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Efficient, safe and cost-effective synthesis of CdTe from elements is rather challenging in silica sealed ampoules due to the high vapor pressure of Cd. In this article, we report on the integrated synthesis and crystal growth of high-purity CdTe using the high pressure Bridgman (HPB) technique that is scalable to large volumes. The process lends itself for cost competitive industrial production of polycrystalline feedstock material for photovoltaics, sensors and electro-optic applications. Cadmium telluride (CdTe) crystals exceeding 1 kg in size were synthesized from elemental Cd and Te sources with purity comparable to state-of-the-art gamma ray detector crystals. In addition, synthesis of highly-doped CdTe feedstock for thin film photovoltaics applications demonstrating effective incorporation of group V (As, Sb) dopants was achieved at growth speeds of ~500 mm/hr. The technique may be applicable to produce other II-VI compounds with volatile components.

The transport, trapping, and subsequent detrapping of charge in single crystals of semi-insulating cadmium zinc telluride (CdZnTe) has been analyzed using multiple-scale perturbation techniques. This method has the advantage of not only treating impulse charge generation typical in spectroscopic analysis, but also a large class of continuous generation sources more relevant to high-flux x-ray imaging applications. We first demonstrate that the multiple-scale solutions obtained for small-current transients induced by an impulse generation of charge are consistent with well-known exact solutions. Further, we use the multiple-scale solutions to derive an analytic generalization of the Hecht equation that incorporates detrapping over times much longer than the carrier transit time (i.e., delayed signal components). The method is then applied to a continuous charge generation source that approximates that of an x-ray source. The space–time solutions obtained are relevant to detector design in high-flux x-ray imaging applications. Throughout this work the multiple-scale solutions are compared with exact solutions as well as full numerical solutions of the fundamental charge conservation equations.

Next generation high-flux X-ray imaging technology is expected to advance towards multi-color or spectroscopic imaging and will significantly expand the capabilities of the technique in a multitude of applications. Spectroscopic X-ray imaging will require energy-sensitive detector arrays. In this work we evaluated the applicability of pulse-mode CdZnTe detector arrays to high-flux spectroscopic imaging. To study the material and device performance limitations of currently available CdZnTe detectors under high-flux X-ray irradiation we designed a 2D monolithic CdZnTe test array and associated test system. The detector arrays were 16 times 16 pixel devices with 0.4 mm times 0.4 mm area pixels on a 0.5 mm pitch and were fabricated using 8.7 mm times 8.7 mm times 3.0 mm CdZnTe single crystals. We measured the high-flux performance of over 1200 such arrays with various bulk CdZnTe crystal properties using a 120 kVp X-ray source and our custom built test system. We studied the various static and dynamic charge collection effects typically not observed in low-flux applications. These included dynamic polarization, static charge steering and dynamic lateral polarization and charge steering. In parallel with the experimental effort we developed a dynamic charge transport and trapping model to describe the experimentally observed static, dynamic and transient phenomena. For the first time we demonstrated > 15 times 106 counts/s/mm2 count-rate for several hundred such CdZnTe detector arrays. In addition we demonstrated good < 1% short term count-rate stability of the detector arrays.

The dynamic lateral polarization and charge steering effect was studied in 2D pixilated CdZnTe monolithic detector arrays designed for high flux X-ray imaging applications. While these detectors have shown the ability to work at 15 times 106 counts s-1 mm-2 and higher count-rates in pulse mode, we observed some detectors that exhibited a dynamic lateral polarization and charge steering effect causing non-uniform spatial response to the radiation field. The dynamic nature of the effect is shown by its flux dependence and reversibility upon changing the X-ray flux without a requirement to turn off the bias voltage. The effect causes the induced charge that would normally move from the cathode towards the anode to instead move laterally causing counts to shift away from a flux boundary. We show that the effect is not related to the physical boundary of the detector but rather related to the boundary of the irradiated area of the device. The dynamic polarization and charge steering effect can be attributed to the limited hole transport in the bulk material causing a buildup of a dynamic space-charge region under the irradiated area. The resulting lateral (perpendicular to the irradiation direction) electric field causes the lateral drift (steering) of the X-ray injected charge clouds. The static version of such lateral steering is often observed for charged structural defects in CdZnTe crystals. The studied 2D CdZnTe monolithic arrays were 16 times16 pixel devices having 0.4 mm times 0.4 mm area pixels on a 0.5 mm pitch and were fabricated using 8.7 mm times 8.7 mm times3.0 mm CdZnTe single crystals grown by the high-pressure electro-dynamic gradient freeze technique. The devices were probe tested in a system consisting of a custom 16 times16 pin probe head, 256 channel read-out electronics utilizing 8-channel fast bipolar ASIC chips, and a computer controlled 120 kVp X-ray source.

The performance of 2-D CdZnTe monolithic detector arrays designed for high flux X-ray imaging applications was studied. For the first time we have obtained 5 times 106 counts/s/mm2 count-rate for a CdZnTe pixelated detector array. This count-rate is more than twice the highest count-rate ever achieved using a CdZnTe detector array. Such excellent performance was demonstrated for more than 600 individual CdZnTe detector arrays. The 2-D CdZnTe monolithic arrays were 16 x 16 pixel devices with 0.4 mm times 0.4 mm area pixels on a 0.5 mm pitch and were fabricated using 8.7 mm times 8.7 mm times 3.0 mm CdZnTe single crystals grown by the high-pressure, electro-dynamic gradient freeze technique. The CdZnTe detector arrays were bonded to a ceramic substrate with the Z-bondtrade technique. This enabled performance testing of the individual detector arrays before bonding to the read-out ASIC chip. The detector arrays were characterized in a custom designed test system. The measurement and data acquisition system consisted of a 16 times 16 pin probe head and 256-channel read-out electronics controlled by a host PC. We utilized our 8-channel fast bipolar ASIC chip and computer controlled 120 kVp X-ray source. In order to measure the true throughput of the CdZnTe devices a counts correction method was developed and implemented that compensates for the counting system non-linearity caused by pile-up and amplifier shaping time effects. Survey of detector array performance as a function of CdZnTe charge transport properties showed that the maximum achievable count-rate of these detectors strongly depends on the hole charge transport properties of the crystals.

We have studied the defect levels in as grown and post growth processed cadmium telluride (CdTe) using thermoelectric effect spectroscopy (TEES) and thermally stimulated current (TSC) techniques. We have extracted the thermal energy (Eth) and trapping cross section (σth) for the defect levels using the initial rise and variable heating rate methods. We have identified 10 different defect levels in the crystals. Thermal ionization energy values obtained experimentally were compared to theoretical values of the transition-energy levels of intrinsic and extrinsic defects and defect complexes in CdTe determined by first-principles band-structure calculations. On the basis of this comparison, we have associated the observed ionization levels with various native defects and impurity complexes.

A multi-scale model has been developed to study the growth of single crystals of cadmium zinc telluride (CZT) in an industrial electrodynamic gradient freeze (EDG) furnace. A global model (CrsyVUn) that computes furnace heat transfer is coupled with a local model (CATS2D) that solves for heat transfer, melt flow, and melt–crystal interface shape in an iterative scheme to compute a self-consistent solution. Details of how the two models are coupled have a strong bearing on the convergence of the iterations. One-way coupling of the models, i.e., providing either flux or temperature boundary conditions from the global model to the local model without iteration, yields solutions that differ significantly from the self-consistent solution of the multi-scale model.

The high-pressure electro-dynamic gradient (HP-EDG) crystal-growth technology has been recently developed and introduced at eV PRODUCTS to grow large-volume, semi-insulating (SI) CdZnTe single crystals for room-temperature x-ray and gamma-ray detector applications. The new HP growth technology significantly improves the downstream CdZnTe device-fabrication yield compared to earlier versions of the HP crystal-growth technology because of the improved structural and charge-transport properties of the CdZnTe ingots. The new state-of-the-art, HP-EDG crystal-growth systems offer exceptional flexibility and thermal and mechanical stability and allow the growth of high-purity CdZnTe ingots. The flexibility of the multi-zone heater system allows the dynamic control of heat flow to optimize the growth-interface shape during crystallization. This flexibility combined with an advanced control system, improved system diagnostics, and realistic heat-transport modeling provides an excellent platform for continuing process development. Initial results on large-diameter (140 mm), SI Cd1−xZnxTe (x=0.1) ingots grown in low temperature gradients with the HP-EDG technique show reduced defect density and complete elimination of ingot cracking. The increased single-crystal yield combined with the improved charge transport allows the fabrication of large-volume, high-sensitivity, high energy-resolution detector devices at increased yield. The CdZnTe ingots grown to date produced large-volume crystals (≥1cm3) with electron mobility-lifetime product (μτe) in the (3–7) × 10−3 cm2/V range. The lower-than-desired charge-transport uniformity of the HP-EDG CdZnTe ingots is associated with the high density of Te inclusions formed in the ingots during crystallization. The latest process-development efforts show a reduction in the Te-inclusion density, an increase of the charge-transport uniformity, and improved energy resolution of the large-volume detectors fabricated from these crystals.

The performance of CdZnTe room-temperature X-ray and gamma-ray detectors is determined by material and device defects that govern carrier transport trough the device. In this contribution, we review common bulk, interface, and surface defects and their effects on charge transport, charge transport uniformity, and device performance. We note that pure CdZnTe grown under Te-rich conditions has an excess of Cd-vacancies and other Te-related native defects and must be electrically compensated in order to obtain high resistivity material. Through the critical analysis of the various compensation schemes it is shown that deep level defects must be introduced with donor doping elements in order to achieve a practical compensation technique. The role of carrier trapping and limitations on detector performance with increasing crystal size are discussed. Based on typical measured carrier lifetimes and the available literature data on carrier capture cross sections, we estimate that the residual acceptor concentration in CdZnTe detector crystals is much lower than widely thought, about 10/sup 11/ cm/sup -3/ instead of 10/sup 15/ cm/sup -3/. The deleterious effects of structural defects within single crystals are also discussed. We also provide a brief overview of the progress in CdZnTe crystal growth and device fabrication technologies aiming at reducing the concentration of the detrimental defects and improving CdZnTe detector performance.

Good detection efficiency and high energy-resolution make Cadmium Zinc Telluride (CdZnTe) and Cadmium Telluride (CdTe) detectors attractive in many room temperature X-ray and gamma-ray detection applications such as medical and industrial imaging, industrial gauging and non-destructive testing, security and monitoring, nuclear safeguards and non-proliferation, and astrophysics. Advancement of the crystal growth and device fabrication technologies and the reduction of bulk, interface and surface defects in the devices are crucial for the widespread practical deployment of Cd 1-x Zn x Te-based detector technology. Here we review the effects of bulk, interface and surface defects on charge transport, charge transport uniformity and device performance and the progress in the crystal growth and device fabrication technologies aiming at reducing the concentration of harmful defects and improving Cd 1-x Zn x Te detector performance.

We present the design of the gamma-ray and neutron spectrometer (GR/NS) for Dawn, which is a NASA Discovery-class mission to explore two of the largest main-belt asteroids, 1 Ceres and 4 Vesta, whose accretion is believed to have been interrupted by the early formation of Jupiter. Dawn will determine the composition and structure of these protoplanetary bodies, providing context for a large number of primitive meteorites in our sample collection and a better understanding of processes occurring shortly after the onset of condensation of the solar nebula. The Dawn GR/NS design draws on experience from the successful Lunar Prospector and Mars Odyssey missions to enable accurate mapping of the surface composition and stratigraphy of major elements, radioactive elements, and hydrogen at both asteroids. Here, we describe the overall design of the GR/NS and compare the expected performance of the neutron spectrometer subsystem to the neutron spectrometer on Mars Odyssey. We also describe radiation damage studies carried out on CdZnTe detectors, which will be components of the primary gamma-ray spectrometer on Dawn. We conclude that provisions for annealing at moderate temperatures (40/spl deg/C to 60/spl deg/C) must be made to ensure that the spectrometer will function optimally over the nine-year mission.

The growth of large-volume semi-insulating CdZnTe single crystals with improved structural perfection has been demonstrated by the electrodynamic gradient (EDG) technique and active control of the Cd partial pressure in the ampoule. The EDG furnace nearly completely eliminates the uncontrolled radiative heat transport commonly encountered in traditional Bridgman systems where the charge and furnace move relative to each other. Since the new furnace utilizes electronically controlled high-precision gradient translation, it achieves superior thermal stability throughout the growth. The control of the Cd partial pressure allowed the solidification and cool-down of the ingots close to the stoichiometric composition. As a result, the formation and incorporation of large-size (/spl ges/1 /spl mu/m diameter) Te inclusions was avoided during crystallization and ingots with high structural perfection were achieved. Adequate electrical compensation has been achieved in most of the crystal growth experiments yielding CdZnTe crystals with bulk electrical resistivity in the 10/sup 9/-10/sup 10/ /spl Omega//spl middot/cm range and electron mobility-lifetime product as high as /spl mu//spl tau//sub e/=1.2/spl times/10/sup -3/ cm/sup 2//V. The materials exhibit good spectral performance in the parallel plate detector configuration.

We present the design and analysis of a new gamma ray spectrometer for planetary science that uses an array of CdZnTe detectors to achieve the detection efficiency needed for orbital measurements. The use of CdZnTe will provide significantly improved pulse height resolution relative to scintillation-based detectors, with commensurate improvement in the accuracy of elemental abundances determined by gamma ray and neutron spectroscopy. The spectrometer can be flown either on the instrument deck of the spacecraft or on a boom. For deck-mounted systems, a BGO anticoincidence shield is included in the design to suppress the response of the CdZnTe detector to gamma rays that originate in the spacecraft. The BGO shield also serves as a backup spectrometer, providing heritage from earlier planetary science missions and reducing the risk associated with the implementation of new technology.

Tl6SI4 is a promising material for room-temperature semiconductor radiation detection applications. The history of the development of semiconductor radiation detection materials has demonstrated that impurities strongly affect the carrier transport and that material purification is a critically important step in improving the carrier transport and thereby the detector performance. Here, we report combined experimental and theoretical studies of impurities in Tl6SI4. Impurity concentrations in Tl6SI4 were analyzed by glow discharge mass spectrometry. Purification of the raw material by multi-pass vertical narrow zone refining was found to be effective in reducing the concentrations of most impurities. Density functional theory calculations were also performed to study the trapping levels introduced by the main impurities detected in experiments. We show that, among dozens of detected impurities, most are either electrically inactive or shallow. In the purified Tl6SI4 sample, only Bi has a significant conce...

This paper will report on additional studies undertaken on a novel electrode geometry, referred to as CAPtureTM technology, wherein the cathode is extended up the sides of a planar CdZnTe detector. The initial presentation of this development focused on a single geometry, and a simple mechanism was proposed to explain the improved performance. In this presentation, we will describe the results of further electric field and charge transport modeling as applied to this detector configuration, and also compare the calculated performance with experimental results.

We experimentally investigate the possible correlation between high hole-trap concentrations in wide-bandgap semiconductors and delayed temporal response of high-flux x-ray detector devices to changing photon fluxes. We show that fast photo-current response can be achieved with (1) CdZnTe detectors with high hole mobility-lifetime products, (2) temperature increased detrapping, and (3) constant below-bandgap energy light illumination that modifies the dark defect occupation towards a steady-state with a reduced concentration of active hole traps. This way, the detector signal stabilizes immediately upon flux onset, independent of details of the semiconductor's point defect structure. Quasi-instantaneous response stabilization ( 107 photons mm-2 s-1 is demonstrated.

Arachidate Langmuir−Blodgett (LB) films of different chemical composition and number of monomolecular layers deposited on silylated silica glass substrates were studied by means of positron annihilation spectroscopy. The applied methods included the measurement of the Doppler broadening of the annihilation photopeak with variable energy positron beams and bulk positron lifetime measurements. The studied samples were 58 monomolecular layers (MML) thick Mg- and Cd-arachidate, arachidic acid (50 MML) and a series of Pb-arachidate samples with 4, 10, 20, 40, and 58 MML. The investigation showed that the variable energy positron beam technique is capable of measuring the thickness of the deposited LB films. The measured positron annihilation parameters are sensitive to the chemical composition of the films and the behavior of the films in a vacuum. The results confirmed the stability of salt base LB films in high vacuum conditions and showed the desorption of pure acid films. These investigations have also sho...

Near-surface damage induced by saw cutting of ingots of Cd1−xZnxTe was investigated by positron-defect depth profiling. The damage extends to several micrometers depth and depends on the cutting apparatus. The samples were polished and etched repeatedly, followed each time by positron-depth profiling. New subsurface damage created during the polishing process is observed. No new damage is observed after etching. Positron-depth profiling is suggested as a diagnostic tool to monitor the quality of sample surfaces.

In this paper, we theoretically investigate the mechanism of polarization in wide-bandgap semiconductor radiation detectors under high-flux x-ray irradiation. Our general mathematical model of the defect structure within the bandgap is a system of balance laws based on carrier transport and defect transition rates, coupled together with the Poisson equation for the electric potential. The dynamical system is self-consistently evolved in time using a high-resolution wave propagation numerical algorithm. Through simulation, we identify and present a sequence of dynamics that determines a critical flux of photons above which polarization effects dominate. Using the experience gained through numerical simulation of the full set of equations, we derive a reduced system of conservation laws that describe the dominant dynamics. A multiple scale perturbation analysis of the reduced system is shown to yield an analytical dependence of the maximum sustainable flux on key material, detector, and operating parameters. The predicted dependencies are validated for $16\ifmmode\times\else\texttimes\fi{}16\phantom{\rule{0.3em}{0ex}}\text{pixel}$ CdZnTe monolithic detector arrays subjected to a high-flux $120\phantom{\rule{0.3em}{0ex}}\mathrm{kVp}$ x-ray source.

The effect of temperature and composition dependences of deep level ionization energies in semi-insulating compound semiconductors is investigated and illustrated in the example of the ternary Cd{sub 1-x}Zn{sub x}Te system. Those dependences are determined by the behavior of the band extrema in an absolute energy scale and the actual nature of a particular defect, which controls its relation to the host crystal states. Examples for the interpretation of experimental temperature and composition dependences of critical charge transport parameters are given.

We report on the material selection, testing and fabrication technology development of high performance CdZnTe quasihemispherical CAPture TM Plus radiation detectors. Quasi-hemispherical CdZnTe detectors offer a cost effective alternative to other single-polarity (electron-only) detector configurations such as co-planar grid, pixilated or Frisch ring CdZnTe detectors with comparable energy resolution both in the high (>500 keV) and low energy range (

An oxygen-related defect was studied in nominally undoped CdTe crystals grown by the high pressure Bridgman technique using thermo-electrical effect spectroscopy and first-principles band structure calculations. Based on the linear relationship between the oxygen concentration and the emitted charge from the defect level with an ionization energy $E=0.184\ifmmode\pm\else\textpm\fi{}0.011\mathrm{eV}$ and a capture cross section $\ensuremath{\sigma}=(7\ifmmode\pm\else\textpm\fi{}4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}{\mathrm{cm}}^{2}$ we show that the defect is an oxygen complex. Using thermal annealing experiments we show that the defect complex contains cadmium vacancies. Using first-principles band structure calculations we assign the defect level to the second ionization level of the isoelectronic oxygen-cadmium vacancy pair $({\mathrm{O}}_{\mathrm{Te}}\ensuremath{-}{\mathrm{V}}_{\mathrm{Cd}}{)}^{\ensuremath{-}/2\ensuremath{-}}.$

The properties of large diameter (140 mm) semi-insulating Cd1-xZnxTe (x = 0.1) ingots grown by the vertical High-Pressure Electro-Dynamic Gradient (HP EDG) technique are discussed. The HP EDG crystal growth technology recently developed and introduced at eV PRODUCTS significantly improves the downstream CdZnTe detector fabrication yield compared to earlier versions of the HP crystal growth technology. These yield improvements stem from the improved structural and charge transport properties of the HP EDG CdZnTe ingots. Improvements were achieved in three areas: a) reduced thermal stress in the ingots, b) improved single crystal yield, and c) improved electron transport properties. The new state-of-the-art HP EDG crystal growth systems offer exceptional flexibility, thermal and mechanical stability and allow the growth of high purity CdZnTe materials. The flexibility of the multi-zone heater system allows the dynamic control of heat flow to optimize the growth-interface shape during crystallization. This flexibility combined with an advanced control system, improved system diagnostics and realistic thermal modeling provides an excellent platform for further process development. Results on the initial HP EDG CdZnTe ingots grown with low temperature gradient show the complete elimination of ingot cracking. The increased single crystal yield combined with the improved electron transport properties allows the fabrication of large volume electron-only devices at higher yield. The CdZnTe ingots regularly contain sections with electron mobility-lifetime product μτe≥5.0x10-3 cm2/V and occasionally yield material with μτe≥8.0x10-3 cm2/V.

The electronic performance of CZT-based gamma radiation spectrometers is governed by a synergism of bulk and surface properties. Compensation is used to increase the bulk resistivity of Cd1-xZnxTe (x~0.1), but the same electronic states that are introduced to increase the material resistivity can also trap charge and reduce the carrier lifetime. Electrical and mechanical surface defects introduced during or subsequent to crystal harvesting are also known to interfere with device performance. Using a contactless, pulsed laser microwave cavity perturbation technique, electronic decay profiles were studied in high pressure Bridgman CZT as a function of temperature. The electronic decay profile was found to depend very strongly on temperature and was modeled using a function consisting of two exponential terms with temperature-dependent amplitudes and time constants. The model was used to relate the observed temperature dependent decay kinetics in CZT to specific trap energies. It was found that, at low temperatures, the electronic decay process is dominated by a deep trap with an energy of approximately 0.69 ± 0.1 eV from the band edge. As the temperature is increased, the charge trapping becomes dominated by a second trap with an energy of approximately 0.60 ± 0.1 eV from the band edge. Surface damage introduces additional charge traps that significantly alter the decay kinetics particularly at low temperatures.

The spectroscopic performance of cadmium zinc telluride (CZT) room temperature radiation detectors is currently limited by both bulk and surface imperfections introduced during the growth, harvesting and fabrication of these devices. Bulk imperfections including impurities, vacancies, interstitials, grain boundaries and dislocations have been relatively well studied and are known to trap charge and reduce detector performance. However, the effect of specific traps on the electronic decay process has been difficult to quantify. Surface imperfections including mechanical damage or adsorbed chemical species are also known to trap charge or increase leakage current, but it has proven difficult to characterize the electronic properties of CZT surfaces prior to electrode deposition. Here it is shown that the pulsed laser microwave cavity perturbation method can provide important information on the electronic decay process both in the bulk and at the surface of high pressure Bridgman CZT crystals. Electronic decay process both in the bulk and at the surface of high pressure Bridgman CZT crystals. Electronic decay time was measured as a function of temperature and surface conditions. It is shown that the electronic decay process in bulk CZT is consistent witha single deep hole trap at an energy between 600meV and 700meV. The effect of surface quality was resolved by analyzing distinct features in the photoconductivity decay curves. Atomic force microscopy was used to characterize the surface roughness. Rough or damaged surfaces exhibited persistent photoconductivity, which could be eliminated by etching with a bromine methanol solution.

technique allowed the growth of large single crystals extending across the entire cross-section of the ingots. The control of theCd partial pressure allowed the solidification and cool-down of the ingots close to the stoichiometric composition. As a result,the formation and incorporation of large size ( 1 jim diameter) Te inclusions was avoided during crystallization and ingotswith high structural perfection were achieved. The improved structural perfection of the material was found to be associatedwith large spatial variation in the compensation conditions in the ingots and a resulting spatial variation of the bulk electrical

Surface processing plays a major role in manufacturing CdZnTe semiconductor devices used for radiation detection. We are conducting a thorough, systematic study of surfaces and contacts and their effect on charge transport and signal formation in CdZnTe devices. We are investigating wet chemical processing techniques as well as treatment of surfaces with energetic neutral atoms. Our goal is to develop and implement improved surface treatment methods and device manufacturing techniques for large-volume CdZnTe detectors. In addition, we will determine how surfaces and electrical contacts affect the performance of CdZnTe devices used for radiation detection. In this paper, we will show how surface electronic properties influence carrier transport and signal formation in devices designed to simulate coplanar grid detectors. By altering the surface using a wet chemical process, we will show that charge collection is significantly effected by the conductivity of the surface.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Thermoelectric Effect Spectroscopy and Thermally Stimulated Current measurements were used to investigate trapping levels in a semi-insulating CdTe and Cd1-xZnxTe crystals from multiple ingots grown by vertical Bridgman with over pressure control and high-pressure Bridgman methods. The crystals from different growth methods have different dislocation densities as well as Zn concentrations. The thermal ionization energies of these levels were extracted using both the variable heating rate and initial rise methods; the trapping cross sections were then calculated using the temperature maximum method. We report here that the shallow levels observed at E1=0.11+/- 0.02 and E2=0.17+/- 0.02 eV are intrinsic and the latter level is most likely related to the dislocation density.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The effects of two intrinsic deep levels on electrical compensation and effects of dislocations on carrier mobility in semi-insulating CdTe and CdZnTe radiation detector crystals are reported here. These levels were found in samples grown by conventional Bridgman and high-pressure Bridgman techniques. The levels were observed with thermoelectric effect spectroscopy at distinct temperatures corresponding to thermal ionization energies of Es1 equals 0.27 +/- 0.07 eV, Ed1 equals Ev + 0.735 +/- 0.005 eV and Ed2 equals Ev + 0.743 +/- 0.005 eV. The shallow level (Es1) is associated with dislocations. The first deep level (Ed1) is associated with the doubly ionized Cd vacancy acceptor and the second deep level (Ed2) is associated with the Te-antisite (TeCd). The second deep level (TeCd) was found to electrically compensate the material to produce high resistivity CdTe and CdZnTe, provided that the Cd vacancy concentration is sufficiently reduced during crystal growth or by post-growth thermal processing. The dislocations were found to affect the mobility of the carriers in the CdTe and CdZnTe crystals.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The charge transport properties and radiation detector performance of semi-insulating CdTe single crystal grown by the conventional vertical Bridgman technique in this paper. The measured room-temperature electrical resistivity of the crystals is below the theoretical maximum allowed by the band gap of CdTe indicating incomplete electrical compensation of the material. The crystals show excellent spectroscopic performance in the 15 keV - 662 keV energy range, with reduced low-energy tailing in the photopeaks. The energy resolution of the best detector was 2.7 keV full width half maximum (FWHM) at 59.5 keV, 4.5 keV FWHM at 122 keV, and 20.1 keV FWHM at 662 keV. This improved performance is attributed to the improved hole transport over the typical HPB CdZnTe. The measured mobility-lifetime product of holes, (mu) (tau) h approximately equals 2.3 X 10 -4 cm 2 /V, is significantly higher than that typical for HPB CdZnTe crystals. The measured electron (mu) (tau) e approximately equals 1.6 X 10 -3 cm 2 /V of these CdTe crystals suggest somewhat poorer electron transport than in a spectroscopic grade HPB material.

Recently, much has appeared in the literature concerning methods to improve the resolution and photopeak efficiency of CdZnTe radiation detectors operating at or around ambient temperature. THese methods generally involve either the use of modified electrode structures or pulse processing techniques, both of which add complexity, and hence cost, to the production and operation of such devices. In this paper, we will report on results obtained with a simpler, modified two-terminal device. The detector structure combines a planar anode with an extended surface cathode, and relies on a standard, single channel preamplifier/shaping amplifier system. The results obtained demonstrate that the charge collection efficiency of the device, as shown by the Peak- to-Valley ratio, is significantly improved when compared to the standard planar geometry, especially at higher photon energies.

The growth and properties of semi-insulating CdZnTe for nuclear radiation detector applications are reviewed. The current state of the high-pressure Bridgman growth and the potentials of the conventional vertical and horizontal Bridgman techniques to grow radiation detector material are discussed. The characteristic macroscopic and microscopic defects of high-pressure Bridgman grown CdZnTe ingots, such as cracks, pipes, inclusions, precipitates, grain boundaries and their effect on the electrical and charge trapping properties of the material are reviewed.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The availability of large-size, detector-grade CdZnTe crystals in large volume and at affordable cost is a key to the further development of radiation-detector applications based on this II-VI compound. The high pressure Bridgman technique that supplies the bulk of semiinsulating CdZnTe crystals used in X-ray, γ-ray detector and imaging devices at present is hampered by material issues that limit the yield of large-size and high-quality crystals. These include ingot cracking, formation of pipes, material homogeneity and the reproducibility of the material from growth to growth. The incorporation of macro defects in the material during crystal growth poses both material quality limitations and technological problems for detector fabrication. The effects of macro defects such as Te inclusions and pipes on the charge-transport properties of CdZnTe are discussed in this paper. Growth experiments designed to study the origin and formation of large defects are described. The importance of material-crucible interactions and control of thermodynamic parameters during crystal growth are also addressed. Opportunities for growth improvements and yield increases are identified.

Under intense x-ray irradiation, wide band gap semiconductor radiation detectors fabricated from crystals with low hole transport properties develop a steady-state space charge distribution that results from a dynamic equilibrium between charge carrier dynamics and the incident photon field. At a high enough x-ray flux, this space charge can collapse the electric field within the detector, resulting in the paralyzation of photon counting (i.e., high-flux polarization). However, well before polarization causes a catastrophic device failure, there can be enough space charge present to significantly modify the electric field. A modified field affects the electron transport and, therefore, signal generation within the sensor, which can ultimately degrade the performance of high-rate photon counting electronics. In this study, we analytically solve the fundamental equation of charge conservation to derive the modified electron transport in the presence of an exponential space charge distribution that results f...

It has been observed that pixillated CdZnTe detectors fabricated from crystals with low hole transport properties (μhτh

A classical method for the accurate measurement of the bulk resistivity and a quantitative separation of bulk and surface leakage currents in semi-insulating CdZnTe radiation detectors is evaluated. We performed an extensive set of experiments on CdZnTe single-crystal test devices to confirm the reliability and reproducibility of the measurements and the validity of the underlying assumptions for data analysis and parameter extraction. The experiments included temperature dependent dual current-voltage measurements on devices with guard electrodes as a function of device thickness, surface preparation, surface passivation, and electrode deposition conditions. We also evaluated the temperature dependence of the bulk resistivity and implemented a general temperature normalization routine to allow a reliable comparison between various crystal samples.

Positron annihilation was studied in a vinyl ester and four epoxy coatings on steel. Measurements were made on coatings in equilibrium with air at approximately 50% relative humidity and on coatings immersed in liquid water for 24 h. Three spectral components were identified: a short-lived component associated with annihilation largely in the steel substrate; a medium-lived component associated with annihilation in small voids in the coating; and a long-lived component associated with annihilation of o-positronium in the polymer matrix and within large voids in the polymer and at the polymer/metal interface. An excellent correlation was observed between the effect of water exposure on the annihilation spectra and the protective properties of the coating in an aggressive H 2 SO 4 environment. Other correlations between the annihilation spectra and the protective properties were also identified.

All three methods of the positron annihilation technique were applied to investigate amorphous metals. Samples of Fe40Ni40Si14B6 and Fe40Ni40P14B6 were prepared by the melt spun technique with constant ejection pressure and orifice size but at varying quenching rate and melt temperature. The influence of the production process leading to different Hc for the samples, the effect of heat treatment, cold rolling, and electron irradiation were studied. The measured dependence of positron annihilation parameters on the above treatments indicates the presence of positron-active trapping centres in the amorphous phase.

The interfacial chemistry and composition of Pt electrodes sputter deposited on bromine-etched CdZnTe surfaces was studied by X-ray photoelectron spectroscopy. The interfacial composition of a functioning and a nonfunctioning CdZnTe detector shows significant differences. The degree of cation out-diffusion into the Pt overlayer and the in-diffusion of Pt into the CdZnTe correlate with the degree of oxidation found at the metal-semiconductor interface. Most of the oxide present at the interface was found to be TeO/sub 2/. The results suggest that the interdiffusion of the atoms and associated charges contribute to stoichiometric variations at the metal-semiconductor interface and influence the electrical performance of the devices.