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Surface resistivity is an important parameter of multi-electrode CZT detectors such as coplanar-grid, strip, or pixel detectors. Low surface resistivity results in a high leakage current and affects the charge collection efficiency in the areas near contacts. Thus, it is always desirable to have the surface resistivity of the detector as high as possible. In the past the most significant efforts were concentrated to develop passivation techniques for CZT detectors. However, as we found, the field-effect caused by a bias applied on the cathode can significantly reduce the surface resistivity even though the detector surface was carefully passivated. In this paper we illustrate that the field-effect is a common feature of the CZT multi-electrode detectors, and discuss how to take advantage of this effect to improve the surface resistivity of CZT detectors.

In the course of developing microcalorimeters as detectors for astronomical X-ray spectroscopy, we have undertaken an empirical characterization of non-ideal effects in the doped semiconductor thermometers used with these detectors, which operate at temperatures near 50 mK. We have found three apparently independent categories of such behavior that are apparently intrinsic properties of the variable-range hopping conduction mechanism in these devices: 1/f fluctuations in the resistance, which seems to be a 2D effect; a departure from the ideal coulomb-gap temperature dependence of the resistance at temperatures below T 0 /24; and an electrical nonlinearity that has the time dependence and extra noise that are quantitatively predicted by a simple hot electron model. This work has been done largely with ion-implanted Si:P:B, but similar behaviors have been observed in transmutation doped germanium.

We present fabrication details of our Mo/Au X-ray microcalorimeters, which are being developed as one of the candidate high resolution spectrometers for the Constellation-X mission. We have reproducibly fabricated Mo/Au transition-edge sensors with Tc's of /spl sim/100 mK on etched silicon nitride membranes and connected via superconducting Nb leads. Our single pixel devices have, so far, attained resolution of 3.7 eV at 3.3 keV. We also discuss our plans for fabrication and testing of fully functional multi-pixel array of X-ray microcalorimeters.

We have made Radio-Frequency Single-Electron Transistors (RF-SETs) with large input gates, and tested performance and modes of operation with the goal of using such devices as on-chip amplifiers for a variety of high impedance cryogenic photodetectors. We achieved /spl ap/100 kHz of closed-loop bandwidth for charge-locked-loop and transimpedance amplifier feedback configurations, and have combined amplifier outputs using a form of wavelength division multiplexing. With our choice of SET junction resistance, a 0.5 fF input gate capacitance gave a cotunneling-degraded charge noise of 1/spl times/10/sup -4/ e//spl radic/Hz, but a fairly low input voltage noise of 30 nV//spl radic/Hz.

We report on progress made so far at NASA Goddard Space Flight Center towards the development of arrays of X-ray microcalorimeters as candidates for the high-resolution X-ray spectrometer on the Constellation-X mission. In the design concept presently under consideration, the microcalorimeter consists of (i) a Bi/Cu multilayer absorber for stopping and thermalizing the incident X-rays, (ii) an e-beam evaporated Mo/Au proximity bilayer with sputtered Nb leads for sensing the resultant temperature rise, and (iii) a silicon nitride membrane to provide a weak thermal link to the sink temperature so that the calorimeter can return to its equilibrium temperature. Fabrication details and preliminary results are reported.

Superconducting bilayers made of thin films of molybdenum and gold show promise as robust transition-edge sensor (TES) thermometers for calorimeters. We present our first X-ray results from experiments with Mo/Au TES calorimeters on silicon-nitride membranes. These results include analysis of the signal pulse shape and noise as functions of the bias point, which is varied through changing the bias voltage for operation at different places within the superconducting transition and changing the heat sink temperature relative to the transition temperature. Ultimately, we determined that the performance of our devices is limited by the slew rate of the SQUID amplifier used to measure the change in current, which restricts the choice of bias. The amplifier must be replaced before further device characterization and optimization can proceed.

We present results from two 1 cm 3 CdZnTe gamma-ray spectrometers with full 3-D position sensitivity. To our knowledge, these are the first reported semiconductor spectrometers that provide independent spectral data for each of over 2000 volume elements. Energy resolutions of 1.5–1.6% FWHM and position resolutions of 0.7×0.7×0.5 mm were obtained at 662 keV gamma-ray energy from the central region of both detectors for single-pixel events. With the 3-D position sensing capability, variations in spectral response over the detector volume were recorded using a 137Cs source. These measurements allow a study of full-energy peak efficiency, mean ionization energy and electron trapping as a function of 3-D position. The effects of material non-uniformity on detector spectroscopic performance are discussed.

We discuss the design for a submillimeter-wave photometer, using a combination of superconducting and single-electron devices, which would have high quantum efficiency, very low noise-equivalent powers, and eventually even submicrosecond timing resolution. The absorption of above-gap photons occurs in a small strip of superconducting Al, whose normal-state resistance can be matched efficiently to an antenna of a higher gap (Nb) superconductor. The quasiparticles produced by photon absorption are then confined via Andreev reflection, and forced to tunnel through a small SIS tunnel junction. The tunneling time is much shorter than the known (>10 /spl mu/s) quasiparticle recombination time, so collection efficiency will be high. The device sensitivity would be limited by the small subgap current in the high-quality Al/AlO/sub x//Al tunnel junction at temperatures (100 mK) well below T/sub c/. Scaling based on the larger junctions used in X-ray detector applications suggests that the total dark current can be

Two 3-D position-sensitive room temperature semiconductor γ-ray spectrometers have been built using 1 cm 3 cubic CdZnTe crystals. The lateral coordinates of γ-ray interaction are obtained from the location of the 11( x )×11( y ) pixellated anodes and the depth ( z ) is obtained from the ratio of the signals coming from the cathode and the anode. Energy spectra from 662 keV incident γ-rays have been collected from each of the 11 (x)×11 (y)×20 (z) voxels in both of the CdZnTe devices. After corrections for electron trapping, the difference of weighting potentials in 3-D, and for the gain variation of the readout circuitry, energy resolutions of 1.70% (11.3 keV) FWHM and 1.84% (12.2 keV) FWHM were obtained at 662 keV γ-ray energy on the first and second detectors, respectively, from the whole bulk for single-pixel events. Possible improvements in the detector performance are discussed.

The authors report on the performance of a CdZnTe (CZT) array readout by an XA (X-ray imaging chip produced at the AMS foundry) application specific readout chip (ASIC). The array was designed and fabricated at NASA/Goddard Space Flight Center (GSFC) as a prototype for the Burst Arc-Second Imaging and Spectroscopy gamma-ray instrument. The XA ASIC was obtained from Integrated Detector and Electronics (IDE), in Norway. Performance characteristics and spectral data for /sup 241/Am are presented both at room temperature and at -20/spl deg/C. The measured noise (/spl sigma/) was 2.5 keV at 60 keV at room temperature. This paper represents a progress report on work with the XA ASIC and CZT detectors. Work is continuing and in particular, larger arrays are planned for future NASA missions.

With the goal of making high-sensitivity bolometers for low-background measurements in the far-infrared or submillimeter, we have made small silicon thermistors, which act as hot-electron bolometers, with integrated single-electron transistors for readout amplifiers. Semiconductors doped just below a metal–insulator transition can make highly sensitive thermistors that have been used as thermometers in X-ray microcalorimeters and FIR bolometer arrays. In such arrays, thermal isolation is engineered by supporting the absorber and thermometer on a membrane. However, electron–phonon decoupling in doped silicon can be made the dominant thermal isolation by reducing device volume, potentially allowing a smaller thermal conductance and a more sensitive bolometer. A key feature is that, while its DC resistance is very high, the thermistor's surface impedance at terahertz frequencies is conveniently low, making feasible efficient antenna coupling of radiation into the electron system. Radio-frequency single-electron transistors integrated with the thermistors have sufficiently low input capacitance to offer high-speed readout of the high impedance detectors.

A CdZnTe detector array is being developed for the proposed gamma-ray Burst ArcSecond Imaging and Spectroscopy (BASIS) spaceflight mission to accurately locate gamma-ray bursts, determine their distance scale, and measure the physical characteristics of the emission region. Two-dimensional strip detectors with 100 μm pitch have been fabricated and wire bonded to readout electronics to demonstrate the ability to localize 60 and 122 keV gamma-rays to less than 100 μm. Radiation damage studies on a CdZnTe detector exposed to MeV neutrons showed a small amount of activation but no detector performance degradation for fluences up to 10 10 neutrons/cm 2 . A 1 × 1 in. CdZnTe detector has also been flown on a balloon payload at 115 000 ft in order to measure the CdZnTe background rates.

The energy resolution of CdTe and CdZnTe detectors is usually limited by the poor transport properties of holes. Devices segmented into small pixels have been observed to exhibit improved energy resolutions. Simulations have shown that this small pixel effect is due to the fact that small pixels are mostly sensitive to carriers moving close to the pixel, within a distance of the order of the pixel size. In this paper, signals are calculated for CdZnTe strip detectors in order to determine to what extent a similar small electrode effect is produced by strips. The free carrier density distributions following the absorption of a /spl gamma/-ray are calculated by solving the continuity equations. Combined with the strip weighting field, this provides the signal induced in the strip. Simulations are made for various detector geometries and for both the anode and cathode strips. Simulated signals are compared with actual signals measured on CdZnTe detectors.

The Advanced Technology Large Aperture Space Telescope (ATLAST) team identified five key technology areas to enable candidate architectures for a future large-aperture ultraviolet/optical/infrared (LUVOIR) space observatory envisioned by the NASA Astrophysics 30-year roadmap, "Enduring Quests, Daring Visions." The science goals of ATLAST address a broad range of astrophysical questions from early galaxy and star formation to the processes that contributed to the formation of life on Earth, combining general astrophysics with direct-imaging and spectroscopy of habitable exoplanets. The key technology areas are internal coronagraphs, starshades (or external occulters), ultra-stable large-aperture telescope systems, detectors, and mirror coatings. For each technology area, we define best estimates of required capabilities, current state-of-the-art performance, and current technology readiness level (TRL), thus identifying the current technology gap. We also report on current, planned, or recommended efforts to develop each technology to TRL 5.

We present results on wavelength division multiplexing of radio-frequency single-electron transistors. We use a network of resonant impedance matching circuits to direct applied rf carrier waves to different transistors depending on carrier frequency. A two-channel demonstration of this concept using discrete components successfully reconstructed input signals with small levels of crosscoupling. A lithographic version of the rf circuits had measured parameters in agreement with electromagnetic modeling, with reduced crosscapacitance and inductance, and should allow 20–50 channels to be multiplexed.

Rapid progress in the AlGaN (E g =3.4-6.2eV), 4H-SiC (E g =3.2eV) and ZnMgO (E g =2.8-7.9eV) material systems over the last five years has led to the demonstration of a number of opto-electronic devices. These wide energy band gap devices offer several key advantages for space applications, over conventional Si (E g =1.1eV) based devices, such as visible-blind detection, high thermal stability, better radiation hardness, high breakdown electric field, high chemical inertness and greater mechanical strength. Furthermore, the shorter cut-off wavelength of these material systems eliminates the need for bulky and expensive optical filtering components mitigating risk and allowing for simpler optical design of instrumentation. In this paper, we report on the development at NASA/Goddard of ultra-sensitive, high quantum efficiency AlGaN and 4H-SiC Schottky barrier UV-EUV photodiodes, 4H-SiC UV single photon avalanche diodes, large format 256x256 AlGaN UV p-i-n photodiode arrays and recent progress in elemental substitution for p-type and enhanced n-type doping of ZnO.

We report our in-depth study of Cd-Zn-Te (CZT) crystals to determine an optimum pixel and guard band configuration for Hard X-ray imaging and spectroscopy. We tested 20x20x5mm crystals with 8x8 pixels on a 2.46mm pitch. We have studied different types of cathode / anode contacts and different pixel pad sizes. We present the measurements of leakage current as well as spectral response for each pixel. Our I-V measurement setup is custom designed to allow automated measurements of the I-V curves sequentially for all 64 pixels, whereas the radiation properties measurement setup allows for interchangeable crystals with the same XAIM3.2 ASIC readout from IDEAS. We have tested multiple crystals of each type, and each crystal in different positions to measure the variation between individual crystals and variation among the ASIC channels. We also compare the same crystals with and without a grounded guard band deposited on the crystal side walls vs. a floating guard band and compare results to simulations. This study was carried out to find the optimum CZT crystal configuration for prototype detectors for the proposed Black-Hole Finder mission, EXIST., To appears in the SPIE 2004 proceedings (5540: Hard X-ray and gamma-ray detector physics V)

We discuss a new type of direct detector, a silicon hot-electron bolometer, for measurements in the far-infrared and submillimeter spectral ranges. High performance bolometers can be made using the electron-phonon conductance in heavily doped silicon to provide thermal isolation from the cryogenic bath. Noise performance is expected to be near thermodynamic limits, allowing background limited performance for many far infrared and submillimeter photometric and spectroscopic applications.

We report detector characteristics and background measurements from two prototype imaging CZT detectors flown on a scientific balloon payload in May 2001. The detectors are both platinum-contact 10mm x 10mm x 5mm CZT crystals, each with a 4 $\times$ 4 array of pixels tiling the anode. One is made from IMARAD horizontal Bridgman CZT, the other from eV Products high-pressure Bridgman material. Both detectors were mounted side-by-side in a flip-chip configuration and read out by a 32-channel IDE VA/TA ASIC preamp/shaper. We enclosed the detectors in the same 40deg field-of-view collimator (comprisinga graded passive shield and plastic scintillator) used in our previously-reported September 2000 flight. I-V curves for the detectors are diode-like, and we find that the platinum contacts adhere significantly better to the CZT surfaces than gold to previous detectors. The detectors and instrumentation performed well in a 20-hour balloon flight on 23/24 May 2001. Although we discovered a significant instrumental background component in flight, it was possible to measure and subtract this component from the spectra. The resulting IMARAD detector background spectrum (from 30 keV to ~450 keV) reaches ~5 x 10^{-3}$ counts/cm^2 -sec-keV at 100 keV and has a power-law index of ~2 at high energies. The eV Products detector has a similar spectrum, although there is more uncertainty in the energy scale because of calibration complications.

After the design of the calorimeter array for the high-resolution x-ray spectrometer (XRS) on the original Astro-E was frozen, new fabrication techniques became available and our understanding of these devices continually increased. We are now able to complete the optimization of this technology and, potentially, to increase the capability of new XRS instrument for Astro-E2, our on-going sounding recket experiments, and possible further applications. The most significant improvement comes from greatly reducing the excess noise of the ion-implanted thermistors by increasing the thickness of the implanted region.

We are developing superconducting direct detectors for submillimeter astronomy that can in principle detectindividual photons. These devices, Single Quasiparticle Photon Counter (SQPC), operate by measuring thequasiparticles generated when single Cooper-pairs are broken by absorption of a submillimeter photon. Thisphotoconductive type of device could yield high quantum efficiency, large responsivity, microsecond responsetimes, and sensitivities in the range of 10 -20 Watts per root Hertz. The use of antenna coupling to a smallabsorber also suggests the potential for novel instrument designs and scalability to imaging or spectroscopicarrays. We will describe the device concept, r ecent results on fabrication and electrical characterization of thesedetectors, issues related to saturation and optimization of the device parameters. Finally, we have developedpractical readout amplifiers for these high-impedance cryogenic detectors based on the Radio-FrequencySingle-Electron Transistor (RF-SET). We will describe results of a demonstration of a transimpedanceamplifier based on closed-loop operation of an RF-SET, and a demonstration of a wavelength-divisionmultiplexing scheme for the RF-SET. These developments will be a key ingredient in scaling to large arrays ofhigh-sensitivity detectors.Keywords: submillimeter, superconducting detector

We report the status of ongoing investigations into cadmium zinc telluride (CZT) strip detectors for application in hard X-ray astronomy. We have instrumented a nine strip by nine strip region of a two sided strip detector made in our detector fabrication facility. In order to measure the position resolution of our detectors, we have implemented a collimated beam that concentrates radiation to a spot size less than 50 /spl mu/m. We have placed a 30 /spl mu/m upper limit on the width of the point spread function for this system. We have also performed charge collection studies as a function of incident photon energy and bias voltage with a single sided, 100 /spl mu/m pitch CZT strip detector wire bonded to an SVX ASIC charge amplifier. The detectors exhibited excellent strip uniformity in terms of photon count rate and spectroscopic information.

There is a great need in hard X-ray and gamma-ray astrophysics for large area, room temperature, lightweight, semiconductor detectors that have comparable energy resolution to germanium without the inconvenient cooling requirements. Recent improvements in the quality of cadmium zinc telluride (CdZnTe) and mercuric iodide (HgI/sub 2/) materials have brought room temperature semiconductors to the point where real astrophysics applications may seriously be considered. Future astrophysics instruments will be described, including PoRTIA, a small engineering prototype balloon instrument that will make material dependent background measurements at balloon altitudes. In addition to application oriented detector development, basic issues such as detector spectral performance, noise, thermal behavior and bulk transport properties in CdZnTe have also been studied. Preliminary results from this CdZnTe detector research will be presented. >

We are optimizing Mo/Au transition-edge-sensor (TES) calorimeters to meet the specifications of NASA’s Constellation-X mission. Our calorimeters have already demonstrated very good energy resolution of X rays (2.4 eV at 1.5 keV). We wish to further improve the energy resolution by reducing excess noise in the calorimeters. Development of a detailed model and understanding of the noise is instrumental to reaching this goal. Towards that end, we employ a linear model that describes the response of a calorimeter to signal and various sources of noise. The model is based on detailed measurements of the parameters that affect the calorimeter’s performance, such as current-voltage characteristics of the TES, thermal conductance of our silicon-nitride membranes, and inductance in the electronic circuit used to bias the TES. We determine the sharpness of the superconducting phase transition by fitting the model to the measured responsivity of the calorimeter. The model relates sources of noise, such as phonon noi...

We report the first results from a high-energy-resolution imaging spectrometer called a Position-Sensitive Transition-Edge Sensor (PoST). A PoST is a quantum calorimeter consisting of two Transition Edge Sensors (TESs) on the ends of a long absorber to do one dimensional imaging spectroscopy. Comparing rise time and energy information, the position of the event in the PoST is determined. Energy is inferred from the sum of the two pulses. We have fabricated 7- and 15-pixel PoSTs using Mo-Au TESs and Au absorbers. We have achieved 32 eV FWHM energy resolution at 1.5 keV with a 7-pixel PoST calorimeter.

We describe work on a wavelength division multiplexing scheme for radio-frequency single electron transistors. We use a network of resonant impedance matching circuits to direct applied rf carrier waves to different transistors depending on carrier frequency. Using discrete components, we made a two-channel demonstration of this concept and successfully reconstructed input signals with small levels of cross coupling. A lithographic version of the rf circuits had measured parameters in agreement with electromagnetic modeling, with reduced cross capacitance and inductance, and should allow 20 to 50 channels to be multiplexed.

Most single grains in cadmium zinc telluride (CdZnTe) grown by the high-pressure Bridgman (HPB) technique contain multiple twin boundaries. As a consequence, twin boundaries are one of the most common macroscopic material defects found in large area (400 to 700 mm2) CdZnTe specimens obtained from HPB ingots. Due to the prevalence of twin boundaries, understanding their effect on detector performance is key to the material selection process. Twin boundaries in several 2 mm thick large area specimens were first documented using infrared transmission imaging. These specimens were then fabricated into either 2mm pixel or planar detectors in order to examine the effect of the twin boundaries on detector performance. Preliminary results show that twin boundaries, which are decorated with tellurium inclusions, produce a reduction in detector efficiency and a degradation in resolution. The extent of the degradation appears to be a function of the density of tellurium inclusions.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In the X-ray astrophysics community, the desire for wide- field, high-resolution, X-ray imaging spectrometers has been growing for some time. We present a concept for such a detector called a Position-Sensing Transition-edge sensor (PoST). A PoST is a calorimeter consisting of two Transition- Edge Sensors (TESs) on the ends of a long absorber to do one dimensional imaging spectroscopy. Comparing the rise time and energy estimates obtained from each TES for a given event, the position of that event in the PoST is determined. Energy is inferred from the sum of the two signals on the TESs. We have designed 7, 15, and 32 pixel PoSTs using our Mo/Au TESs and bismuth absorbers. We discuss the theory, modeling, operation and readout of PoSTs and the latest results from our development.

The XRS instrument has an array of 32 micro-calorimeters at the focal plane. These calorimeters consist of ion-implanted silicon thermistors and HgTe thermalizing x-ray absorbers. These devices have demonstrated a resolution of 9 eV at 3 keV and 11 eV at 6 keV. We will discuss the basic physical parameters of this array, including the array layout, thermal conductance of the link to the heat sink, operating temperature, thermistor size, absorber choice, and means of attaching the absorber to the thermistor bearing element. We will present representative performance data, though a more detailed presentation of the results of the instrument calibration is presented elsewhere in these proceedings. A silicon ionization detector is located behind the calorimeter array and serves to reject events due to cosmic rays. We will briefly describe this anti-coincidence detector and its performance in conjunction with the array.

One of the critical challenges for large area cadmium zinc telluride (CdZnTe) detector arrays is obtaining material capable of uniform imaging and spectroscopic response. Two complementary nondestructive techniques for characterizing bulk CdZnTe have been developed to identify material with a uniform response. The first technique, infrared transmission imaging, allows for rapid visualization of bulk defects. The second technique, x-ray spectral mapping, provides a map of the material spectroscopic response when it is configured as a planar detector. The two techniques have been used to develop a correlation between bulk defect type and detector performance. The correlation allows for the use of infrared imaging to rapidly develop wafer mining maps. The mining of material free of detrimental defects has the potential to dramatically increase the yield and quality of large area CdZnTe detector arrays.

We are developing novel photodetector arrays based on superconducting transition-edge sensor (TES) and pop-up detector(PUD) technologies. The TES has the potential for a new generation of high sensitivity photodetectors from the infrared tothe x-ray. This is directly due to the sharpness of the resistance change with temperature at the superconducting transition.The TESs are deposited on the PUD arrays and serve as the sensing elements. The PUDs are close-packed, folded membranearrays that provide the TES substrate and the thermal isolation required by the bolometers and microcalorimeters. This paperpresents the processing-related characterization results of preliminary TES and PUD designs. The goal of this work is tofabricate a new generation of x-ray calorimeters and infrared bolometers for space flight projects.Al-Ag and Mo-Au TES bilayers were fabricated by electron-beam deposition in an ultra-high vacuum system. To achieve thedesired transition temperatures and stability, materials-related properties such as adhesion, surface morphology, film stress,and electrochemical reaction ofthe bilayers were studied. Mo-Au bilayers showed better chemical and electrical stability thanAl-Ag bilayers. The PUD arrays were fabricated with a 0.2mm pitch on silicon wafers. MEMS techniques were used in thefabrication of PUD arrays. Mechanical folding tests were conducted to inspect the mechanical stress distribution for the arraystructures. The test results showed yields for fourteen designs. Mechanical modeling was conducted using non-linear staticand dynamic simulations. The simulations agreed with the trends of mechanical folding tests. Scanning electron microscopywas utilized to examine the flatness of detectors and the alignment of detectors in arrays.Key Words: Superconducting transition edge sensor, Pop-up detector, Mo-Au bilayer, Al-Ag bilayer, bolometer,microcalorimeter, Anisotropic etching, FEM simulation.

Cadmium zinc telluride (CsZnTe) has been applied as a X-ray and gamma-ray radiation detector due to its large bandgap and high atomic number which gives low leakage current and good absorption efficiency. In addition, the CdZnTe detectors can be operated at room temperature with very good energy resolution and excellent spatial resolution. The main challenges in fabricating a large area CdZnTe strip detector array are the requirements of low leakage current in the individual strips of each detector and the good adhesion of the pad metal to the CdZnTe surface for wire bonding. The Detector Systems Branch at Goddard Space Flight CEnter has successfully fabricated the double sized CdZnTe strip detectors for a 6 X 6 array. The strip detector array has 36 double sided detectors with each detector having 127 strips and a guard ring on each side with a spatial resolution < 100 micrometers . The total area of the array is 60 cm2 with 762 X 762 strips to give 580,000 pixels - a factor of 100 better than other CdZnTe or scintillator arrays. In this paper, we will present our novel processing for fabricating such high performance strip detectors. The leakage current, interstrip resistance and energy resolution were studied as a function of different etchants/time and post-annealing temperature. The effects of chemical etching and post annealing on the CdZnTe surface and interface property were also discussed.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

CdZnTe detector technology has been developed at NASA Goddard for imaging and spectroscopy applications in hard x-ray and gamma ray astronomy. A CdZnTe strip detector array with capabilities for arc second imaging and spectroscopy has been built as a prototype for a space flight gamma ray burst instrument. CdZnTe detectors also have applications for medical imaging, environmental protection, transportation safety, nuclear safeguards and safety, nuclear non-proliferation, and national security. This can be accomplished from space and also from portable detectors on earth. One of the great advantages of CdZnTe is that the detectors can be operated at room temperature which eliminates the need for cryogenic cooling. CdZnTe detectors have good energy resolution (3.6 keV at 60 keV) and excellent spatial resolution (

The burst and all sky imaging survey (BASIS) is a proposed mission to provide plus or minus 3 arc-second locations of an estimated 90 gamma-ray bursts (GRBs) per year. The BASIS coded aperture imaging system requires a segmented detector plane able to detect the position of photon absorption to less than 100 microns. To develop prototype detector arrays with such fine position resolution we have fabricated many 15 mm by 15 mm by 2 mm 100 micron pitch CdZnTe strip detectors. A 2 by 2 prototype 100 micron CdZnTe strip detector array has been fabricated and has been used to test the capabilities of the BASIS imaging system. Preliminary shadowgrams of a 1 mm wide gap between two tungsten straight edges indicate that our position resolution is on the order of 69 micrometers. Both the array and imaging tests are described. A 6 by 6 element CdZnTe detector array is also being fabricated at GSFC. The assembly of this flight prototype array is discussed as well as applications for BASIS.

A CdZnTe strip detector array with capabilities for arc second imaging and spectroscopy has been built as a prototypefor a space flight gamma ray burst instrument. Two dimensional orthogonal strip detectors with 100 im pitch have beenfabricated and tested for a large area array ( 60 cm2 with 36 detectors). Details for the fabrication and evaluation of thedetectors will be presented. Critical issues to be addressed include fabricating metal contacts with low leakage current andwith excellent wire bonding yield, achieving high yield for good strips, and surface cleaning and passivation.Keywords: CdZnTe, strip detector, gamma ray, hard x-ray, imaging, spectroscopy, wire bonding, fabrication, testing, array

One of the critical issues in CdZnTe detector fabrication is the surface treatment. This will not only affect the electrical properties, such as leakage current, but also influence the physical properties, such as smoothness and adhesion between the metal and the semiconductor. The latter will determine the wire bonding yield. Historically, there has been a problem in achieving both low leakage current and excellent wire bonding yield. In this paper, we report our new approach to fabricate high performance doubled sided CdZnTe strip detectors. The new surface treatments involve chemical etching and post-annealing. The leakage current, interstrip resistance and energy resolution were studied as a function of different etchants/time and post-annealing temperature. It was found that a chemical etch with bromine in ethylene glycol (Br/EG) is suitable for the double sided strip detector process. Keeping a relatively smooth surface is critical for achieving a high yield of good strips. To improve the adhesion of the metal to CdZnTe for wire bonding, the detectors were annealed from 100 to 175°C for 10 hours. It has been observed that after annealing, not only has the strip leakage current decreased, but the interstrip resistance is increased for a temperature less than 150°C.

The Burst and All Sky Imaging Survey (BASIS) is a proposed mission to provide ∼3 arc second locations of approximately 90 Gamma-Ray Bursts (GRBs) per year. The BASIS coded aperture imaging system requires a segmented detector plane able to detect the interaction position of (10 - 150 keV) photons to less than 100 μm. To develop prototype detector arrays with such fine position resolution we have fabricated many 15 mm × 15 mm × 2 mm 100 μm pitch CdZnTe strip detectors. We have assembled these fine pitch CdZnTe strip detectors into prototype 2 × 2 and 6 × 6 element arrays read out by ASIC electronics. The assembly and electronics readout of the 6 × 6 flight prototype array will be discussed, and preliminary data illustrating the uniformity and efficiency of the array will be presented.

A CdZnTe strip detector large area array (∼ 60 cm2 with 36 detectors) with capabilities for high resolution imaging and spectroscopy has been built as a prototype for a space flight gamma ray burst instrument. The detector array also has applications in nuclear medical imaging. Two dimensional orthogonal strip detectors with 100 gm pitch have been fabricated and tested. Details for the array design, fabrication and evaluation of the detectors will be presented.

Because of its high atomic number and convenient room temperature operation, CdZnTe has great potential for use in both balloon and space borne hard x-ray (5 - 200 keV) astrophysics experiments. Here we present preliminary results from the first CdZnTe background measurements made by a balloon instrument. Measurements of the CdZnTe internal background are essential to determine which physical processes make the most important background contributions and are critical in the design of future scientific instruments. The PoRTIA CdZnTe balloon instrument was flown three times in three different shielding configurations. PoRTIA was passively shielded during its first flight from Palestine, Texas and actively shielded as a piggyback instrument on the GRIS balloon experiment during flights 2 and 3 from Alice Springs, Australia. PoRTIA flew twice during the Fall 1995 Alice Springs, Australia campaign using the thick GRIS NaI anticoincidence shield. A significant CdZnTe background reduction was achieved during the third flight with PoRTIA placed completely inside the GRIS shield and blocking crystal, and thus completely surrounded by 15 cm of NaI. These background results are presented and contributions from different background processes are discussed.

CdZnTe strip detectors have been fabricated and tested to show the ability for arc second imaging and spectroscopy. Two dimensional CdZnTe strip detectors with 100 micron pitch have been fabricated and wire bonded to readout electronics to demonstrate the ability to localize 22 to 122 keV photons to less than 100 microns. Good spectral resolution has also been achieved. The uniformity and relative efficiency of the strip detector are discussed. Radiation damage effects by intermediate energy neutrons and low energy protons on the surface and bulk performance of CdZnTe devices have been investigated and are presented. Activation and annealing of radiation effects have been seen and are discussed.

A CdZnTe strip detector array with capabilities for arc second imaging and spectroscopy is being developed for a space flight gamma-ray burst instrument. Two dimensional strip detectors with 100 micrometers pitch have been fabricated and wire bonded to readout electronics to demonstrate the ability to localize 22 to 122 keV photons to less than 100 micrometers. In addition, good spectral resolution has been achieved. The uniformity of response and relative efficiency of the strip detector will be discussed. Results form electrical characterization which include strip leakage current and strip capacitance will be presented.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The James Webb Space Telescope (JWST) is the successor to the Hubble Space Telescope. JWST will be an infrared-optimized telescope, with an approximately 6.5 m diameter primary mirror, that is located at the Sun-Earth L2 Lagrange point. Three of JWST’s four science instruments use Teledyne HgCdTe HAWAII-2RG (H2RG) near infrared detector arrays. During 2010, the JWST Project noticed that a few of its 5 μm cutoff H2RG detectors were degrading during room temperature storage, and NASA chartered a “Detector Degradation Failure Review Board” (DD-FRB) to investigate. The DD-FRB determined that the root cause was a design flaw that allowed indium to interdiffuse with the gold contacts and migrate into the HgCdTe detector layer. Fortunately, Teledyne already had an improved design that eliminated this degradation mechanism. During early 2012, the improved H2RG design was qualified for flight and JWST began making additional H2RGs. In this article, we present the two public DD-FRB “Executive Summaries” that: (1) determined the root cause of the detector degradation and (2) defined tests to determine whether the existing detectors are qualified for flight. We supplement these with a brief introduction to H2RG detector arrays, some recent measurements showing that the performance of the improved design meets JWST requirements, and a discussion of how the JWST Project is using cryogenic storage to retard the degradation rate of the existing flight spare H2RGs.

Room temperature cadmium zinc telluride (CdZnTe) and mercuric iodide (HgI2) semiconductor hard X-ray detectors are currently being evaluated at NASA Goddard Space Flight Center for use in future balloon and satellite applications. PoRTIA, a small engineering prototype hard X-ray (20 - 150 keV) balloon instrument will contain both a CdZnTe and a HgI2 detector, each 6.5 cm2 x .15 - .2 cm and sharing the same 5 degree(s) field-of-view. PoRTIA will be launched from Alice Springs, Australia in the Spring of 1995 as a piggyback instrument aboard the GRIS balloon payload. PoRTIA will provide valuable information about detector efficiency, durability and material dependent detector background components at balloon altitudes as it observes the Crab Nebula. In addition, a CdZnTe research and development program has been initiated to develop the capability to produce improved CdZnTe detectors for astrophysics applications. The work at Goddard continues in an effort to develop CdZnTe detectors with improvements in electronics, contacts and packaging methods.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Semiconductor thermistors have been used for several years and their ideal behavior is well known both experimentally and theoretically. Their current performance is limited by non-ideal behaviors. These include 1/f noise and non-ohmic effects. We find that the 1/f noise appears to be a 2-D effect, and can be greatly reduced by fabricating thicker thermistors. Eliminating this noise could improve the intrinsic detector resolution as much as 40%. It also allows us to study other sources of excess noise in the thermometer. The non-ohmic behavior can be empirically explained using a hot-electron model. Although this model does not seem suitable for semiconductors in the variable range-hopping regime, where the electrons are localized, it fits the experimental data quite well. We measured an excess white noise at low frequencies consistent with the predicted thermodynamic fluctuations between electrons and phonons. We also measured a characteristic time of the non-ohmic behavior that is consistent with a C/G ...

We have characterized the 1/f noise in standard ion-implanted silicon thermistors, which are about 250 nm thick. We find that it is associated with the bulk of the implant, and is interpretable as a ∆R/R fluctuation that is independent of the bias and depends only on the doping density and resistivity, or electron temperature. This excess noise is large enough that it has a significant effect on the energy resolution or NEP of a detector using these thermistors. The very steep temperature dependence of the 1/f noise suggested that it might be related to the conduction becoming two- dimensional, and we have fabricated thicker detectors to test this hypothesis. Similar doped silicon thermistors that are 1500 nm thick show negligible 1/f noise, but otherwise behave identically to the thinner thermistors of the same volume. This simple change could provide a 40% improvement in resolution for some existing X-ray detectors.