R2D  CdZnTe3

Tuesday, Nov. 3  16:30-18:25  California

Session Chair:  Aleksey Bolotnikov, Brookhaven National Laboratory, United States

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(16:30) R2D-1, invited, CdZnTe Materials Properties and Detector Performance for Gamma Spectroscopy and Imaging or High-Flux Spectral X-Ray

M. Prokesch

eV Products division, Kromek USA, Saxonburg, PA, USA
CdZnTe single crystals grown by the traveling heater method (THM) are engineered to optimize electron and hole transport, space charge, and device characteristics for both high-resolution gamma spectroscopy and imaging and for high-flux spectral x-ray applications. Fundamental semiconductor materials and device characterization results for different formulations of CdZnTe will be presented and compared with the corresponding spectroscopic and high-flux performance of the detector devices.

(16:50) R2D-2, invited, High Precision Medium Flux Rate CZT Spectroscopy

K. Iniewski1, J. Greenberg2, R. Crestani1, A. Densmore1, A. Grosser1, D. Brady2

1redlen technologies, bc, canada
2duke university, nc, usa
Abstract— CZT detectors are primary candidates for many next-generation X-ray imaging systems. They are typically used in two modes of operations: high precision, low flux in spectroscopy and low precision, high flux in photon counting. We will demonstrate that a new emerging way of operating those detectors in a high precision, medium flux spectroscopy mode opens a window for a variety of new applications in medical imaging, non-destructive testing and baggage scanning. In particular we will show an implementation of a coherent scattering X-ray system that offers superior performance and unparalleled capabilities in object/material discrimination.

(17:10) R2D-3, Response Non-Uniformity in Large-Volume Detector-Grade CZT Crystals

A. M. Hossain, A. E. Bolotnikov, G. S. Camarda, Y. Cui, U. N. Roy, G. Yang, R. B. James

Nonproliferation and National Security, Brookhaven National Laboratory, Upton, NY, USA
There is a high demand for large-volume CZT detectors for applications in the fields of nonproliferation and national security. However, their usage is limited by non-uniformities in the detector�s response that eventually, degrade the performance of the fabricated detectors. This issue of non-uniformity in the detector�s response mainly is due to the occurrence of various secondary-phase defects in the crystals. Over the past decade, many problems were identified and resolved, so taking this technology to a more advanced stage. Presently, the remaining critical defects in CZT crystals mostly are attributable to crystallographic defects, e.g., dislocations and micro-grains. In this study, we characterized these structural defects in detector-grade CdZnTe crystals, employing a variety of advanced techniques, including micro-scale fluorescence and diffraction, along with energy dispersive x-ray spectroscopy (EDS), and selective chemical etchants, to characterize the structural defects and other related ones in the crystals. We also measured the electron beam induced currents (EBICs) to understand their role on the electrical properties of the actual devices. By combining these techniques we were able to acquire comprehensive information on the quality and compositional variation of the crystals and their influence on the performance of the CZT detectors.

(17:25) R2D-4, Optical and Electrical Properties of CdZnTe and CdZnTeSe Crystals for Radiation Detector Applications

G. Yang, U. Roy, A. E. Bolotnikov, Y. Cui, G. S. Camarda, A. Hossain, R. Gul, R. B. James

Brookhaven National Laboratory, Upton, NY, USA
The II-VI compound CdZnTe (CZT) has been investigated in the last few years as one of the leading materials for fabricating room-temperature X-ray and gamma-ray detectors. Currently a new quaternary compound semiconductor CdZnTeSe is being developed at BNL as a promising candidate for this application. Here we use different optical and electrical characterization tools to study the properties of CdZnTe and CdZnTeSe crystals and relate the variation of these properties to the presence of different types of material defects. In particular, we focus on the low-temperature photoluminescence measurements to understand the effects of Se and other dopants/impurities. We also present and analyze the test data from infrared (IR) transmission microscopy, polarized light microscopy and current-voltage measurements. Our efforts offer a better understanding of the material defects in both CdZnTe and CdZnTeSe and help to take steps to mitigate the effects of these defects, thus improving the opto-electronic properties of detectors.

(17:40) R2D-5, Ion Implantation and Plasma Etching of CZT for Surface and Bulk Diodes

L. F. Voss1, A. M. C1, E. L. Swanberg1, A. J. Nelson1, R. J. Nikolic1, S. A. Payne1, A. Burger2

1Lawrence Livermore National Lab, Livermore, CA, USA
2Fisk University, Knoxville, TN, USA
High performance, room temperature gamma detectors are required for both medical imaging and defense applications. The leading candidate semiconductor material is CdZnTe, which in the best case has demonstrated excellent energy resolution, with a full width half maximum of less than 0.6% for the 137Cs 662 keV gamma ray using pixelated detectors and less than 2% for co-planar grid detectors. In order to enable further improvements to energy resolution and improve the performance of non-spectroscopic grade crystals, higher electric fields are required to improve charge collection to mitigate the tail to lower energy. However, the optimal field is currently limited due to increasing electronic charge injection and thus noise, especially for co-planar grid detectors. Towards this end, we have been developing advanced process techniques including plasma etching of surfaces for damage removal and current reduction, surface passivation, and ion implantation and activation of p- and n-type dopants to enable formation of p-n and p-i-n diodes. These techniques have been shown to be capable of reducing surface and bulk current of CZT by >10x compared to carefully polished surfaces. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, LLNL-ABS-670259. This research has been supported by the Defense Threat Reduction Agency.

(17:55) R2D-6, Effects of Te Precipitates in CdZnTe as Radiation Detector Materials

W. Jie

School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, Shaanxi, China

On behalf of the Zha Gangqiang, Wang Tao, Xu Yadong and Xu Lingyan
Te nano-size precipitates are often observed in telluride II-VI stoichiometry compounds due to the high solubility of Te at high temperature in the crystal, which decrease greatly with lowering the temperature. Since II-VI compound semiconductors are normally used for several important opto-electronic devices and Te precipitates apply significant negative effects, the behavior as well as the underlining principles of Te precipitates in this kind of materials are of great importance for both the basic researches and the engineering applications. In the present talk, the origination and the evolution of Te precipitates will discussed based on the experimental observation taking CdZnTe as example. The precipitates first appear as disks through the accumulation of the point defects, especially Cd vacancies and Te anti-sites, which then grow into particles of nano-size as a kind of coherent precipitates during the post growth cooling process. In the annealing process, these precipitates will grow in size and evolution in morphologies through migration, combination and Ostwald ripening. At high temperature over the melting point of Te, these precipitates will be melted and change their form into polyhedrons depending mainly on the properties of the surrounding matrix. In the inhomogeneous thermal field, they will migrate upward the temperature gradient. The effects of the precipitates on the physical properties of some typical II-VI compounds will be also discussed.

(18:10) R2D-7, Development of a Multi-Energy Photon-Counting Readout ASIC for CZT Detectors

Z. Deng1,2, X. Zhu1,2, Y. Chen1,2, Y. Liu1,2, Y. Xing1,2, C. Feng1,2, G. Wang3, H. Liu3, S. Wu3

1Engineering Physics, Tsinghua University, Beijing, China
2Key Laboratory of Particle & Radiation Imaging, Ministry of Education, Beijing, China
3Imdetek Corporation LTD, Xian/Shaanxi, China
The paper presents the development of a multi-energy photon-counting readout ASIC for CZT detectors for spectral X-ray imaging applications. It integrated 32 identical channels of signal processing circuits, including a charge sensitive preamplifier, a high-order CR-(RC)6 shaper with 100 ns peaking time and five discriminators with fine threshold adjustment and corresponding window logics and 16-bit counters. The ASIC was optimized for linear or pixel CZT detectors with sub-millimeter size and was fabricated in a 0.18 �m CMOS process. The first prototype was implemented and verified in two separated parts: the analog front-end and the discriminators with counters. Preliminary test results showed the ASICs worked well. The overall gain of the analog front-end was measured to be 170-260 mV/fC and the ENC was measured to be <400 electrons with ~3 pF input capacitance. The discriminator and counting function were also verified up to ~1MHz, including the fine adjustment of the thresholds and the data readout. Detailed circuit design and test results will be depicted in the paper.