New approaches in order to enlarge the grain size of bulk CdZnTe (CZT) crystals

For the few decades, II-VI compound semiconductors are gaining attention because of its numerous applications in the field of detector technology, photovoltaic, nuclear medicine, astronomy etc. In the recent past, materials scientists focused their attention for the growth of CdTe/CdZnTe single crystals because it doesn\u27t require any specialized cooling and detects higher energy photos as in comparison with the existing Ge, Si and Hgl(2) detectors. In the present study, we are going to discuss five main approaches in order to get good quality CZT crystal and we have successfully grown the CZT crystal by adopting these approaches. They are: i) oscillatory Bridgman technique previous to the growth process, ii) modifying the thermal environments in a Bridgman geometry using a Pt tube as a cold finger in order to reduce the growth velocity iii) growth from the vapour phase using Bridgman geometry with a pyrolitic boron nitride (PBN) crucible to locate the feed material, and with a special temperature profile, iv) microgravity experiments in the FOTON M3 mission using magnetic field prior to the growth process and v) growth by a boron oxide encapsulation. The detailed discussions are given in the following sections

Dewetting During Crystal Growth of (Cd,Zn)Te:In under Microgravity

The phenomenon of "Dewetting" during crystal growth has been observed in several microgravity experiments for different semiconductor crystals. The results of these experiments showed an improvement of the material quality due to the contact-less growth of the crystals. A number of crystal growth techniques have been used to grow CZT. The most widely used is the growth from the melt by the Bridgman method. However the crucible, which is generally made of carbon-layered silica glass, causes a number of problems: solid-liquid interface curvature, spurious nucleation of grains and twins, thermal stresses during the cooling of the crystal. This work is concentrated on the growth of high resistivity (Cd,Zn)Te:In (CZT) crystals by using the phenomenon of dewetting and its application in the processing of CZT detectors. Two Cd0.9Zn0.1Te:In crystals were grown under microgravity on the Russian FOTON satellite in the Polizon facility in September 2007. One crystal was grown under a rotating magnetic field during the phase of homogenization to destroy the typical tellurium clusters in the melt. The other crystal was superheated with 20 K above the melting point. A third crystal has been grown on the ground in similar thermal conditions. Inspection of the surface of the space grown crystals gave the evidence of successful dewetting during the crystal growth. The influence of the dewetting on the material properties is shown by the results of optical and electrical characterization methods. Finally, CZT detectors have been processed from the grown part of the different crystals. The influence of dewetting on their performance will be studied by means of the detector measurements with X- and Gamma-ray sources

Characterization of Structural Defects in (Cd,Zn)Te Crystals Grown by the Travelling Heater Method

Structural defects and compositional uniformity remain the major problems affecting the performance of (Cd, Zn)Te (CZT) based detector devices. Understanding the mechanism of growth and defect formation is therefore fundamental to improving the crystal quality. In this frame, space experiments for the growth of CZT by the Travelling Heater Method (THM) under microgravity are scheduled. A detailed ground-based program was performed to determine experimental parameters and three CZT crystals were grown by the THM. The structural defects, compositional homogeneity and resistivity of these ground-based crystals were investigated. A ZnTe content variation was observed at the growth interface and a high degree of stress associated with extensive dislocation networks was induced, which propagated into the grown crystal region according to the birefringence and X-ray White Beam Topography (XWBT) results. By adjusting the growth parameters, the ZnTe variations and the resulting stress were efficiently reduced. In addition, it was revealed that large inclusions and grain boundaries can generate a high degree of stress, leading to the formation of dislocation slip bands and subgrain boundaries. The dominant defects, including grain boundaries, dislocation networks and cracks in the interior of crystals, led to the resistivity variation in the crystals. The bulk resistivity of the as-grown crystals ranged from 109 Ωcm to 1010 Ωcm

Overview of GaAs und CdTe Pixel Detectors Using Medipix Electronics

GaAs and CdTe pixel detectors have been developed over the last few decades. The applications of these detectors include X- and gamma-ray detectors working at room temperature. Fundamental properties such as detection efficiency and noise are determined by the material properties of the sensor material. Different materials have been evaluated over the years in search of the best choice for different types of radiation. This article describes the properties of GaAs and CdTe materials for single photon processing pixel detectors using the Medipix electronics.

How spectroscopic x-ray imaging benefits from inter-pixel communication

Spectroscopic x-ray imaging based on pixellated semiconductor detectors can be sensitive to charge sharing and K-fluorescence, depending on the sensor material used, its thickness and the pixel pitch employed. As a consequence, spectroscopic resolution is partially lost. In this paper, we study a new detector ASIC, the Medipix3RX, that offers a novel feature called charge summing, which is established by making adjacent pixels communicate with each other. Consequently, single photon interactions resulting in multiple hits are almost completely avoided. We investigate this charge summing mode with respect to those of its imaging properties that are of interest in medical physics and benchmark them against the case without charge summing. In particular, we review its influence on spectroscopic resolution and find that the low energy bias normally present when recording energy spectra is dramatically reduced. Furthermore, we show that charge summing provides a modulation transfer function which is almost independent of the energy threshold setting, which is in contrast to approaches common so far. We demonstrate that this property is directly linked to the detective quantum efficiency, which is found to increase by a factor of three or more when the energy threshold approaches the photon energy and when using charge summing. As a consequence, the contrast-to-noise ratio is found to double at elevated threshold levels and the dynamic range increases for a given counter depth. All these effects are shown to lead to an improved ability to perform material discrimination in spectroscopic CT, using iodine and gadolinium contrast agents. Hence, when compared to conventional photon counting detectors, these benefits carry the potential of substantially reducing the imaging dose a patient is exposed to during diagnostic CT examinations

Charge Summing in Spectroscopic X-Ray Detectors With High-Z Sensors

The spectroscopic performance of photon counting detectors is limited by the effects of charge sharing between neighboring pixels and the emission of characteristic X-rays. For these reasons, an event can be either missed or counted more than once. These effects become more and more of a concern when pixel pitches are reduced, and for the technology available so far, this meant that there would always be a trade-off between a high spatial and a high spectral resolution. In this work, we present first measurements obtained with the new Medipix3RX ASIC, which features a network of charge summing circuits establishing a communication between pixels which helps to mitigate these effects. Combined with cadmium telluride sensors, we show that this new technology is successful at improving a detector's spectroscopic capabilities even at pixel pitches as small as 55 mu m. At this pitch, we measure an energy response function similar to that observed for a pixel pitch of 165 mu m in the absence of a charge summing circuitry. This amounts to an effective reduction of the pixel area by at least one order of magnitude at a comparable energy response. Additionally, we present synchrotron measurements at high X-ray fluxes, where significant pulse pile-up occurs, and provide first experimental evidence for a net benefit when balancing spectroscopic performance and high flux tolerance in charge summing mode

Energy weighted x-ray dark-field imaging

The dark-field image obtained in grating-based x-ray phase-contrast imaging can provide information about the objects’ microstructures on a scale smaller than the pixel size even with low geometric magnification. In this publication we demonstrate that the dark-field image quality can be enhanced with an energy-resolving pixel detector. Energy-resolved x-ray dark-field images were acquired with a 16-energy-channel photon-counting pixel detector with a 1 mm thick CdTe sensor in a Talbot–Lau x-ray interferometer. A method for contrast-noise-ratio (CNR) enhancement is proposed and validated experimentally. In measurements, a CNR improvement by a factor of 1.14 was obtained. This is equivalent to a possible radiation dose reduction of 23%