CdZnTe Semiconductor Research Articles

Recent progress in CdTe and CdZnTe detectors

Cadmium telluride (CdTe) and cadmium zinc telluride (CdZnTe) have been regarded as promising semiconductor materials for hard X-ray and /spl gamma/-ray detection. The high atomic number of the materials (Z/sub Cd/=48, Z/sub Te/=52) gives a high quantum efficiency in comparison with Si. The large bandgap energy (Eg/spl sim/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 paper we summarize 1) advantages and disadvantages of CdTe and CdZnTe semiconductor detectors and 2) the technique for improving energy resolution and photopeak efficiencies. Applications of these imaging detectors in future hard X-ray and /spl gamma/-ray astronomy missions are briefly discussed.

Development of gamma-ray monitor using CdZnTe semiconductor detector

In this study, we aimed to develop a new X-ray and gamma-ray monitor using the CdZnTe semiconductor detector, which has high sensitivity at room temperature. The pulse height spectra and the detection efficiencies of 10 mm/spl times/10 mm by 2 mm thick CdZnTe detector were measured in the energy range of 10 keV to 1.8 MeV by using monoenergetic X-ray and gamma-ray sources. The measured results showed very good agreement with the results calculated using the EGS4 Monte Carlo code taking into account the charge collection efficiency in the detector. By using two CZT detectors of 10 mm/spl times/10 mm/spl times/2 mm and 3 mm /spl times/3 mm/spl times/2 mm coupled with a filter, the weighted sum of a few energy channels with different cutoff energies was finally found to realize a flat energy response with an equivalent dose (counts per /spl mu/Sv) within /spl plusmn/30% or /spl plusmn/10% deviation.

A physics-based model of pixellated semiconductor gamma camera

A solid state CdZnTe gamma camera simulator, taking into account the gamma-ray emission and attenuation in the examined phantom, a geometric collimator model and the CdZnTe semiconductor detector response, has been developed. In a first step, the energy dependent spectrometric response of the planar semiconductor pixels is computed from first physical principles (gamma-ray interaction with the crystal and charge collection in the semiconductor detector), as well as from specific acquisition parameters (front-end electronics signal filtering, pulses classification in a pulse height versus rise time biparametric spectrum and bi-dimensional window based pulses selection). In a second step, the uncollided and once scattered photon fluxes, incident to each gamma camera's pixel, are computed by the means of ray tracing through the phantom's CAD model. They are then combined with a geometric collimator model. The energy dependent pixel response is finally used to generate the simulated image. Each component of the model has been compared to experimental data. Finally, a correct qualitative agreement was found between simulated images and measurements performed on a prototype platform.

Response calculation of a stacked CdZnTe detector for 16N γ-ray measurement

A stacked CdZnTe semiconductor detector, which is able to detect the 6.13 MeV γ-rays from 16N, was prototyped and the response calculation was carried out by a Monte-Carlo simulation. The detector response was simulated by using the electron and photon transport code, EGS4 and taking into account the carrier trapping phenomena. The results of the response calculation agreed with the experimental data from the checking source of 137Cs and 60Co. The prototype's response and sensitivity to 16N were calculated by simulating an incident energy of 6.13 MeV . The source spectra were unfolded with the detector's response function obtained by simulation, and it indicated that the incident γ-ray energy and its intensity ratio was identified. We have acquired the potential to measure the γ-ray energy in the high energy region about 6 MeV with the stacked CdZnTe detector.

Energy Spectrum Improvement for CdZnTe Semiconductor Detector Based on Euclidian Distance with Digital-Analog Hybrid Signal Processing System

The application of a ‘digital-analog hybrid signal processing system’ combined with a Euclidian distance method is presented. The proposed method is used to improve gamma-ray energy spectrum characteristics of a CdZnTe semiconductor detector. A digital and an analog system are simultaneously applied to the detector output after passing through a preamplifier, where the digital system digitizes the signal pulse shape for the Euclidian distance determination, while the analog system measures the signal pulse height with a conventional analog method. In this paper, the digital and the analog data are combined to improve the energy spectrum characteristics; giving the increase of the photopeak area, the peak-to-valley ratio, and the photopeak sharpness.

Pulse Shape Analysis on Mixed Beta Particle and Gamma-ray Source Measured by CdZnTe Semiconductor Detector by means of Digital-Analog Hybrid Signal Processing Method

We present a study on pulse shapes of a CdZnTe semiconductor detector for beta particles and gamma-rays emitted by a 137Cs-90Sr mixed source. When beta and gamma radiations enter a semiconductor detector, two kinds of pulses are produced according to the type of radiations. The characteristics of the pulses are determined by their energy and depth of interaction. The energy deposited on the detector basically determines the pulse height and the depth of interaction affects the pulse shape. Because of the different characteristics of beta and gamma radiations, their pulse shapes are also different. The digital-analog hybrid signal processing method was applied to separation of the beta spectrum from the beta-gamma mixed one. The separation of beta spectrum was done based on a neural network algorithm, a leading edge area method, and a falling continuum tail value method. However, the perfect separation could not be obtained. The separation was also done by reconstructing the spectra on the basis of subtraction of gamma-ray component. The spectra of beta particles and gamma-rays from 137Cs-90Sr mixed source could be separated well by using the reconstruction method.

Pulse shape analysis based on similarity with digital–analog fusion method

We present the application of a “fusion method” combined with a “similarity” method to improve γ-ray energy spectrum characteristics of a CdZnTe semiconductor detector. A digital and an analog system are simultaneously applied to the detector output after passing through a preamplifier, where the digital system digitizes the signal pulse shape for the similarity determination, while the analog measures the signal pulse heights with a conventional analog method. In this paper, the digital and the analog data are fused to improve the energy spectrum characteristics, giving an increase of the photopeak area, the peak-to-valley ratio, and the photopeak sharpness.

Medical applications of CdTe and CdZnTe detectors

This review on the medical applications of CdTe and CdZnTe semiconductors is an update on the 1992 and 1996 papers [1]. In addition, future trends in the application of these semiconductors to the medical field are presented and discussed. Several solid-state detectors, based on direct or indirect detection principles, have been proposed for digital radiography and radionucleide imaging applications, although there is no single technology of choice that addresses all the issues for optimal imaging. CdTe/CdZnTe semiconductor substrates are promising digital sensors for medical imaging applications. The widespread use of these detectors depends on the large-scale production of low-cost, large-size semiconductor substrates.

Timing performance of pixelated CdZnTe detectors

In comparison with most medical imaging techniques, Nuclear Medical imaging has traditionally been plagued by inferior image quality as a result of the limitations imposed by mechanical collimation, which is necessary to form an image using single-photon emitters. To a large extent, this basic limitation may be overcome with the use of positron-emitting radiotracers to detect the two photons that are emitted following positron annihilation. Although CdZnTe semiconductor detectors are renowned for their increased count-rate capabilities and improved energy and spatial resolution, they have some limitations regarding timing and charge collection efficiency. The CdZnTe rise-time has two components: electrons, and hole-induced charge collection. Since the holes move much slower than the electrons, and because there is an equal absorption probability at all depths and energies (in the region of 511 keV), the different slopes between the rise time and the timing signal, vary dramatically. These slopes are mostly influenced by the ‘small pixel effect’. This effect is relative to the amount of charge collected and therefore, a different pulse shape slope will be obtained for different charges collected. The current study directly examines the timing limitations of measuring coincidence events in CdZnTe-pixelated detectors. The results show that it is possible to use these detectors for positron emission applications.

Prototype of the Stacked CdZnTe Semiconductor Detector for <sup>16</sup>N Measurement

The prototype model of the stacked CdZnTe semiconductor detector, which is able to measure the 6.13MeV γ-ray from 16N, was fabricated. The prototype's response calculation was carried out by Monte-Carlo method. The result of the response calculation agreed with the experiment data of check sources of 137Cs and 60Co, and 16N which was measured at vicinity of the primary cooling water pipe of the nuclear reactor. The source spectra were unfolded with detector's response function obtained by simulation, and it is indicated that the incident γ-ray energy and its intensity ratio was identified and that the energy of 6MeV γ-ray could be measured by the prototype of the stacked detector.

Detected contrast and dynamic range measurements of CdZnTe semiconductors for flat-panel digital radiography

The detected contrast and dynamic ranges of Cd/sub 1-x/Zn/sub x/Te semiconductor detectors have been measured, within the X-ray diagnostic energy range, using a contrast sensitivity phantom. The aim of this study is to optimize the image quality parameters of these solid state ionization devices for flat panel digital radiographic applications. The experimental results of this study indicate that Cd/sub 1-x/Zn/sub x/Te detectors have excellent detected contrast response and large dynamic range.

Pulse Shape Analysis on Mixed Beta Particle and Gamma-ray Source Measured by CdZnTe Semiconductor Detector by means of Digital-Analog Hybrid Signal Processing Method.

We present a study on pulse shapes of a CdZnTe semiconductor detector for beta particles and gamma-rays emitted by a 137Cs-90Sr mixed source. When beta and gamma radiations enter a semiconductor detector, two kinds of pulses are produced according to the type of radiations. The characteristics of the pulses are determined by their energy and depth of interaction. The energy deposited on the detector basically determines the pulse height and the depth of interaction affects the pulse shape. Because of the different characteristics of beta and gamma radiations, their pulse shapes are also different. The digital-analog hybrid signal processing method was applied to separation of the beta spectrum from the beta-gamma mixed one. The separation of beta spectrum was done based on a neural network algorithm, a leading edge area method, and a falling continuum tail value method. However, the perfect separation could not be obtained. The separation was also done by reconstructing the spectra on the basis of subtraction of gamma-ray component. The spectra of beta particles and gamma-rays from 137Cs-90Sr mixed source could be separated well by using the reconstruction method.

The Use of Genetic Algorithms for the Improvement of Energy Characteristics of CdZnTe Semiconductor Detectors

A new charge loss correction method using genetic algorithms (GA) has been proposed to improve gamma ray energy spectrum characteristics of CdZnTe detectors. The correction method is based on the analysis of signal waveform shapes taking into account the contribution of multiple interaction processes to pulse shape generation. A GA recognizes the charge deposition places for each signal and provides the related corrective factors of the pulse heights; the corrected pulse height spectrum was obtained by summing up the corrected pulse heights for each signal. An enhanced energy spectrum characteristic was obtained after the correction process for 662 keV photons. This method is simple and useful for pulse shape analysis; the results demonstrate promise for the successful application of GAs for digital signal processing data analysis.

Improvement of energy spectrum characteristics of CdZnTe semiconductor detector with a digital–analog fusion method

We present a new application of a `Fusion Method’ to improve gamma-ray energy spectrum characteristics of a CdZnTe semiconductor detector. A digital and an analog system are simultaneously applied for the detector output; The digital system digitizes the signal pulse shapes for pattern recognition based on a neural network algorithm. The analog measures the signal pulse heights with a conventional analog system. The digital and the analog output data are fused and then used for improvement of the energy spectrum characteristics, i.e. the photopeak area, the peak-to-valley ratio, and the FWHM of photopeaks.

Growth and structural and magnetic characterization of the FePd ordered alloy on CdZnTe II–VI semiconductor

We have studied growth conditions and structural and magnetic properties of FePd ferromagnetic layers deposited on (001) CdZnTe semiconductors. By using Au and Pd as buffer layers between the II–VI compound and the FePd, a two dimensional growth of the alloy is obtained. The alloy is well chemically ordered with the L10 tetragonal structure and is perpendicularly magnetized with a perpendicular magnetic anisotropy constant of about 6.6.106 erg cm−3. Magnetic force microscopy images showed the perpendicular magnetic configuration with up and down domains whose size is about 70 nm. The results compare favorably with FePd layers elaborated in the same run on an MgO substrate.

CdTe and CdZnTe semiconductor gamma detectors equipped with ohmic contacts

Semiconductor gamma detectors, equipped with ohmic contacts, are uniform and fast response devices that are not sensitive to hole trapping. Gamma generated charges flow within the detector bulk towards the ohmic contacts, and induce additional charge flow from the contacts towards them. The additional flow stems from the fundamental principles of Poisson and the continuity equations. Electrons flow from the negative contacts towards the holes and recombine with them, therefore, they overcome hole trapping. The ohmic contact effect transforms the detector into a single carrier device. Good quality ohmic contact detectors are achieved from a crystal grown by standard methods, that initially has too many traps, by adjustment of the Fermi level position within the forbidden band. The device design and its principle of operation are discussed.

Pulse shape recognition for CdZnTe semiconductor detector by using multi-shaping amplifiers method with neural network algorithm

A new pulse shape recognition method with multi-shaping amplifiers, combined with a neural network algorithm, has been developed, where four pulse heights are sampled from one signal pulse through four linear amplifiers with different shaping time constants. The four pulse heights are used as characteristic parameters to recognize the pulse shape with a neural network. This method has been applied to signal processing for a CdZnTe semiconductor detector to improve the deteriorated energy spectra caused by pulse height deficits due to the different mobilities of electrons and holes in the detector. The neural network recognizes the pulse shape patterns and provides the corrective magnification factors of the pulse heights. After the corrective procedure, the energy spectrum for /sup 137/Cs gamma-rays is improved from 9.3 keV to 7.4 keV in the energy resolution (FWHM) of the 662 keV gamma rays photopeak. The photopeak becomes a considerably symmetrical shape without a low-energy tail. It has been verified that this method is simple and useful for pulse shape analyses, which can be used for many other applications.

New pulse-shape analysis method with multi-shaping amplifiers

A novel pulse-shape analysis method that uses similarity to recognize an individual pulse shape is presented in this paper. We obtain four pulse heights by using four linear amplifiers with different shaping time constants. We treat a combination of the four pulse heights as a pattern vector. A similarity of the pulse shape can be obtained by comparison between the pattern vector and a discriminant vector which was given in advance. Each pulse shape is analyzed by using the similarity. The method has been applied for the improvement of characteristics of a CdZnTe semiconductor detector. The characteristics of the energy spectrum of the CdZnTe detector such as the photopeak efficiency or the peak-to-valley ratio are improved after the correction procedure with the similarity.

Direct measurement of product of the electron mobility and mean free drift time of CdZnTe semiconductors using position sensitive single polarity charge sensing detectors

This article describes novel techniques to directly measure the electron mobility and mean free drift time product μeτe in semiconductor detectors. These methods are based on newly developed single polarity charge sensing and depth sensing techniques. Compared with conventional methods based on the Hecht relation, the new methods do not involve curve fitting, are less sensitive to the variation of pulse rise times, and allow the use of higher energy γ rays typical of many applications.

Novel pulse-shape-analysis method by using similarity

A novel pulse-shape-analysis method that uses the similarity of patterns to recognize individual pulse shapes is presented. The principle of this method is described. The method has been applied to improve the gamma-ray energy spectrum of a CdZnTe semiconductor detector. The energy spectrum has been improved with increased photopeak efficiency and better energy resolution.