CdZnTe Research Articles

The mechanism of defects effect on carrier transport by employing multi-wavelength sub-bandgap lights on CdZnTe crystal

The effect of different point defects in CdZnTe (CZT) crystals on carrier transport performance were discussed. The energy level and concentration of the traps were determined by the photo-induced current transient spectroscopy (PICTS). By adding the infrared light illumination into the PICTS experiment, the effect of sub-bandgap light on defects was observed. Meanwhile, several DC photoconductive experiments were done under different wavelength lights were used to explore the effect of the different defects on carrier transport properties of CZT crystals. Furthermore, the energy spectrum test was used to investigate the effect of sub-bandgap light on the energy resolution of CZT detectors. The data results showed that sub-bandgap light could effectively change the ionization state of defects. The doubly ionized Te antisite (TeCd2+) had the greatest effect on mobility-lifetime product(μτ) of electrons owing to its stronger ability of capturing electrons and energy level located around the midgap. While the secondary ionized Cd vacancy (VCd2−), as considered as a hole trap, had the greatest effect on μτ-product of holes. Consequently, choosing suitable infrared light can improve the detection performance of CZT crystal effectively.

Simulation of Coplanar Proximity Charge Sensing Electrodes in CZT Detectors

The concept of applying proximity charge sensing electrodes to semiconductor radiation detectors is a novel technique that has distinct advantages over directly deposited electrodes. This paper evaluates the application of proximity charge sensing onto a CdZnTe (CZT) semiconductor crystal using ANSYS Maxwell simulation software to calculate the weighting potential across the detector depth, the weighting potential across the detector width, and the electric field (E) generated inside the detector volume for multiple designs. To accomplish this goal, several variables are studied in the simulated designs: (1) a high resistivity thin-film material that is applied to the detector proximity surface from the anode side, (2) an appropriate metal that acts as an Ohmic contact to dissipate generated charges on the CZT anode side, and finally (3) an insulating layer (dielectric) to isolate the CZT detector body from the proximity electrodes. The results of the generated weighting potentials for both directly deposited electrodes and proximity-sensing electrodes, having the same electrode width and pitch, have been quantitatively compared using a figure of merit (FOM). The FOM compares the weighting potential created by each simulated design for weighting potential uniformity and weighting potential similarity.

Improvement of Surface Defects in CdZnTe Crystals by Rapid Thermal Annealing

We have investigated the influence of rapid thermal annealing (RTA) on the surface defects of CdZnTe (CZT) crystals by using low-temperature photoluminescence (PL) measurements. In the low-temperature PL spectrum for the as-grown CZT crystals, the donor–acceptor pair (DAP) recombination peak is dominant. Overlapping of the free-exciton (FE) and the neutral donor bound exciton (D0,X) luminescence peaks leads to broadening of the (D0,X) peak. In the low-temperature PL spectra for CZT crystals annealed at 523 K and 623 K for 5 min, the A-center peak is dominant. The deterioration of the intensity and FWHM of the (D0,X) peak indicates worsening of the crystal quality under the two RTA conditions. In the low-temperature PL spectra for CZT crystals annealed at 723 K, 773 K and 873 K for 5 min, an FE peak gradually becomes evident as the annealing temperature increases. The full-width half-maximum (FWHM) of the (D0,X) peak decreases from 7.95 meV before annealing to 2.85 meV after annealing at 873 K. The neutral acceptor bound exciton (A0,X) peak intensity increases with the annealing temperature increasing. The peak intensity ratio I(D0,X)/I(DAP) increases from ∼ 0.24 before annealing to ∼ 1.62 after annealing at 873 K. The A-center peak intensity decreases with the increase in annealing temperature. These phenomena demonstrate better surface structure and higher crystal quality under the three RTA conditions, especially for the annealing at 873 K. RTA treatment can be an effective way to improve the surface properties of CZT crystals by modifying the technical parameters.

Sensitive Direct Converting X‐Ray Detectors Utilizing Crystalline CsPbBr3 Perovskite Films Fabricated via Scalable Melt Processing

AbstractHere the fabrication of an inorganic metal‐halide perovskite CsPbBr3 based X‐ray detector is reported utilizing a simple, scalable, and cost‐sensitive melt processing directly on substrate of any size. X‐ray diffraction analysis on the several 100 mm thick melt processed films confirms crystalline domains in the cm2 range. The CsPbBr3 film features a resistance of 8.5 GΩ cm and a hole mobility of 18 cm2 V−1 s−1. An X‐ray to current conversion rate of 1450 µC Gyair−1 cm−2 at an electric field of 1.2 × 104 V cm−1 and a detection limit in the sub µGyair s−1 regime is demonstrated. The high crystallinity and chemical purity of the melt processed CsPbBr3 films are suggested to be responsible for a performance which is on par to current state‐of‐the‐art Cd(Zn)Te based X‐ray detector technology.

Novel CdZnTe micro pillar films deposited by CSS method

CdZnTe (CZT) micro pillar films with cubic zinc blend phase have been deposited on single layer graphene coated quartz substrate by close-spaced sublimation technique. Structural and phase purities of deposited micro pillar CZT films were investigated using X-ray diffraction and Raman spectroscopy. Scanning electron microscopy characterizations indicated that the diameters of CZT micro pillars were about 5 µm and their lengths were close to 57 µm. The band-gap of CZT micro pillar films was about 1.53 eV, which was confirmed by UV–Vis–NIR diffuse reflection spectra and PL spectra. The structure of Au/BGZO/CZT/graphene/quartz was fabricated to study its optoelectronic properties. Under the illumination of AM 1.5, CZT micro pillar films had good photo response, which indicated their great potential in the application of optoelectronic devices.

Characterization of large-volume Frisch grid detector fabricated from as-grown CdZnTeSe

CdZnTe (CZT) has been under continuous development as a room-temperature radiation detector for the past 2–3 decades. Due to its intrinsic defects, such as a high concentration subgrain boundary network, it has been very challenging to consistently produce high-quality CZT detectors with high yield, particularly for detector volumes exceeding a few cubic centimeters. In this paper, we report characterization of a relatively large-volume Frisch-grid detector (dimensions: 11 × 10.8 × 19.4 mm3) fabricated from a new material Cd0.9Zn0.1Te0.985Se0.015. The detector was fabricated from an as-grown ingot with a 2-in. diameter grown by the Traveling Heater Method. The detector sample was evaluated by infrared transmission microscopy to study the presence of Te inclusions. X-ray topography using a synchrotron light source was deployed to measure the presence of subgrain boundaries and their networks. The detector performance was measured for an optimized Frisch-grid length to extract the best energy resolution at room temperature.

The Effect of Indium Doping on Deep Level Defects and Electrical Properties of CdZnTe

CdZnTe (CZT) ingots doped with different concentrations of indium (2 ppm, 5 ppm, 8 ppm, and 11 ppm) were grown by the Vertical Bridgman Method. The charge transport behaviors of CZT wafers were characterized by Thermally Stimulated Current (TSC), Time of Flight technique (TOF) and Current–Voltage measurements (I–V). TSC results indicate that the concentration of deep donor defects $$ {\hbox{Te}}_{\rm{Cd}}^{{ 2 { + }}} $$ is reduced significantly by increasing indium dopant content from 2 ppm to 8 ppm, while that of indium related traps, $$ {\hbox{In}}_{\rm{Cd}}^{ + } $$ and A-centers, is sharply increased. Hecht fitting and TOF results indicate that the electron mobility keeps nearly unchanged for different dopant concentrations in the region between 2 ppm and 5 ppm, but the lifetime increased greatly with increasing indium dopant concentration. Therefore, (μτ)e value was increased with higher indium dopant. The up-shift of Fermi level is also observed in the temperature-dependent I–V result with the increasing of indium dopant content. Large Schottky barriers are found in detectors with higher indium concentration. High voltage x-ray response results show that the channel number shifts to the low energy side for 2 ppm dopant samples compared with best performance 5 ppm dopant samples, while the full-energy peaks are broadened for 8 ppm and 11 ppm dopant samples.

Recent Advances in Lead Halide Perovskites for Radiation Detectors

Research interest in lead halide perovskites has shown a spurt of growth in the last few years due to their high absorption coefficient, large carrier mobility, and long diffusion length. Besides their wide applications in solar cells, LEDs, lasers, and photodetectors, lead halide perovskites are demonstrated as excellent candidate materials for radiation detectors with comparable performance to commercial Si and CdZnTe (CZT) detectors. Herein, the essential results on perovskite semiconductor‐based radiation conductors are summarized. Furthermore, a brief outlook for the further development of lead halide perovskites‐based radiation detectors is proposed.

Defects and the Formation of Impurity ‘Hot Spots’ in HgCdTe/CdZnTe

The formation of impurity ‘hot spot’ macro-defects—localized high impurity level contaminates—is examined. The evolution of macro-defects through their critical stages: as-received CdZnTe substrate, molecular beam epitaxy (MBE) prep etch, Te/oxide desorption and stabilization at deposition temp in MBE chamber, thin (200 nm) HgCdTe deposition, and thick (> 4 μm) MBE HgCdTe deposition, is analyzed. As-received CdZnTe wafers have small and large agglomerations of residual SiO2 and Al2O3 polishing grit. Additionally, macro-defects composed of organic residue, Cd(Zn)Te particulates, and additional impurities are also observed. The MBE preparation etch does not significantly alter the types of defects found on the wafer. The Te/oxide desorption process adds a small number of defects due to colloidal graphite particles. HgCdTe deposition introduces MBE spit defects and Hg droplet residue. There is significant movement of polishing grit and organic macro-defects during the various processing steps. Energy dispersive X-ray spectroscopy (EDS) analysis of MBE spit defects shows they are Cd rich with some of the spit defects containing high concentrations of impurities. The vertical gradient in Te concentration associated with the overgrowth on a metal rich MBE spit defect acts as an impurity pipe for Cu to travel toward the surface to become a ‘hot spot’ defect.

High-resolution virtual Frisch grid gamma-ray detectors based on as-grown CdZnTeSe with reduced defects

X- and gamma-ray detectors are increasingly becoming essential tools for science and technology in various fields. These detectors offer broad applications such as homeland security, nonproliferation, nuclear security, medical imaging, astrophysics, and high energy physics. All these applications demand high-resolution detectors operable at room temperature and available at a reasonable cost. CdZnTe (CZT) is the material of choice for this purpose; however, the material still suffers from intrinsic defects such as highly decorated subgrain boundary networks and a high concentration of secondary phases. These defects not only hinder the charge transport but also create a spatial inhomogeneity in the charge transport properties, subsequently causing substantial degradation in detector response particularly for relatively thick (>1 cm) detectors. Some of the material deficiencies suffered by CZT have been addressed by adding selenium into the CZT matrix. Selenium was found to be very effective in producing material that is principally free from a subgrain boundary network with the occasional appearance of subgrain boundaries with reduced secondary phases decorating these boundaries. The resulting quaternary compound CdZnTeSe (CZTS) showed excellent material quality mitigating some major deficiencies suffered by CZT. Virtual Frisch grid detectors were fabricated from the as-grown CZTS ingots, and they demonstrated high resolution spectroscopic grade. The excellent CZTS material contained very low defects and was found to potentially increase the yield of high-quality detectors as compared to CZT.

Detector performance and defect densities in CdZnTe after two-step annealing

Defects both microscale and nanoscale are play an important role in CdZnTe (CZT) device performance. Typical micro-scale defects such as Te inclusions were removed via a two-step annealing process, and their concentration was analyzed via IR transmission microscopy. In addition, transmission electron microscopy (TEM) measurement was employed to investigate the evolution of nano-scale defects after the annealing process. Dislocation and stacking faults were commonly observed defects in as-grown and annealed CZT. The line shape defects, which are possibly related to the stress field around dislocations, disappeared during the in-situ the annealing at 200–220 °C. A Frisch-grid CZT detector made via the two-step annealing process exhibited improved energy resolution and low backscattering counts in Cs-137 gamma spectra.

Interface regulation and photoelectric performance of CdZnTe/AlN composite structure for UV photodetector

AlN ceramic substrates (c-AlN), with excellent thermal conductivity, were used to deposit high-quality polycrystalline CdZnTe (CZT) thick films for the first time. Meanwhile, AlN transition layers, made by magnetron sputtering method, have been introduced and annealed to modulate the interface properties of CZT/c-AlN composite structures for UV photodetectors. The relationship between the structure as well as performance of CZT films and the anneal temperature of AlN transition layers was studied by SEM, XRD, EDS, UV–vis spectra, XPS and current–voltage (I-V) characteristic. The CZT films deposited on the AlN transition layer annealed at 1200 °C have the best properties, with the full width at half maximum (FWHM) and leakage current of 0.163°and 2.15 × 10−9 A, respectively. The interface regulation can effectively promote the crystallinity of CZT films and lead to optimum UV detection performance of CZT/AlN composite structures.

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.

FPGA-Based Interface of Digital DAQ System for Double-Scattering Compton Camera.

The double-scattering Compton camera (DSCC) is a radiation imaging system that can provide both unknown source energy spectra and 3D spatial source distributions. The energies and detection locations measured in coincidence with three CdZnTe (CZT) detectors contribute to reconstructing emission energies and a spatial image based on conical surface integrals. In this study, we developed a digital data acquisition (DAQ) board to support our research into coincidence detection in the DSCC. The main components of the digital DAQ board were 12 ADCs and one field programmable gate array (FPGA). The ADCs digitized the analog 96-channel CZT signals at a sampling rate of 50MHz and transferred the serialized ADC samples and the bit and frame clocks to the FPGA. In order to correctly capture the ADC sample bits in the FPGA, we conducted individual sync calibrations for all the ADC channels to align the bit and frame clocks to the right positions of the ADC sample bits. The FPGA logic design was composed of IDELAY and IDDR components, six shift registers, and bit slip buffer resources. Using a Deskew test pattern, the delay value of the IDELAY component was determined to align the bit clock to the center of each sample bit. We determined the bit slip in the 12-bit ADC sample using an MSB test pattern by checking where the MSB value of one is located in the captured parallel data. After sync calibration, we tested the interface between the ADCs and the FPGA with a synthetic analog Gaussian signal. The 96 ADC channels yielded a mean R 2 goodness-of-fit value of 0.95 between the Gaussian curve and the captured 12-bit parallel data.

Innovative 3D sensitive CdZnTe solid state detector for dose monitoring in Boron Neutron Capture Therapy (BNCT)

A BNCT-dedicated SPECT system to monitor in real time the 10B dose is under development focusing on CdZnTe (CZT) solid state detectors as photon sensors. Since BNCT facilities are characterised by a (n+γ) radiation field and considering the high value of cadmium neutron absorption cross section, we evaluated the response of a CZT detector using the irradiation thermal neutron facility of the Pavia University research nuclear reactor. The reported measurements showed that, despite the thermal neutron and γ background, the CZT detector is able to discriminate the 10B signal from the neutron capture peaks of 113Cd.

Low temperature growth and extrinsic doping of mono-crystalline and polycrystalline II-VI solar cells by MBE

An investigation of low temperature MBE growth of crystalline Cd(1-x)Zn(x)Te with 0 ≤x ≤ 1, in the range 220 ℃≤Ts ≤ 320 ℃, and extrinsic doping is conducted. This results in the construction of homo-junction crystalline solar cells on lattice-mismatched 211B and 111B GaAs substrates. Doping and composition studies of crystalline layers are used as a guide for low-temperature, high growth-rate, MBE grown polycrystalline CdTe solar cells on various n-type emitters and different back-contacts. The best resulting devices reach efficiencies of 14.7% under AM 1.5 g, without an anti-reflection coating.

Performance Evaluation of Compton Micro-PET for Detector Modalities: A Monte Carlo Study

Performance of Compton positron emission tomography (PET) is studied in this paper using qualitative and quantitative methods. Lutetium-yttrium oxyorthosilicate (LYSO), lutetium-gadolinium oxyorthosilicate (LGSO), and CdZnTe (CZT) materials are used for Compton PET. LYSO is widely used for conventional PET, and LGSO is a prospective scintillator material for PET detectors. CZT is one of the semiconductor materials that have high energy and position resolution. For conventional PET, only the photoelectric effect is considered a valid interaction for image reconstruction. However, Compton scattering tracing technology is applied for our Compton PET to additionally use Compton scattering events for image reconstruction. It is relatively difficult to use multiple layers for PET made of scintillators, as electronic circuits must be attached to each layer. For this reason, conventional PET generally uses only one layer for each detector module and limits the spatial resolution in the depth direction. In contrast, it is possible for a CZT detector to measure a depth of interest based on the cathode-to-anode signal ratio or electron drift time with relatively simple electronic circuits. Furthermore, CZT materials have high spatial and energy resolutions. Therefore, the position and energy information of the radiation interactions in the detector module can be precisely calculated to determine the interaction sequence, and hence, the information from the Compton scattering can be used for image reconstruction in PET. For this reason, the reconstructed image of CZT PET can show better quality than those of scintillator PETs. The detection efficiency and quality of the reconstructed image are significantly increased by including the Compton scattering effect as a valid interaction process for image reconstruction because Compton scattering has twice the interaction probability of the photoelectric effect at 511 keV. In this paper, the effectiveness of including Compton scattering events for PET reconstruction was evaluated for scintillators and CZT semiconductor detectors. The maximum likelihood expectation and maximization reconstruction method was applied for conventional and Compton PET reconstruction, and the qualities of the reconstructed images were evaluated.

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.

Preliminary characterization of a CdZnTe photon detector for BNCT-SPECT

The effectiveness of Boron Neutron Capture Therapy (BNCT) treatment depends on the amount of radiation dose deposited locally by the reaction10B(n,α)7Li in the tumour; however, the local and real time measurement of this quantity during the neutron irradiation is a big challenge. To evaluate the dose it is necessary to know the spatial distribution of the10B concentration and thermal neutron flux in the tissue during the irradiation.One technique to solve this problem relies on the on-line detection of the 478 keV7Li de-excitation photons that can be used to map the spatial distribution of the reaction rate of the10B(n,α)7Li. The present work has been developed within a project aiming to develop a Single Photon Emission Computed Tomography (SPECT) system based on CdZnTe (CZT) semiconductor detector.This work studied the performances of a 5×5×20mm3 CZT prototype detector focusing on the energy resolution and detection efficiency inside the energy range 200–1400 keV. The energy resolution at 478 keV is ∼3% and the detection efficiency is around 6%. These preliminary results confirms the feasibility of a CZT detector as a sensitive unit for a BNCT-SPECT imaging system.