Mark E. Gramer

Lateral Diffusion Length Changes in HgCdTe Detectors in a Proton Environment

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.

Nonlinear response of quantum well infrared photodetectors under low-background and low-temperature conditions

Quantum well IR photodetectors (QWIPs) have been pro- posed for use in space-based sensing applications. These space sys- tems place stringent performance requirements on IR detectors due to low-irradiance environments and the associated requirement for low- temperature operation. We demonstrate that under these conditions, the responsivity of a QWIP detector depends on frequency and that the shape of the frequency response varies with operational conditions. This nonlinear frequency response is empirically similar to dielectric relax- ation effects observed in bulk extrinsic silicon and germanium photocon- ductors under similar operational conditions. Radiometric characteriza- tion data demonstrate how the frequency response varies with temperature, photon irradiance, and bias voltage. These data also show that at low irradiances and temperatures, the detector response is ex- tremely slow, with response times on the order of seconds. The perfor- mance of the QWIP detector is described using standard figures of merit including responsivity, noise, and specific detectivity (D*). We also de- scribe the performance of an IR focal plane array (IRFPA) made with QWIP detectors, under operational conditions that result in long re- sponse times. The dependence of this time constant on photon irradi- ance and operating temperature is also described.

Nonlinear Response of QWIP Detectors: Summary of Data from Four Manufacturers

Quantum Well Infrared Photodetectors (QWIPs) have been proposed for use in space based remote sensing applications. These space systems place stringent performance requirements on infrared detectors due to the low irradiance environments and the associated requirement for low temperature operation. This study demonstrates that, under these space conditions, the responsivity of a QWIP detector depends on frequency and that the shape of the frequency response depends on the operational conditions. The non-flat frequency response is empirically similar to dielectric relaxation effects observed in bulk extrinsic silicon and germanium photoconductors under similar operational conditions. Data from four QWIP detectors, obtained from four independent sources, demonstrate how the frequency response of QWIP detectors vary with temperature, photon irradiance, and bias voltage, and how the shape of the frequency response depends on the dynamic resistance of the detector. This QWIP frequency response results in a detector signal that is nonlinear with irradiance for some combinations of detector bias, photon irradiance, and operating temperature.

Measurement of the radiometric and polarization characteristics of a microgrid polarizer infrared focal plane array

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.

Total ionizing dose and proton radiation characterization of Si P-i-N visible hybrid focal plane arrays

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 1012 protons/cm2 (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.

The Impact of Radiation Hardened By Design (RHBD) Techniques on the Performance of Readout Integrated Circuits In Radiation Environments

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.

Impact of excess low-frequency noise (ELFN) in Si:As impurity band conduction (IBC) focal plane arrays for astronomical applications

Long wavelength, infrared focal plane arrays (IRFPAs) fabricated with arsenic doped silicon (Si:As), impurity band conduction (IBC) detectors are being utilized in astronomical applications. In these systems, long integration times and/or the co-addition of consecutive frames are typically used to increase the signal-to-noise ratio. Some of the IBC detectors used in these IRFPAs have exhibited Excess Low Frequency Noise (ELFN) which limits their performance under some operational conditions. Data are presented on two Si:As IRFPAs which exhibit ELFN. These data illustrate the parametric dependence of ELFN on detector bias, photon irradiance, and integration time. Additionally, noise spectra from a single detector with ELFN illustrate the frequency dependence of ELFN at several photon irradiances. Finally, the effectiveness of the co- addition of frames on improving the signal-to-noise ratio when using an IRFPA with ELFN is quantified.

Radiation effects characterization of infrared focal plane arrays using the Mosaic Array Test System

The Mosaic Array Test System (MATS) has been developed to perform radiation effects characterization of single-element infrared detectors, readout devices, and infrared flocal plane arrays (IRFPAs). MATS has been used to perform radiation effects characterization of low background IRFPAs in various radiation environments. In this paper, the authors describe the components and capabilities of the MATS and present representative data to demonstrate the testing capabilities of the MATS.

Measurement of the spectral response of long-wavelength infrared focal plane arrays

A system for measuring the average relative spectral response of long wavelength infrared focal plane arrays (IRFPA) has been developed. Typically, the spectral response of sister detectors, fabricated on the same wafer as the FPA detectors are characterized in lieu of characterizing the actual IRFPA and it is generally assumed that the IRFPA detectors are equivalent to the sister detectors. On occasion, however, this assumption has proven to be incorrect. The spectral capability described here was implemented by interfacing an IRFPA test station to a long wavelength monochrometer, allowing the spectral response of the detectors in the IRFPA to be measured while the IRFPA operates in a normal manner. The spectral response measurements made with this system are validated by determining the peak wavelength responsivity of a long wavelength IRFPA, whose spectral response was measured with the system, using various blackbody temperatures. Sample spectral response data are presented along with the responsivities determined. With this spectral response measurement and a blackbody IRFPA measurement capability, complete radiometric characterization of IRFPAs can be performed.

Radiometric characterization of a Si:As scanning hybrid array

A radiometric characterization of a Si:As impurity band conduction hybrid array has been performed. The characterization included measurements of the responsivity and noise as functions of detector bias, photon irradiance, and integration time. Conversion gain, linearity, dynamic range, and NEI were calculated from measured data. The results are discussed in terms of optimal operating parameters for the hybrid array.