CdZnTeSe: A path forward for room-temperature radiation detector applications (#3304)
U. N. Roy1, G. S. Camarda1, Y. Cui1, R. Gul1, A. Hossain1, G. Yang1, R. B. James2, J. Zazvorka3, V. Dedic3, J. Franc3, V. Lordi4, J. Varley4
1 Brookhanev National Laboratory, Nonproliferation and National Security, Upton,, New York, United States of America
Cadmium zinc telluride (CdZnTe/CZT) is the most widely used semiconductor material for room-temperature X-and gamma ray detection, ranging from medical imaging to non-proliferation and homeland security and gamma telescope applications. Because of the steady advancement in the material quality of large bulk crystals of CZT over the last two and a half decades, many advanced instruments have been developed to realize these applications. However, issues associated with the material properties of CZT remain a challenge. Further improvements of the material quality of melt-grown CZT may have stagnated due to inherently poor thermo-physical properties near and below the melting point of the material. Thus, the search for alternative materials with greater opportunity for much improved material quality is timely, with the goal to produce higher yield large volume crystals without compromising the detector performance. In this work, we will report advances in growth of the new material system CdZnTeSe (CZTS). CZTS was found to successfully mitigate many of the disadvantages present in today’s CZT material without conceding the detector performance. Detailed results of growth, characterization, and device performance will be discussed.
Keywords: CdZnSeTe, Room temperature radiation detector, II-VI semiconductor
Development of Fine-Pixel X-ray Imaging Arrays using CdTe/n+-Si Epitaxial Layers (#1475)
M. Niraula1, K. Yasuda1, S. Tsubota1, T. Yamaguchi1, J. Ozawa1, T. Mori1, Y. Agata1
1 Nagoya Institute of Technology, Graduate School of Engineering, Nagoya, Aichi, Japan
We are developing room-temperature X-ray, gamma ray detectors for spectroscopy and imaging applications with working photon energies up to 100 keV, using metalorganic vapor-phase epitaxy (MOVPE) grown thick single crystal CdTe epilayers on Si substrates. We have previously reported on the epitaxial CdTe crystals growth, and details on p-CdTe/n-CdTe/n+-Si heterojunction diode-type detector fabrications1. We further presented fabrication details and performances of (8x8) as well as (20x20) pixel imaging arrays, where the pixel size was typically 1.12 x 1.12 mm2 with a 1.27 mm pitch2 .
We recently developed very fine pixels X-ray imaging sensor using our epitaxially grown CdTe layer on Si substrate and CMOS read-out ASIC. The detector consists of (127x127) heterojunction diode-type pixels array developed on the CdTe side, where each pixel is 60 micron2 in area with a 80 micron pixel pitch. The detector array was bump-bonded to the read-out ASIC. The basic performance of this sensor was evaluated by measuring the array dark currents and taking the X-ray images. The results were promising which confirmed that the sensor can be applied in high spatial resolution X-ray imaging. Details on the array fabrication and basic results on array evaluations will be presented.
1. M. Niraula et al., IEEE RTSD Workshop 2008. 2. M. Niraula et al., IEEE RTSD Workshop 2013, 2016.
Keywords: X-ray imaging, CdTe epitaxial layer, Fine pixel array
Real-time monitoring of resistivity in CdZnTe during the two-step annealing. (#4127)
K. Kim3, A. E. Bolotnikov1, R. B. James2
1 Brookhaven National Laboratory, Nonproliferation and national security, Upton, United States of America
Charge carrier trapping at Te secondary phase defect in CdZnTe (CZT) is the limiting factor in the performance of CZT detector in term of the charge collection efficiency (CCE). Commercial detector grade CZT crystals were grown Te-rich condition regardless of growth method, so the generation of Te secondary phase defects (i.e. Te inclusion, Te precipitates) is inevitable. There are several reports on the post growth annealing of detector grade CZT crystals under Cd/Te overpressure to eliminate those Te secondary defects. Annealing of CZT under Cd overpressure eliminate Te secondary phase defects completely. However, it leaves the prismatic punching defects over entire CZT volume and decreases a resistivity of CZT to 1-100 Ohm·cm. The prismatic punching defect is a kind of defect which deteriorate the uniformity of charge transport and affect more severely on the uniformity of charge transport considering its size. Usually prismatic punching defects are larger than Te inclusions by 10-1000 times. Low resistivity of CZT which annealed under Cd overpressure can be explained by annihilation of (TeCd+VCd) complex by in-diffusion of Cdi in to the CZT lattice during Cd overpressure annealing, which lead to loss of the compensation. Te overpressure annealing with temperature gradient can remove Te inclusions by the thermo-migration but also gives low resistivity CZT of 104-6 Ohm·cm. In our annealing experiment, two-step annealing (i.e. the first in Cd pressure, the second in Te pressure) demonstrated to be effective in order to preserve the resistivity of detector grade CZT.
In summary, we trying to get rid of Te secondary phase defects of CZT crystals through post-growth two-step annealing without losing spectroscopic properties. Also, the variation of resistivity in CZT during the two-step annealing was monitored in real time.
Keywords: CdZnTe, Two-step annealing, resistivity, real-time monitoring, Te secondary phase defects
Performance of position-sensitive virtual Frisch-grid CdZnTe detector arrays and a new readout ASIC (#4177)
A. E. Bolotnikov1, G. S. Cmarda1, G. De Geronimo1, J. Fried1, R. Gul1, A. Hossain1, L. Ocampo1, E. Vernon1, G. Yang1, R. B. James2
1 BNL, Upton, New York, United States of America
We present the results from testing of small arrays of CdZnTe (CZT) position-sensitive virtual Frisch-grid detectors (PSVFG) that offer good capabilities for spectroscopy and imaging of gamma-ray sources and correcting the response non-uniformities caused by the detector’s geometry and crystal defects. The 3D position sensitivity allows for virtual segmenting the detector’s volume into large numbers of small voxels and individually adjusting their gains. Arrays of such detectors can be integrated into compact hand-held instruments for many practical applications, including international safeguards, nonproliferation, arms control, and homeland security. Here, we present the results from testing a 4x4 array coupled with a new front-end ASIC chip, which can capture signals from 16 anodes, 32 position sensing pads and 4 cathodes. Each detector is encapsulated into an ultra-thin polyester shell with 5-mm-wide charge-sensing pads placed near the anode. The virtually grounded pads provide electrostatic shielding of the anode from the slow-moving holes. In addition, the pads and cathodes signals are processed to provide the 3D coordinates of the locations of interaction points inside the detector. Alternatively, the sum of the pads signals can be used to substitute the cathode signals, which simplify the array design. In addition, we report on testing of a linear array of 6 PSVFG detectors used for low-energy gamma rays. The above feature of PSVFG detectors can significantly improve the performance of CZT detectors fabricated from unselected standard-grade CZT crystals and reduce the overall costs of instruments based on PSVFG detectors.
Keywords: CdZnTe detectors, crystal defects
Material identification CT by accumulation type CdTe imaging sensor (#3843)
T. Aoki1, 2, A. Koike1, 2, K. Nozawa1, H. Morii1, 2, T. Okunoyama2
1 Shizuoka University, Research Institute of Electronics, Hamamatsu, Japan
An accumulation type CdTe X-ray image sensor used in this study is just 2D image sensor, which can used under high dose irradiation condition in practical use. Usually, it face is irradiated with X-rays and material identification is impossible in this way. We propose a method of irradiating X-rays to the side of the image sensor which is laid flat. In this case, the image sensor was used as a line sensor for detecting the penetration length of X-ray and that material identification is possible by utilizing the difference in penetration length. In this paper, we discuss a new method of X-ray CT scan using an accumulation type image sensor for material identification.
Keywords: CdTe image sensor, X-ray CT, Non Destructive Inspection