Search

The traditional method for electron lifetime measurements of CdZnTe (CZT) detectors relies on using the Hecht equation. The procedure involves measuring the dependence of the detector response on the applied bias to evaluate the μτ product, which in turn can be converted into the carrier lifetime. Despite general acceptance of this technique, which is very convenient for comparative testing of different CZT materials, the assumption of a constant electric field inside a detector is unjustified. In the Hecht equation, this assumption means that the drift time would be a linear function of the distance. This condition is not fulfilled in practice at low applied biases, where the Hecht equation is most sensitive to the μs product. As a result, researchers usually take measurements at relatively high biases, which work well in the case of the low μτ-product material, <10-3 cm²/V, but give significantly underestimated values for the case of high μτ-product crystals. In this work, we applied the drift-time method to measure the electron lifetimes in long-drift-length (4 cm) standard-grade CZT detectors produced by the Redlen Technologies. We found that the electron μτ product of tested crystals is in the range 0.1-0.2 cm²/V, which is an order of the magnitude higher than any value previously reported for a CZT material. In comparison, using the Hecht equation fitting, we obtained μτ=2.3 × 10-2 cm²/V for a 2-mm thin planar detector fabricated from the same CZT material. [ABSTRACT FROM AUTHOR]

CdZnTe crystals often exhibit nonuniformities due to the presence of Te inclusions and dislocations. High concentrations of such defects in these crystals generally entail severe charge-trapping, a major problem in ensuring the device's satisfactory performance. In this study, we employed a high-intensity, high-spatial-resolution synchrotron x-ray beam as the ideal tool to generate charges by focusing it over the large Te inclusions, and then observing the carrier's response at room- and at low-temperatures. A high spatial 5-μm resolution raster scan revealed the fine details of the presence of extended defects, like Te inclusions and dislocations in the CdZnTe crystals. A noticeable change was observed in the efficiency of electron charge collection at low temperature (1 °C), but it was hardly altered at room-temperature. [ABSTRACT FROM AUTHOR]

We measured the dependence of the energy gap in Bridgman-grown Cd1-xMnxTe crystals, 0≤x≤0.25, on the Mn mole fraction and temperatures from 40 to 300 K. We determined the Mn mole fraction and energy gap, respectively, from electron probe microanalysis and near-infrared Fourier-transform infrared transmission spectra. The energy gap increased linearly with an increase in the Mn content in the crystal and with a decrease in temperature. We formulated new equations from these experimental results, wherein we expressed the energy gap as a function of Mn mole fraction and temperature. Also, we compare our findings with published results. [ABSTRACT FROM AUTHOR]

Te secondary phase defects are known as major obstacle for improving charge carriers transport properties of CdTe compounds material. The enhanced detector performance was reported for Te inclusions free CZT detector implemented through two-step annealing (i.e., first in Cd and second in Te). We applied the time-of-flight (TOF) technique to measure the electron lifetime in as-grown and two-step annealed CZT material. TOF has several advantages over Hecht equation fitting such as independence of the electric field distribution, direct measurement of carrier lifetime, and high accuracy for high mobility-lifetime product CZT material. The detector performance improved drastically for the two-step annealed CZT detector except which containing inordinate number and size of prismatic punching defects. The average electron lifetime increased more than 10 times after successful two-step annealing. • Electron lifetime increased more than 10 times after successful two-step annealing. • The aggregate of small-sized Te inclusions also generate prismatic punching defect. • TOF technique provide more reliable μ τ products than Hecht equation for > 10−2 cm 2 /V. [ABSTRACT FROM AUTHOR]

Cloud and bubble chambers have historically been used for particle detection, capitalizing on supersaturation and superheating, respectively. Here, we present new results from a prototype snowball chamber, in which an incoming particle triggers the crystallization of a purified, supercooled liquid. We demonstrate, for the first time, simulation agreement with our first results from 5 years ago: the higher temperature of the freezing of water and significantly shorter time spent supercooled compared to the control in the presence of a Cf-252 fission neutron source. This is accomplished by combining Geant4 modeling of neutron interactions with the Seitz nucleation model used in superheated bubble chambers, including those seeking dark matter. We explore the possible implications of using this new technology for GeV-scale WIMP searches, especially in terms of spin-dependent proton coupling, and report the first supercooling of WbLS (water-based liquid scintillator). [ABSTRACT FROM AUTHOR]

Position-sensitive virtual Frisch-grid (VFG) CdZnTe (CZT) detectors offer a unique capability for correcting the response nonuniformities caused by crystal defects. This allowed us to achieve high energy resolution, while using typical-grade commercial CZT crystals with relaxed requirements to their quality, thus reducing the overall cost of detectors. Another advantage of the VFG detectors is that they can be integrated into arrays and used in small compact hand-held instruments or large-area gamma cameras that will enhance detection capability for many practical applications, including nonproliferation, medical imaging, and gamma-ray astronomy. Here, we present the results from testing small array prototypes coupled with front-end application-specified integrated circuit. Each detector in the array is furnished with 5-mm-wide charge-sensing pads placed near the anode. The pads signals are converted into XY coordinates, which combined with the cathode signals (for Z coordinates) provide 3-D position information of all interaction points. The basic array consists of a number of detectors grouped into $2\times 2$ subarrays, each having a common cathode made by connecting together the cathodes of the individual detectors. These features can significantly improve the performance of detectors while using typical-grade low-cost CZT crystals to reduce the overall cost of the proposed instrument. [ABSTRACT FROM PUBLISHER]

We investigated the feasibility of long-drift-time CdZnTe (CZT) gamma-ray detectors, fabricated from CZT material produced by Redlen Technologies. CZT crystals with cross-section areas of 5 × 5mm² and 6 × 6mm² and thicknesses of 20-, 30-, 40-, and 50-mm were configured as 3D position-sensitive drift detectors and were read out using a front-end ASIC. By correcting the electron charge losses caused by defects in the crystals, we demonstrated high performance for relatively thick detectors fabricated from unselected CZT material. [ABSTRACT FROM AUTHOR]

The longer electron lifetime of today’s CdZnTe (CZT) crystals allows for free carriers to travel longer distances in the crystals, which means that, in principle, thicker devices could be fabricated. These thicker CZT devices would offer greater detection efficiency for high-energy gamma-ray detectors. However, up to now, the thicknesses and sizes of actual detectors have still been limited by the nonuniform detector response, and the biggest devices reported in the literature are (20\times 20\times 15)\-mm^3 pixelated detectors with a drift distance of 15 mm. Although thicker and bigger single crystals are becoming available today, the high requirements on their crystal quality drastically reduce their acceptance yield and increase their cost. Fortunately, in many cases, the inhomogeneity in response can be corrected by segmenting the active volumes of the detectors and correcting the responses generated from each of the voxels. Such high-granularity position-sensitive detectors open up the opportunity for using thicker and less-expensive CZT crystals. The goal of this work is to demonstrate that today’s commercial high electron mobility-lifetime CZT material is suitable for a new class of detectors with 20-25-mm drift distances and even larger in the near future, provided that the detectors’ response nonuniformities can be corrected on a scale comparable to or larger than the sizes of the electron clouds, which is \sim 100~\mu\m. [ABSTRACT FROM AUTHOR]

We studied the post-growth annealing of CdZnTe crystals in a temperature-gradient field and compared the changes of Te inclusions in CdZnTe crystals before and after annealing. The infrared (IR) transmission microscopy shows that Te inclusions can migrate towards the high-temperature end and leave some smaller dark spots behind. These dark spots may be voids within the original inclusions left behind after the migration of the Terich solid. Such thermo-migration depends on the specific locations of the Te inclusions within the CdZnTe matrix. Grain boundaries could have a strong trapping effect on the migration of Te inclusions, probablythrough the high density of dislocation networks in the grain- boundary region. In addition, we also observed new Te inclusions in the grain-boundary regions, which were formed during the annealing process. This phenomenon is attributed to the fast diffusion and gettering behavior of excess Te atoms, or possibly Cd vacancies, along the grain boundaries. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

CdZnTe (CZT) is a promising medium for room-temperature gamma-ray detectors. However, the low production yield of acceptable quality crystals hampers the use of CZT detectors for gammaray spectroscopy. Significant efforts have been directed towards improving quality of CZT crystals to make them generally available for radiation detectors. Another way to address this problem is to implement detector designs that would allow for more accurate and predictable correction of the charge loss associated with crystal defects. In this work, we demonstrate that high-granularity position-sensitive detectors can significantly improve the performance of CZT detectors fabricated from CZT crystals with wider acceptance boundaries, leading to an increase of their availability and expected decrease in cost. [ABSTRACT FROM AUTHOR]

We present our results from testing 15\times 15\times 10~\mm^3 CdZnTe pixelated detectors using a readout system based on the H3D ASIC. Data obtained with an uncollimated ^137Cs source helped reveal details of the operational principle of such devices, and how the pulse-processing electronics may influence their performance. The responses of individual pixels were compared by using two different pulse processing approaches based on the analog shaping (H3D ASIC) and fitting of the waveforms as read out from hybrid preamplifiers. [ABSTRACT FROM PUBLISHER]

We discussed the design implementation and results from testing 2\,\times\,2-, 3\,\times\,3-, and 2\,\times\,4-arrays of 6\times 6 \times 15~mm^3 CdZnTe virtual Frisch-grid detectors. In these measurements we employed a data acquisition system based on the H3D ASIC developed by BNL's Instrumentation Division in collaboration with the University of Michigan for 3D position-sensitive detectors. We used CZT crystals with a range of performance attributes to evaluate practical array configurations and detector-assembling procedures. The detector ratings were assigned based on the pulse-height spectra and correlated with data from X-ray diffraction topography measurements and X-ray response mapping obtained at BNL's National Synchrotron Light Source. The results helped us to better understand the performance limits of these detectors, and to identify future improvements in the array's design and requirements for the new readout ASIC. [ABSTRACT FROM PUBLISHER]

We conducted low-temperature annealing experiments at temperatures slightly above and below the melting point of Te to clarify the effects of the state of Te inclusions (solid or liquid) upon the formation of 'star-like' defects in Cd-annealed CdZnTe (CZT). We also carried out post-growth annealing experiments with and without using Cd vapor to clarify the mechanism of formation of such defects. We demonstrated that these 'star-like' defects are due to the reaction between in-diffused Cd atoms and the molten Te inclusions, but we found no observable 'one-to-one' correlation between 'star-like' defects and Te inclusions. The non-uniform distribution of Te inclusions in the CZT matrix could account for this phenomenon since the punching distance of the dislocations depends on the volume fraction of inclusions within the matrix. [ABSTRACT FROM AUTHOR]

Although CdZnTe (CZT) and CdMnTe (CMT) materials are leading contenders for room-temperature semiconductor detectors, nonetheless, both materials have limitations hindering their full usage in producing economical, uniform, large-volume devices due to their grain/twin boundaries, material purity, secondary-phase Te defects and material segregation. We tried to prevent the generation of twin and subgrain boundaries to achieve large-volume CZT crystals by means of local temperature control between the CZT melt and quartz crucible. Also, we have expanded the understanding of the electrical and structural properties of coherent/incoherent twin boundaries. The high residual impurities in the starting source materials, especially in manganese, were identified as obstacles against obtaining high-performance CMT detectors. We found that purifying manganese telluride (MnTe) via a floating Te melt-zone very effectively removes impurities, leading to better detectors. CMT detectors fabricated with purified material give a 2.1% energy resolution for 662 keV with a ^137Cs gamma source without any electron-loss corrections. Secondary-phase Te defects deteriorate detector performance due to incomplete charge collection caused by charge trapping. In situ growth interface studies reveal the thermo-migration of Te inclusions to CZT melts and the dependence of Te-inclusion size on the cooling rate. The effective segregation coefficient of Zn in the CdTe host is nearly 1.3, so about 5%–6% of Zn deviation was reported in Bridgman-grown CZT (Zn\,=10% ingots. Such uncontrolled Zn variations cause a significant variation of the band-gap throughout the ingot and, consequently, affect the nonuniformity of the detectors' responses. Practically, this means that manufacturers cannot cut the ingot parallel to the crystal growth direction. We also demonstrated that the segregation of Zn can be controlled by creating particular thermal environments after growth. [ABSTRACT FROM AUTHOR]

New results from testing an array of 6\,\times\,6\,\times\,15 mm^3 virtual Frisch-grid CdZnTe (CZT) detectors with common-cathode readout for charge signals correction and rejection of incomplete charge collection events (ICC) are presented. The array employs parallelepiped-shaped crystals of a large geometrical aspect ratio with two planar contacts on the top and bottom surfaces (anode and cathode) and an additional shielding electrode placed on the sides to create the virtual Frisch-grid effect. The detectors are arranged in 2\,\times\,2 or 3\,\times\,3 detector modules with the common cathode readout by a single electronic channel. Because of the common cathode, the length of the shielding electrode can be further reduced with no adverse effects on the device performance. By implementing a novel technique for rejecting ICC events caused by the extended defects, we can achieve good spectral responses from ordinary CZT crystals, which can be produced with higher yield and at lower cost. For such crystals, the resolution of individual detectors is expected to be in the range of 0.8–1.5% FWHM at 662 keV with an average value of 1.3%. Arrays of virtual Frisch-grid detectors offer a robust and low-cost approach for making large-area detection modules that can potentially substitute for more advanced, but also more expensive and less available, pixel detectors in applications with slightly relaxed requirements on position- and energy-resolution (e.g., for coded aperture telescopes). In addition, such virtual Frisch-grid arrays will require a comparably smaller number of readout channels, which allows for lower power consumption. [ABSTRACT FROM AUTHOR]

Poor crystallinity remains a major problem affecting the availability and cost of CdZnTe (CZT) detectors. Point defects are responsible for small gradual charge loss and correlated with the electron clouds' drift times, which allows electronic correction of the output signals to achieve high spectral-resolution even with large-volume CZT detectors. In contrast, extended defects causes significant charge losses, which typically are uncorrelated, and, thus, result in much greater fluctuations of the output signals that cannot be corrected. Although extended defects do not affect all the interaction events, their fraction rapidly increases with the crystal's thickness and volume. In this paper, we summarize our recent results from testing CZT material and detectors that emphasize the particular roles of two types of extended defects, and their contributions to the device's overall performance. [ABSTRACT FROM AUTHOR]

Both Te inclusions and point defects can trap the charge carriers generated by ionizing particles in CdZnTe (CZT) detectors. The amount of charge trapped by point defects is proportional to the carriers' drift time and can be corrected electronically. In the case of Te inclusions, the charge loss depends upon their random locations with respect to the electron cloud. Consequently, inclusions introduce fluctuations in the charge signals, which cannot be easily corrected. In this paper, we describe direct measurements of the cumulative effect of Te inclusions and its influence on the response of CZT detectors of different thicknesses and different sizes and concentrations of Te inclusions. We also discuss a means of partially correcting their adverse effects. [ABSTRACT FROM AUTHOR]

The ideal operation of CdZnTe devices entails having a uniformly distributed internal electric field. Such uniformity especially is critical for thick long-drift-length detectors, such as large-volume CPG and 3-D multi-pixel devices. Using a high-spatial resolution X-ray mapping technique, we investigated the distribution of the electric field in real devices. Our measurements demonstrate that in thin detectors, < 5 mm, the electric field-lines tend to bend away from the side surfaces (i.e., a focusing effect). In thick detectors, > 1 cm, with a large aspect ratio (thickness-to-width ratio), we observed two effects: the electric field lines bending away from or towards the side surfaces, which we called, respectively, the focusing field-line distribution and the defocusing field-line distribution. In addition to these large-scale variations, the field-line distributions were locally perturbed by the presence of extended defects and residual strains existing inside the crystals. We present our data clearly demonstrating the non-uniformity of the internal electric field. [ABSTRACT FROM AUTHOR]

High densities of impurities and defects lead to severe charge-carrier trapping that can be major issues in assuring the high performance of CZT detectors. For some medical-imaging applications, the typical X-ray flux can be very high. Under such high irradiation conditions, the trapped charge builds up inside the detector affecting its stability. This phenomenon generally is termed the polarization effect. We conducted detailed studies on polarization in CZT crystals employing a highly collimated synchrotron X-ray radiation source available at Brookhaven's National Synchrotron Light Source (NSLS). We were able to induce polarization effects by irradiating specific areas within the detector. These measurements allowed us to make, for the first time, a quantitative comparison between areas where polarization is induced, and the electron- and hole-collection X-ray maps obtained at low flux, where no polarization is induced. We discuss the results of these polarization studies. [ABSTRACT FROM AUTHOR]

Te inclusions existing at high concentrations in CdZnTe (CZT) material can degrade the performance of CZT detectors. These microscopic defects trap the free electrons generated by incident radiation, so entailing significant fluctuations in the total collected charge and thereby strongly affecting the energy resolution of thick (long-drift) detectors. Such effects were demonstrated in thin planar detectors, and, in many cases, they proved to be the dominant cause of the low performance of thick detectors, wherein the fluctuations in the charge losses accumulate along the charge's drift path. We continued studying this effect using different tools and techniques. We employed a dedicated beam-line recently established at BNL's National Synchrotron Light Source for characterizing semiconductor radiation detectors, along with an IR transmission microscope system, the combination of which allowed us to correlate the concentration of defects with the devices' performances. We present here our new results from testing over 50 CZT samples grown by different techniques. Our goals are to establish tolerable limits on the size and concentrations of these detrimental Te inclusions in CZT material, and to provide feedback to crystal growers to reduce their numbers in the material. [ABSTRACT FROM AUTHOR]

Frisch-ring CdZnTe detectors have demonstrated good energy resolution for identifying isotopes, <1% FWHM at 662 keV, and good efficiency for detecting gamma rays. To facilitate the application of this detector in radiation detection, we are designing a portable device that can incorporate with an 8 × 8 Frisch-ring detector array. A prototype system has been designed, which includes detector modules, front-end electronics, signal processing circuit, USB interface and high-voltage power supply. This paper describes the design and assembly of the detector modules and the structure of the prototype system. Some test results are also reported. [ABSTRACT FROM AUTHOR]

We studied the effects of small, <20 µm, Te inclusions on the energy resolution of CdZnTe gamma-ray detectors using a highly collimated X-ray beam and gamma-rays, and modeled them via a simplified geometrical approach. Previous reports demonstrated that Te inclusions of about a few microns in diameter degraded the charge-transport properties and uniformity of CdZnTe detectors. The goal of this work was to understand the extent to which randomly distributed Te-rich inclusions affect the energy resolution of CZT detectors, and to define new steps to overcome their deleterious effects. We used a phenomenological model, which depends on several adjustable parameters, to reproduce the experimentally measured effects of inclusions on energy resolution. We also were able to bound the materials-related problem and predict the enhancement in performance expected by reducing the size and number of Te inclusions within the crystals. [ABSTRACT FROM AUTHOR]

The performance characteristics of Frisch-ring CdZnTe (CZT) detectors are described and compared with other types of CZT devices. The Frisch-ring detector is a bar-shaped CZT crystal with a geometrical aspect ratio of ~1 : 2. The side surfaces of the detector are coated with an insulating layer followed by a metal layer deposited directly upon the insulator. The simple design operates as a single-carrier device. Despite the simplicity of this approach, its performance depends on many factors that are still not fully understood. We describe results of testing several detectors fabricated from CZT material produced by different vendors and compare the results with numerical simulations of these devices. [ABSTRACT FROM AUTHOR]

A rapid increase of the fluctuation of ionization yield in Xe + 3 %³He gas mixture irradiated with thermal neutrons is observed at density above 0.18 ± 0.01 g/cm³ (8.3 × 1020 cm-3). The significance and possible explanation of this experimental observation is discussed. [ABSTRACT FROM AUTHOR]

Commercial 15 × 15 × 7.5 mm² coplanar grid CdZnTe detectors were studied on the micron-scale using a collimated high-energy X-ray beam provided by Brookhaven's National Synchrotron Light Source. This powerful tool enables simultaneous studies of detector response uniformity, electronic properties of the material, and effects related to the device's contact pattern and electric field distribution. The availability of a front-end Application Specific Integrated Circuit, developed at Brookhaven's Instrumentation Division, providing low noise amplification of grids and cathode signals, corresponding timing signal and adjustable relative gain, allowed to correlate performance mapping and fluctuations in collected charge. We observed the effect of the strip contacts comprising the coplanar grids on the energy resolution of the coplanar-grid device. [ABSTRACT FROM AUTHOR]

In the past few years, significant progress has been achieved in the development of room-temperature semiconductor detectors, particularly those based on CdZnTe (CZT) crystals. Several types of electron-transport-only detectors have been developed: pixel, coplanar-grid, cross-strip, drift-strip, orthogonal coplanar strip, and virtual Frisch grid, many of which are now commercially available. Despite all these varieties in the detector designs, they have many common features and problems. This review summarizes the common detector design constraints and related factors limiting performance of CZT detectors: bulk and surface leakage currents, surface effects, properties of Schottky contacts and surface interfacial layers, charge sharing and loss in multielectrode devices, charge transport nonuniformities, and fluctuations in the pulse height for long-drift-length devices. We also describe unique capabilities at Brookhaven National Laboratory, Upton, NY, for CZT device characterization and recent progress utilizing these tools. [ABSTRACT FROM AUTHOR]

We investigated the performance of the modification of a Frisch-ring device design applied to the bar shaped HgI2 detector. In this device, the ring electrode, separated from the detector surface by thin insulating layer, is extended to cove the entire area of side surfaces of the crystal, and connected to the cathode. To fabricate this device the side surfaces of 3 × 3 × 6 mm³ HgI2 crystals were wrapped in a Cu foil separated from the crystal by a thin layer of a Teflon tape. These bar-shaped HgI2 detectors can be used in large-area high-efficiency detector arrays that are in demand for medical imaging and nuclear materials detection. We obtained an energy resolution of ∼5% FWHM at 511 keV with our tested detectors. This is a very good result for the HgI2 detector of such thickness. However, this resolution is far away from the theoretical limit predicted for HgI2 material. This paper analyzes several factors affecting the energy resolution of this type of the virtual Frisch-grid devices, of which the fluctuation of charge loss due to electron trapping by local defects with high concentration of traps seems is a dominant factor. [ABSTRACT FROM AUTHOR]

Examines the effects of bulk and surface conductivity on the performance of high-resistivity CdZnTe pixel detectors. Comparison of the mechanisms of bulk and surface conductivity; Effect of surface layer on charge-collection efficiency; Results of the leakage current measurements.

In this paper, we present studies of the I–V characteristics of CdZnTe (CZT) detectors with Pt contacts fabricated from high-resistivity single crystals grown by the high-pressure Bridgman process. We have analyzed the experimental I–V curves using a model that approximates the CZT detector as a system consisting of a reversed Schottky contact, in series with the bulk resistance. Least-square fit to the experimental data yields 0.78–0.79 eV for the Pt–CZT Schottky barrier height, and <20 V for the voltage required to deplete a 2 mm thick CZT detector. We demonstrate that, at high bias, the thermionic current over the Schottky barrier, the height of which is reduced due to an interfacial layer between the contact and CZT material, controls the leakage current of the detectors. In many cases, the dark current is not determined by the resistivity of the bulk material, but rather the properties of the contacts; namely, by the interfacial layer between the contact and CZT material. [Copyright &y& Elsevier]

In this work, we studied the free-carrier transport properties and space-charge formation/polarization in a CdZnTe bar-shaped radiation detector using the Laser-Induced Transient Current technique. We found out that a steady-state space charge ranging from 8 × 108 to 2.1 × 109 cm−3 is formed throughout the detector at about 0.5 s following biasing. The measured current waveforms were modeled by Monte Carlo simulations after taking into account the electric and weighting fields inside the detector and shielding box. Modeling of the unpolarized waveforms revealed an exceptionally high electron mobility-lifetime product μ e τ e ≥ 0.095 c m 2 V − 1 . The observed formation of positive space charge in the biased detector and linear scaling of the current waveform shapes on applied bias revealed that the detector polarization is attributed to carrier injection at the anode. Measurements in pulsed and DC bias in modified electrode geometry proved the surface-charge formation. [ABSTRACT FROM AUTHOR]

We utilized a low-temperature spatially resolved micro-photoluminescence mapping technique to investigate the spatial variation of photoluminescence- and electronic-defect states in areas of CdZnTe (CZT) single crystals containing structural-imperfections. Photoluminescence mapping of the donor-bound-exciton emission reveals an unexpected blue-shift of the CZT bandgap at Te inclusions, which indicates that for optical measurements the localized strain field needs to be considered for accurate calculation of Zn composition and energy levels near micro-scale defects. We observed that the line widths of the donor-bound-exciton peak and defect-related D band are broadened in regions with a high density of dislocations; in contrast, the donor-acceptor-pair peak is narrowed. [ABSTRACT FROM AUTHOR]

Measurements using the National Synchrotron Light Source provided a detailed comparisons of the microscale detector response and infrared microscopy images for CdZnTe Frisch-ring x-ray and gamma detectors. Analysis of the data showed conclusively that local deteriorations of the electron charge collection and x-ray device response fully correlate with the presence of Te precipitates as seen in the IR images. Effects of the surface processing conditions on the detector performance were also clearly observed. [ABSTRACT FROM AUTHOR]

By analyzing photoconductive decay curves, we compared the surface recombination velocities of semi-insulating CdMnTe:In crystals grown by the vertical Bridgman method with or without surface passivation. Sulfur passivation effectively prevents the formation of a conductive Te oxide layer on the CdMnTe surface and reduces the surface recombination velocities by about one third. We demonstrated, from IR observations of the distribution maps of Te precipitates, that their configuration affects the anomalous photoconductive decay curves and the gamma-ray spectrum in some areas of the CdMnTe crystal. Notably, not only the size but also the spatial configuration of the Te precipitates modulates the carrier-transport properties. [ABSTRACT FROM AUTHOR]

The effect of selenium in CdZnTe was investigated by characterizing the Cd0.9Zn0.1Te0.98Se0.02 crystal. The bandgap of Cd0.9Zn0.1Te0.98Se0.02 is smaller than Cd0.9Zn0.1Te. In addition, its value in ingot follows the zinc-segregation tendency than that of selenium. There was a recognizable difference in the amounts of compensating dopants, the distribution of Te inclusions, and the performance of the detector. The amounts of indium doping for the compensation were reduced by the addition of selenium, which provided the highest partial pressure in the melts. Thus, the generation of Cd vacancies, which is a native defect in CdZnTe, was prevented. In addition, the size of Te inclusions varied from the tip, middle, and heel of the CdZnTeSe ingot. The addition of selenium may change the thermal conductivity of CdZnTeSe melts, which is responsible for reducing the bulging of the retrograde solidus line near stoichiometry. The planar CdZnTeSe detector showed a 59.5-keV gamma peak for Am-241 with an 11% of energy resolution. [ABSTRACT FROM AUTHOR]

The addition of 2 % of selenium in CdTe and CdZnTe (CZT) exhibited an additional acceptor-bound exciton in photoluminescence (PL). From the PL and inductively coupled plasma mass spectrometer (ICP/MS) analyses, the additional acceptor-bound exciton emission is associated with a Cu impurity in the relatively low-purity (5N) precursor CdSe starting material. In addition, thermal quenching of the acceptor/donor bound exciton emissions do not require a shift to any higher excited states or to the free exciton state. ICP/MS analysis also showed a higher concentration of Sn in CdZnTeSe (CZTS) compared to CdTe and CZT which is known to form a deep level trap at around E C – 0.83 eV. The trapping and de-trapping times indicate that the Sn-related deep trap acts as an electron trapping center. The relatively low electron mobility-lifetime product of CZTS might be correlated with the concentration of Sn in the impure CdSe material. Pulse height spectra using an Eu-152 gamma-ray source was taken for a 2-mm-thick planar CZTS detector. The spectra clearly showed all the expected gamma peaks with energy lower than 344.3 keV. • Additional (A 0 X) 1 found in all Se added CdTe/CdZnTe. • Isolated Cu impurity responsible for additional (A 0 X) 1 . • Sn impurity might responsible for electron trapping. [ABSTRACT FROM AUTHOR]

With the global shortage of ³He gas, researchers worldwide are looking for alternative materials for detecting neutrons. Among the candidate materials, semiconductors are attractive because of their light weight and ease in handling. Currently, we are looking into the suitability of boron arsenide (B12As2) for this specific application. As the first step in evaluating the material qualitatively, the photo-response of B12As2 bulk crystals to light with different wavelengths was examined. The crystals showed photocurrent response to a band of 407- and 470- nm blue light. The maximum measured photoresponsivity and the photocurrent density at 0.7 V for 470 nm blue light at room temperature were 0.25 A·W-1 and 2.47 mA⋅cm-2, respectively. In addition to photo current measurements, the electrical properties as a function of temperature (range: 50-320 K) were measured. Reliable data were obtained for the low-temperature I-V characteristics, the temperature dependence of dark current and its density, and the resistivity variations with temperature in B12As2 bulk crystals. The experiments showed an exponential dependence on temperature for the dark current, current density, and resistivity; these three electrical parameters, respectively, had a variation of a few nA to µA, 1-100 µA⋅cm-2 and 7.6x105-7.7x10³ δ⋅cm, for temperature increasing from 50 K to 320 K. The results from this study reported the first photoresponse and demonstrated that B12As2 is a potential candidate for thermal-neutron detectors. [ABSTRACT FROM AUTHOR]

Presents information on charge collection in the cadmium-zinc-tellurium pixel detectors with emphasis on events split between multiple pixels. Phenomenology of charge sharing; Uses of the Gauss-Seidel algorithm in the study; Comparison of charge loss scenarios as predicted by the model and as observed in the experiment.

In this research, we assessed the abundance of point defects and their influence on the resistivity, the electron mobility-lifetime (μτe) product, and the electron trapping time in CdTeSe crystals grown under different conditions using the traveling heater method. We used current-deep level transient spectroscopy to determine the traps' energy, their capture cross-section, and their concentration. Further, we used these data to determine the trapping and de-trapping times for the charge carriers. The data show that detectors with a lower concentration of In-dopant have a higher density of A-centers and Cd double vacancies (VCd--). The high concentrations of VCd-- and A-centers, along with the deep trap at 0.86 eV and low density of 1.1 eV energy traps, are the major cause of the detectors' low resistivity, and most probably, a major contributor to the low μτe product. Our results indicate that the energy levels of point defects in the bandgap, their concentrations, capture cross-sections, and their trapping and de-trapping times play an important role in the detector's performance, especially for devices that rely solely on electron transport. [ABSTRACT FROM AUTHOR]

Electro-migration of ionized/electrically active impurities in CdZnTe (CZT) was successfully demonstrated at elevated temperature with an electric field of 20 V/mm. Copper, which exists in positively charged states, electro-migrated at a speed of 15 μm/h in an electric field of 20 V/mm. A notable variation in impurity concentration along the growth direction with the segregation tendency of the impurities was observed in an electro-migrated CZT boule. Notably, both Ga and Fe, which exist in positively charged states, exhibited the opposite distribution to that of their segregation tendency in Cd(Zn)Te. A CZT detector fabricated from the middle portion of the electromigrated CZT boule showed an improved mobility-lifetime product of 0.91×10-2 cm2/V, compared with that of 1.4×10-3 cm2/V, observed in an as-grown (non-electro-migrated) CZT detector. The optimum radiation detector material would have minimum concentration of deep traps required for compensation. [ABSTRACT FROM AUTHOR]

The detector performance and presence of Te secondary-phase defects distribution were investigated in CdMnTe (CMT) crystals prepared with different cooling rates. Detectors fabricated from fast-cooled CMT crystals exhibit a relatively poor detector performance, although IR transmission microscopy measurements show that the Te secondary-phase defects have a lower concentration and smaller size compared to slow-cooled crystals. Current deep-level transient spectroscopy (I-DLTS) measurements for both CMT detectors reveal the same trap levels, but there is a clear difference in the densities for the 0.26- and 0.42-eV traps for the two different cooling schemes. These two traps are probably attributed to Cd vacancies and Te anti-site defects, respectively. In addition, there is some likelihood that the traps are anti-correlated with respect to each other. [ABSTRACT FROM AUTHOR]

The focus of this paper is to evaluate thick, 20×20×10 and 10×10×10 mm3, cadmium zinc telluride (CZT), Cd0.9Zn0.1Te, crystals grown using the traveling heater method (THM). The phenomenal spectral performance and small size and low concentration of Te inclusions/precipitates of these crystals indicate that the THM is suitable for the mass production of CZT radiation detectors that can be used in a variety of applications. Our result also proves that with careful material selection using IR and high-quality fabrication processes, the theoretical energy resolution limit can be achieved. [ABSTRACT FROM AUTHOR]

The presence of Te secondary-phase defects (i.e., Te inclusions and Te precipitates) is a major factor limiting the performance of CdZnTe (CZT) X-ray and gamma-ray radiation detectors. We find that Te secondary-phase defects in CZT crystals can be removed through postgrowth two-step annealing without creating new trapping centers (i.e., prismatic punching defects). Two-step annealing (with the first under a Cd pressure and the second one under a Te pressure) was demonstrated to be effective in removing the Te secondary-phase defects, while preserving the electrical resistivity of the CZT detector. The first step involves annealing of semi-insulating CZT under a Cd overpressure at 700 °C/600 °C (CZT/Cd) for 24 h, which completely eliminated the Te-rich secondary-phase defects (Te inclusions). However, it resulted in a lower resistivity of the samples (down to $2\times 10^{4-6}\,\,\Omega \cdot \text {cm}$). A subsequent annealing step involves processing CZT under a Te ambient condition at 540 °C/380 °C (CZT/Te) for 120 h, which restored the crystal’s resistivity to $6.4 \times 10^{10}\,\,\Omega \cdot \text {cm}$ without creating new Te secondary-phase defects. However, Te inclusions reappeared in the case of unnecessarily long Te ambient annealing. Pulse-height spectra taken with the two-step annealed CZT detectors showed an improved detector performance due to a reduced concentration and the size of Te secondary-phase defects. [ABSTRACT FROM AUTHOR]

An improved detector performance for 2% selenium-added CdZnTe(CZT) was observed and explained by the reduced concentration of a deep-level hole trap, which was confirmed by photoluminescence measurements of CdZnTeSe(CZTS) and CdTeSe(CTS). The hole lifetime in CZTS was found to be 2. 18 μ s in the selenium-containing material, which is much longer than that typically measured by 10–100 times. The prolonged lifetime of hole carriers is likely caused by the disappearance of hole trapping at a specific binding energy of 1.1-eV below the conduction band. Also, the enhanced pulse height spectra of CZTS gamma-ray detectors were explained in terms of the ratio of electron and hole mobility-lifetime products. Cu-migrated patterns are used for the first time to reveal sub-grains and their networks in CZT and CZTS. Based on qualitative analysis, the Cu-migrated patterns in CZT and CZTS showed similar results. • 2% of selenium-added to CdZnTe material exhibited an improved energy resolution. • 1.1-eV defect level disappeared in selenium-added CdTe and CdZnTe. • Cu-migrated patterns match with sub-grain boundaries and extended defect structure. [ABSTRACT FROM AUTHOR]

The electrical properties of CdZnTe(CZT) above the melting point of tellurium (Te) inclusions were determined during in situ annealing. The thermal annealing cycles of the CZT detectors were 490 °C, 530 °C, and 570 °C continuously, which were higher than the melting points of elemental Te and Te inclusions and lower than the sublimation temperature of CZT. Unexpectedly, the CZT detectors exhibited very low leakage current at room temperature after the thermal annealing cycles due to the formation of rectifying contacts. The activation energy of high-resistivity CZT was 0.81 eV indicating pinning of Fermi level nearly in the middle of bandgap. At room temperature, CZT detectors with rectifying contacts showed clearly the 59.5-keV gamma-ray peak of Am-241. Observed fluctuations of the leakage current at about 470 °C might have originated from a mixed conductivity of liquid and solid CZT due to the melting of Te inclusions. [ABSTRACT FROM AUTHOR]

We investigated the temperature dependences (TDs) in the range of 290–423 K for the Hall constant R\mathbf {H} and the Hall carrier mobility \mu \mathbf {n} ( \sigma R\mathbf {H} ) in semi-insulating Cd0.9 Zn0.1 Te:In (CZT) crystals. As-grown, CZT material has nonequilibrium distributions of native and impurity-related defects. Thus, before taking any measurements, the samples were kept inside the test chamber in the dark at 423 K to reach an equilibrium state at T < 423 K. For all the tested samples, the R\mathbf {H} TD could be described by two activation energies. At the transitional point, the TD of the carrier mobility also changes from “normal” at high temperatures to “exponential” at low temperatures. The latter is a result of the collective effect of drift barriers due to microinhomogeneity. Therefore, only the high-temperature activation energies can be assigned to the ionization energies of the compensated deep donors ( \varepsilon \mathbf {D} ). For different samples, the values for \varepsilon \mathbf {D}^{\mathbf {0}} (at absolute zero) were found to be in the range of 0.50–0.78 eV, and the degree of donor compensation [ D^\mathbf + ]/[D] is between 0.3 and 0.98. The low-temperature region, where there are strong effects of crystal microinhomogeneities, cannot be used to characterize the ionization energy of donors. Therefore, we describe the activation energy as \varepsilon \mathbf {1} = \varepsilon \mathbf {D}-\alpha \varepsilon \mathbf {b} , where \varepsilon \mathbf {b} is the drift barrier height found from the TD of the carrier mobility and $\alpha $ takes a value close to unity. Values of \varepsilon \mathbf {b} for our studied samples lie within (0.05–0.35) eV. [ABSTRACT FROM AUTHOR]

Defects located at the \text E\rm C - 1.1 (eV) level, which are electro-optically active deep traps, generally have been overlooked since their presence cannot be detected except for those in highly resistive CdTe compounds. The origin of this trap is still debated on whether it is from Te vacancies or from dislocations induced by secondary phase defects in Te. We have grown high-resistivity Te-rich CZT ingots to clarify the origin of the 1.1 eV defects and to analyze the defect levels in the CZT samples by current deep level transient spectroscopy (I-DLTS)and photoluminescence (PL). From the analysis, defect levels such as shallow acceptor/donor, A-centers, Cd vacancies and Te antisite appeared to be in both high and low concentrations of Te inclusions, but the level of 1.1-eV defects exhibited dependence on the density of Te inclusion in the CZT samples. We also evaluated the effect of the 1.1-eV deep-level defects on the detector’s performance in point view of carrier trapping and de-trapping. [ABSTRACT FROM PUBLISHER]

It is known that low temperature annealing at below 200 °C decreases the leakage current in a CdTe and CdZnTe (CZT) detector. However, only CZT detectors which have electrodes made by electroless method and low temperature annealing in the air showed diminished leakage current after annealing. With the aid of an in-situ annealing monitoring system, we measured the leakage current of a CZT detector while carrying out low-temperature annealing. A decrease in the leakage current resulted from the presence of thin insulating Te oxide layers, TeO2 and CdTeO3, instead of CZT/electrode interface enhancement by diffusion of Au. Other measurement results of Auger electron spectroscopy, micro TEM analysis, and pulse height spectrum were in good agreement with our new interpretation of the low temperature annealing effects of CZT. [ABSTRACT FROM PUBLISHER]

CdTe crystals are used widely for manufacturing gamma-ray radiation sensors, and we can improve their properties if we eliminate as many as possible of their Te inclusions. In this paper, we describe our two modes of removing them; first, we used the traveling heater method for growing them, and then we applied post-growth cooling, for which we used a special slow-cooling program. Here, we placed the CdTe ingot, containing inclusions, into a quartz container, and moved a narrow heater zone along it. The molten inclusions moved together with the hot zone, and, at end of the process, they were concentrated heavily at the ingot’s surface. Consequently, very few inclusions were observed in much of the CdTe ingot. Hall effect measurements showed that after such annealing the value of the carrier mobility was increased significantly, and the concentration of ionized centers was reduced. One major advantage of this treatment, compared to thermal annealing alone, is that both the inclusions and impurities are eliminated simultaneously. [ABSTRACT FROM PUBLISHER]

Cd0.9Zn0.1Te ingots were synthesized from pure components (6N purity Cd, Zn, and Te with In as the dopant) and subsequently grown from the melt under an argon overpressure. Graphite crucibles (with and without an inner coating of pyrolytic BN) were used. The temperature gradient in the solidification zone was 7–30 K/cm, and the growth rate was 0.6-1.0 mm/hour. We investigated the chemical composition, structure, and electrical properties of the as-grown crystals, and established relationships between the crystal properties and the growth conditions. The bottom, middle, and top of the ingots had n-type conductivity, but slightly different properties. Resistivity reached a maximum in the middle of the ingots ((2.5 - 5.0) \times 10^10~\Ohm-cm), and was less at the edges \sim 0.8 \times 10^10~\Ohm-cm. The value of the bandgap was minimal in the middle of the ingots (\sim 1.5~\eV), and 1.53-1.55 eV at the edges. The compensation degree (Nd/Na) of the energy level responsible for the low dark conductivity showed a maximum value at the bottom of the ingots (\sim 60 - 90\% ), and a minimum in the middle part (1-2%). The crystals were then used to fabricate Cd(Zn)Te detectors for gamma-ray radiation. [ABSTRACT FROM AUTHOR]

We observed a novel unanticipated effect in CdTe, \bf Cd1-{\bf x}{\bf Zn}\bf x{\bf Te} and \bf Cd1-{\bf x}{\bf Mn}\bf x{\bf Te} crystals whilst we were measuring the dependences of several electrical properties (e.g., specific conductivity and free-carrier density) over time. During isothermal annealing under constant thermodynamic conditions (temperatures of 450-500^\circ C and under maximal cadmium-vapor pressure), we recorded a jump-like increase in the conductivity after some \sim 1 - 2 hours of heating required to stabilize the sample’s electric parameters. The values of specific conductivity and free-carrier density suddenly increased by up to tenfold, and they persisted at those levels during further ageing. At the same time, the sample’s conductivity became insensitive to stoichiometric changes in the crystal. We explain this effect as reflecting a sudden reformatting of the sample’s native/foreign point-defect structure. This transformation is evaluated and mathematically approximated within the framework of our model of the melting of Te-containing second-phase particles; this process releases impurities from within the particles. The respective diffusion “clouds” grow, and at the moment of their mutual percolation (infiltration), a peculiar “short circuit” is observed with striking changes in the crystals’ electrical parameters. [ABSTRACT FROM PUBLISHER]