Single-crystal perovskite devices 2–3 mm thick exhibit 16.4% X-ray detection efficiency with sensitivity four times higher than α-Se X-ray detectors.

Sensitive X-ray detectors made of methylammonium lead tribromide perovskite single crystals

The ability of dual- and multi-energy CT to differentiate materials of different effective atomic numbers makes possible several new and clinically relevant CT applications.

Dual- and Multi-Energy CT: Principles, Technical Approaches, and Clinical Applications

Photon counting detectors (PCDs) with energy discrimination capabilities have been developed for medical x-ray computed tomography (CT) and x-ray (XR) imaging. Using detection mechanisms that are completely different from the current energy integrating detectors and measuring the material information of the object to be imaged, these PCDs have the potential not only to improve the current CT and XR images, such as dose reduction, but also to open revolutionary novel applications such as molecular CT and XR imaging. The performance of PCDs is not flawless, however, and it seems extremely challenging to develop PCDs with close to ideal characteristics. In this paper, the authors offer our vision for the future of PCD-CT and PCD-XR with the review of the current status and the prediction of (1) detector technologies, (2) imaging technologies, (3) system technologies, and (4) potential clinical benefits with PCDs.

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Vision 20/20: Single photon counting x-ray detectors in medical imaging

We present a comprehensive review of the material properties of cadmium zinc telluride (CZT, Cd1ˇxZnxTe) with zinc content xa 0:1‐0.2. Particular emphasis is placed on those aspects of this material related to room temperature nuclear detectors. A review of the structural properties, charge transport, and contacting issues and how these are related to detector and spectrometer performance is presented. A comprehensive literature survey and bibliography are also included. # 2001 Elsevier Science B.V. All rights reserved.

Cadmium zinc telluride and its use as a nuclear radiation detector material

Cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) have been regarded as promising semiconductor materials for hard X-ray and /spl gamma/-ray detection. The high atomic number of the materials (Z/sub Cd/=48, Z/sub Te/=52) gives a high quantum efficiency in comparison with Si. The large bandgap energy (Eg/spl sim/1.5 eV) allows us to operate the detector at room temperature. However, a considerable amount of charge loss in these detectors produces a reduced energy resolution. This problem arises due to the low mobility and short lifetime of holes. Recently, significant improvements have been achieved to improve the spectral properties based on the advances in the production of crystals and in the design of electrodes. In this paper we summarize 1) advantages and disadvantages of CdTe and CdZnTe semiconductor detectors and 2) the technique for improving energy resolution and photopeak efficiencies. Applications of these imaging detectors in future hard X-ray and /spl gamma/-ray astronomy missions are briefly discussed.

Recent progress in CdTe and CdZnTe detectors

Good detection efficiency and high energy-resolution make Cadmium Zinc Telluride (CdZnTe) and Cadmium Telluride (CdTe) detectors attractive in many room temperature X-ray and gamma-ray detection applications such as medical and industrial imaging, industrial gauging and non-destructive testing, security and monitoring, nuclear safeguards and non-proliferation, and astrophysics. Advancement of the crystal growth and device fabrication technologies and the reduction of bulk, interface and surface defects in the devices are crucial for the widespread practical deployment of Cd 1-x Zn x Te-based detector technology. Here we review the effects of bulk, interface and surface defects on charge transport, charge transport uniformity and device performance and the progress in the crystal growth and device fabrication technologies aiming at reducing the concentration of harmful defects and improving Cd 1-x Zn x Te detector performance.

CdZnTe and CdTe materials for X-ray and gamma ray radiation detector applications

The performance of CdZnTe room-temperature X-ray and gamma-ray detectors is determined by material and device defects that govern carrier transport trough the device. In this contribution, we review common bulk, interface, and surface defects and their effects on charge transport, charge transport uniformity, and device performance. We note that pure CdZnTe grown under Te-rich conditions has an excess of Cd-vacancies and other Te-related native defects and must be electrically compensated in order to obtain high resistivity material. Through the critical analysis of the various compensation schemes it is shown that deep level defects must be introduced with donor doping elements in order to achieve a practical compensation technique. The role of carrier trapping and limitations on detector performance with increasing crystal size are discussed. Based on typical measured carrier lifetimes and the available literature data on carrier capture cross sections, we estimate that the residual acceptor concentration in CdZnTe detector crystals is much lower than widely thought, about 10/sup 11/ cm/sup -3/ instead of 10/sup 15/ cm/sup -3/. The deleterious effects of structural defects within single crystals are also discussed. We also provide a brief overview of the progress in CdZnTe crystal growth and device fabrication technologies aiming at reducing the concentration of the detrimental defects and improving CdZnTe detector performance.

Advances in the crystal growth and device fabrication technology of CdZnTe room temperature radiation detectors

In this paper, we theoretically investigate the mechanism of polarization in wide-bandgap semiconductor radiation detectors under high-flux x-ray irradiation. Our general mathematical model of the defect structure within the bandgap is a system of balance laws based on carrier transport and defect transition rates, coupled together with the Poisson equation for the electric potential. The dynamical system is self-consistently evolved in time using a high-resolution wave propagation numerical algorithm. Through simulation, we identify and present a sequence of dynamics that determines a critical flux of photons above which polarization effects dominate. Using the experience gained through numerical simulation of the full set of equations, we derive a reduced system of conservation laws that describe the dominant dynamics. A multiple scale perturbation analysis of the reduced system is shown to yield an analytical dependence of the maximum sustainable flux on key material, detector, and operating parameters. The predicted dependencies are validated for $16\ifmmode\times\else\texttimes\fi{}16\phantom{\rule{0.3em}{0ex}}\text{pixel}$ CdZnTe monolithic detector arrays subjected to a high-flux $120\phantom{\rule{0.3em}{0ex}}\mathrm{kVp}$ x-ray source.

Nature of polarization in wide-bandgap semiconductor detectors under high-flux irradiation: Application to semi-insulating Cd 1 − x Zn x Te

Next generation high-flux X-ray imaging technology is expected to advance towards multi-color or spectroscopic imaging and will significantly expand the capabilities of the technique in a multitude of applications. Spectroscopic X-ray imaging will require energy-sensitive detector arrays. In this work we evaluated the applicability of pulse-mode CdZnTe detector arrays to high-flux spectroscopic imaging. To study the material and device performance limitations of currently available CdZnTe detectors under high-flux X-ray irradiation we designed a 2D monolithic CdZnTe test array and associated test system. The detector arrays were 16 times 16 pixel devices with 0.4 mm times 0.4 mm area pixels on a 0.5 mm pitch and were fabricated using 8.7 mm times 8.7 mm times 3.0 mm CdZnTe single crystals. We measured the high-flux performance of over 1200 such arrays with various bulk CdZnTe crystal properties using a 120 kVp X-ray source and our custom built test system. We studied the various static and dynamic charge collection effects typically not observed in low-flux applications. These included dynamic polarization, static charge steering and dynamic lateral polarization and charge steering. In parallel with the experimental effort we developed a dynamic charge transport and trapping model to describe the experimentally observed static, dynamic and transient phenomena. For the first time we demonstrated > 15 times 106 counts/s/mm2 count-rate for several hundred such CdZnTe detector arrays. In addition we demonstrated good < 1% short term count-rate stability of the detector arrays.

CdZnTe Semiconductor Detectors for Spectroscopic X-ray Imaging

The growth of large-volume semi-insulating CdZnTe single crystals with improved structural perfection has been demonstrated by the electrodynamic gradient (EDG) technique and active control of the Cd partial pressure in the ampoule. The EDG furnace nearly completely eliminates the uncontrolled radiative heat transport commonly encountered in traditional Bridgman systems where the charge and furnace move relative to each other. Since the new furnace utilizes electronically controlled high-precision gradient translation, it achieves superior thermal stability throughout the growth. The control of the Cd partial pressure allowed the solidification and cool-down of the ingots close to the stoichiometric composition. As a result, the formation and incorporation of large-size (/spl ges/1 /spl mu/m diameter) Te inclusions was avoided during crystallization and ingots with high structural perfection were achieved. Adequate electrical compensation has been achieved in most of the crystal growth experiments yielding CdZnTe crystals with bulk electrical resistivity in the 10/sup 9/-10/sup 10/ /spl Omega//spl middot/cm range and electron mobility-lifetime product as high as /spl mu//spl tau//sub e/=1.2/spl times/10/sup -3/ cm/sup 2//V. The materials exhibit good spectral performance in the parallel plate detector configuration.

Csaba Szeles

Papers

CdZnTe and CdTe materials for X-ray and gamma ray radiation detector applications

Advances in the crystal growth and device fabrication technology of CdZnTe room temperature radiation detectors

Nature of polarization in wide-bandgap semiconductor detectors under high-flux irradiation: Application to semi-insulating Cd 1 − x Zn x Te

CdZnTe Semiconductor Detectors for Spectroscopic X-ray Imaging

Advances in the crystal growth of semi-insulating CdZnTe for radiation detector applications