John Edward Hubbs

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.

Proton-induced transients and charge collection measurements in a LWIR HgCdTe focal plane array

We compare measurements and modeling of 27 and 63 MeV proton-induced transients in a large-format HgCdTe long wavelength infrared (LWIR) focal plane assembly operating at 40 K. Charge collection measurements describe very limited diffusion of carriers to multiple pixels showing significantly reduced particle induced cross-talk for the lateral diffusion structure.

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.

Bidirectional reflectance distribution function of the Infrared Astronomical Satellite solar-shield material

The BRDF values of a special, striated gold sample at 10.6 and 118 microm were measured. For 118-microm radiation the values decrease from beta - beta(0) = 0.1 to 1 for both orientations, but the vertical orientation of stria always produces a higher value. At 10.6 microm the horizontal orientation produces BRDF values that start at approximately 10(-2) at 0.1 and decrease to approximately 5 x 10(-4) at 1. The vertical orientation generates curves that are considerably higher.

Bidirectional transmittance distribution function measurements on ZnSe

The bidirectional transmittance distribution function (BTDF) is used to evaluate the scattering properties of chemically vapor-deposited (CVD) zinc selenide (ZnSe). Conceptually, the BTDF is a logical extension of the bidirectional reflectance distribution function (BRDF) relation. A working equation for BTDF based on the practical limitations of the instrumentation has been developed. The practical limitations inherent in making BTDF measurements are the finite detector size, the two scattering surfaces of the sample, and the linear radiometric responses of the detector. The instrumentation used to measure BTDF and the data collection procedure are described. Plots of BTDF vs angle from the sample normal are presented for CVD ZnSe at room temperature. The plots are for three thicknesses of ZnSe (6, 19, and 25 mm) at three laser wavelengths (0.6328, 3.39 and 10.6 microm).

Scattering characteristics of Martin Black at 118 μm

BRDF (bidirectional reflectance distribution function) values for 0.000118 m radiation at different angles of incidence and different scattering angles from the Infrared Astronomical Satellite telescope baffle coated with Martin Black are presented. Data from scatterometer experiments are collected and the BRDF and beta - beta sub 0 (sin theta sub s - sin theta sub 0) values are calculated based on the geometry, the voltage readings, the attenuators in the beam, and the calculated reference levels. A composite curve of forward and backward scattering data for several angles of incidence shows a peak near the specular direction (beta - beta sub 0 = 0), which is the instrument profile reduced by the 20% specular reflection of the Martin Black. The nonspecular part of the reflectivity indicates the slightly specular but largely Lambertian character of the coating. Data for the specular reflectivity as a function of the incidence angle unexpectedly shows a decrease in the specular reflectance with increasing angle of incidence.

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.

Method to validate relative spectral response curves

A methodology for validating the measured relative spectral response function of infrared detectors is described. Typically, the spectral response of sister detectors, fabricated on the same wafer as the detectors for infrared focal plane arrays (IRFPAs), are characterized in lieu of measuring the spectral response of the actual IRFPA. It is then generally assumed that the spectral characteristics of the IRFPA detectors are equivalent to the spectral characteristics of the sister detectors. To validate this assumption, a measurement methodology has been developed to assess the accuracy the measured relative spectral response curves. This methodology is based on the premise that infrared detectors measured peak responsivity is independent of the spectral content of the irradiance at the detector. The peak wavelength responsivity of the infrared detector is measured as a function of spectral photon irradiance to evaluate the accuracy of the measured detector relative spectral response. If the measured peak responsivity of the infrared detector is independent of spectral irradiance, then the measured spectral response accurately represents the spectral characteristics of the infrared detector. However, if the peak responsivity varies with spectral irradiance, then the measured spectral response is in error. The shape of the peak responsivity versus spectral irradiance curve provides insight into the spectral region where the measured spectral response is in error. Sample relative spectral response data are presented along with analysis of the spectral response curves.

Measurements of Martin Black at ~10 μm

The BRDF values of several samples of Martin Black have been measured at 10.6 μm and several angles of incidence. The values are in good agreement with data taken recently on an identical sample with a different instrument and with data from an identical sample taken several years ago with a similar instrument.The BRDF values of several samples of Martin Black have been measured at 10.6 microm and several angles of incidence. The values are in good agreement with data taken recently on an identical sample with a different instrument and with data from an identical sample taken several years ago with a similar instrument.