R1C  CdZnTe1

Monday, Nov. 2  14:00-15:35  California

Session Chair:  Zhong He, The University of Michigan, United States

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(14:00) R1C-1, invited, Mechanically Stable Metal Layers for Ohmic and Blocking Contacts on CdZnTe Detectors by Electroless Deposition

M. Bettelli1, G. Benassi1,2, L. Nasi1, N. Zambelli1,2, D. Calestani1, E. Gombia1, L. Abbene3, F. Principato3, A. Zappettini1

1IMEM-CNR, Parma, Italy
2due2lab srl, Parma, Italy
3Dept. of Physics and Chemistry, University of Palermo, Palermo, Italy
Deposition of contacts is a major issue for the preparation of CdZnTe-based detectors. In particular, due to CdZnTe thermal instability above 150 degree Celsius, thermal annealing of contacts after deposition is not possible. Also for this reason, mechanical stability of contacts is usually poor. Blocking contacts are usually adopted to realize high resolution spectrometers, where minimization of dark current is required. On the other hand, one of the main exploitation fields for CdZnTe detectors is related to high flux applications. In this case, the use of ohmic contacts is preferred due to their capability to extract with high efficiency photo-generated carriers. Indium is traditionally employed to realize ohmic contacts on high resistivity CdZnTe crystals. However, indium electroless deposition is not easy, and evaporated Indium contacts usually show a poor mechanical stability. In this work, we describe a procedure to obtain gold and nickel contacts by electroless deposition. Deposited layer thickness up to 1 micron has been obtained. Current-voltage characterization has been carried out, paying attention to cut surface conduction by using a guard-ring. A ohmic behavior was found for nickel contacts. The contact resistivity seemed to be negligible with respect to material resistivity. Gold contacts showed blocking characteristic. The obtained layers show good adhesion and can withstand standard tape test. EDS-TEM investigations demonstrate that in both the cases, the good adhesion is correlated with the good CdZnTe stoichiometry under the contact layer . CdZnTe planar detectors have been realized and spectroscopic characteristics have been obtained. Detectors with symmetric nickel contacts have been compared with detectors realized with nickel and gold contacts. In this work, the response of detectors with nickel contacts under high flux conditions is tested and reported.

(14:20) R1C-2, (Withdrawn), Electrical and Optical Properties of Cd0.9Zn0.1Te with Indium and Lead.

Y. Zaman, W. Jie, T. Wang, L. Xu, Y. He, Y. Xu, G. Zha, R. Guo, F. Xu

Northwestern Polytechnical University, Xi'an, P.R China
Abstract withdrawn

(14:35) R1C-3, Material and Device Engineering for High Performance CZT Detectors

S. Taherion, P. Lu, E. Chen

Redlen Technologies, British Columbia, Canada
We report the necessary steps needed to achieve high performance and robust radiation detection systems. In particular, we address the need for spectral performance stability in gamma imaging cameras and major factors responsible for short and long-term detector performance stabilities. The theoretical background and our hypothesis on stability are described. We argue that the properties of the semiconductor metal contact, the CZT material type and quality, the thermal induced stress from adhesive layers from detector-carrier board attachment, the effect of underfill dielectric properties and passivation all affect short and long-term spectral stability. In this paper, we have utilized in-house material characterization techniques such as Photo Induced Current Transition Spectroscopy (PICTS), Pockels, Fourier Transform Infrared spectroscopy (FTIR), and Ellipsometer for oxide thickness measurement along with experimental data from thermal cycling, and computational methods to support our findings. Finally, we show that by carefully optimizing all material, fabrication, and attachment parameters, detector efficiencies about 70% (using 13% window for 122KeV photon) are achievable with thermal spectral peak stability of 0.05KeV/șC and temporal stability of 2% per year.

(14:50) R1C-4, Detecting Ionizing Radiation with the Pockels Effect in Cadmium Based Zinc-Blende Materials

S. Parsons1, A. Lohstroh1, I. Della-Rocca1, A. Langley2, C. Shenton-Taylor2, D. Blackie2

1Department of Physics, University of Surrey, Guildford, United Kingdom
2AWE Aldermaston, Reading, United Kingdom
A new technique is being developed which uses a Mach-Zehnder interferometer to detect ionizing radiation via the Pockels effect within cadmium based alloys. The charge carriers created in an ionization event distort the electric field and hence the refractive index of the crystal which can then be observed by measuring the relative phase shift between two interferometer beam arms. One of the main advantages of this approach is that the sensitive electronics do not need to be close to the detector material as the signal is not formed by charge collection. This means a potential system using this technology could be left in-situ within a high flux environment. An added benefit is that the system is does not suffer from dead time as the signal processing equipment usually used in solid state radiation detectors is absent. Preliminary results acquired using a free-space system have shown that the interferometer design is capable of detecting variations in electric field strength caused by both varying the bias voltage applied to a 7x7x2 mm cadmium zinc telluride (CZT) crystal with planar contacts and when irradiating with 50 keV x-rays at constant bias. An upgraded design has been developed which incorporates fiber optic components in order to reduce the inherent mechanical and thermal instabilities. This report presents an overview of the system including data to determine whether the detected changes in electric field are caused solely by a localised release of charge carriers or from polarization. ? British Crown Owned Copyright 2015/AWE

(15:05) R1C-5, Influence of Infrared Illumination on Spectroscopy Characteristics of Different CdZnTe Detectors

V. Ivanov, V. Fjodorovs, A. Loutchanski, P. Dorogov, S. Hinoverov

ZRF RITEC SIA, Riga, Latvia
It was previously found that illumination with monochromatic infrared (IR) light with wavelengths close to the absorption edge of the CdZnTe exert significant positive influence on the spectrometric characteristics of quasi-hemispherical CdZnTe detectors at room temperature. In this paper, results of IR stimulation on the spectrometric characteristics of planar, coplanar-grid, Frish-grid and pixelate CdZnTe detectors as well as results of further studies of quasi-hemispherical detectors are presented. Methods based on the transient current and the transient charge pulses waveforms analysis from alpha particle, alpha and gamma–radiation spectra analysis were used to investigate the effect of IR illumination on detectors characteristics. Significant improvement of spectrometric characteristics under the IR illumination was obtained with hemispherical detectors. The main processes caused by IR light in CdZnTe detectors is the holes detrapping form the deep levels, which causes increasing of the positive space charge in the detector sensitive volume. Presence of this positive space charge leads to the increasing electric field near the cathode. In the case of quasi-hemispherical detectors the increased electric field at the cathode is a positive factor improving electrons collection efficiency. Spectrometric characteristics improvements of other detectors types were much less. The waveforms of output pulses, alpha and gamma-radiation spectra at different intensity of IR illumination, operation voltages and temperatures measured with various CdZnTe detectors are shown in the paper.

(15:20) R1C-6, Orientation Dependence of Apparent Resistivity in CdZnTe Crystals

J. M. MacKenzie, F. J. Kumar, L. Burgess, G. Prekas, A. Densmore, U. El-Hanany

Redlen Technologies, Victoria, Canada
Contactless resistivity measurements based on the capacitive relaxation time (Time Dependent Charge Measurements (TDCM)) were conducted using (111) oriented CdZnTe (CZT) and orientations away from (111). The results show that the calculated resistivity value can change by an order of magnitude in (111) material depending on which polar face is towards the collection electrode. Specifically, in (111) oriented CZT, calculated resistivity is always higher with the Cd-rich (A-face) facing the collecting electrode. This behavior has also been observed in (111) oriented materials fabricated using electrodless Au-Au contacts in a planar configuration at voltages between 100 and 1000V depending on the relative polarity of the IV measurement. Conversely, un-oriented CZT does not show variation in resistivity when flipped from one side to the other. In (111) CZT, electron mobility lifetime products (”te) do not show significant variation with current direction. Mechanisms for variations in TDCM values in polar CZT are described.