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Effects of Bulk and Surface Conductivity on the Performance of CdZnTe Pixel Detectors  [PDF]
Aleksey E. Bolotnikov,C. M. Hubert Chen,Walter R. Cook,Fiona A. Harrison,Irfan Kuvvetli,Stephen M. Schindler
Physics , 2001, DOI: 10.1109/TNS.2002.801673
Abstract: We studied the effects of bulk and surface conductivity on the performance of high-resistivity CdZnTe (CZT) pixel detectors with Pt contacts. We emphasize the difference in mechanisms of the bulk and surface conductivity as indicated by their different temperature behaviors. In addition, the existence of a thin (10-100 A) oxide layer on the surface of CZT, formed during the fabrication process, affects both bulk and surface leakage currents. We demonstrate that the measured I-V dependencies of bulk current can be explained by considering the CZT detector as a metal-semiconductor-metal system with two back-to-back Schottky-barrier contacts. The high surface leakage current is apparently due to the presence of a low-resistivity surface layer that has characteristics which differ considerably from those of the bulk material. This surface layer has a profound effect on the charge collection efficiency in detectors with multi-contact geometry; some fraction of the electric field lines originated on the cathode intersects the surface areas between the pixel contacts where the charge produced by an ionizing particle gets trapped. To overcome this effect we place a grid of thin electrodes between the pixel contacts; when the grid is negatively biased, the strong electric field in the gaps between the pixels forces the electrons landing on the surface to move toward the contacts, preventing the charge loss. We have investigated these effects by using CZT pixel detectors indium bump bonded to a custom-built VLSI readout chip.
On the issue of ohmicity of Schottky contacts  [PDF]
A. V. Sachenko,A. E. Belyaev,R. V. Konakova
Physics , 2015,
Abstract: An analysis is made of the conditions for ohmic contacts realization in the case of Schottky contacts. Based on the classical notions about the mechanisms of current flow, we consider the generalized model of Schottky contact that takes into account the thermionic current of majority charge carriers and recombination current of minority charge carriers in Schottky contacts with a dielectric gap. An analysis of the results given by that model made it possible to obtain ohmicity criteria for Schottky contacts and compare the conditions for low injection level and ohmicity of Schottky contacts in the case of silicon-based contacts. It is shown that conditions for Schottky contact ohmicity do not coincide with those for p-n junctions.
Development of Prototype Pixellated PIN CdZnTe Detectors  [PDF]
T. Narita,P. Bloser,J. Grindlay,R. Sudharsanan,C. Reiche,C. Stenstrom
Physics , 1998, DOI: 10.1117/12.312894
Abstract: We report initial results from the design and evaluation of two pixellated PIN Cadmium Zinc Telluride detectors and an ASIC-based readout system. The prototype imaging PIN detectors consist of 4X4 1.5 mm square indium anode contacts with 0.2 mm spacing and a solid cathode plane on 10X10 mm CdZnTe substrates of thickness 2 mm and 5 mm. The detector readout system, based on low noise preamplifier ASICs, allows for parallel readout of all channels upon cathode trigger. This prototype is under development for use in future astrophysical hard X-ray imagers with 10-600 keV energy response. Measurements of the detector uniformity, spatial resolution, and spectral resolution will be discussed and compared with a similar pixellated MSM detector. Finally, a prototype design for a large imaging array is outlined.
CdZnTe detectors with parallel capacitive Frisch grid

HUANG Jian,LIANG Xiao-Yan,MIN Jia-Hu,QIN Kai-Feng,TANG Ke,WANG Lin-Jun,XIA Yi-Ben,ZHANG Ji-Jun,SHI Zhu-Bin,

红外与毫米波学报 , 2011,
Abstract: The crystal size and energy resolution of CdZnTe (CZT) detectors are limited by crystal growth technology. In this paper, capacitive Frisch grid detector structure was used to reduce the leakage current and overcome the charge carriers' recombination by the geometric of unipolar charge. The electrode contact of the detector was made of Schottky contact which can further compress the weight of potential and reduce the leakage current noise. A large volume multi-element parallel detector was made by capacitive Frisch grid structure and Schottky contact electrode (In-p/CZT-AuCl3). The energy resolution of multi-element detectors is not limited to the worst energy resolution unit and the detection efficiency is 2.45 ~ 7.31 times more than that of the single unit.
On the calculation of Schottky contact resistivity  [PDF]
Yang Liu
Physics , 2008,
Abstract: This numerical study examines the importance of self-consistently accounting for transport and electrostatics in the calculaiton of semiconductor/metal Schottky contact resistivity. It is shown that ignoring such self-consistency results in significant under-estimation of the contact resistivity. An explicit numerical method has also been proposed to efficiently improve contact resistivity calculations.
Study of Chemical Etching and Chemo-Mechanical Polishing on CdZnTe Nuclear Detectors  [PDF]
Aaron L. Adams, Stephen U. Egarievwe, Ezekiel O. Agbalagba, Rubi Gul, Anwar Hossain, Utpal N. Roy, Ralph B. James
Journal of Materials Science and Chemical Engineering (MSCE) , 2019, DOI: 10.4236/msce.2019.78005
Abstract: Cadmium zinc telluride (CdZnTe) semiconductor has applications in the detection of X-rays and gamma-rays at room temperature without having to use a cooling system. Chemical etching and chemo-mechanical polishing are processes used to smoothen CdZnTe wafer during detector device fabrication. These processes reduce surface damages left after polishing the wafers. In this paper, we compare the effects of etching and chemo-mechanical polishing on CdZnTe nuclear detectors, using a solution of hydrogen bromide in hydrogen peroxide and ethylene glycol mixture. X-ray photoelectron spectroscopy (XPS) was used to monitor TeO2 on the wafer surfaces. Current-voltage and detector-response measurements were made to study the electrical properties and energy resolution. XPS results showed that the chemical etching process resulted in the formation of more TeO2 on the detector surfaces compared to chemo-mechanical polishing. The electrical resistivity of the detector is of the order of 1010 Ω-cm. The chemo-mechanical polishing process increased the leakage current more that chemical etching. For freshly treated surfaces, the etching process is more detrimental to the energy resolution compared to chemo-mechanically polishing.
Recent Progress in CdTe and CdZnTe Detectors  [PDF]
Tadayui Takahashi,Shin Watanabe
Physics , 2001, DOI: 10.1109/23.958705
Abstract: Cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) have been regarded as promising semiconductor materials for hard X-ray and Gamma-ray detection. The high atomic number of the materials (Z_{Cd} =48, Z_{Te} =52) gives a high quantum efficiency in comparison with Si. The large band-gap energy (Eg ~ 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 overview talk, we summarize (1) advantages and disadvantages of CdTe and CdZnTe semiconductor detectors and (2) technique for improving energy resolution and photopeak efficiencies. Applications of these imaging detectors in future hard X-ray and Gamma-ray astronomy missions are briefly discussed.
Analysis of Te and TeO2 on CdZnTe Nuclear Detectors Treated with Hydrogen Bromide and Ammonium-Based Solutions  [PDF]
Mebougna L. Drabo, Stephen U. Egarievwe, Ifechukwude O. Okwechime, Dominique E. Jones, Anwar Hossain, Ralph B. James
Journal of Materials Science and Chemical Engineering (MSCE) , 2017, DOI: 10.4236/msce.2017.54002
Abstract: Surface defects caused during cutting and polishing in the fabrication of cadmium zinc telluride (CdZnTe) nuclear detectors limit their spectral performance. Chemical treatments are often used to remove surface damages and defects. In this paper, we present the analysis of Te and TeO2 species on the surfaces of CdZnTe nuclear detectors treated with hydrogen bromide and ammonium-based solutions. The CdZnTe wafers were chemo-mechanically polished in a mixture of hydrogen bromide in hydrogen peroxide and ethylene glycol, followed by a chemical passivation in a mixture of ammonium fluoride and hydrogen peroxide solution. X-ray photoelectron spectroscopy showed significant conversion of Te to TeO2, thus producing a more chemically stable surface. The resistivity of the CdZnTe samples is in the order of 1010 ohms-cm. The current for a given applied voltage increased following the passivation and decreased after a 3-hour period. Results from spectral response measurements showed that the 59.5-keV gamma-peak of Am-241 was stable under the same channel for the surface treatment processes.
Progress in the Development of CdTe and CdZnTe Semiconductor Radiation Detectors for Astrophysical and Medical Applications  [PDF]
Stefano Del Sordo,Leonardo Abbene,Ezio Caroli,Anna Maria Mancini,Andrea Zappettini,Pietro Ubertini
Sensors , 2009, DOI: 10.3390/s90503491
Abstract: Over the last decade, cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) wide band gap semiconductors have attracted increasing interest as X-ray and gamma ray detectors. Among the traditional high performance spectrometers based on silicon (Si) and germanium (Ge), CdTe and CdZnTe detectors show high detection efficiency and good room temperature performance and are well suited for the development of compact and reliable detection systems. In this paper, we review the current status of research in the development of CdTe and CdZnTe detectors by a comprehensive survey on the material properties, the device characteristics, the different techniques for improving the overall detector performance and some major applications. Astrophysical and medical applications are discussed, pointing out the ongoing Italian research activities on the development of these detectors.
Progress in the Development of CdZnTe Unipolar Detectors for Different Anode Geometries and Data Corrections  [PDF]
Qiushi Zhang,Congzhe Zhang,Yanye Lu,Kun Yang,Qiushi Ren
Sensors , 2013, DOI: 10.3390/s130202447
Abstract: CdZnTe detectors have been under development for the past two decades, providing good stopping power for gamma rays, lightweight camera heads and improved energy resolution. However, the performance of this type of detector is limited primarily by incomplete charge collection problems resulting from charge carriers trapping. This paper is a review of the progress in the development of CdZnTe unipolar detectors with some data correction techniques for improving performance of the detectors. We will first briefly review the relevant theories. Thereafter, two aspects of the techniques for overcoming the hole trapping issue are summarized, including irradiation direction configuration and pulse shape correction methods. CdZnTe detectors of different geometries are discussed in detail, covering the principal of the electrode geometry design, the design and performance characteristics, some detector prototypes development and special correction techniques to improve the energy resolution. Finally, the state of art development of 3-D position sensing and Compton imaging technique are also discussed. Spectroscopic performance of CdZnTe semiconductor detector will be greatly improved even to approach the statistical limit on energy resolution with the combination of some of these techniques.
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