CdZnTe Research Articles

Enhanced hole mobility-lifetime product in selenium-added CdTe compounds

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 2.18μs 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.

Feasibility study of CdZnTe and CdZnTeSe based high energy X-ray detector using linear accelerator

CdZnTeSe (CZTS) has attracted attention for applications in X- and gamma-ray detectors owing to its improved properties compared to those of CdZnTe (CZT). In this study, we grew and processed single crystals of CZT and CZTS using the Bridgeman method to confirm the feasibility of using a dosimeter for high-energy X-rays in radiotherapy. We evaluated their linearity and precision using the coefficient of determination (R2) and relative standard deviation (RSD). CZTS showed sufficient RSD values lower than 1.5% of the standard for X-ray dosimetry, whereas CZT's RSD values increased dramatically under some conditions. CZTS exhibited an R2 value of 0.9968 at 500 V/cm, whereas CZT has an R2 value of 0.9373 under the same conditions. The X-ray response of CZTS maintains its pulse shape at various dose rates, and its properties are improved by adding selenium to the CdTe matrix to lower the defect density and sub-grain boundaries. Thus, we validated that CZTS shows a better response than CZT to high-energy X-rays used for radiotherapy. Further, the applicability of an onboard imager, a high-energy X-ray (>6 MV) image, is presented. The proposed methodology and results can guide future advances in X-ray dose detection.

Imaging effects due to pixel distortions in CdZnTe detectors—results from the HREXI Calibration Facility

ProtoEXIST2 (P2) was a prototype imaging X-ray detector plane developed for wide-field Time Domain Astrophysics (TDA) in the 5 - 200 keV energy band. It was composed of an 8 $\times$ 8 array of 5 mm thick, 2cm $\times$ 2cm pixelated (32 $\times$ 32) CdZnTe (CZT) detectors with a 0.6 mm pitch that utilize the NuSTAR ASIC(NuASIC) for readout. During the initial detector development process leading up to post-flight examination of the entire detector plane, distortions in expected pixel positions and shapes were observed in a significant fraction of the detectors. The HREXI (High Resolution Energetic X-ray Imager) Calibration Facility (HCF) was designed and commissioned to improve upon these early experiments and to rapidly map out and characterize pixel non-uniformities and defects within CZT detector planes at resolutions down to 50 $\rm \mu$m. Using this facility, the sub-pixel level detector response of P2 was measured at 100 $\rm \mu$m resolution and analyzed to extract and evaluate the area and profile of individual pixels, their morphology across the entire P2 detector plane for comparison with previous measurements and to provide additional characterization. In this article, we evaluate the imaging performance of a coded-aperture telescope using the observed pixel morphology for P2 detectors. This investigation will serve as an initial guide for detector selection in the development of HREXI detector planes, for the future implementation of the 4pi X-Ray Imaging Observatory (4piXIO) mission which aims to provide simultaneous and continuous imaging of the full sky ($\rm 4\pi$ sr) in the 3-200 keV energy band with $\rm \simeq$ 2 arcmin angular resolution and $\simeq$ 10 arcsec source localization, as well as other, future coded-aperture instruments.

Evaluation of CZT Drift Strip Detectors for Use in 3-D Molecular Breast Imaging

X-ray mammography is a widely used technique for breast cancer screening. However, the technique is imprecise when it comes to radiographically dense breast tissue, and therefore a supplemental screening technique, such as molecular breast imaging (MBI), could increase the cancer detection rate. Emerging technologies within MBI require excellent detector performance, preferably with a submillimeter intrinsic spatial resolution. A collaboration between Kromek and DTU Space aims to advance the DTU Space developed 3-D CdZnTe (CZT) drift strip technology, for application in new emerging MBI systems. This collaboration has resulted in ten compact 3-D MBI test modules with the goal of producing high-performance and high yield detectors. In this article, we present overall excellent detector performance, submillimeter position resolution at both 122 and 661.6 keV, and good energy resolution for the applied electrode deposition and contact optimization. Although the current experimental setup and results suffer from high electronic noise (using discrete NIM standard charge sensitive preamplifiers), we conclude that the measured spatial and spectral performance fulfills the expected requirements for the modules, with room for improvement, especially within limiting electronic noise. The detector test modules indicate a promising future for the 3-D CZT drift strip technology within future emerging MBI systems.

Tuning of nuclear spectroscopic telescope array application specific integrated circuits to improve low energy threshold of future hard x-ray imaging detectors

Detector commanding, processing and readout of spaceborne instrumentation is often accomplished with application specific integrated circuits (ASICs). The ASIC designed for the nuclear spectroscopic telescope array (NuSTAR) mission enables future tiled CdZnTe (CZT) detector array readout for x-ray detectors, such as the high resolution energetic x-ray imager (HREXI). Modified NuSTAR ASIC (NuASIC) gain settings have been implemented for HREXI’s broader targeted imaging energy range (3 to 300 keV) compared with NuSTAR (2 to 79 keV), which may require updated NuASIC internal parameters for optimal energy resolution. To reach HREXI’s targeted low energy threshold, we have also enabled the NuASIC’s “charge pump mode,” which introduces an additional tuning parameter. We describe the mechanics of the NuASIC’s adjustable parameters and use our recently developed ASIC test stand to probe a “bare” NuASIC using its internal test pulser. We record the effects of parameter tuning on the device’s electronics noise and low energy threshold and report the optimal set of parameters for HREXI’s updated gain setting. We detail a semiautomated procedure to derive the optimal parameters for each of HREXI’s large area closely tiled NuASIC/CZT detectors to expedite instrument integration.

Development of a high energy resolution and wide dose rate range portable gamma-ray spectrometer

In this paper, a portable gamma-ray spectrometer for real-time and in-situ gamma-ray detection applications is presented. By combining a quasi-hemispherical CdZnTe (CZT) semiconductor detector and a Geiger-Muller (GM) counter together, a wide dose rate range is achieved, ranging from 0.1 μSv/h to 100 mSv/h with a relative error of less than 10%. The GM counter is used to measure dose rate from 1 mSv/h to 100 mSv/h. With CZT, the spectrometer can provide a high energy resolution spectrum for nuclide identification and a high precision dose rate at low dose rates. The full width half maximum (FWHM) resolution is 2.2% at 662 keV below 70 μSv/h and is better than 3.3% at 3.8 mSv/h. The weight of the spectrometer is 3.2 kg for handheld and the runtime is up to 12 h without charging. For preliminary applications, the spectrometer was used to measure the gamma radiation around the Back-n white neutron beam line at China Spallation Neutron Source and around the steam generator in the nuclear power plant at Daya Bay Nuclear Power Station.

Performance Study of Virtual Frisch Grid CdZnTeSe Detectors

Nuclear detectors for x-ray and gamma-ray spectroscopy and imaging are a vital tool in many homeland security, medical imaging, astrophysics and other applications. Most of these applications require room-temperature operation due to the operational constraints imposed by a cryogenic cooling system. CdZnTe (CZT) has been the main material with the desired detection properties, and CZT crystals have been used commercially for three decades. However, CdZnTe still suffers from long-standing issues of high densities of performance-limiting intrinsic defects such as Te inclusions and networks of dislocation walls (sub-grain boundaries). A recently invented new quaternary material CdZnTeSe showed excellent material properties for radiation detection. The material was found to be free from dislocation networks, possess reduced Te inclusions, and have better compositional homogeneity. Virtual Frisch grid detectors were fabricated from crystals taken from a CdZnTeSe ingot that was grown by the traveling heater method. The detectors were fabricated from an as-grown ingot, bypassing the post-growth annealing process commonly practiced for industrial-grade CZT. The performances of the detectors were studied with different Frisch grid lengths using an amplifier shaping time ranging from 1–6 µs. The detectors showed high-quality spectroscopic performance with an as-measured energy resolution of ~1.1% at 662 keV for an optimum Frisch grid length of 3 mm. The charge collection was observed to enhance for longer Frisch grids.

Effects of annealing in Te2 atmosphere on photoelectric properties and carrier transport properties of CdZnTe films

CdZnTe (CZT) epitaxial films were deposited on GaAs substrates by closed spaced sublimation. In order to reduce the adverse effects of point defects and dislocations in the film on its properties, CZT films were annealed at 400 °C in Te2 atmosphere at different times. The I–V test and energy spectrum show that the resistivity of CZT films increased from 109 Ω cm to 1010 Ω cm, and the electron mobility lifetime product (μτ)e increased from 10−5 cm2 V−1 to 10−4 cm2 V−1. According to the etch pit density and photoluminescence tests, the results show that annealing reduced the dislocation density from 3.28 × 105 cm−2 to 2.52 × 105 cm−2 in the CdZnTe films. In this study, the annealing process which can stably improve the properties of CdZnTe films is obtained, that is, annealing at 400 °C in Te2 atmosphere for 2–4 h.

Performance optimization of CZT thick films by CdTe buffer layers for solar-blind ultraviolet photoelectric detectors

CdZnTe (CZT) thick films with CdTe buffer layers (CZT/CdTe) were studied for high-performance ultraviolet photoelectric detector. CZT thick films were prepared on AlN/Si substrates via close-spaced sublimation (CSS) and CdTe buffer layers were introduced to optimize the interface quality of CZT thick films. Results of x-ray diffraction showed that the full width at half maximum (FWHM) and dislocation density (δ) of interface of optimized samples were 0.156 ° and 2.543 × 1014 lin/m2, respectively. Interface detect analysis of free-standing CZT and CZT/CdTe thick films was studied by x-ray photoelectron spectroscopy (XPS) and low-temperature photoluminescence (PL) spectroscopy, and mechanism of in-situ annealing in CdTe atmosphere to reduce interfacial defect states was discussed. Moreover, the photo-dark current ratio of the CZT/CdTe thick film metal-semiconductor-metal (MSM) structural photodetector was as high as 5530, which was nearly double that of the CZT thick film detector, and the carrier mobility lifetime product could reach 5.59 × 10−4 cm2/V at 100 V bias, confirming the improvement of carrier transport capacity and detection efficiency. CZT-based detectors have inestimable prospect in the field of solar-blind ultraviolet detection with the participation of CdTe buffer layers.

Effectiveness of parylene coating on CdZnTe surface after optimal passivation

Parylene coating was adopted on CdZnTe (CZT) detector as a mechanical protection layer after wet passivation with hydrogen peroxide (H2O2) and ammonium fluoride (NH4F). Wet chemical passivant lose their effectiveness when exposed to the ambient conditions for a long time. Parylene coating could protect the effectiveness of passivation, by mechanically blocking the exposure to the ambient conditions. Stability of CZT detector was tested with the measurement of leakage current density and response to radio-isotopes. When the enough thickness of parylene (>100 μm) is adopted, parylene is a promising protection layer thereby ensuring the performance and long-term stability of CZT detectors.

Surface Planarization of CdZnTe Wafers: Effect of Slurry Formulation and CMP Processing Parameters on Surface Planarity

This study investigates the effect of chemical mechanical planarization (CMP) processing parameters such as platen velocity, the concentration of the oxidizer and abrasive nanoparticle, slurry pH and surfactant types on the surface roughness of cadmium zinc telluride (CdZnTe) substrate. It was found that these parameters have a significant effect on the quality of the polished surfaces. It was found that lower platen velocity, lesser concentration of abrasive particles, basic slurry pH, and addition of anionic surfactant (SDS) into the CMP slurry solution showed improved surface planarity. Optical Surface Profiler and atomic force microscopy (AFM) techniques were used to monitor the surface topography before and after polishing. A notable root-mean-square surface roughness, (Rq), ∼0.9 nm, has been obtained on the polished CdZnTe (CZT) surface over a scan area of 481 × 361 μm2 under the optimized conditions of 60 rpm relative velocity, slurry pH of 9, 3.75 vol% of oxidizer (H2O2) and 1.25 wt% of abrasive (SiO2 nanoparticle). A probable mechanism of the present CMP surface planarization of CZT substrate has been proposed. Unlike the conventional surface planarization processes, which involve two-step lapping followed by CMP for the CZT surfaces, we have developed a single step CMP process to obtain good surface planarity.

Lithium Chloride-Substituted Methylammonium Lead Tribromide Perovskites for Dual γ/Neutron Sensing.

Dual γ/neutron radiation sensors are a critical component of the nuclear security mission to prevent the proliferation of a special nuclear material (SNM). While high-performing semiconductors such as high purity germanium (HPGe) and CdZnTe (CZT) already exist in the nuclear security enterprise, their high cost and/or logistical burdens make widespread deployment difficult to achieve. Metal lead halide perovskites (MHPs) have attracted interest in recent years to address this challenge. In particular, methylammonium lead tribromide (CH3NH3PbBr3, MAPbBr3, or MAPB) has been widely evaluated for its radiation sensing capabilities. While previous studies have demonstrated low-energy X-ray and α particle sensing of MAPB-based detectors and several studies discuss the potential for γ ray sensing, neutron sensing of this material has been rarely explored. Here, we explore the incorporation of lithium in the form of LiCl into the MAPB structure to add thermal neutron sensitivity. Characterizations of the lithium-doped MAPB crystals demonstrate that quality growths are achievable with single crystals that exhibit high crystallinity, no phase change, and high macroscopic bulk quality. Finally, we report on the first demonstrated γ ray and thermal neutron sensing based on lithium-doped MAPB single crystals, which is a significant milestone in the development of 3D dual γ/neutron MHP sensors.

Effect of different annealing conditions on CdZnTe thin films for absorber layer applications

The Silicon based single junction and perovskite solar cells have demonstrated high efficiency but Silicon based cells suffer from the drawback of higher cost and perovskite solar cells are not stable and degradable in environment. The thin film based CIGS and CdTe cells have achieved efficiency of more than 22% but, particularly, the CdTe thin film solar cells require a suitable back contact which can be obtained by doping of Zn element in CdTe. The grain growth in CdTe-based films is attained by chloride treatment and therefore, in order to seek applicability of chloride treatment on CdZnTe (CZT) thin films, the present communication reports an evolution of different annealing conditions (in air, CdCl2 and MgCl2 environments) on physical properties to electron beam evaporated CdZnTe thin films. The XRD diffractograms revealed to the dominance of cubic phased peak corresponding to (111) plane and augmentation in grain size with treatment temperature. The optical spectra indicated variation in absorbance and transmittance with temperature of treatments and Tauc's plots demonstrated band gap of all the films in 1.41–1.55 eV range. Current-voltage characteristics divulged Ohmic character of films where conductivity and resistivity are eventually modified with annealing conditions. The FESEM micrographs of pristine films represented bouncy grains with some vacant spaces whereas chloride treated films showed larger grains. The EDS patterns confirmed the deposition of CdZnTe thin films and treatments concerned. The AFM maps demonstrated deep valley-like and hill-like configurations of processed films. The obtained results signify that the films annealed at 300 °C aver their suitability for absorber layer roles in development of efficient solar cells with single junction and tandem architectures.

Minimal Detection Time for Localization of Radioactive Hot Spots in Low and Elevated Background Environments Using a CZT Gamma-Ray Spectrometer

This study determines the minimal detection time (MDT) needed for successful localization of radioactive hot spots during nuclear decommissioning work. An automated XY stage, equipped with a CdZnTe (CZT) spectrometer, was used to identify and localize hot spots of 241Am, 137Cs, and 60Co in a 1.7 × 1.7-m area. The stage served as a preliminary test platform for the development of an automated robotic characterization platform [Autonomous Robotic platform for CHaractERization (ARCHER) robot]. The dependence of the MDT on the detector efficiency and background (BKG) level was examined. For low BKG environments, the MDT for 137Cs was 871 ms and resulted in an error of the source localization of 14.21 mm and an error of the activity of 6.85%. For elevated BKG levels, the MDT increased to 15 526 ms. The 137Cs source was localized with an error of 34.13 mm and an error of the source activity of −7.04%. The MDT determination method used here offers a valuable approach for decreasing total scanning times while avoiding missing the presence of hot spots.

Evaluation of crystalline quality of traveling heater method (THM) grown Cd0.9Zn0.1Te0.98Se0.02 crystals

Because of its excellent opto-electronic properties, CdZnTe (CZT) has been the material of choice for x- and gamma-ray detectors operable at room temperature. CZT is the leading commercially available room-temperature radiation detector material today. Although much progress has been made over the past three decades, today's CZT crystals still face certain challenges, especially the presence of the performance-limiting materials defects and the associated relatively high production cost. In this regard, CdxZn1−xTeySe1−y (CZTS) is emerging as a next-generation compound semiconductor, which overcomes some of the limitations of CZT technology for the stated applications. Here, we conducted a study to evaluate the crystalline quality of the traveling heater method grown CZTS with an optimized alloy composition, i.e., Cd0.9Zn0.1Te0.98Se0.02. The as-grown samples were evaluated by low-temperature photoluminescence (PL) spectroscopy and high-resolution x-ray diffraction using the synchrotron light source at Brookhaven National Laboratory. The full width at half maximum of both the PL and x-ray rocking curves was observed to be broadened due to the lattice disorder of the quaternary compound, eventually degrading the crystalline quality. This was consistent with density functional theory calculations.

Polishing of CdTe, Cd(Zn)Te, Cd(Mn)Te Single Crystals by Iodine in Dimethylformamide

The process of chemical-mechanical polishing of CdTe, Cd0,9Zn0,1Te and Cd0.95Mn0.05Te surface by etchants, based on solutions of the I2-dimethylformamide system, has been studied. Ethylene glycol and lactic acid were used as modifying components. The dependences of chemical-mechanical polishing rates on the content of ethylene glycol and lactic acid in the composition of the basic solution have been studied. The technological stability of the developed etchants has been determined. Qualitative characteristics of the surface are established by atomic force microscopy and non-contact profilographic analysis. Based on the obtained results, the compositions of etching solutions and technological modes of chemical-mechanical treatment of CdTe, Cd0,9Zn0,1Te and Cd0.95Mn0.05Te single crystals were optimized. Etchants modified with an organic component are promising for use in the semiconductor materials technology in the case when the main goal is to obtain high-quality, clean from contaminants and impurities, structurally perfect surface.

Radiation effects induced by the energetic protons in 8x8x32 mm3 CdZnTe detectors

CdZnTe (CZT) detectors have been used in the past and are being actively considered for future gamma-ray space telescopes. One of the challenges of operating CZT detectors in low Earth orbit is radiation damage caused by energetic protons. Previous studies concluded that fluences of 108 -109 p/cm 2 are sufficient to cause shifting of peak positions and degradations of energy resolution. Degradation of detectors after proton irradiation has been extensively investigated to determine their radiation resistance limits and performance recovering procedures for their applications in space and nuclear nonproliferation. Here we present the study of radiation effects induced by 100 MeV protons in 3-cm long CdZnTe detectors that we recently proposed for the gamma-ray telescope GECCO. The goal of this study is to evaluate the feasibility of using such long CZT detectors in future space missions.

Bandgap engineering of [formula omitted

CdZnTe (CZT) detectors with more than 10% zinc content did not show any remarkable improvement in the detector performance due to the additional defects introduced by the higher zinc content. However, recent research showed that the formation of defects was suppressed effectively by adding a small amount of selenium (at. 2%) in CZT. On this basis, we attempted to enhance the detector performance through the bandgap engineering by increasing the zinc content up to 25%, while adding 2% of selenium. Multiple CdZnTeSe (CZTS) ingots with Zn = 10, 12.5, 15, and 20%, while keeping the Se contents at 2%, were grown by the Bridgman method. The bandgap of CZTS for the different Zn and Se contents was analyzed and then introduced modified equation for predicting more accurately the bandgap of other alloy compositions. Also, the crystallinity of CZTS was evaluated by photoluminescence measurements. The pulse height spectra for Am-241 and Co-57 sources were used to evaluate the detector performance for the CZTS samples.

Experimental Evaluation of Scattered X-Ray Spectra due to X-Ray Therapeutic and Diagnosis Equipment for Eye Lens Dosimetry of Medical Staff

Background: For proper monitoring of the eye lens dose, an appropriate calibration factor of a dosimeter and information about the mean energies of X-rays are indispensable. The scattered X-ray energy spectra should be well characterized in medical practices where eye lenses of medical staffs might be high.Materials and Methods: Scattered X-ray energy spectra were experimentally derived for three different types of X-ray diagnostic and therapeutic equipment, i.e., the computed tomography (CT) scan, the angiography and the fluoroscopy. A commercially available CdZnTe (CZT) spectrometer with a lead collimator was employed for the measurement of scattered X-rays, which was performed in the usual manner.Results and Discussion: From the obtained energy spectra, the mean energies of the scattered X-rays lied between 40 and 60 keV. This also agreed with that obtained by the conventional half value layer method.Conclusion: The scattered X-rays to which medical workers may be exposed in the region around the eyes were characterized by means of spectrometry. The obtained mean energies of the scattered X-rays were found to match the flat region of the dosimeter response.

Identification of twin and nanoscale Te precipitations in CdZnTe crystals grown by vertical gradient method with HRTEM

CdZnTe (CZT) has attracted much attention in nuclear detection field attribute to its high detection efficiency, high energy resolution and room-temperature operation. In this work, the CdZnTe samples were prepared by Vertical gradient method. High-resolution transmission electron microscopy (HRTEM) was used to investigate the nanoscale defects in CZT crystals. It is found that the twins are symmetric about the {111} plane, and the dislocations and stacking faults locate around the twin boundary. Te precipitations with hexagonal crystal structure is coherent with the CZT matrix. Te precipitated phases have three shapes of hexagon, polygon and irregularity, which determined by the interface energy between precipitations and matrix. The ordered phase with acicular shapes was found along [011] zone axis. The two variants of the ordered phase is corresponding to 1/211¯1 and 1/31¯11 ultrasound lattice, respectively. Besides the small angle grain boundaries with semi-coherent interface are observed. It is important to explore the behavior of nanoscale defects in the growth of CZT crystals in order to enlighten a path to regulate the nanoscale defects. The stability of the temperature interface is crucial to the preparation of high quality CZT crystals.