CdZnTe Research Articles

PVT growth of exfoliated CdZnTe polycrystalline thick films based on stress mismatch mechanism

The exfoliated CdZnTe (CZT) films were grown on quartz substrates by closed tube physical vapor transport method based on stress mismatch mechanism. The structure and morphology of the exfoliated CZT thick films were characterized by X-ray diffraction and scanning electron microscope. The as-grown CZT films are polycrystalline with (111) preferential orientation and exhibit a dense pyramid surface structure and a thickness of more than 600 μm. The coplanar and planar X-ray detectors were fabricated based on the exfoliated CZT film. The discrepant dark resistivity is 3.17 × 109 and 5.14 × 1010 Ω cm with regard to the coplanar and planar structure detector, respectively. The planar structure detector exhibits higher quantum efficiency at low doses X-ray radiation.

Characterization of HgCdTe Films Grown on Large-Area CdZnTe Substrates by Molecular Beam Epitaxy

Recent advances in growth of Hg1−xCdxTe films on large-area (7 cm × 7.5 cm) CdZnTe (CZT) substrates is presented. Growth of Hg1−xCdxTe with good uniformity on large-area wafers is achieved using a Riber 412 molecular beam epitaxy (MBE) tool designed for growth of Hg1−xCdxTe compounds. The reactor is equipped with conventional CdTe, Te, and Hg sources for achieving uniform exposure of the wafer during growth. The composition of the Hg1−xCdxTe compound is controlled in situ by employing a closed-loop spectral ellipsometry technique to achieve a cutoff wavelength (λco) of 14 μm at 78 K. We present data on the thickness and composition uniformity of films grown for large-format focal-plane array applications. The composition and thickness nonuniformity are determined to be <1% over the area of a 7 cm × 7.5 cm wafer. The films are further characterized by Fourier-transform infrared spectroscopy, optical microscopy, and Hall measurements. Additionally, defect maps show the spatial distribution of defects generated during the epitaxial growth of the Hg1−xCdxTe films. Microdefect densities are in the low 103 cm−2 range, and void defects are below 500 cm−2. Dislocation densities less than 5 × 105 cm−2 are routinely achieved for Hg1−xCdxTe films grown on CZT substrates. HgCdTe 4k × 4k focal-plane arrays with 15 μm pitch for astronomical wide-area infrared imagers have been produced using the recently developed MBE growth process at Teledyne Imaging Sensors.

135Xe measurements with a two-element CZT-based radioxenon detector for nuclear explosion monitoring

Measurement of elevated concentrations of xenon radioisotopes (131mXe, 133mXe, 133Xe and 135Xe) in the atmosphere has been shown to be a very powerful method for verifying whether or not a detected explosion is nuclear in nature. These isotopes are among the few with enough mobility and with half-lives long enough to make their detection at long distances realistic. Existing radioxenon detection systems used by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) suffer from problems such as complexity, need for high maintenance and memory effect. To study the response of CdZnTe (CZT) detectors to xenon radioisotopes and investigate whether it is capable of mitigating the aforementioned issues with the current radioxenon detection systems, a prototype detector utilizing two coplanar CZT detectors was built and tested at Oregon State University. The detection system measures xenon radioisotopes through beta-gamma coincidence technique by detecting coincidence events between the two detectors. In this paper, we introduce the detector design and report our measurement results with radioactive lab sources and 135Xe produced in the OSU TRIGA reactor. Minimum Detectable Concentration (MDC) for 135Xe was calculated to be 1.47 ± 0.05 mBq/m3.

Development of new CdZnTe detectors for room-temperature high-flux radiation measurements.

Recently, CdZnTe (CZT) detectors have been widely proposed and developed for room-temperature X-ray spectroscopy even at high fluxes, and great efforts have been made on both the device and the crystal growth technologies. In this work, the performance of new travelling-heater-method (THM)-grown CZT detectors, recently developed at IMEM-CNR Parma, Italy, is presented. Thick planar detectors (3 mm thick) with gold electroless contacts were realised, with a planar cathode covering the detector surface (4.1 mm × 4.1 mm) and a central anode (2 mm × 2 mm) surrounded by a guard-ring electrode. The detectors, characterized by low leakage currents at room temperature (4.7 nA cm-2 at 1000 V cm-1), allow good room-temperature operation even at high bias voltages (>7000 V cm-1). At low rates (200 counts s-1), the detectors exhibit an energy resolution around 4% FWHM at 59.5 keV (241Am source) up to 2200 V, by using commercial front-end electronics (A250F/NF charge-sensitive preamplifier, Amptek, USA; nominal equivalent noise charge of 100 electrons RMS). At high rates (1 Mcounts s-1), the detectors, coupled to a custom-designed digital pulse processing electronics developed at DiFC of University of Palermo (Italy), show low spectroscopic degradations: energy resolution values of 8% and 9.7% FWHM at 59.5 keV (241Am source) were measured, with throughputs of 0.4% and 60% at 1 Mcounts s-1, respectively. An energy resolution of 7.7% FWHM at 122.1 keV (57Co source) with a throughput of 50% was obtained at 550 kcounts s-1 (energy resolution of 3.2% at low rate). These activities are in the framework of an Italian research project on the development of energy-resolved photon-counting systems for high-flux energy-resolved X-ray imaging.

Exchange couplings for Mn ions in CdTe: Validity of spin models for dilute magnetic II-VI semiconductors

We employ density-functional theory (DFT) in the generalized gradient approximation (GGA) and its extensions GGA+$U$ and GGA+Gutzwiller to calculate the magnetic exchange couplings between pairs of Mn ions substituting Cd in a CdTe crystal at very small doping. DFT(GGA) overestimates the exchange couplings by a factor of three because it underestimates the charge-transfer gap in Mn-doped II-VI semiconductors. Fixing the nearest-neighbor coupling $J_1$ to its experimental value in GGA+$U$, in GGA+Gutzwiller, or by a simple scaling of the DFT(GGA) results provides acceptable values for the exchange couplings at 2nd, 3rd, and 4th neighbor distances in Cd(Mn)Te, Zn(Mn)Te, Zn(Mn)Se, and Zn(Mn)S. In particular, we recover the experimentally observed relation $J_4>J_2,J_3$. The filling of the Mn 3$d$-shell is not integer which puts the underlying Heisenberg description into question. However, using a few-ion toy model the picture of a slightly extended local moment emerges so that an integer $3d$-shell filling is not a prerequisite for equidistant magnetization plateaus, as seen in experiment.

Carrier transport performance of Cd0.9Zn0.1Te detector by direct current photoconductive technology

In this paper, based on the derivation of the Hecht model and Many model, we establish a new theoretical model and deduce its mathematical equation by considering an even-distributed charged center of certain concentration under constant illumination. This model helps us deeply understand the carrier transport performance of Cd0.9Zn0.1Te (CZT) under various illuminations and non-uniform distribution of the internal electric field in CZT. In our research, direct current photoconductive technology (DCPT) is applied to assess the electrical transport properties of carriers in CZT crystals, which is verified by room temperature Am-241 alpha-particle responses and charge collection efficiency test. The mobility-lifetime product (μτ) for carriers is extracted from fitting the Hecht model by DCPT for CZT radiated from the cathode and anode by a constant laser, respectively. Moreover, the carrier transport properties of CZT under various light intensities and during a wide range of temperatures are also taken and analyzed in detail, which enable us to develop the best performance of CZT. In addition, we figure out a method for estimating μeτe and μhτh on different positions of CZT wafer on the basis of the pixel detector that collects single polarity charge, and several corrective actions have improved the accuracy of the measurement.

Accumulative dose response of CdZnTe detectors to 14.1 MeV neutrons

The accumulative dose response of CdZnTe (CZT) detectors to 14.1MeV neutrons is discussed experimentally in this paper. The Cockcroft–Walton Accelerator is used to obtain a steady neutron beam of 14.1MeV neutrons. A pulsed X-ray source is used to test the response parameters of the neutron-exposed CZT detectors under the pulse mode. The irradiation time (hours) is shorter relative to the time scales (years) where annealing effects occur. Time and linearity response is analyzed to evaluate the maximum dose rate of the CZT detectors and the pulse shape. The result shows that the experimental CZT detectors maintain stable response behaviors, while the maximum dose rate and the total accumulative dose are less than 106neutrons/(cm2·s) and 1010neutrons/cm2, respectively.

A comparison of emerging gamma detector technologies for airborne radiation monitoring

This paper presents a comparison of new and emerging gamma detector technologies that have the potential to improve in-situ dose and radioactivity-in-air measurements for national monitoring networks. Five detectors were chosen for investigation; LaBr3(Ce), CeBr3, SiPM-CsI(Tl), Cd(Zn)Te and electromechanically-cooled HPGe. These detectors represent the full range of the price-performance matrix. Comparisons have been made of energy resolution, detection efficiency and minimum detectable activity by exposing each detector to a mixed radionuclide source drop-deposited across a filter. Other factors, such as internal radioactivity, linearity, size and cost have also been considered.

Use of the drift-time method to measure the electron lifetime in long-drift-length CdZnTe detectors

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 μτ product. As a result, researchers usually take measurements at relatively high biases, which work well in the case of the low μτ-product material, &amp;lt;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.

Efficiency calibration of a portable CZT detector for nondestructive activation assessment of a cyclotron bunker

ABSTRACTDuring the operational life of a PET Cyclotron, the concrete walls of the cyclotron vault are activated by the secondary neutron flux interacting with rare earths and metals present in the concrete or in reinforcement bars. For this reason when considering dismantling of such accelerators, the amount of radioactive waste has to be evaluated in advance to identify any critical issues or possible countermeasures to be taken to define an optimum decommissioning strategy. The aim of this work is to define a non-destructive in situ measurement methodology for a preliminary activation assessment of a cyclotron bunker with no need for core drilling. A very compact, USB-powered, CdZnTe (CZT) detector for gamma-ray spectrometry was used for the activation assessment of the site of installation of a GE PETtrace (16.5 MeV) cyclotron, routinely used in the production of positron-emitting radionuclides. Because of the complexity of measurement geometry, the efficiency calibration of the detector was performed via Monte Carlo (MC) simulations. The detector was accurately modelled using FLUKA, including a 5 cm lead shielding set-up. The MC model of the detector was validated for a wide range of energies and different source geometries, showing discrepancies below 5% for all tested sources. The efficiency curve for wall activation measurements was calculated, allowing a quantitative evaluation of activity concentration.

X-ray response of CdZnTe detectors grown by the vertical Bridgman technique: Energy, temperature and high flux effects

Nowadays, CdZnTe (CZT) is one of the key materials for the development of room temperature X-ray and gamma ray detectors and great efforts have been made on both the device and the crystal growth technologies. In this work, we present the results of spectroscopic investigations on new boron oxide encapsulated vertical Bridgman (B-VB) grown CZT detectors, recently developed at IMEM-CNR Parma, Italy. Several detectors, with the same electrode layout (gold electroless contacts) and different thicknesses (1 and 2.5mm), were realized: the cathode is a planar electrode covering the detector surface (4.1×4.1mm2), while the anode is a central electrode (2×2mm2) surrounded by a guard-ring electrode. The detectors are characterized by electron mobility-lifetime product (µeτe) values ranging between 0.6 and 1·10−3cm2/V and by low leakage currents at room temperature and at high bias voltages (38nA/cm2 at 10000V/cm). The spectroscopic response of the detectors to monochromatic X-ray and gamma ray sources (109Cd, 241Am and 57Co), at different temperatures and fluxes (up to 1 Mcps), was measured taking into account the mitigation of the effects of incomplete charge collection, pile-up and high flux radiation induced polarization phenomena. A custom-designed digital readout electronics, developed at DiFC of University of Palermo (Italy), able to perform a fine pulse shape and height analysis even at high fluxes, was used. At low rates (200cps) and at room temperature (T=25°C), the detectors exhibit an energy resolution FWHM around 4% at 59.5keV, for comparison an energy resolution of 3% was measured with Al/CdTe/Pt detectors by using the same electronics (A250F/NF charge sensitive preamplifier, Amptek, USA; nominal ENC of 100 electrons RMS). At high rates (750 kcps), energy resolution values of 7% and 9% were measured, with throughputs of 2% and 60% respectively. No radiation polarization phenomena were observed at room temperature up to 1 Mcps (241Am source, 60keV), while a detector collapse characterizes the samples at T<10°C.Comparison with two traveling heater method (THM) grown CZT detectors (REDLEN, Canada), fabricated with the same electrode layout, is also presented.These activities are in the framework of an Italian research project on the development of energy-resolved photon counting (ERPC) systems for high flux energy-resolved X-ray imaging.

Investigation of dislocation migration in substrate-grade CdZnTe crystals during post-annealing

The migration of dislocations in substrate-grade CdZnTe (CZT) single crystals during temperature gradient annealing under Cd/Zn vapor has been investigated. The etch pit density (EPD) and configuration of dislocations have been evaluated before and after annealing in CZT crystals with and without Cd-rich second phase (Cd-SP) particles, respectively. After Cd/Zn overpressure annealing, dislocation reduction in CZT crystals was observed. However, dislocation walls with 120° intervals along <211> crystalline direction were observed in the both types of CZT crystals. The formation of these dislocation walls can be attributed to the reaction of <110> dislocations. Moreover, it is considered that the release of the restored stress during annealing act as the domain driving force for dislocation migration, by comparing the variation of dislocation configuration in CZT crystals with and without Cd-SP particles.

The Effect of Low-Temperature Annealing on a CdZnTe Detector

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.

Al-doped ZnO contact to CdZnTe for x- and gamma-ray detector applications

The poor adhesion of common metals to CdZnTe (CZT)/CdTe surfaces has been a long-standing challenge for radiation detector applications. In this present work, we explored the use of an alternative electrode, viz., Al-doped ZnO (AZO) as a replacement to common metallic contacts. ZnO offers several advantages over the latter, such as having a higher hardness, a close match of the coefficients of thermal expansion for CZT and ZnO, and better adhesion to the surface of CZT due to the contact layer being an oxide. The AZO/CZT contact was investigated via high spatial-resolution X-ray response mapping for a planar detector at the micron level. The durability of the device was investigated by acquiring I–V measurements over an 18-month period, and good long-term stability was observed. We have demonstrated that the AZO/CZT/AZO virtual-Frisch-grid device performs fairly well, with comparable or better characteristics than that for the same detector fabricated with gold contacts.

Analysis of Etched CdZnTe Substrates

State-of-the-art as-received (112)B CdZnTe substrates have been examined for surface impurity contamination and polishing residue. Two 4 cm × 4 cm and one 6 cm × 6 cm (112)B state-of-the-art as-received CdZnTe wafers were analyzed. A maximum surface impurity concentration of Al = 1.7 × 1015 atoms cm−2, Si = 3.7 × 1013 atoms cm−2, Cl = 3.12 × 1015 atoms cm−2, S = 1.7 × 1014 atoms cm−2, P = 1.1 × 1014 atoms cm−2, Fe = 1.0 × 1013 atoms cm−2, Br = 1.2 × 1014 atoms cm−2, and Cu = 4 × 1012 atoms cm−2 was observed on the as-received CdZnTe wafers. CdZnTe particulates and residual SiO2 polishing grit were observed on the surface of the as-received (112)B CdZnTe substrates. The polishing grit/CdZnTe particulate density on CdZnTe wafers was observed to vary across a 6 cm × 6 cm wafer from ∼4 × 107 cm−2 to 2.5 × 108 cm−2. The surface impurity and damage layer of the (112)B CdZnTe wafers dictate that a molecular beam epitaxy (MBE) preparation etch is required. The contamination for one 4 cm × 4 cm and one 6 cm × 6 cm CdZnTe wafer after a standard MBE Br:methanol preparation etch procedure was also analyzed. A maximum surface impurity concentration of Al = 2.4 × 1015 atoms cm−2, Si = 4.0 × 1013 atoms cm−2, Cl = 7.5 × 1013 atoms cm−2, S = 4.4 × 1013 atoms cm−2, P = 9.8 × 1013 atoms cm−2, Fe = 1.0 × 1013 atoms cm−2, Br = 2.9 × 1014 atoms cm−2, and Cu = 5.2 × 1012 atoms cm−2 was observed on the MBE preparation-etched CdZnTe wafers. The MBE preparation-etched surface contamination consists of Cd(Zn)Te particles/flakes. No residual SiO2 polishing grit was observed on the (112)B surface.

A CZT-based radioxenon detection system in support of the Comprehensive Nuclear-Test-Ban Treaty

In this work, a prototype radioxenon detection system was designed, developed and tested at Oregon State University to study the response of CdZnTe (CZT) detectors to xenon radioisotopes for monitoring nuclear explosions. The detector utilizes two coplanar CZT detectors and measures xenon radioisotopes through beta–gamma coincidence detection between the two detection elements. The CZT-based detection system offers excellent energy resolution and background count rate compared with scintillator-based beta–gamma coincidence detectors currently in operation at the IMS stations. In this paper, we briefly discuss the detector design and report our recent measurement results with 131mXe, 133mXe, and 133Xe produced in the TRIGA reactor at OSU.

Start from Scratch: the Prospect of Nuclear Cardiology

The future is always hard to forecast but the prospect of nuclear cardiology has never been more unobtainable than these days. Myocardial perfusion single-photon emission computed tomography (MPS) has been one of the major nuclear medicine studies for decades, but the annual number of MPS is stagnant or steadily decreasing in Korea and other countries [1]. The challenge from coronary computed tomography (CCT) and the concern of radiation exposure of MPS [2] were the main reasons for the stalemate of nuclear cardiology. Compared to the rapid technological progress of CCT, enabling greater image resolution in conjunction with lower radiation exposure to the patients, development of new radiopharmaceuticals or scintillation imaging techniques has been at a relatively slow pace. Positron emission tomography (PET) has many advantages over MPS in terms of image resolution and radiation exposure. N-13 ammonia, O-15 H2O, and Rb-82 are the PET perfusion agents, which are widely used all over the world. However, the capacity of cyclotrons for N-13 ammonia and O-15 H2O usually limits the number of PET perfusion studies, and the generator for Rb-82 is not imported to Korea yet. F-18 labeled myocardial perfusion agent is under clinical trial in the U.S. [3] but even after the approval for clinical application, the cost of F-18 agent may not be readily affordable for the time being. When it comes to the development of imaging techniques, semi-conductor detector is worthy of mention [4]. The CdZnTe (CZT) detector-based cardiac gamma cameras from two manufacturers are already on the market, and two national institutes installed each model from the two vendors. The capabilities of the CZT gamma cameras are reported to exceed those of conventional Anger type gamma cameras: >2 times the spatial resolution, >1.5 times the energy resolution, and 5∼10 times the sensitivity, which will make it possible to reduce the patient dose or the imaging acquisition time [4]. However, it remains to be seen whether the diagnostic efficacy for coronary artery disease is improved using those CZT gamma cameras compared to the traditional gamma cameras because the unique L-type gantry structure of CZT gamma camera may lead to spatial inhomogeneity regarding the signal detection efficiency [5]. The challenges often lead to the innovation, and the innovation usually starts from scratch. Nuclear medicine features the functional imaging that is complementary to the anatomical imaging. For example, the coronary stenosis detected by CCT is not always the stigma of symptomatic coronary artery disease. Functional significance can still be effectively assessed using the MPS. The presence of atheromatic plaque confirmed by high resolution imaging studies needs to be further characterized regarding the vulnerability. F-18 sodium fluoride holds promise for the identification of such a vulnerable plaque [6]. Quantitative imaging is also an unparalleled strength of nuclear imaging. PET is an inherently robust quantitative imaging, thanks to the coincidence detection principle. Furthermore, SPECT, in the prototype of SPECT/CTwith CTbased attenuation correction, scatter correction, and resolution recovery, is now proven as a promising quantitative imaging tool [7]. Therefore, the future of nuclear cardiology is really dependent on the application of the genuine nuclear medicine principle to patient’s management. The review for current update of nuclear cardiology will ensue in the next issue of Nuclear Medicine and Molecular Imaging. * Won Woo Lee wwlee@snu.ac.kr

Overcoming Zn segregation in CdZnTe with the temperature gradient annealing

The availability of large volume crystals with the same energy gap in melt-grown CdZnTe (CZT) is restricted due to the Zn segregation in CdTe hosts. We observed the migration of Zn in the solid phase along the positive temperature gradient direction both in situ and post-growth temperature gradient annealing (TGA) of CZT. Diffusivity of Zn obtained from the in situ TGA was approximately 10−5cm2/s order and completely different mechanism with that of post-growth. The CZT ingots obtained through in situ TGA have uniform Zn and resistivity of 1010Ωcm orders. The CZT detectors fabricated from in situ TGA applied ingots exhibit 10% of energy resolution for 59.5keV peak of 241Am.

CdZnTe position-sensitive drift detectors with thicknesses up to 5 cm

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 × 5 mm2 and 6 × 6 mm2 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.

Improvement of the energy resolution in a pixellated CdZnTe detector using depth sensing based on pulse rise-time correlation

In this work we present the pulse rise-time method by using signals from one electrode to determine the interaction depth in a 16-pixellated CdZnTe (CZT) detector manufactured by eV Products. Pulse shapes were theoretically calculated by varing the depth in steps of 0.5 mm with fixing the in-plane position. The electric field in the CZT detector was calculated numericallly by using the weighting potential method. The depth information was extracted by the rise-time correlation using calculations for anode pulse shapes. In order to check the accuracy of the pulse shape calculation, we performed a maximum-likelihood fitting process by comparing measured pulse shapes with calculated ones. The pulse shapes were digitally recorded by using a 64-channel 62.5 MS/s digitizer (v1740B) manufactured by CAEN . Gamma rays emitted from a 22Na standard source were bombarded on the negatively biased contact of the CZT detector. The low energy tail of a photo peak attributed to the hole trapping in the energy spectrum was corrected by using the rise-time correlation. The energy resolution was improved from 9.3% to 6.5% for the 511-keV photo peak with the T50 rise-time correlation.