bias. The 1/f noise was not a direct function of the applied bias.

The 1/f noise in photovoltaic (PV) molecular-beam epitaxy (MBE)-grown Hg1−xCdxTe double-layer planar heterostructure (DLPH) large-area detectors is a critical noise component with the potential to limit sensitivity of the cross-track infrared sounder (CrIS) instrument. Therefore, an understanding of the origins and mechanisms of noise currents in these PV detectors is of great importance. Excess low-frequency noise has been measured on a number of 1000-µm-diameter active-area detectors of varying “quality” (i.e., having a wide range of I-V characteristics at 78 K). The 1/f noise was measured as a function of cut-off wavelength under illuminated conditions. For short-wave infrared (SWIR) detectors at 98 K, minimal 1/f noise was measured when the total current was dominated by diffusion with white noise spectral density in the mid-10−15A/Hz1/2 range. For SWIR detectors dominated by other than diffusion current, the ratio, α, of the noise current in unit bandwidth in(f = 1 Hz, Vd = −60 mV, and Δf = 1 Hz) to dark current Id(Vd = −60 mV) was αSW-d = in/Id ∼ 1 × 10−3. The SWIR detectors measured at 0 mV under illuminated conditions had median αSW-P = in/Iph ∼ 7 × 10−6. For mid-wave infrared (MWIR) detectors, αMW-d = in/Id ∼ 2 × 10−4, due to tunneling current contributions to the 1/f noise. Measurements on forty-nine 1000-µm-diameter MWIR detectors under illuminated conditions at 98 K and −60 mV bias resulted in αMW-P = in/Iph = 4.16 ± 1.69 × 10−6. A significant point to note is that the photo-induced noise spectra are nearly identical at 0 mV and 100 mV reverse bias, with a noise-current-to-photocurrent ratio, αMW-P, in the mid 10−6 range. For long-wave infrared (LWIR) detectors measured at 78 K, the ratio, αLW-d = in/Id ∼ 6 × 10−6, for the best performers. The majority of the LWIR detectors exhibited αLW-d on the order of 2 × 10−5. The photo-induced 1/f noise had αLW-P = in/Iph ∼ 5 × 10−6. The value of the noise-current-to-dark-current ratio, α appears to increase with increasing bandgap. It is not clear if this is due to different current mechanisms impacting 1/f noise performance. Measurements on detectors of different bandgaps are needed at temperatures where diffusion current is the dominant current. Excess low-frequency noise measurements made as a function of detector reverse bias indicate 1/f noise may result primarily from the dominant current mechanism at each particular bias. The 1/f noise was not a direct function of the applied bias.

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 [lambda(c)(98 K) similar to 5 mum], MWIR [lambda(c)(98 K) similar to 9 mum] and LWIR [lambda(c)(81 K) similar to 15 mum]. 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.1/f noise in photovoltaic Hg1-xCdTe detectors is a critical parameter that limits the sensitivity of the CrIS instrument. Therefore, an understanding of the mechanisms that impact noise currents in a photovoltaic detector is of great importance. MBE grown Hg1-xCdxTe Double Layer Planar Heterostructure (DLPH) photovoltaic detectors have been characterized, to determine the dominant mechanisms impacting detector I-V performance. Excess low frequency noise has been measured on a number of 1000 gm diameter active area detectors of varying "quality" (i.e. having a wide range of I-V performance at 78 K). The 1/f noise was measured as a function of bias, cutoff wavelength, and under illuminated conditions. For SWIR [lambdac(98 K) similar to 5 mum] detectors at 98 K, minimal 1/f noise was measured. The noise was white and in the mid 10-(15) A/Hz(1/2) range. For MWIR [lambda(c)(98 K) similar to 9 mum] detectors, the most important point to note is that the photo-induced noise spectra are nearly identical at 0 mV and 100 mV reverse bias, with a noise current to photocurrent ratio of alpha(P) in the mid x 10(-6) range. For the LWIR [lambda(c)(81 K) similar to 15 mum] detectors measured at 78 K, the ratio, alpha, of the noise current in unit bandwidth i,(f = 1 Hz, V-d = -0.1 V, Deltaf = 1 Hz) to the dark current I-d(V-d = -0.1 V) is alpha(DLW) = i(n)/I-d similar to 6 x 10(-6) for the best performers. The majority of the detectors had alpha(DLW) similar to 2 x 10(-5) (Our alpha(DLW) is equivalent to rootalpha(H)/N which appears in the well-known Hooge expression.). The photo-induced 1/f noise had alpha(PLW) = i(n)/I-photo similar to 5 x 10(-6). For the MWIR detectors measured at 98 K, alpha(DMW) = i(n)/I-d = 1.59 +/- 0.69 x 10(-4). The value of alpha increases as the bandgap, increases. Excess low frequency noise measurements made as a ftinction of detector reverse bias indicate 1/f noise appears to result from the dominant current at each particular bias. 1/f noise was not a direct function of the applied bias.

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 lambda(c) = 16 mum, 4-mum-radius, diffusion-limited detectors at 78 K held at reverse bias, the average and standard deviation in dark current is Id = 9.76 +/- 1.59 x 10(-8) A while the average and standard deviation in noise current at 1 Hz in a 1 Hz bandwidth is in = 1.01 +/- 0.63 x 10(-12) A. For all detector areas measured at 100 mV reverse bias, the average and standard deviation in dark current to noise current ratio is alpha(D) = i(n) / I-d = 1.39 +/- 1.09 x 10(-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 lambda(c) = 16 mum detectors from 55 K to 100 K. The average and standard deviation in the noise current to dark current ratio is OLD = i(n) / I-d = 2.36 +/- 0.83 x 10(-5) for the 26-mum-diameter detector and aD = i(n) / I-d = 1.71 +/- 0.69 x 10(-5) for the 16-mum-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 alphaD = i(n) / Id = 1.51 +/- 0.12 x 10-5. When tunneling currents dominated, alpha(T) = i(n) / I-d = 5.21 +/- 0.83 x 10(-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 alpha(D) 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.

This paper investigates 1/f noise performance of very-long-wavelength infrared (VLWIR) Hg1−xCdxTe (cutoff wavelengths λc=15 µm and λc=16 µm) photodiodes at 78 K, where detector current is varied by changing detector area, detector bias, and illumination conditions. Holding detector bias and temperature constant, the 1/f noise current is proportional to the detector current. Significant nonuniformity is observed in the noise data for each detector area because of the varying detector quality. Defects are presumed resident in the detectors to produce greater nonuniformity in 1/f noise as compared to dark current at 100-mV reverse bias. For λc=16 µm, 4-μ-radius, diffusion-limited diodes at 78 K and 100-mV reverse bias, the average dark current is Id=9.76±1.59×10−8 A, while the average noise current measured at 1 Hz is in=1.01±0.63×10−12 A/Hz1/2. For all detector areas measured, the average ratio in 1-Hz bandwidth is α D =in/Id=1.39±1.09×10−5. The 1/f noise was also measured on one diode as a function of detector-dark current as the applied bias is varied. In the diffusion-limited portion of this detector’s current-voltage (I-V) curve, to about 130 mV, the 1/f noise was independent of bias. For this diode, the ratio αD=in/Id=1.51±0.12×10−5. The 1/f noise associated with tunneling currents is a factor of 3 greater than the 1/f noise associated with diffusion currents, αT=in/IT=5.21±0.83×10−5. In addition, 1/f noise was measured on detectors held at −100 mV and 78K under dark and illuminated conditions. The measured ratios αP ∼αD ∼1.5×10−5 were about the same for the dark and photon-induced diffusion currents. Therefore, the diffusion current appears to have a unique value of α as compared to the tunneling current. This may be indicative of unique noise-generation mechanisms associated with each current.

State-of-the-art large-area photovoltaic (PV) detectors fabricated in HgCdTe grown by molecular beam epitaxy (MBE) have been demonstrated for the Crosstrack Infrared Sounder (CrIS) instrument. Large-area devices (1 mm in diameter) yielded excellent electrical and optical performance operating at 81 K for λc ∼ 15 µm, at 98 K for λc ∼ 9 µm, and λc ∼ 5-µm spectral cutoffs. Fabricated detectors have near-theoretical electrical performance, and Anti Reflection coated quantum efficiency (QE) is greater than 0.70. Measured average R0A at 98 K is 2.0E7 Ωcm2, and near-theoretical QEs greater than 0.90 were obtained on detectors with λc ∼ 5-µm spectral cutoffs. These state-of-the-art large-area PV detector results reflect high-quality HgCdTe grown by MBE on CdZnTe substrates in all three spectral bands of interest.

VLWIR ( c ∼15 m to 17 m at 78 K) detectors have been characterized as a function of temperature to determine the dominant current mechanisms impacting detector performance. I d −V d curves indicate that VLWIR detectors are diffusion limited in reverse and near zero bias voltages down to temperatures in the 40 K range. At 30 K the detectors are limited by tunneling currents in reverse bias. Since the detectors are diffusion limited near zero bias down to 40 K, the R 0 A imp versus temperature data represents the diffusion current performance of the detector as a function of temperature. The detector spectral response measurement and active layer thickness are utilized to calculate the HgCdTe layer x value and the optical activation energy E a optical . The activation energy, E a electrical , obtained from the measured diffusion limited R 0 A imp versus temperature data is not equal to the activation energy, E a optical , obtained from the spectral response measurement for all x values measured. E a electrical = * E a optical , where ranges between 0.64 and 1.0 For cutoff wavelengths in the 9 m at 78 K, E a electrical =E a optical . E a electrical =0.65 * E a optical have been measured for c =17 m at 78 K detectors. As the band gap energy decreases to values in the range of 70 meV and lower, it is reasonable to expect a more dominant role of band tailing effects on the transport properties of the material system. In such a picture, one would expect the optical band gap to be unmodified, whereas the intrinsic concentration could be enhanced from its value for the ideal semiconductor. Such a picture could explain the observed behavior. Further probing experiments and modeling efforts will help clarify the physics of this behavior.

The National Polar-orbiting Operational Environmental Satellite System (NPOESS) Cross-track Infrared Sounder (CrIS) is an interferometric sensor that measures earth radiances at high spectral resolution, using the data to provide pressure, temperature and moisture profiles of the atmosphere. The pressure, temperature and moisture sounding data are used in weather prediction models that track storms, predict levels of precipitation etc. The CrIS instrument contains SWIR (gamma(c) similar to 5 mum at 98K), MWIR (gamma(c) similar to 9 mum at 98K) LWIR (gammac similar to 16 mum at 81K) Focal Plane Array (FPA) modules. A critical CrIS design selection was the use of photovoltaic (PV) detectors in all three spectral bands. PV detectors have the important benefits of high sensitivity and linearity. Each FPA modules consists of nine large (1000 mum diameter) photovoltaic detectors with accompanying cold preamplifiers. This paper describes the performance for all the modules forming the CrIS Detector Preamplifier Module (DPM).Molecular Beam Epitaxy (MBE) is used to grow the appropriate bandgap n-type Hg1-xCdxTe on lattice matched CdZnTe. SWIR, MWIR and LWIR 1000 mum diameter detectors have been manufactured using the Lateral Collection Diode (LCD) architecture. Custom pre-amplifiers ' have been designed to interface with the large SWIR, MWIR and LWIR detectors. The operating temperature is above 78K, permitting the use of passive radiators in spacecraft to cool the detectors. Recently fabricated 1000 mum diameter photovoltaic detectors have the measured performance parameters listed in the Table below. Expected D* performance from the detector/pre-amplifier models are also listed in the table. The D* values are calculated at the CrIS program peak wavelength specified for each spectral band.[GRAPHIC].

Very Long Wavelength InfraRed (VLWIR; lambda (c) similar to 15 to 17 mum at 78K) photovoltaic detectors operating in the 78K 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) similar to 15 to 17 mum at 78K) photovoltaic detectors in a range of sizes (8 mum diameter to 1000 mum 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.I-d - V-d versus temperature curves for 8 and 1000 mum diameter, lambda (c) = 17 mum at 78K detectors indicate that the 8 mum diameter detector is diffusion limited for temperatures greater than 63K even at a -200 mV bias. There is no appreciable tunneling at T = 50K and at -200 mV applied bias. At T = 40K tunneling commences at a bias similar to -80 mV. Below T = 30K, the diode is tunneling limited. The 1000 mum diameter detector is diffusion limited at bias values less than -50 mV at 78K. 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 78K. The 1000 mum diameter, lambda (c) similar to 17 mum at 78K detectors have dark currents similar to 160 muA at a -100mV bias and at 78K. Detector non-AR coated quantum efficiency > 60% was measured at -100mV bias in these large detectors and the response was constant across the lambda = 7 mum to 15 mum spectral band. With AR-coating the quantum efficiency will be > 70%. Response was measured and non-linearity < 0.15% was calculated for the 1000 <mu>m detectors. The flux values were in the 10(17) ph/cm(2)/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 mum detectors. This excellent response uniformity measured as a function of spot size implies that, low frequency spatial response variations are absent, for the 1000 mum detectors.

Very long wavelength infrared (VLWIR; 15 to 17 µm) detectors are required for remote sensing sounding applications. Infrared sounders provide temperature, pressure and moisture profiles of the atmosphere used in weather prediction models that track storms, predict levels of precipitation etc. Traditionally, photoconductive VLWIR (λ c >15 µm) detectors have been used for sounding applications. However, photoconductive detectors suffer from performance issues, such as non-linearity that is 10X – 100X that of photovoltaic detectors. Radiometric calibration for remote sensing interferometry requires detectors with low non-linearity. Photoconductive detectors also suffer from non-uniform spatial optical response. Advances in molecular beam epitaxy (MBE) growth of mercury cadmium telluride (HgCdTe) and detector architectures have resulted in high performance detectors fabricated in the 15 µm to 17 µmm spectral range. Recently, VLWIR (λ c ∼ 17 µm at 78 K) photovoltaic large (1000 µm diameter) detectors have been fabricated and measured at flux values targeting remote sensing interferometry applications. The operating temperature is near 78 K, permitting the use of passive radiators in spacecraft to cool the detectors. Detector non-AR coated quantum efficiency >60% was measured in these large detectors. A linear response was measured, while varying the spot size incident on the 1000 µm detectors. This excellent response uniformity, measured as a function of spot size, implies that low frequency spatial response variations are absent. The 1000 µm diameter, λ c ∼ 17 µm at 78 K detectors have dark currents ∼160 µA at a −100 mV bias and at 78 K. Interfacing with the low (comparable to the contact and series resistance) junction impedance detectors is not feasible. Therefore a custom pre-amplifier was designed to interface with the large VLWIR detectors operating in reverse bias. A breadboard was fabricated incorporating the custom designed preamplifier interfacing with the 1000 µm diameter VLWIR detectors. Response versus flux measurements were made on the large VLWIR detectors and non-linearity <0.15% was measured at high flux values in the 2.5×10 17 to 3.5×10 17 ph-cm −2 sec −1 range. This non-linearity is an order of magnitude better than for photoconductive detectors.

Excess low frequency noise is investigated for the first time in infrared MBE grown LWIR Hg1−xCdxTe double layer planar heterostructure (DLPH) detectors grown on lattice matched substrates. LWIR detectors having R0Aopt values at 40K in the 101–107 Θ-cm2 range have been characterized as a function of temperature between 120 and 20K. Detectors with R0Aopt≥103Θ-cm2 at 40K have theoretical diffusion limited performance down to 78K and detectors with R0Aopt ≥105 Θ-cm2 at 40K are within a factor of two of theoretical diffusion limited performance for T>65K. Activation energies extracted from noise (Vd=−100 mV) and dark current (Vd=−100 mV) vs temperature measurements were detector dependent. The activation energy for detectors with R0Aopt≈106 Θ-cm2 at 40K is ∼0.90*Eg to 0.99*Eg. The noise measured between 78 and 105K in the intermediate performance (R0Aopt∼103–104 Θ-cm2 at 40K) detectors are higher than the noise measured in the higher performance (R0Aopt∼105–107 Θ-cm2) detectors. In addition, the excess low frequency noise and the dark current at −100 mV in the intermediate and poor (R0Aopt∼101 Θ-cm2) performance detectors are temperature independent. For each detector measured, the activation energy extracted from noise (Vd=−100 mV) vs temperature measurements is equal to the activation energy extracted from the total dark current (Vd=−100 mV) vs temperature measurements. For different dark current mechanisms, the excess low frequency noise varies with temperature and also with area within statistical accuracy in the same manner as the total dark current through the detector. At 78K, the Tobin14 expression holds in the general sense for equal area detectors dominated by different current mechanisms and also for detectors with a wide range of implant dimensions (Aimp=3.85×10−7 cm2 to Aimp=6.25×10−4 cm2). Following measurements, the detectors were stripped of the passivation and overlaying metal layers and dressed by a defect etch to reveal defects in each detector. A correlation among noise, leakage current and defect type has been determined for each detector.