CZT detectors fabricated from horizontal and vertical Bridgman-grown crystals (original) (raw)
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Journal of Crystal Growth, 2002
A review of growth methods used to produce Cd 1Àx Zn x Te (CZT) (0.0oxo0.20) crystals for radiation detector applications is presented. Most of the results emphasize the high-pressure Bridgman (HPB) method. For selected meltgrown HPB ingots, the liquid/solid segregation coefficients of some impurities were measured. The correlation of the impurity content and nuclear detector performance will be discussed. Extended defects and surface and bulk crystallinity were measured using triple and double axis X-ray diffraction techniques (TAD and DAD XRD), X-ray topography, and infrared microscopy. X-ray diffraction maps and IR images were generated and compared to gammaray detector tests to correlate macroscopic defects with the nuclear detector responses. Defects states of CZT were also investigated using low-temperature photoluminescence spectroscopy. Comparisons between the material and detector properties for different CZT growth methods will be discussed.
IEEE Transactions on Nuclear Science, 2000
CdZnTe crystals were grown by the vertical Bridgman method in closed quartz ampoules. The crystalline quality and the impurity content of these crystals were studied. Several X-ray detectors were cut out of these crystals. The resistivity, emission spectra, product, and spectroscopic characteristics of these detectors were extensively measured and compared with the characteristics of detectors obtained from CdZnTe crystals grown by the boron oxide encapsulated vertical Bridgman technique. The detectors prepared from crystals grown without boron oxide show good value, spectroscopic resolution, and higher reproducibility. The influence of growth method on impurity content and on detector response was discussed.
Journal of Electronic Materials, 1998
The material showing the greatest promise today for production of large-volume gamma-ray spectrometers operable at room temperature is cadmium zinc telluride (CZT). Unfortunately, because of deficiencies in the quality of the present material, high-resolution CZT spectrometers have thus far been limited to relatively small dimensions, which makes them inefficient at detecting high photon energies and ineffective for weak radiation signals except in near proximity. To exploit CZT fully, it will be necessary to make substantial improvements in the material quality. Improving the material involves advances in the crystallinity, purity, carrier lifetimes, and control of the electrical compensation mechanism. A more detailed understanding of the underlying material problems limiting the performance of CZT gamma-ray detectors is required; otherwise, problems with supply, delivery times, and unit cost of large-volume (>5 cm3 active volume) CZT spectrometers are expected to continue. A variety of analytical and numerical techniques have been employed to quantify crystallinity, strain, impurities, compositional and stoichiometric variations, bulk and surface defect states, carrier mobilities and lifetimes, electric field distributions, and surface passivation. Data from these measurements were correlated with spatial maps of the gamma-ray and alpha particle spectroscopic response, and feedback on the effectiveness of crystal growth and detector fabrication procedures has been generated. The results of several of these analytical techniques will be presented in this paper.
Characterization of Extended Defects Observed in Cadmium Zinc Telluride (CZT) Crystal
MRS Proceedings, 2015
Cadmium Zinc Telluride (CZT) semiconductor crystal properties have been studied extensively with a focus on correlations to their radiation detector performance. The need for defect-free CZT crystal is imperative for optimal detector performance. Extended defects like Tellurium (Te) inclusions, twins, sub-grain boundaries, and dislocations are common defects found in CZT crystals; they alter the electrical properties and, therefore, the crystal's response to high energy radiation. In this research we studied the extended defects in CZT crystals from two separate ingots grown using the low-pressure Bridgman technique. We fabricated several detectors cut from wafers of two separate ingots by dicing, lapping, polishing, etching and applying gold metal contacts on the main surfaces of the crystals. Using infrared (IR) transmission microscope we analyzed the defects observed in the CZT detectors, showing three dimensional scans and plot size distributions of Te inclusions, twins and sub-grain boundaries observed in particular regions of the CZT detectors. We characterized electrical properties of the detectors by measuring bulk resistivity and detector response to gamma radiation. We observed that CZT detectors with more extended defects showed poor opto-electrical properties compared to detectors with fewer defects.
Characterization of CdZnTe Crystals Grown Using a Seeded Modified Vertical Bridgman Method
IEEE Transactions on Nuclear Science, 2009
In this paper, several 60 mm diameter CdZnTe crystal ingots, containing large-size single crystals, were grown using h111i or h211i orientation seeds by the modified vertical Bridgman method. The Zn concentration distribution along the axial direction of the ingots was measured by near-infrared transmission spectroscopy at room temperature. The partition coefficient of Zn during the growth was calculated to be about 1.2. Zinc distribution uniformity measurements were carried out using photoluminescence mapping measurement at 80 K, which showed that the band gap variation in the CdZnTe wafers is less than 0.004 eV. IR microscopy showed that the diameters of the Te inclusions present in the material are in the range 6-9 m, and the density of the inclusions is 1 3 2 10 5 cm 03. IR transmission measurements in the wave number region from 500 to 4000 cm 01 demonstrate that the IR transmittance of CdZnTe wafers is higher than 60%. Current-voltage measurements were performed on test structures fabricated using thermally evaporated Au contacts deposited on as-grown crystals, which revealed bulk resistivity values of 2 5 2 10 10 1 cm. Typical leakage currents for the planar devices were 4 nA at a field strength of 1500 Vcm 01. The electron and hole mobility-lifetime products were evaluated using alpha particle irradiation. The obtained typical ()e and () h values for the as-grown CdZnTe were 2:3 2 10 03 cm 2 V 01 and 1:5 2 10 04 cm 2 V 01 , respectively. Index Terms-Mobility lifetime product, seeded growth, Te inclusions, Zinc uniformity. I. INTRODUCTION C ADMIUM ZINC TELLURIDE (or CZT)-based room temperature nuclear radiation detectors are gradually gaining acceptance in many applications due to their favorable properties [1], [2]. Recently, research has concentrated on increasing the size and singularity of CZT
Influence of thermal environments on the growth of bulk cadmium zinc telluride (CZT) single crystals
Journal of Crystal Growth, 2009
The II-VI compound semiconductor crystal cadmium zinc telluride (CZT) is very important in the field of room-temperature radiation detectors and medical imaging applications. In the present study, bulk CZT single crystal has been grown by (i) oscillatory Bridgman technique, (ii) from vapour phase using pyrolytic boron nitride ampoule in the Bridgman geometry, and (iii) by using a Pt tube used for the ampoule support as a cold finger. Several improvements were found in the thermal environments such as the effects of superheating and reduced growth velocity, as well as improvements in the grain size and zinc composition along the ingot. The compositional homogeneity and its current-voltage characteristic behaviour have been analysed using energy dispersive X-ray analysis and I-V method, respectively.
Journal of Synchrotron Radiation
In this work, the spectroscopic performances of new cadmium–zinc–telluride (CZT) pixel detectors recently developed at IMEM-CNR of Parma (Italy) are presented. Sub-millimetre arrays with pixel pitch less than 500 µm, based on boron oxide encapsulated vertical Bridgman grown CZT crystals, were fabricated. Excellent room-temperature performance characterizes the detectors even at high-bias-voltage operation (9000 V cm−1), with energy resolutions (FWHM) of 4% (0.9 keV), 1.7% (1 keV) and 1.3% (1.6 keV) at 22.1, 59.5 and 122.1 keV, respectively. Charge-sharing investigations were performed with both uncollimated and collimated synchrotron X-ray beams with particular attention to the mitigation of the charge losses at the inter-pixel gap region. High-rate measurements demonstrated the absence of high-flux radiation-induced polarization phenomena up to 2 × 106 photons mm−2 s−1. These activities are in the framework of an international collaboration on the development of energy-resolved pho...
Characterization of large cadmium zinc telluride crystals grown by traveling heater method
Journal of Applied Physics, 2008
The focus of this paper is to evaluate thick, 20ϫ 20ϫ 10 and 10ϫ 10ϫ 10 mm 3 , cadmium zinc telluride ͑CZT͒, Cd 0.9 Zn 0.1 Te, crystals grown using the traveling heater method ͑THM͒. The phenomenal spectral performance and small size and low concentration of Te inclusions/precipitates of these crystals indicate that the THM is suitable for the mass production of CZT radiation detectors that can be used in a variety of applications. Our result also proves that with careful material selection using IR and high-quality fabrication processes, the theoretical energy resolution limit can be achieved.
Characterization of Cd1−xZnxTe crystals grown from a modified vertical bridgman technique
Journal of Electronic Materials, 2006
Cd 1ÿ x Zn x Te (CZT) crystals grown from a modified vertical Bridgman technique were characterized by means of an optical polarized transmission technique using the Pockels effect, low-temperature direct current (DC) photoconductivity technique, low-temperature photoluminescence (PL) spectroscopy, room-temperature PL mapping technique, and detector performance measurements. Electric field mapping indicates that an approximation of a uniform electric field distribution approximation is generally satisfied for CZT detectors operated at room temperature under typical working conditions. A nonuniform electric field distribution is observed under intense infrared (IR) light illumination, and a model is proposed based on charge generation of defects, trapping, and space-charge effects. The largest hole mobility-lifetime product (mt) h of the CZT detector measured by DC photoconductivity is 7.0 3 10 ÿ 4 cm 2 /V. The detector treated with 2% bromine in methanol chemical etch has a relatively small surface recombination velocity at room temperature, which was obtained from DC photocurrent and detector performance tests, as measured by irradiation of 5.5-MeV a particles and 59.6-keV grays , respectively. We have clearly shown the equivalence of charge collection efficiency results measured by both DC photocurrent and a particle response. Lowtemperature DC photocurrent measurements show that surface recombination velocity increases significantly with decreasing temperature from 300 K to 250 K. The effective electron mobility-lifetime product-combination effects of bulk and surface of CZT crystal-increases with increment of temperature. Room-temperature PL mapping measurements indicate uniformity of zinc concentration within CZT crystals. Low-temperature PL spectroscopy shows that the dominant emission peaks are excitons, which are bound to either shallow neutral donors (D 0 , X) or neutral acceptors (A 0 , X), depending on the temperature, concentration of donors and acceptors, and the incident light intensity. It was found that the luminescence of (D 0 , X) depends linearly on the incident laser intensity, while (A 0 , X) has a nonlinear dependence.