Edwin Y. Lee

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

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.

Optical studies of the internal electric field distributions, crystal defects, and detector performance of CdZnTe radiation detectors

Polarized transmission optical profiles were employed to characterize the CdZnTe (CZT) room-temperature radiation detectors. 2D images reflecting the internal electric field intensity changes were obtained utilizing the Pockels electro-optic effect. Varieties of different types of CZT detectors, i.e., planar and P-I-N detectors, were investigated under different operating bias voltages, respectively. Single crystal and polycrystalline CZT detectors were also studied and compared. Nonuniform internal electric field distributions throughout the detector volumes were observed and analyzed. The grain- boundary effects to the internal electric fields will be presented and discussed, along with a theoretical simulation. A semiconductor energy band model associated with depletion layer width will be emphasized and discussed.

Detection of electron and hole traps in CdZnTe radiation detectors by thermoelectric emission spectroscopy and thermally stimulated conductivity

The electric properties of CdZnTe radiation detectors are largely determined by the electron and hole traps in this material. The traps, in addition to degrading the detector performance, can function as dopants and determine the resistivity of the material. Thermoelectric emission spectroscopy and thermally stimulated conductivity are used to detect these traps in a commercially available spectrometer-grade CdZnTe detector, and the electrical resistivity is measured as a function of temperature. A deep electron trap having an energy of 695 meV and cross section of 8 X 10-16 cm(superscript 2

CZT detectors fabricated from horizontal and vertical Bridgman-grown crystals

is detected and three hole traps having energies of 70 +/- 20 meV, 105 +/- 30 meV and 694 +/- 162 meV are detected. A simple model based on these traps explains quantitatively all the data, including the electrical properties at room temperature and also their temperature dependence.

Compensation and trapping in semi-insulating CdZnTe

Characterizations of Cd1−xZnxTe (0.04<x<0.24) detector crystals grown by vertical high-pressure Bridgman (VHPB), vertical ambient pressure Bridgman (VB), horizontal ambient pressure Bridgman (HB) and vapor-grown crystals obtained from various sources were compared. The following methods were applied: (1) Triaxial double crystal X-ray diffraction (TADXRD) to determine the crystal homogeneity and Zn content. (2) Sensitivity to radiation from high-flux X-rays to investigate detector efficiency and contacts. (3) Laser-induced transient charge technique (TCT) for measuring the carrier lifetimes. (4) Thermoelectric voltage spectroscopy (TEVS) and thermal-stimulated current spectroscopy, (TSC) to study the carrier traps. (5) IR imaging to characterize macroscopic crystalline defects. We compared cadmium zinc telluride crystals grown by different methods in order to understand better the nature of defects, which influence their nuclear spectroscopic response, and how the defects are affected by the growth technique.

Device Simulation of a Unipolar Gamma-Ray Detector

Semi-insulating CdZnTe radiation detectors from five leading crystal growers and universities were characterized by thermally stimulated conductivity (TSC), thermoelectric voltage spectroscopy (TEVS), dark conductivity, current- voltage, and variable temperature time-resolved and spatially-resolved photoluminescence (PL). By TEVS, which is an extension of the hot-probe method, all of the samples were found to have n-type electrical conductivities at room temperature and this implied that the dominant deep level is a donor level. The TSC and TEVS spectra showed that all of the samples had a dominant deep electron trap, a series of shallow electron traps, and a deep hole trap. Some of the samples showed large concentrations of shallow hole traps. A two level model of compensation is proposed which is consistent with the observed resistivities, electrical conductivities at room temperature, observed trap level energies, and observed trapping behavior. It consists of a dominant deep donor level compensating a smaller concentration of a hole acceptor level that may be shallow or deep. The model showed that the electrical conductivity type of the stably compensated materials at RT is determined by the dominant level of the compensation, which is a deep donor level for CdZnTe. Preliminary results from the variable temperature time-resolved and spatially-resolved PL showed that the emission from the traps dominate the photoluminescence spectra from these materials and that there is much spatial variation in the trap concentrations.

Low-cost cadmium zinc telluride radiation detectors based on electron-transport-only designs

The pulse height spectra from a new kind of unipolar gamma-ray detectors were predicted using a new three-dimensional simulation program developed at Sandia National Laboratories. The detectors were fabricated at Sandia and RMD Inc., and tested at Sandia. They were fabricated from Cd{sub 1{minus}x}Zn{sub x}Te crystals and they were electron-transport-only devices. For the simulation, a successive overrelaxation method was used to determine the three-dimensional internal electric field within a detector, and to find the weighting potentials for the anode and the cathode. Uniform irradiation and ionization from a {sup 137}Cs source was assumed, and the charge transport and the signal induction within the detector were numerically computed using the appropriate materials and design parameters. The simulation gave excellent agreement with experimental pulse height spectra, and it demonstrated the power of such a simulation to correlate the materials parameters and the device design to the actual detector performance.

Material properties of large-volume cadmium zinc telluride crystals and their relationship to nuclear detector performance

The goal of this project was to utilize a novel device design to build a compact, high resolution, room temperature operated semiconductor gamma ray sensor. This sensor was constructed from a cadmium zinc telluride (CZT) crystal. It was able to both detect total radiation intensity and perform spectroscopy on the detected radiation. CZT detectors produced today have excellent electron charge carrier collection, but suffer from poor hole collection. For conventional gamma-ray spectrometers, both the electrons and holes must be collected with high efficiency to preserve energy resolution. The requirement to collect the hole carriers, which have relatively low lifetimes, limits the efficiency and performance of existing experimental devices. By implementing novel device designs such that the devices rely only on the electron signal for energy information, the sensitivity of the sensors for detecting radiation can be increased substantially. In this report the authors describe a project to develop a new type of electron-only CZT detector. They report on their successful efforts to design, implement and test these new radiation detectors. In addition to the design and construction of the sensors the authors also report, in considerable detail, on the electrical characteristics of the CZT crystals used to make their detectors.