Enormous effort has been exerted on research and development of CdZnTe (CZT) over the past two decades, as well as the pursuit of an alternative material to mitigate the disadvantages in today’s CZT material or provide comparable device performance at a lower cost of production. Although the quality of CdZnTe crystals has been improved drastically over the past few years and the material cost has steadily decreased, the yield of large-volume high-quality detector-grade CZT continues to be an issue due to its poor thermo-physical properties. TlBr was found to be a promising material to compete with CZT, but the contact degradation and device stability are still big issues and severely hinder the deployment of commercial TlBr-based devices for nonproliferation and national security applications. At BNL, we are developing a new compound Cd1-xZnx Te1-y Sey (CZTS) that holds promise as a potentially viable crystal for the replacement of CZT for some radiation detection and imaging applications. The addition of Se in the CZT compound has been found to be very effective in a drastic reduction of the sub-grain boundary network, leading to better compositional and charge-transport homogeneity. The new material has tremendous potential to increase the yield of high-quality detectors at a much lower cost of production. The reduction of the sub-grain boundary network can result in detectors with a lower voltage operation and increased detector thickness. Our efforts to develop CZTS for X- and gamma-ray radiation detector applications will be discussed in detail.
Intrinsic materials can offer advantages over doped materials for some important applications. The doped material might suffer from non-uniform distribution of the dopant, such as fine-scale striations and larger scale segregation, which might affect the overall device response, especially for large-volume detectors such as those in demand for homeland security applications for gamma spectroscopy. Cs2LiCeCl6 (CLCC), being an intrinsic scintillator, can be grown in large volume to produce large detectors with good performance, provided the crystals are free from unwanted scattering centers. CLCC belongs to the elpasolite family and the structure is cubic, so large-volume ingots can be grown without the strains resulting from anisotropic thermal expansion coefficients. In this presentation, we will discuss extensive material characterization and device response of CLCC for gamma and thermal neutron detector applications.
Alloying of CdZnTe (CZT) with selenium has been found to be very promising and effective in reducing the overall concentration of secondary phases (Te precipitates/inclusions) and sub-grain boundary networks in the crystals. These two types of defects are the main causes for incomplete charge collection, and hence they affect the yield of high-quality CZT, resulting in a very high cost for large-volume, high-quality detector-grade CZT detectors. The addition of selenium was also found to very effective in increasing the compositional homogeneity along the growth direction of the CdZnTeSe (CZTS) ingots grown by the traveling heater method (THM) technique. The compositional homogeneity along the growth direction can enhance the overall yield of detector-grade CZTS, which should therefore be possible to produce at a lower cost compared to CZT. The electrical properties and detector performance of the CZTS crystals will be presented and discussed.
We present new results from testing a small array of position-sensitive virtual Frisch-grid gamma-ray detectors. Such arrays provide high-detection efficiency, excellent energy and position resolution. They can be used in compact hand-held instruments or in large-area gamma ray imaging cameras. The high granularity position sensing enables these detectors to correct the response non-uniformity caused by crystal defects. This important feature allows one to achieve high detection performance while using standard-grade (unselected) CZT crystals, which is expected to reduce the overall cost of field deployable high-resolution CZT gamma ray detection instruments. Here, we report the results of testing several array prototypes with configurations designed for different applications.
High-resolution position-sensing has been proposed to correct response non-uniformities in Cadmium Zinc Telluride (CZT) gamma ray detectors by virtually subdividing the area into small voxels and equalizing responses from each voxel. 3D pixelated detectors coupled with multichannel readout electronics are the most advanced type of CZT devices offering many options in signal processing and enhancing detector performance. The main hurdle in achieving high sub-pixel position resolution is the relatively low signal induced on the neighboring pixels because of the electrostatic shielding effect caused by the collecting pixel. In addition, to achieve high position sensitivity one should rely on time-correlated transient signals, which means that digitized output signals must be used. Previous results have shown the benefit of using a focused laser beam to study position resolution in 3D pixelated detectors. We present the results of our studies to measure the amplitude of the pixel signals so that these can be used to measure positions of the interaction points. This is done with the processing of digitized correlated time signals measured from several adjacent pixels taking into account rise-time and charge-sharing effects. In these measurements we used a focused pulsed laser to generate a 10-micron beam at one milliwatt (650-nm wavelength) over the detector surface while the collecting pixel was moved in cardinal directions. The results include measurements that present the benefits of combining conventional pixel geometry with digital pulse processing for the best approach in achieving sub-pixel position resolution with different pixel dimensions ranging from 0.5 mm to 1.72 mm.
High-resolution position-sensitive detectors have been proposed to correct response non-uniformities in Cadmium Zinc Telluride (CZT) crystals by virtually subdividing the detectors area into small voxels and equalizing responses from each voxel. 3D pixelated detectors coupled with multichannel readout electronics are the most advanced type of CZT devices offering many options in signal processing and enhancing detector performance. One recent innovation proposed for pixelated detectors is to use the induced (transient) signals from neighboring pixels to achieve high sub-pixel position resolution while keeping large pixel sizes. The main hurdle in achieving this goal is the relatively low signal induced on the neighboring pixels because of the electrostatic shielding effect caused by the collecting pixel. In addition, to achieve high position sensitivity one should rely on time-correlated transient signals, which means that digitized output signals must be used. We present the results of our studies to measure the amplitude of the pixel signals so that these can be used to measure positions of the interaction points. This is done with the processing of digitized correlated time signals measured from several adjacent pixels taking into account rise-time and charge-sharing effects. In these measurements we used a focused pulsed laser to generate a 10-micron beam at one milliwatt (650-nm wavelength) over the detector surface while the collecting pixel was moved in cardinal directions. The results include measurements that present the benefits of combining conventional pixel geometry with digital pulse processing for the best approach in achieving sub-pixel position resolution with the pixel dimensions of approximately 2 mm. We also present the sub-pixel resolution measurements at comparable energies from various gamma emitting isotopes.
KEYWORDS: Sensors, Electrodes, 3D metrology, Data acquisition, Crystals, Germanium, Hard x-rays, Gamma radiation, Physics, Current controlled current source
In this work, we reconfigured the design of the electrodes, incorporating the high-granularity position-sensitive 3D concept into a larger geometrical form factor, e.g., hemispheric detectors, to improve the uniformity of charge collection and the energy resolution. We designed and fabricated new position-sensitive hemispheric detectors and measured the pulse-height spectra and acquired charge transport data and other electrical measurements with different sealed radioactive sources before and after modifying the design, and compared their performance to identify the optimum configuration. We then applied charge-loss corrections by utilizing the x-y-z positional information from the charge-sensing pads for each event. Based on our simulations and experimental data, we optimized a new configuration for our position-sensitive hemispheric detectors that can effectively be fabricated as large as 20x20x15-mm3 size. Furthermore, our simulation suggests that we can achieve an energy resolution of <1% (FWHM) at 662 keV from even 10x10x5 mm3 sized position-sensitive hemispheric detectors using average-grade CZT crystals.
The traditional method for electron lifetime measurements of CdZnTe (CZT) detectors relies on using the Hecht equation. The procedure involves measuring the dependence of the detector response on the applied bias and applying the Hecht equation to evaluate the mu-tau product, which in turn can be converted into the carrier lifetime if the mobility is known. Despite general acceptance of this technique, which is very convenient for comparative testing of different CZT materials, the assumption of a constant electric field inside a detector is unjustified. In the Hecht equation, this assumption means that the drift time would be a linear function of the drift distance. This condition is rarely fulfilled in practice at low applied biases where the Hecht equation is most sensitive to the mu-tau product. As a result, researchers usually take measurements at relatively high biases, which work well in the case of the low mu-tau material, <10-3 cm2/V, but give significantly underestimated values for the case of high mu-tau crystals. In this work, we applied the time-of-flight (TOF) technique to measure the electron lifetimes in long-drift-length (3 cm) standard-grade CZT detectors produced by Redlen Technologies. The TOF-based techniques are traditionally used for monitoring the electronegative impurity concentrations in noble gas detectors by measuring the electron lifetimes. We found the electron mu-tau product of tested crystals is in the range 0.1-0.2 cm2/V, which is an order of the magnitude higher than any value previously reported for CZT material. In this work, we reported the measurement procedure and the results. We will also discuss the applicability criteria of the Hecht equation for measuring the electron lifetime in high mu-tau product CZT.
During the transition period between closure of Beamline X27B at BNL’s NSLS and the opening of Beamline MID at NSLS-II, we began operation of LBNL’s ALS Beamline 3.3.2 to carry out our radiation detection materials RD. Measurements performed at this Beamline include, X-ray Detector Response Mapping and White Beam X-ray Diffraction Topography (WBXDT), among others. We will introduce the capabilities of the Beamline and present the most recent results obtained on CdZnTe and scintillators. The goal of the studies on CdZnTe is to understand the origin and effects of subgrain boundaries and help to visualize the presence of a higher concentration of impurities, which might be responsible for the deterioration of the energy resolution and response uniformity in the vicinity of the sub-grain boundaries. The results obtained in the second year of measurements will be presented.
Point defects and their concentrations play an important role in limiting the electrical and spectral properties of crystals. It is observed that the crystal-growth process causes the generation of different types of point defects, and these defects create non-uniformities that can be detrimental to device performance. In this research Cd1-xZnxTe1-ySey (CZTS) crystals grown by Bridgman and Travelling heater methods are studied for their point defects. The focus is on the types of defects, their concentrations and the variations with the selected growth method. In addition the effects of growth-related medium and deep energy traps and their corresponding densities are related to the resistivity, life-time of charge carriers and -product for electrons.
In our prior research we investigated room-temperature radiation detectors (CZT, CMT, CdMgTe, CTS, among other compound semiconductors) for point defects related to different dopants and impurities. In this talk we will report on our most recent research on newly grown CZT crystals doped with In, In+Al, In+Ni, and In+Sn. The main focus will be on the study of dopant-induced point defects using deep-level current transient spectroscopy (i-DLTS). In addition the performance, product, gamma-ray spectral response and internal electric field of the detectors were measured and correlated with the dopant-induced point defects and their concentrations. Characterization of the detectors was carried out using i-DLTS for the point defects, Pockels effect for the internal electric-field distribution, and -ray spectroscopy for the spectral properties.
Aluminum (Al) doped ZnO with very high Al concentration acts as metal regarding its electrical conductivity. ZnO offers many advantages over the commonly-known metals being used today as electrode materials for nuclear detector fabrication. Often, the common metals show poor adhesion to CdZnTe or CdTe surfaces and have a tendency to peel off. In addition, there is a large mismatch of the coefficients of thermal expansion (CTE) between the metals and underlying CdZnTe, which is one of the reasons for mechanical degradation of the contact. In contrast ZnO has a close match of the CTE with CdZnTe and possesses 8-20 times higher hardness than the commonly-used metals. In this presentation, we will explore and discuss the properties of CdZnTe detectors with ZnO:Al contacts.
The self-activated novel scintillating material, Cs2LiCeCl6, was grown and evaluated at BNL for dual gamma and thermal-neutron detector applications. Cs2LiCeCl6 belongs to the elpasolite family. Because of its cubic structure, Cs2LiCeCl6 has good potential for growth of large-volume ingots. The emission spectra showed doublet emission bands peaking at 384 nm and 402 nm, similar to CLYC. An energy resolution of ~ 6% at 662 keV was measured. Thermal neutrons were also detected with a resolution of ~ 4%. Results on the grown ingots using natural Li and enriched 6Li source materials will be presented and discussed.
Our prior investigations showed that alloying CdTe with selenium results in improved material characteristics, such as a reduction in the concentration of secondary-phase particles, better compositional uniformity and less sub-grain boundary networks, as compared to CdTe/CdZnTe. However, by alloying with Se, the band-gap of CdTeSe is significantly reduced from the value for CdTe, which is the main drawback for high-resistivity CdTeSe compounds useful for radiation detection. In order to increase the band-gap, we are now growing Cd1-xZnxSeyTe1-y crystals for detector applications. The effect of Se alloying with CdZnTe will be discussed in terms of the concentration of secondary phases, stress-related defects such as sub-grain boundaries and their networks. Characterization results for the transport properties of the as-grown materials will also be discussed.
Recently, Cadmium Manganese Telluride (CMT) emerged as a promising material for roomtemperature X- and gamma-ray detectors. However, our studies revealed several material defects primarily related to growth processes that are impeding the production of large single crystals with high resistivity and high mobility-lifetime product. In this work, we characterized various defects in materials grown by the floating zone method, including twins, Te inclusions, and dislocations, using our unique facilities. We also fabricated detectors from selected CMT crystals and tested their performance. This paper discusses our detailed findings on the material’s properties and the performance of fabricated CMT detectors.
Data obtained with BNL's National Synchrotron Light Source (NSLS) has helped to elucidate, in detail, the roles of
non-uniformity and extended defects on the performance of CZT detectors, as well as the root cause of device
polarization during exposure to a high flux of incident X-rays. Measurements of carrier traps will be reported, including
their nature and relationships to different growth methods (conventional Bridgman, high-pressure Bridgman, traveling
heater, and floating zone methods). Most findings will be correlated with the performance of spectrometer-grade CZT Xray
and gamma detectors, and new directions to resolve the material deficiencies will be offered.
Although cadmium zinc telluride (CZT) is one of leading materials for fabricating room-temperature nuclear-radiation-
detectors, different defects in the crystals can degrade the performance of CZT detectors. Post-growth thermal
annealing potentially offers a satisfactory way to eliminate the deleterious influence of these defects. Here, we report that
the annealing of CZT in Cd vapor effectively lowers the density of Te inclusions. It takes a much longer annealing time
to eliminate separate large Te inclusions than small ones; however, the annealing time is greatly reduced when the large
Te inclusions are distributed along grain boundaries. We found that sub-grain boundaries still exist after the annealing at
500 °C, indicating that a higher annealing temperature might be needed.
KEYWORDS: Sensors, X-rays, X-ray detectors, Electrodes, Electric field sensors, Metals, Signal attenuation, Crystals, Signal detection, Gamma radiation
In our previous design of virtual Frisch-grid CdZnTe (CZT) detectors, the charge drift-lines can be terminated at the side
surfaces before the carriers reach the collecting anode; this results in a loss of signal from the interacting events near the
detector's edges. Here, we describe our new design for the anode contact that reduces these edge effects by focusing the
electric field towards the detectors' central axes. Four detectors were fabricated with the new hybrid anode contact, and
their performances were evaluated and compared to those from the previous design for our virtual Frisch-grid detectors.
The results obtained for all four showed similar improvement: therefore, we illustrate them with the findings from one
detector.
CdZnTe (CZT) crystals used for nuclear-radiation detectors often contain high concentrations of
subgrain boundaries and networks of poligonized dislocations that can significantly degrade the
performance of semiconductor devices. These defects exist in all commercial CZT materials,
regardless of their growth techniques and their vendor. We describe our new results from examining
such detectors using IR transmission microscopy and white X-ray beam diffraction topography. We
emphasize the roles on the devices' performances of networks of subgrain boundaries with low
dislocation densities, such as poligonized dislocations and mosaic structures. Specifically, we
evaluated their effects on the gamma-ray responses of thick, >10 mm, CZT detectors. Our findings
set the lower limit on the energy resolution of CZT detectors containing dense networks of subgrain
boundaries and walls of dislocations.
Cadmium Zinc Telluride (CdZnTe or CZT) is a very attractive material for room-temperature semiconductor detectors
because of its wide band-gap and high atomic number. Despite these advantages, CZT still presents some material
limitations and poor hole mobility. In the past decade most of the efforts developing CZT detectors focused on
designing different electrode configurations, mainly to minimize the deleterious effect due to the poor hole mobility. A
few different electrode geometries were designed and fabricated, such as pixelated anodes and Frisch-grid detectors
developed at Brookhaven National Lab (BNL). However, crystal defects in CZT materials still limit the yield of
detector-grade crystals, and, in general, dominate the detector's performance. In the past few years, our group's
research extended to characterizing the CZT materials at the micro-scale, and to correlating crystal defects with the
detector's performance. We built a set of unique tools for this purpose, including infrared (IR) transmission microscopy,
X-ray micro-scale mapping using synchrotron light source, X-ray transmission- and reflection- topography, current deep
level transient spectroscopy (I-DLTS), and photoluminescence measurements. Our most recent work on CZT detectors
was directed towards detailing various crystal defects, studying the internal electrical field, and delineating the effects of
thermal annealing on improving the material properties. In this paper, we report our most recent results.
We present our new results from testing 15-mm-long virtual Frisch-grid CdZnTe detectors with a common-cathode
readout for correcting pulse-height distortions. The array employs parallelepiped-shaped CdZnTe (CZT) detectors of a
large geometrical aspect ratio, with two planar contacts on the top and bottom surfaces (anode and cathode) and an
additional shielding electrode on the crystal's sides to create the virtual Frisch-grid effect. We optimized the geometry of
the device and improved its spectral response. We found that reducing to 5 mm the length of the shielding electrode
placed next to the anode had no adverse effects on the device's performance. At the same time, this allowed corrections
for electron loss by reading the cathode signals to obtain depth information.
We characterized samples cut from different locations in as-grown CdZnTe (CZT) ingots, using Automated Infrared (IR)
Transmission Microscopy and White Beam X-ray Diffraction Topography (WBXDT), to locate and identify the extended
defects in them. Our goal was to define the distribution of these defects throughout the entire ingot and their effects on
detectors' performance as revealed by the pulse-height spectrum. We found the highest- and the lowest- concentration of
Te inclusions, respectively, in the head and middle part of the ingot, which could serve as guidance in selecting samples.
Crystals with high concentration of Te inclusions showed high leakage current and poor performance, because the
accumulated charge loss around trapping centers associated with Te inclusions distorts the internal electric field, affects
the carrier transport properties inside the crystal, and finally degrades the detector's performance. In addition, other
extended defects revealed by the WBXDT measurements severely reduced the detector's performance, since they trap
large numbers of electrons, leading to a low signal for the pulse-height spectrum, or none whatsoever. Finally, we fully
correlated the detector's performance with our information on the extended defects gained from both the IR- and the
WBXDT-measurements.
Cadmium Zinc Telluride (CZT) has attracted increasing interest with its promising potential as a room-temperature
nuclear-radiation-detector material. However, different defects in CZT crystals, especially Te inclusions and
dislocations, can degrade the performance of CZT detectors. Post-growth annealing is a good approach potentially to
eliminate the deleterious influence of these defects. At Brookhaven National Laboratory (BNL), we built up different
facilities for investigating post-growth annealing of CZT. Here, we report our latest experimental results. Cd-vapor
annealing reduces the density of Te inclusions, while large temperature gradient promotes the migration of small-size Te
inclusions. Simultaneously, the annealing lowers the density of dislocations. However, only-Cd-vapor annealing
decreases the resistivity, possibly reflecting the introduction of extra Cd in the lattice. Subsequent Te-vapor annealing is
needed to ensure the recovery of the resistivity after removing the Te inclusions.
In this work we measured the crystal defect levels and tested the performance of CdZnTe detectors by diverse
methodologies, viz., Current Deep Level Transient Spectroscopy (I-DLTS), Transient Current Technique (TCT),
Current and Capacitance versus Voltage measurements (I-V and C-V), and gamma-ray spectroscopy. Two important
characteristics of I-DLTS technique for advancing this research are (1) it is applicable for high-resistivity materials (>106
Ω-cm), and, (2) the minimum temperature for measurements can be as low as 10 K. Such low-temperature capability is
excellent for obtaining measurements at shallow levels.
We acquired CdZnTe crystals grown by different techniques from two different vendors and characterized them for point
defects and their response to photons. I-DLTS studies encompassed measuring the parameters of the defects, such as the
energy levels in the band gap, the carrier capture cross-sections and their densities. The current induced by the laser-generated
carriers and the charge collected (or number of electrons collected) were obtained using TCT that also
provides the transport properties, such as the carrier life time and mobility of the detectors under study. The detector's
electrical characteristics were explored, and its performance tested using I-V, C-V and gamma-ray spectroscopy.
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