Space radiation damage and proton-induced transient effects were evaluated on 4.4-mu m cutoff HgCdTe avalanche photodiode (APD) arrays developed by Leonardo DRS. Device performances as a function of total dose up to 100 krad (Si) were measured with similar to 60-MeV protons on three types of APD samples: 4 x 4 pixel APD fanout arrays with and without connection to a read-out integrated circuit (ROIC) and a 2 x 8 pixel photoncounting APD focal plane array (FPA). A gamma-ray test was also conducted to study ionization effects. Both APD arrays exhibited a small decrease in the quantum efficiency and a linear increase in the dark current with the proton fluence. The 2 x 8 pixel photon-counting FPA also exhibited an increase in the dark count rate with proton dose. After the proton irradiation and an overnight room-temperature warm-up, the APD dark currents at 80 K increased significantly in both types of APD arrays. All radiation damage to these HgCdTe APD arrays annealed out after baking them at >85 degrees C for several hours. Transient protons through the devices were found to cause large pulses at the detector output, but recover within 1 mu s.

We report the results from proton and gamma ray radiation testing of HgCdTe avalanche photodiode (APD) arrays developed by Leonardo DRS for space lidar receivers. The devices were tested with ~60 MeV protons up to 100 krad(Si) without the read-out integrated circuit (ROIC) and 30 krad(Si) with the ROIC. We also measured the transient responses with the device fully powered and the APD gain set to >1000. The detectors output a large current impulse in response to each proton hit, which could saturate the ROIC but recovered within 1 μs. The APD dark currents increased linearly with the proton dose. The quantum efficiency and APD gain decreased slightly with dose. The dark currents due to the radiation damage went up many times after the devices were warmed to room temperature and cooled to 80K again. The radiation damage was found to completely anneal after baking the device at 85°C or higher. These results showed the HgCdTe APD arrays are suitable for use in space lidar for typical Earth orbiting and planetary missions provided that provisions are made to heat the detector chip to 85°C when the system performance is impacted.

This paper will report on the proton, and total ionizing dose characterizations of an MWIR nBn detector array. The FPA was fabricated using MWIR nBn infrared detectors, which have a cutoff wavelength of approximately 5.0 mu m at 120 K. Full radiometric characterizations were performed at multiple total ionizing dose levels to determine the impact of the radiation on the FPA noise, responsivity, NEI, and dynamic range. Displacement damage effects on the MWIR nBn detectors was evaluated at a proton energy of 63 MeV. These displacement damage effects primarily degrade the performance of the detector array through increased dark current, generation of "hot pixels" due to elevated dark current that form a tail to the dark current distribution, reduction in responsivity, and degraded uniformity. The majority of the performance degradation of the MWIR nBn detectors due to proton interactions can be attributed to a reduction in the minority carrier lifetime in the absorber region of the nBn detector.

An examination of the collective results from recent experiments quantifying the performance degradation rates of III-V-based, unipolar barrier infrared detectors with various designs and materials, cutoff wavelengths and operating conditions due to 63 MeV proton irradiation is presented. Empirical relationships were established between the radiation damage factors for dark current density, lateral optical collection length, and quantum efficiency and the inverse product of the detectors' cutoff wavelength and operating temperature. Fitting the dark current density damage factor's empirical relationship reflected these detectors' tendency to remain diffusion-limited during irradiation, which was previously established using Arrhenius-analysis of the post-irradiation, temperature-dependent dark current measurements on each. Collectively, the results affirmed the performance degradation stemmed from a reduction of the minority carrier recombination lifetime via generation of additional defects by proton-induced displacement damage. For comparing detector's radiation-tolerance, the results indicated that damage factors alone were not ideal, but their empirical relationships would serve as heuristics in this role.

Optical cooling of solids is a promising and innovative method to provide cryogenic cooling to infrared sensors. Currently insulator crystals, specifically ytterbium-doped yttrium-lithium-fluoride (Yb:YLF), have shown the most promise for cooling to low temperatures. This method has demonstrated cooling below the National Institute of Standards and Technology (NIST) cryogenic temperature definition of less than 123 K. Optical refrigeration utilizes a phenomenon called anti-Stokes fluorescence to generate cooling power. Incident laser light is absorbed by the cooling crystal and photons are spontaneously emitted at a higher, and thus more energetic, frequency. The difference in frequency is proportional to the cooling power of the crystal. Anti-Stokes cooling is highly dependent on doping percentages and YLF crystal purity and structure. Space based infrared sensors and their coolers are operated in a radiation environment where protons, gamma rays, heavy ions, and other radiation species are common and of varying severities depending on operational orbit. To ensure that radiative effects on cooling crystal performance is minimal, we irradiated two samples with 63 MeV protons to a total ionizing dose of 100 Krad (Si) and 1 Mrad (Si), and compared cooling crystal efficiency parameters before and after dosing.

We describe an integrated sensor assembly serving as both a component technology demonstration and a potential means of detecting distant point sources of infrared radiation. The objective of the demonstration was to show that usefully long integration times could be achieved with a low-background and well capacity, LWIR focal plane array optimized for use with cooled optics in space. The system controls extraneous background radiation with a small (150 μm) cooled pinhole that nevertheless transmits all the radiation of a point source collected by the fore-optic. Broad waveband response (~3 to 12 μm) results from optimization of the fore-optic for both MW and LWIR, as well as from a broadband anti-reflection coating on the field lens that is used at the pinhole to reimage the entrance aperture and its surrounding cold stop. Integration times in excess of 10 msec have been achieved for room temperature backgrounds with the FPA cold stage operated at 50 Kelvin, and noise performance has been bracketed with single frames of data collected over several integration times and over several minutes duration. However, anomalous signal behavior has been observed as the temperature of a remote blackbody increases. Although operation to date has been with a lower operability, engineering grade FPA, plans are to eventually upgrade to a higher quality device.

We present the results of the radiometric characterization of an "M" structure long wavelength infrared Type-II strained layer superlattice (SLS) infrared focal plane array (IRFPA) developed by Northwestern University (NWU). The performance of the M-structure SLS IRFPA was radiometrically characterized as a function of photon irradiance, integration time, operating temperature, and detector bias. Its performance is described using standard figures of merit: responsivity, noise, and noise equivalent irradiance. Assuming background limited performance operation at higher irradiances, the detector quantum efficiency for the SLS detector array is approximately 57%. The detector dark density at 80 K is 142 mu A/cm(2), which represents a factor of seven reduction from previously measured devices. (C) 2012 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.OE.51.6.064002]

The radiation tolerance characterization of dual band InAs/GaSb type-II strain-layer superlattice pBp detectors of varying size using 63 MeV proton irradiation is presented. The detectors' mid-wave infrared performance degraded with increasing proton fluence ΦP up to 3.75 × 1012 cm−2 or, equivalently, a total ionizing dose = 500 kRad (Si). At this ΦP, an ∼31% drop in quantum efficiency η, ∼2 order increase in dark current density JD, and consequently, >1 order drop in calculated detectivity D* were observed. Proton damage factors were determined for η and D*. Arrhenius-analysis of temperature-dependent JD measurements reflected significant changes in the activation energies following irradiation.

A large format 1k×1k focal plane array (FPA) is realized using type-II superlattice photodiodes for long wavelength infrared detection. Material growth on a 3 in. GaSb substrate exhibits a 50% cutoff wavelength of 11 μm across the entire wafer. The FPA shows excellent imaging. Noise equivalent temperature differences of 23.6 mK at 81 K and 22.5 mK at 68 K are achieved with an integration time of 0.13 ms, a 300 K background and f/4 optics. We report a dark current density of 3.3×10−4 A cm−2 and differential resistance-area product at zero bias R0A of 166 Ω cm2 at 81 K, and 5.1×10−5 A cm−2 and 1286 Ω cm2, respectively, at 68 K. The quantum efficiency obtained is 78%.

The effects of materials surrounding the active devices on single event-induced charge generation in a proton-irradiated Si diode array are shown be significant. Particle scatters in layers underlying the device have an impact on the response of a focal plane array. This effect is likely to be important in a variety of semiconductor devices.

Recent developments for long-wave infrared (LWIR) imaging polarimeters include incorporating a microgrid polarizer array onto the focal plane array. Inherent advantages over other classes of polarimeters include rugged packaging, inherent alignment of the optomechanical system, and temporal synchronization that facilitates instantaneous acquisition of both thermal and polarimetric information. On the other hand, the pixel-to-pixel instantaneous field-of-view error that is inherent in the microgrid strategy leads to false polarization signatures. Because of this error, residual pixel-to-pixel variations in the gain-corrected responsivity, the noise-equivalent input, and variations in the pixel-to-pixel micropolarizer performance are extremely important. The degree of linear polarization is highly sensitive to these parameters and is consequently used as a metric to explore instrument sensitivities. We explore the unpolarized calibration issues associated with this class of LWIR polarimeters and discuss the resulting false polarization signature for thermally flat test scenes. (C) 2008 Society of Photo-Optical Instrumentation Engineers.

The tolerance of a hybrid array (HA) to total ionizing dose (TID) radiation continues to be a major performance consideration for space based imaging systems. In an effort to improve TID performance, HA manufacturers have begun to utilize circuit design techniques to enhance the TID tolerance of readout integrated circuits (ROICs). This paper will report on the radiometric and TID radiation characterizations of a HA that utilizes radiation-hardened-by-design (RHBD) techniques. This paper will not describe the design techniques used. Instead, characterization data are presented that demonstrate a HA TID tolerance of over 25 units of total ionizing dose (UTID). This result is compared with the performance of devices with ROICs processed at commercial foundries that do not make use of RHBD techniques. The HA described in this paper represents a state-of-the-art device; the ROIC was designed to be low noise, high gain, and radiation tolerant. While design techniques were utilized to enhance its TID hardness, no special fabrication processes were used.

Proton-induced energy deposition in a silicon P-i-N focal plane array is analyzed with Monte Carlo based simulations. These simulations include all physical processes, including events resulting from multiple particles incident on a single pixel, to describe the experimental data accurately. Post-processing of Monte Carlo simulations is done to account for the effects of pile up (multiple hits on a single pixel during one integration time) and non-radiation-induced noise in experiment. The results are compared with experimental data, and demonstrate how direct ionization dominates the cross section, yet fluctuations in dE/dx cause a broad range of energy depositions not addressed by an average LET calculation. An event rate is predicted for a full space proton flux and the dominance of direct ionization is shown and compared to computation using constant LET methods in CREME96. This comparison shows that at lower energies, CREME96 sufficiently predicts the event rate, but at higher energies a high fidelity simulation method is needed to capture the distribution.

This paper presents a study of the performance degradation in a proton environment of long wavelength infrared (LWIR) HgCdTe detectors. The energy dependence of the Non-Ionizing Energy Loss (NIEL) in HgCdTe provides a framework for estimating the responsivity degradation in LWIR HgCdTe detectors due to on-orbit exposure from protons. Banded detector arrays of different detector designs were irradiated at proton energies of 7, 12, and 63 MeV. These banded detector arrays allowed insight into how the fundamental detector parameters degraded in a proton environment at the three different proton energies. Measured data demonstrated that the detector responsivity degradation at 7 MeV is 5 times larger than the degradation at 63 MeV. Comparison of the responsivity degradation at the different proton energies suggests that the atomic Columbic interaction of the protons with the HgCdTe detector is likely the primary mechanism responsible for the degradation in responsivity at proton energies below 30 MeV.

Remote sensing applications make use of the optical polarization characteristics of a scene to enhance target detection and discrimination. Imaging polarimeters typically utilize polarizing arrays located in front of a focal plane array as a means of extracting polarization information from the optical scene. Over the last few years, technology development efforts have resulted in FPAs that integrate the polarizer with the infrared focal plane array (FPA). This paper will report on the radiometric and polarization characterization of a micro-grid polarizer FPA from DRS Infrared Technologies, L.P. (DRS). These measurements were performed to evaluate the radiometric performance and the polarization characteristics of the FPA.

Recent developments for Long Wave InfraRed (LWIR) imaging polarimeters include incorporating a microgrid polarizer array onto the focal plane array (FPA). Inherent advantages over typical polarimeters include packaging and instantaneous acquisition of thermal and polarimetric information. This allows for real time video of thermal and polarimetric products. The microgrid approach has inherent polarization measurement error due to the spatial sampling of a non-uniform scene, residual pixel to pixel variations in the gain corrected responsivity and in the noise equivalent input (NEI), and variations in the pixel to pixel micro-polarizer performance. The Degree of Linear Polarization (DoLP) is highly sensitive to these parameters and is consequently used as a metric to explore instrument sensitivities. Image processing and fusion techniques are used to take advantage of the inherent thermal and polarimetric sensing capability of this FPA, providing additional scene information in real time. Optimal operating conditions are employed to improve FPA uniformity and sensitivity. Data from two DRS Infrared Technologies, L.P. (DRS) microgrid polarizer HgCdTe FPAs are presented. One FPA resides in a liquid nitrogen (LN2) pour filled dewar with a 80°K nominal operating temperature. The other FPA resides in a cryogenic (cryo) dewar with a 60° K nominal operating temperature.

The results of total ionizing dose and proton fluence characterization of hybrid Si P-i-N focal plane arrays are reported. The focal plane arrays consist of a silicon P-i-N detector array bump bonded to 128 x 128 CMOS readout integrated circuit (ROIC). The FPAs were characterized in total ionizing dose and proton fluence radiation environments. Full radiometric characterizations were performed at each radiation dose level to determine the impact of the radiation on dark current, noise, responsivity, sensitivity, and dynamic range. Results from the total ionizing dose experiment demonstrate an unexpected increase in the visible P-i-N detector dark current. The median dark current increased more than two orders of magnitude from pre-radiation to 300 krad(Si) and the magnitude of the dark current was found to be a strong function of detector bias. No appreciable change in responsivity or noise was observed for wavelengths above 400 nm up to a total ionizing dose of 750 krad(Si). Results from the proton radiation experiment show no appreciable change in responsivity was observed up to a 63 MeV proton fluence of 3 x 10 12 protons/cm 2 (400 krad(Si) of total ionizing dose). The median dark current increased approximately two orders of magnitude, but even at this higher level, the dark current did not contribute significantly to the median noise at an integration time of 10 ms. The dominant degradation mechanism, in both the total ionizing dose and proton fluence environments, is an increase in dark current in the Si P-i-N detectors.

Quantum-well infrared photocletectors (QWIPs) are trapping-mode photodetectors, and it is trapping that leads to an inherent problem: QWIPs' responsivity can be background-dependent under low-background conditions. We have determined that the source of the dependence is a relaxation process due solely to dielectric effects (dielectric relaxation), which results in a long time constant for recovery to steady state after a pulse of radiation is detected. We have analyzed responsivity-versus-frequency measurements on QWIPs published separately by Arrington et al. We have determined the value of the background flux for which significant background dependence occurs. For a given sensor integration time tau(i) (frame time for a staring sensor, dwell time for a scanning sensor), we calculated the response as a function of an exponential time constant. Fitting the calculated response to the measured data, we extracted values for the exponential time constant. We then determined the lower bound of background flux, (Phi(BL), above which the total responsivity will be less than 1% background-dependent. The value Of Phi(BL) for a typical OWIP detector is such that Phi(BL)tau(i) lies between 10(12) and 10(13) photons cm(-2). Practically, when significant background dependence occurs, it is essentially impossible to calibrate a QWIP-based focal plane array. And this, together with their the inherently low sensitivity and low required operating temperature (compared with HgCdTe detectors), makes QWIPs a poor choice for many low-background spaceborne sensors. (c) 2005 Society of Photo-Optical Instrumentation Engineers.