CdZnTe Crystals Research Articles

A monitor solution to characterize a fissile material in a glove box

Handling fissionable material requires special know-how in managing irradiation and contamination risks while controlling any potential chain reaction, i.e. maintaining sub-criticality. If the risks associated with cleanup and dismantling operations had to be ranked in order of seriousness, keeping control of sub-criticality would probably be number one. The control modes for sub-criticality management are often reduced to the measurement of the fissile mass. This subject represents an opportunity for the development of fissile material mass control. More precisely, D&D operations require the design and deployment of innovative ways to ensure accurate measurement of the mass of fissile material in a hostile environment. The technical proposal described in this document is based on gamma spectrometry applied to the characterization of fissile material holdup in a glove box. Gamma spectrometry is adapted to monitoring the evolution of the main radionuclides: 241Am and 241Pu. The work has confirmed that detectors equipped with CdZnTe crystals are devices which are well-adapted to on-line monitoring of glove box cleanup.

Revealing Te inclusions on VGF-grown CdZnTe crystals by using low energy argon ion irradiation: Influence of the substrate temperature

Cd1−xZnxTe (CZT) crystals find interesting uses in a variety of detector-type applications. However, it is well known that the concentration and distribution of Te inclusions within a device are one of the major contributions to the degradation of CZT detectors. In addition, Te precipitates on the surface can also increase the leakage current, thereby deteriorating the device performance.Our previous studies clearly revealed a pronounced effect when CZT surfaces were irradiated with Ar ions at room temperature. Even for small fluences, a removal of polishing scratches on the sample surfaces was observed. Correlated to this effect, an important enhancement in the luminescence intensity of the irradiated samples was observed. Te inclusions also tend to aggregate after the irradiation process and are also eliminated from the surfaces for the highest ion fluences.In this work we continue our previous work by studying the effect of the temperature of the sample in the range 300–600 K during Ion Beam Sputtering of CZT crystals surfaces at different fluences, by means of Scanning Electron Microscopy, Atomic Force Microscopy, and μ-Photoluminescence. It is expected that increasing the temperature of CZT samples during the irradiation process will enhance the beneficial effects this technique has shown when trying to reduce surface Te inclusions and to increase the luminescence properties of CZT crystal surfaces.

Effect of point defects trapping characteristics on mobility-lifetime (μτ) product in CdZnTe crystals

The trapping characteristics of point defects in CdZnTe crystals grown by the vertical Bridgman method (VB) and the traveling heater method (THM) were analyzed. The photo-induced current transient spectroscopy (PICTS) was used to determine the traps’ energy, their capture cross-section and concentration. Moreover, the trapping and de-trapping times of point defects were obtained. Furthermore, the photoconductive technology under sub-bandgap illumination were employed to study the influence of the trapping characteristics on mobility-lifetime (μτ) product of CdZnTe crystal. The quantitative results indicated that sub-bandgap illumination can effectively improve carrier mobility-lifetime. The deep levels (TeCd), which occupy the position around the midgap, with high de-trapping time and low trapping time have greater effect on μτ product for the crystals than Cd vacancies. The Te secondary-phase related defects, which can be observed in the VB method grown crystals, have less influence on μτ product than Te antisite.

Surface analysis and electrical measurement of the ohmic contact on p-CdZnTe (111)B face with Au/Cd composite electrode

CdZnTe crystal has proved to be an excellent material to detect X-ray and gamma ray. Ohmic electrode on CdZnTe wafer is one of the key factors during the fabrication of CdZnTe detector. In this work, the effect of Au/Cd composite electrode on the (111)B (Te-rich surface) of CdZnTe wafers with p-type conductivity (p-CdZnTe) was investigated. The Au/Cd electrode was deposited on the (111)B of CdZnTe wafers by vacuum evaporation method. For comparison, the Au/Zn and Au electrodes were also deposited on CdZnTe (111)B surfaces. The surface and structural properties of the composite electrodes on the (111)B surface of CdZnTe were characterized by the AFM, XPS and SEM. The electrical properties of the electrodes were evaluated by Current-Voltage (I-V) test. The results show that the surfaces of the Au/Cd and Au/Zn electrodes on (111)B surface were more smooth with less foreign impurities among the interface of the electrode and CdZnTe, as compared to the Au electrode on (111)B surface. The Au/Cd composite electrode can obtain a more ideal ohmic contact than Au/Zn composite electrode on the (111)B surface of CdZnTe wafer. The Au/Cd composite electrode on (111)B surface has the lowest Schottky barrier height, which is attributed to the reduction of the influence of Te enriched surface on the metal-semiconductor contact.

Relationship of the shape and size between etch pits and corresponding Te inclusions in CdZnTe crystals

The etch pits (EPs) corresponding to Te inclusions with shapes of triangle, hexagon and irregular polygon were investigated by etch pit real-time observation (EPRTO) method, and the relationship of size between EPs and inclusions was established. Although the evolution process of the EPs corresponding to triangular, hexagonal and irregular Te inclusions is obviously different during etching, the final characteristic is flat bottom triangular EP. There is a relationship between the ratio of the size of EPs to inclusions (SEP/SI) and the initial size of Te inclusions (SI).

Improvement to the Carrier Transport Properties of CdZnTe Detector Using Sub-Band-Gap Light Radiation.

The effects of sub-band-gap light radiation on the performance of CdZnTe photon-counting X-ray detectors were studied using infrared light with different wavelengths in the region of 980–1550 nm. The performance of the detectors for X-ray detection was improved by the radiation of infrared light with the wavelengths of 1200 nm and 1300 nm. This was because the increase of the electron indirect transition, and the weakening of the built-in electric field induced by the trapped holes, reduced the drift time of the carrier, and increased the charge collection efficiency. To further analyze the intrinsic behavior of the trapped charge, the deep-level defects of CdZnTe crystal were measured by thermally stimulated current spectroscopy (TSC). The deep-level defect indicated by the trap named T4 in TSC spectra with the ionization energy of 0.43 eV should be responsible for the performance deterioration of CdZnTe detectors.

Upgraded CdZnTe Based on the Infrared Attenuation Spectrum

According to the infrared attenuation spectrum, we can know the conduction type and major point defect of CdZnTe crystals. Based on point defect modelling, an appropriate modus of atmosphere annealing was described in this paper. In this paper, annealing with Cd pressure control technology was used to upgrade p-CdZnTe and n-CdZnTe, respectively. Characterizations reveal that the optical and electrical performance of the CZT crystals is patently improved after annealing. Employment of the infrared attenuation spectrum as the annealing guideline has been verified to be feasible and effectual in this paper.

Study on a co-doped CdZnTe crystal containing Yb and In

The electron mobility of CZT:(ln,Yb) was increased by about 26% over that of the CZT:In crystal.

Capabilities of gamma-spectrometry methods for on-line monitoring of nitride SNF pyrochemical reprocessing

The paper reports the first test results for detectors of various types and equipment of gamma-spectrometry channels under external radiation originating from pyrochemical reprocessing of spent mixed nitride uranium-plutonium (MNUP) fuel. Testing was carried out on a solid-state detector with a CdZnTe crystal, a scintillation detector with a LaBr3crystal, and an ionization chamber based on compressed xenon. Simulated external gamma-radiation was created by means of a Co-based scattered gamma-radiation source. The paper also describes an experimental facility and a measurement technique, and presents the facility testing results for the above three detectors. The solid-state detector was proved to have the best performance. However, achieving the design characteristics of the gamma-spectrometry channel requires new solutions for protection and collimation of gamma-radiation produced by a real MNUP SNF reprocessing facility. What is meant here is the influence of the detectors’ geometry on the configuration of the protective collimator which is proposed to be used in real conditions. Thus, if a Xe-based detector is used, the calculated mass of the protective collimator is 900 kg, while it is possible to use less massive protection for the other detectors. In addition, when manufacturing neutron shielding for detectors based on CdZnTe and LaBr3, it is necessary to consider the neutron radiation factor in MNUP SNF processing. It is possible to surround the collimator with a moderating layer (for example, polyethylene) and create inside it a skin from a thermal neutron absorber (for example, based on cadmium).

High-Performance Compton SPECT Using Both Photoelectric and Compton Scattering Events

In conventional single-photon emission computed tomography (SPECT), only the photoelectric events in the detectors are used for image reconstruction. However, if the 131I isotope, which emits high-energy radiations (364, 637, and 723 keV), is used in nuclear medicine, both photoelectric and Compton scattering events can be used for image reconstruction. The purpose of our work is to perform simulations for Compton SPECT by using the Geant4 application for tomographic emission (GATE). The performance of Compton SPECT is evaluated and compared with that of conventional SPECT. The Compton SPECT unit has an area of 12 cm × 12 cm with four gantry heads. Each head is composed of a 2-cm tungsten collimator and a 40×40 array of CdZnTe (CZT) crystals with a 3×3 mm2 area and a 6-mm thickness. Compton SPECT can use not only the photoelectric effect but also the Compton scattering effect for image reconstruction. The correct sequential order of the interactions used for image reconstruction is determined using the angular resolution measurement (ARM) method and the energies deposited in each detector. In all the results of simulations using spherical volume sources of various diameters, the reconstructed images of Compton SPECT show higher signal-to-noise ratios (SNRs) without degradation of the image resolution when compared to those of conventional SPECT because the effective count for image reconstruction is higher. For a Derenzo-like phantom, the reconstructed images for different modalities are compared by visual inspection and by using their projected histograms in the X-direction of the reconstructed images.

Macroscopic effects and microscopic origins of gamma-ray irradiation on In-doped CdZnTe crystal

The effects of gamma-ray irradiation, exposed to a 60Co source with a dose of 2.7 kGy, on In-doped CdZnTe (CdZnTe:In) crystal were investigated. We combined the “macroscopic” electrical properties of CdZnTe:In sample, evaluated by current–voltage (I–V) measurements at different temperature, with the “microscopic” origins of electrically active defects induced by gamma-irradiation, characterized by thermally stimulated current spectroscopy. It reveals that the bulk resistivity at room temperature have increased from 2.7 × 109 Ω cm for the as-grown CdZnTe:In sample to 5.9 × 109 Ω cm for the irradiated sample. Since the microscopic origins of these macroscopic effects are linked to the electrically active defects within the material, five main defect states (I, II, III, IV and V) were characterized and identified in the CdZnTe:In crystal. In particular, the introduction of gamma-irradiation altered the trap concentrations of these defect states, such as the rapidly decreasing concentration of region I. Besides, the gamma-ray irradiation caused a further deepening of EDD level (region V) from the value of 0.717 ± 0.004 eV for the as-grown sample to the value of 0.749 ± 0.004 eV for the irradiated sample. The microscopic origin of EDD level was identified with TeCd2+ below the conduction band minimum, which is responsible for the pinning of EF level near the mid-gap, and thus for the observed high-resistivity performance of CdZnTe:In.

Two-Step Annealing to Remove Te Secondary-Phase Defects in CdZnTe While Preserving the High Electrical Resistivity

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.

A linear array of position-sensitive virtual Frisch-grid CdZnTe for low-energy gamma rays

Large-area arrays of position-sensitive virtual Frisch-grid CdZnTe (CZT) detectors with enhanced energy resolution and high efficiency have been proposed for spectroscopy and imaging of gamma-ray sources. Here, we present the design and test results of a linear array of such detectors optimized for use in handheld instruments. There are several application areas for such detectors including uranium enrichment measurements, storage monitoring, dosimetry and other nuclear safeguards related tasks. The array configuration provides a large area with minimum number of readout channels, which potentially allows the developers to avoid using ASIC-based readout electronics by substituting it with hybrid preamplifiers followed by digitizers. The array prototype consists of 6 5×7×25 mm3 CZT detectors side-oriented with respect to the source to achieve the maximum area. Each detector is furnished with 5 mm-wide charge-sensing pads placed near the anode. As an option, the pads can be grouped together to reduce the number of readout channels. The same can be done with the cathodes. The pads signals are converted into X–Y coordinates and combined with the cathode signals (for the Z coordinates) to give 3D positional information of each interaction point. This information is used to correct the response non-uniformity caused by material inhomogeneity, which allows the developers to use standard grade (unselected) CZT crystals, while retaining high spectroscopic performance.

Development of a position-sensitive CZT detector with coplanar grid electrode

The finite–element analysis was done to optimize position-sensing coplanar grid electrode geometry. CdZnTe crystals manufactured by PAM-XIAMEN with customized electrode geometry have been tested. After digitizing pre-amplifier signals, trapezoidal digital filter was adopted to enhance the signal to noise ratio. Energy resolution of 2.7% FWHM at 662 keV γ-ray energy was obtained. The measured results show the dramatic improvement with position-sensing technique.

Experimental and simulation research on the screening effect of the pixelated CZT imaging detector

Massive efforts have been made to investigate the characteristics of the CdZnTe detector under different extreme conditions. In this paper, we experimented with different radiation sources to investigate the imaging screening effect of the pixelated CdZnTe detector under ultrahigh irradiance. A donut-shaped irradiation image was obtained due to the increase in the X-ray tube current or the tube voltage. Moreover, statistical data revealed that the total count of all pixels was not significantly different, while the event-count of irradiated area pixels decreased significantly. A consequence of the screening effect was that the event counts redistributed among pixels that are located in the irradiated area, while the inner electric field was distorted. This leads to the catastrophic performance degradation of the central pixels. According to the Poisson equation, we developed a theoretical model of the CdZnTe detector using the finite element software COMSOL to enable an in-depth investigation of carrier collection in the CdZnTe crystal. A comparison between the simulations and the test results showed that pixels in the central irradiated area are completely screened under the ultrahigh irradiance because of the emergence of the relatively higher potential region, which can distort the electron drift path. Furthermore, the photon-generated signal can be collected only partly at the edge of the irradiated area, which was relatively stable because the collection area in the CdZnTe volume was relatively limited. The imaging results deduced from the simulations are well consistent with the experimental data.

Preparation of indium tin oxide contact to n-CdZnTe gamma-ray detector

The nonmetal electrode material Indium Tin Oxide (ITO) has advantages of excellent conductivity, higher adhesion, and interface stability, showing potential to replace the metallic contacts for fabrication of CdZnTe (CZT) X/γ-ray detectors. In this work, high quality ITO electrodes for n-type CZT crystals were prepared by magnetron sputtering under a sputtering power of 75 W and a sputtering pressure of 0.6 Pa. A low dark current of ∼1 nA is achieved for the 5 × 5 × 2 mm3 ITO/CZT/ITO planar device under 100 V bias. The characteristics of Schottky contact are presented in the room temperature I-V curves, which are similar to those of the Au contact detectors. Based on the thermoelectric emission theory, the contact barrier and resistance of ITO electrodes are evaluated to be 0.902–0.939 eV and 0.87–3.56 × 108 Ω, respectively, which are consistent with the values of the Au electrodes. The ITO/CZT/ITO structure detector exhibits a superior energy resolution of 6.5% illuminated by the uncollimated 241Am @59.5 keV γ-ray source, which is comparable to the CZT detector with Au electrodes.

Observations on Nanoscale Te Precipitates in CdZnTe Crystals Grown by the Traveling Heater Method Using High Resolution Transmission Electron Microscopy

Te precipitates in CdZnTe (CZT) crystals grown by the traveling heater method (THM) are investigated using high-resolution transmission electron microscopy (HRTEM). The results show that in THM-grown CZT crystals, Te precipitates are less than 10 nm in size—much smaller than those in Bridgman-grown CZT. They have hexagonal structure and form a coherent interface with zinc blend structure CZT matrix in the orientation relationship [ 1 ¯ 12 ] M / / [ 0001 ] P and ( 1 1 ¯ 1 ) M / / ( 1 ¯ 100 ) P . A ledge growth interface with the preferred orientation along the [ 1 1 ¯ 1 ] M and [ 110 ] M was found near Te precipitates. The growth and nucleation mechanism of Te precipitates are also discussed.

The Characteristics of Detectors Based on Cadmium−Zinc−Tellurium Crystals

The parameters of detectors based on CdZnTe crystals with a sensitive volume of approximately 1500 mm3 are investigated. It is shown that at room temperature the characteristics of these detectors are high enough for spectrometry of γ rays from 241Am, 57Co, 137Cs, and 60Co sources.

Study on the effects of indentation-induced dislocations on the detector performance of CdZnTe crystals

The effects of indentation-induced dislocations on spectroscopic performances of CdZnTe detectors and on the deep-level defects of CdZnTe crystals are studied. The mobility-lifetime product (μτ) for electrons of the as-grown sample is determined to be 1.2 × 10−3 cm2/V by the γ-ray energy spectral measurement. However, the value reduced to 4.5 × 10−4 cm2/V after the dislocation is artificially increased by indentation. Meanwhile, the energy resolution is deteriorated from 7.45% to 28.15%. The ion beam induced charge (IBIC) measurements showed quite low carrier collection efficiency of electrons in the dislocation enriched areas. Moreover, the carrier recombination rate at dislocations was reduced with the increase in applied voltage. The thermally stimulated current measurements showed that the concentration of dislocation related defect levels observed at 0.107 eV and 0.159 eV increased almost twice after indentation. These defect levels could probably result in the carrier loss during the γ-ray energy spectrum and IBIC measurements.

Distribution of microscopic defects in Al-doped CdZnTe crystal

We investigated the distribution of microscopic defects in an Al-doped CdZnTe crystal (CdZnTe:Al), grown under Cd-rich condition by the modified vertical Bridgman method, and their influence on the electrical transport behaviors. These microscopic defects were characterized by photoluminescence (PL), thermally stimulated current spectroscopy and thermoelectric effect spectroscopy. The results show that a low intensity and wide FWHM of (D0, X) peak observed in the low-temperature PL spectrum of CdZnTe:Al sample implies the non-ideal high crystal quality, which is in good agreement with its results of electrical transport properties. The bulk resistivity of CdZnTe:Al sample was evaluated to be in the magnitude of 108 Ω cm with I–V and Hall measurements. Simultaneously, high leakage currents were observed to produce the high noise, which deteriorated the detection performance. Besides, the shallow donor Al Cd + with the activation energy of 13 meV and the relatively shallow acceptor A-center complex (VCd–AlCd) of 112 meV were identified to dominate the low- and high-temperature quenching processes in the temperature-dependent PL spectra, respectively. These microscopic defects as trapping or recombination centers for the charge carriers in CdZnTe:Al detector are responsible for its electrical transport properties, leading to the incomplete charge collection. A further comprehensive investigation on the distribution of microscopic defects was proved to achieve the balance between acceptors and donors in the defects compensation process of CdZnTe:Al sample. In particular, the E DD level at E c − 0.554 eV was attributed to the doubly ionized Cd interstitial (Cd i 2+ ), which pinned the E F level near the mid-gap to achieve the high resistivity performance.