Room-temperature Radiation Detectors Research Articles

Identifying Defects without a priori Knowledge in a Room-Temperature Semiconductor Detector Using Physics Inspired Machine Learning Model.

Room-temperature semiconductor radiation detectors (RTSD) such as CdZnTe are popular in Computed Tomography (CT) imaging and other applications. Transport properties and material defects with respect to electron and hole transport often need to be characterized, which is a labor intensive process. However, these defects often vary from one RTSD to another and are not known a priori during characterization of the material. In recent years, physics-inspired machine learning (PI-ML) models have been developed for the RTSDs which have the ability to characterize the defects in a RTSD by discretizing it volumetrically. These learning models capture the heterogeneity of the defects in the RTSD-which arises due to the fabrication process and the energy bands of elements in the RTSD. In those models, the different defects of RTSD-trapping, detrapping and recombination for electrons and holes-are present. However, these defects are often unknown. In this work, we show the capabilities of a PI-ML model which has been developed considering all the material defects to identify certain defects which are present (or absent). Additionally, these models can identify the defects over the volume of the RTSD in a discretized manner.

Investigation of polarization mechanism in MSM TlBr crystal room-temperature nuclear radiation detector

Thallium Bromide (TlBr) single crystal detector is widely recognized as an excellent semiconductor nuclear radiation detector. And its detection performance has already reached commercial level of the majority semiconductor detector. However, the long-term stability of TlBr detector always exhibited a serious polarization effect and the commercial application of TlBr detector is heavily limited. In this paper, the polarization phenomenon, polarization mechanism and the influence of polarization effect on detection performance are systematic investigated. As the metal-semiconductor interface plays an important role in detector performance, two typical metal-semiconductor-metal structure, Au/TlBr/Au and Ti/TlBr/Ti detectors are designed and fabricated in this paper. The XPS in-situ analysis results reveal that different oxidation/reduction reactions would happen at the metal-semiconductor interface of Au/TlBr/Au and Ti/TlBr/Ti detectors. The serious polarization effect in Au/TlBr/Au detectors resulted from the aggregated [AuBr4]- ion at the Au/TlBr interface. Meanwhile, the relative weaken polarization effect in Ti/TlBr/Ti detectors is ascribed to the combined effects of oxidic titanium ions (Ti3+, Ti4+) and metallic Ti–Tl layer at the Ti/TlBr interface. Thus, the detector Ti/TlBr/Ti has a relative better energy resolution and long-term stability compared to detector Au/TlBr/Au. This paper systematic investigate the polarization mechanism and the influence of polarization effects on TlBr detector performance, and this will provide a potential solution to suppress the polarization problem by electrode architecture designing in the near future.

Growth and characterization of large-size CdMgTe single crystals doped with different in amounts

Cadmium magnesium telluride (CdMgTe) is a very perspective material for room temperature radiation detectors. In this paper, two large-size Cd0.95Mg0.05Te ingots doped with different In amounts were grown by vertical Bridgman method under Cd-compensated condition. It was found that the impurities were distributed inhomogeneously and In dopant mainly concentrated in the tail of two ingots. Compared with the ingot grown by doping with 5 ppm In, the ingot grown by doping with 10 ppm In had more uniform distribution of Mg element, higher IR transmittance, less Te inclusions and better crystal quality. FIB-TEM and HRTEM analysis showed that the size of Te precipitates was less than 10 nm, the composition of both crystals was rich in Cd and was single crystal with sphalerite structure. The type of dislocation was edge dislocation and the density was high. In addition, the maximum IR transmittance and highest resistivity were 64 % and 1.53 × 1010 Ω cm, respectively. The planar detector prepared with 10 ppm In doped crystal had an energy resolution of 19.6 % and a (μτ)e value of 1.38 × 10−3 cm2/V, which could meet the requirements for high performance room-temperature radiation detector.

A review on emerging materials with focus on BiI3 for room-temperature semiconductor radiation detectors

A review on emerging materials with focus on BiI3 for room-temperature semiconductor radiation detectors

Extreme γ-Ray Radiation Tolerance of Spectrometer-Grade CsPbBr3 Perovskite Detectors.

The perovskite compound CsPbBr3 has recently been discovered as a promising room-temperature semiconductor radiation detector, offering an inexpensive and easy-to-manufacture alternative to the current benchmark material Cd1-x Znx Te (CZT). The performance of CsPbBr3 sensors is evaluated under harsh conditions, such as high radiation doses often found in industrial settings and extreme radiation in space. Results show minimal degradation in detector performance after exposure to 1Mrad of Co-60 gamma radiation, with no significant change to energy resolution or hole mobility and lifetime. Additionally, many of the devices are still functional after being exposed to a 10Mrad dose over 3 days, and those that do not survive can still be refabricated into working detectors. These results suggest that the failure mode in these devices is likely related to the interface between the electrode and material and their reaction, or the electrode itself and not the material itself. Overall, the study suggests that CsPbBr3 has high potential as a reliable and efficient radiation detector in various applications, including those involving extreme fluxes and energies of gamma-ray radiation.

Purification and Improved Photoelectric Properties of Lead-Free Perovskite Cs3Bi2Br9 Crystals

The lead-free halide perovskite Cs3Bi2Br9 is a promising semiconductor material for room-temperature X-ray detection due to its excellent properties. However, material purity and crystal quality still limit the use of Cs3Bi2Br9 crystals as detectors. In this work, we present a highly efficient purification method using continuous vacuum extraction to sublimate BiBr3 precursors for Cs3Bi2Br9. Impurity analysis via inductively coupled plasma mass spectroscopy showed that the purification method successfully removed most of the impurities in BiBr3 precursors and improved the purity by at least one order of magnitude. Centimeter-sized Cs3Bi2Br9 single crystals were grown by the vertical Bridgman method. The improved properties after purification were confirmed by UV-Vis-NIR spectra, infrared transmittance, and current–voltage (I–V) measurements. The results showed that the average transmittance of Cs3Bi2Br9 crystals significantly increased from 62% to 75% in the 0.5–20 μm spectral range. Additionally, the resistivity increased by nearly three orders of magnitude from 5.0 × 109 Ω·cm to 2.2 × 1012 Ω·cm, meaning the material will have low leakage currents and be suitable for developing applications for room temperature radiation detection.

Development of Single Crystal CsPbBr3 Radiation Detectors from Low-Cost Solution Synthesized Material

The all-inorganic perovskite CsPbBr3 is a strong candidate for room-temperature, semiconducting radiation detecting applications. With a high stopping power, a large bandgap, and a high mobility-lifetime (μτ) product for both holes and electrons, CsPbBr3 contains all the desirable properties of a room temperature radiation detector. Unfortunately, the production of detector-grade single crystal samples requires high quality starting materials, which must be further processed to achieve the desired purity for semiconducting operation. We have developed a modified zone refining method combining the continuous purification of ternary CsPbBr3 and the subsequent crystal growth step. Taking advantage of this technique, low-cost polycrystalline CsPbBr3 synthesized via solution chemistry processes can be directly used in the production of high-purity, detector-grade crystals. Semi-cylindrical CsPbBr3 single crystal ingots up to 120 mm in length and 22 mm in diameter were obtained. The final product exhibited good chemical stoichiometry and high trace metal purity (2.34 ppm across 73 elements). Detector devices fabricated from the crystal wafers displayed resistivities > 2.0 × 108 Ω·cm and high photocurrent responses. The radiation detectors were able to produce spectroscopy responses to 241Am α-particle. The hole mobility-lifetime (μτH) product of the detectors was determined to be in the range of 1.45 × 10−3 cm2/V.

Towards Extended Gate Field Effect Transistor-Based Radiation Sensors: Impact of Thicknesses and Radiation Doses on Al-Doped Zinc Oxide Sensitivity

Radiation measurements are critical in radioanalytical, nuclear chemistry, and biomedical physics. Continuous advancement in developing economical, sensitive, and compact devices designed to detect and measure radiation has increased its capability in many applications. In this work, we presented and investigated the performance of a cost-effective X-ray radiation detector based on the extended gate field effect transistors (EGFET). We examined the sensitivity of Al-doped Zinc oxide (AZO) of varying thicknesses, fabricated by chemical bath deposition (CBD), following X-ray irradiation with low and high doses. EGFETs were used to connect samples for their detection capabilities. As a function of the absorbed dose, the response was analyzed based on the threshold voltage shift, and the sensitivity of each device was also evaluated. We demonstrated that thin films are less sensitive to radiation than their disk-type EG devices. However, performance aspects of the devices, such as radiation exposure sensitivity and active dosage region, were found to be significantly reliant on the composition and thickness of the materials used. These structures may be a cost-effective alternative for real-time, room-temperature radiation detectors.

Positional correction of a 3D position-sensitive virtual Frisch-grid CZT detector for gamma spectroscopy and imaging based on a theoretical assumption

The virtual Frisch-grid method for room-temperature radiation detectors has been widely used because of its simplicity and high performance. Recently, side electrodes were separately attached to each surface of the detectors instead of covering the entire detector surface with a single electrode. The side-electrode structure enables the measurement of the three-dimensional (3D) gamma-ray interaction in the detector. The positional information of the interaction can then be utilized to precisely calibrate the response of the detector for gamma-ray spectroscopy and imaging. In this study, we developed a 3D position-sensitive 5 × 5 × 12 mm3 cadmium-zinc-telluride (CZT) detector and applied a flattening method to correct detector responses. Collimated gamma-rays incident on the surface of the detector were scanned to evaluate the positional accuracy of the detection system. Positional distributions of the radiation interactions with the detector were imaged for quantitative and qualitative evaluation. The energy spectra of various radioisotopes were measured and improved by the detector response calibration according to the calculated positional information. The energy spectra ranged from 59.5 keV (emitted by 241Am) to 1332 keV (emitted by 60Co). The best energy resolution was 1.06% at 662 keV when the CZT detector was voxelized to 20 × 20 × 10.

Learning-based physical models of room-temperature semiconductor detectors with reduced data

Room-temperature semiconductor radiation detectors (RTSD) have broad applications in medical imaging, homeland security, astrophysics and others. RTSDs such as CdZnTe, CdTe are often pixelated, and characterization of these detectors at micron level can benefit 3-D event reconstruction at sub-pixel level. Material defects alongwith electron and hole charge transport properties need to be characterized which requires several experimental setups and is labor intensive. The current state-of-art approaches characterize each detector pixel, considering the detector in bulk. In this article, we propose a new microscopic learning-based physical models of RTSD based on limited data compared to what is dictated by the physical equations. Our learning models uses a physical charge transport considering trapping centers. Our models learn these material properties in an indirect manner from the measurable signals at the electrodes and/or free and/or trapped charges distributed in the RTSD for electron–hole charge pair injections in the material. Based on the amount of data used during training our physical model, our algorithm characterizes the detector for charge drifts, trapping, detrapping and recombination coefficients considering multiple trapping centers or as a single equivalent trapping center. The RTSD is segmented into voxels spatially, and in each voxel, the material properties are modeled as learnable parameters. Depending on the amount of data, our models can characterize the RTSD either completely or in an equivalent manner.

Effects of electrode fabrication on electrical properties of CdMgTe room temperature radiation detectors

Cd1-xMgxTe (CdMgTe) is an ideal alternative material for room temperature radiation detectors. In this paper, the effects of different electrode structures on the performance of CdMgTe detectors were studied. The electrodes were prepared by vacuum evaporation and chemical precipitation method using Au and Al materials. The results indicated that CdMgTe detectors with Au/CdMgTe/Al electrode structure prepared by vacuum evaporation method had the best electrical properties. For the crystals grown by Cd-excess and Te-rich conditions, the resistivity were 2.89 × 1011 Ω cm and 3.8 × 1011 Ω cm, the leakage current (under 100 V bias voltage) were 4 nA and 3 nA, and the Hall mobility were 2.73 × 103 cm2 V−1 s−1 and 3.69 × 102 cm2 V−1 s−1, respectively. The conductive type of the crystals were weak n-type. 241Am @ 5.48 MeV α particle spectra of the detectors showed that the optimal energy resolution were 8.66% and 8.62%, and the carrier mobility-lifetime products (μτ)e were 1.47 × 10−3 cm2/V and 1.03 × 10−3 cm2/V, respectively. Moreover, the carrier mobility obtained by time-of-flight (TOF) test were 15.01 cm2/(V·s) and 77.79 cm2/(V·s), respectively.

Vertical gradient freeze growth of detector grade CdZnTeSe single crystals

We report the growth of detector grade Cd0.9Zn0.1Te0.97Se0.03 (CZTS) single crystals, a recently discovered quaternary semiconductor for room temperature radiation detection, by a vertical gradient freeze (VGF) method. VGF is a comparatively low-temperature growth method and avoids relative motion between the heater and the ampoule containing the precursor materials which minimizes any thermal drift or temperature fluctuations. As a result, CZTS single crystals with superior charge transport properties has been obtained. Growth of detector-grade CZTS single crystals using VGF method has not been reported yet. X-ray spectroscopy based elemental analysis showed that the grown crystals demonstrated the desired stoichiometry required for high resolution radiation detection. Planar detectors fabricated by deposition of gold contacts (∼0.07 cm2) demonstrated high bulk resistivity ∼1010 Ω-cm and a very low leakage current density of 2.8 × 10−8 A/cm2 at a bias of 100 V when measured at room temperature. The detectors showed excellent radiation response with 100 % charge collection efficiency when exposed to 5486 keV alpha particles. The electron mobility-lifetime (μτ) product was measured to be 3 × 10−3 cm2/V using a single polarity Hecht analysis which is at par with the recently reported values measured in CZTS grown using conventional Bridgman or travelling heater method. The electron mobility has been calculated to be 964 cm2V−1s−1 using a time-of-flight (TOF) method, a substantial improvement over that obtained from conventionally grown CZTS single crystals.

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.

A Learning-Based Physical Model of Charge Transport in Room-Temperature Semiconductor Detectors

Room-temperature semiconductor radiation detectors (RTSDs) such as CdTe are becoming popular in computed tomography (CT) imaging. These detectors are often pixelated, requiring cumbersome postinteraction 3-D event reconstruction, which can benefit from detailed material characterization at the micron level. Transport properties and material defects with respect to electrons and holes are to be characterized, which is a labor-intensive process. Current state-of-the-art characterization is done either as a whole or at most pixel-by-pixel over the detector material. In this article, we propose a novel learning-based physical model to infer material properties at the microscopic level for RTSD. Our approach uses a novel physics-inspired learning model based on physical transport of charges with trapping centers for electrons and holes in the detector. The proposed model learns these material properties from known or measured input charges to the detector along with known or measured output signals and distributed charges in the bulk of the RTSD. The actual physical detector is divided into voxels in space and takes into account different material properties (such as drift, trapping, detrapping, and recombination) in each voxel as learnable model parameters. The model is based on a physics-inspired recurrent neural network model instead of traditional convolutional or fully connected networks. The advantage of our approach is the one-to-one relationship between the actual physical parameters of the voxels and learnable weights in the model, far fewer trainable parameters compared to traditional neural network approaches and less training time. The performance of our model has been evaluated on cadmium zinc telluride (CdZnTe), with voxels of three sizes, 25, 50, and <inline-formula> <tex-math notation="LaTeX">$100~ \mu{\mathrm {m}}$ </tex-math></inline-formula>, for single charge input as well as multiple charge inputs at different voxel positions. Our learning-based model provides material properties with higher spatial resolution and performs well in all scenarios and matches the actual physical parameters better than state-of-the-art classical approaches.

Synthesis of CdZnTeSe single crystals for room temperature radiation detector fabrication: mitigation of hole trapping effects using a convolutional neural network

Synthesis of CdZnTeSe single crystals for room temperature radiation detector fabrication: mitigation of hole trapping effects using a convolutional neural network

Investigations on low energy X-ray induced strong radiation-matter interaction phenomena in polymer-BiI3 hybrid materials for room temperature radiation detectors

Investigations are reported for response of soft X-rays with applications such as X-ray crystallography and mammography, for Polymer-BiI3 (Bismuth triiodide) composite pellets at low biased voltage. The pellets were prepared with solvent free synthesis technique to avoid impurities and charge collection by electrodes has been studied with resistivity. Polymer with low resistivity showed better charge collection and has been further improved by incorporating conductive filler (MWCNT) within the composite. The results suggest that incorporation of MWCNT provide conductive channel and improves the photocurrent of the composite detector by one order.

Characterization of vertical Bridgman grown Cd0.9Zn0.1Te0.97Se0.03 single crystal for room-temperature radiation detection

Characterization of vertical Bridgman grown Cd0.9Zn0.1Te0.97Se0.03 single crystal for room-temperature radiation detection

Quaternary Semiconductor Cd1−xZnxTe1−ySey for High-Resolution, Room-Temperature Gamma-Ray Detection

The application of Cd0.9Zn0.1Te (CZT) single crystals, the primary choice for high-resolution, room-temperature compact gamma-ray detectors in the field of medical imaging and homeland security for the past three decades, is limited by the high cost of production and maintenance due to low detector grade crystal growth yield. The recent advent of its quaternary successor, Cd0.9Zn0.1Te1−ySey (CZTS), has exhibited remarkable crystal growth yield above 90% compared to that of ~33% for CZT. The inclusion of Se in appropriate stoichiometry in the CZT matrix is responsible for reducing the concentration of sub-grain boundary (SGB) networks which greatly enhances the compositional homogeneity and growth yield. SGB networks also host defect centers responsible for charge trapping, hence their reduced concentration ensures minimized charge trapping. Indeed, CZTS single crystals have shown remarkable improvement in electron charge transport properties and energy resolution over CZT detectors. However, our studies have found that the overall charge transport in CZTS is still limited by the hole trapping. In this article, we systematically review the advances in the CZTS growth techniques, its performance as room-temperature radiation detector, and the role of defects and their passivation studies needed to improve the performance of CZTS detectors further.

Elucidation of LiCl encapsulated Bridgman growth of AlSb crystal

Aluminum antimonide (AlSb) possessing an indirect bandgap of 1.6 eV at room temperature offers a good option as a room-temperature dual-carrier radiation detector. However, the growth of AlSb crystal suffers from high reactivity of Al, causing sticking problems and polycrystallinity. Our work demonstrates experimentally that LiCl fully encapsulated Bridgman growth of AlSb is possible from near-stoichiometric AlSb melt. This leads to avoiding the interaction between the pyrolysis Boron Nitride (pBN) crucible and the melt, drastically decreasing nucleation on the peripheral crucible. Additionally, optimizing the melt composition and growth parameters provided a full understanding of LiCl encapsulated Bridgman growth of AlSb. Adding a slight amount of extra Sb into the stoichiometric AlSb melt can reduce the surface tension of the melt and enhance the stability of LiCl encapsulation efficiency. Conversely, it can also raise the risk of constitutional supercooling and promotes macroscopic defect formation. Accordingly, an optimized process was employed to obtain inclusion-free AlSb crystals with large grains (over 5 × 5 mm2) and homogeneous stoichiometric composition.

Impact of selenium addition to the cadmium-zinc-telluride matrix for producing high energy resolution X-and gamma-ray detectors

Both material quality and detector performance have been steadily improving over the past few years for the leading room temperature radiation detector material cadmium-zinc-telluride (CdZnTe). However, although tremendous progress being made, CdZnTe still suffers from high concentrations of performance-limiting defects, such as Te inclusions, networks of sub-grain boundaries and compositional inhomogeneity due to the higher segregation coefficient of Zn. Adding as low as 2% (atomic) Se into CdZnTe matrix was found to successfully mitigate many performance-limiting defects and provide improved compositional homogeneity. Here we report record-high performance of Virtual Frisch Grid (VFG) detector fabricated from as-grown Cd0.9Zn0.1Te0.98Se0.02 ingot grown by the Traveling Heater Method (THM). Benefiting from superior material quality, we achieved superb energy resolution of 0.77% at 662 keV (as-measured without charge-loss correction algorithms) registered at room temperature. The absence of residual thermal stress in the detector was revealed from white beam X-ray topographic images, which was also confirmed by Infra-Red (IR) transmission imaging under cross polarizers. Furthermore, neither sub-grain boundaries nor their networks were observed from the X-ray topographic image. However, large concentrations of extrinsic impurities were revealed in as-grown materials, suggesting a high likelihood for further reduction in the energy resolution after improved purification of the starting material.