Two-step annealing (i.e. first in Cd vapor and second in Te vapor) is essential process for obtaining high quality CdZnTe (CZT) materials. During the post-growth two-step annealing, main defects in CZT, that is, Te inclusions, were successfully removed while maintaining its resistivities. Additionally, commonly observed nano-scale defects, that is, dislocation and stacking faults in CZT were also disappeared. A Frisch-grid CZT detector made via the two-step annealing process exhibited improved energy resolution and low backscattering counts in Cs-137 gamma spectra. In this presentation, we will present the evolution of nano-scale defects in annealing process through in-situ transmission electron microscopy (TEM) measurement.
We evaluated the effect of high-temperature treatment of Cd0.9Zn0.1Te:In single crystals using Hall-effect measurements,
medium- and high-temperature annealing under various deviations from stoichiometry, and infra-red (IR) transmission
microscopy Annealing at ~730 K sharply increased the electrical conductivity (by ~1-2 orders-of-magnitude). Plots of
the temperature- and cadmium-pressure dependences of the electrical conductivity, carrier concentration, and mobility
were obtained. Treating previously annealed Cd-samples under a Te overpressure at 1070 K allowed us to restore their
resistance to its initial high values. The main difference in comparing this material with CdTe was its lowered electron
density. We explained our results within the framework of Kröger’s theory of quasi-chemical reactions between point
defects in solids.
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.
Dark currents, including those in the surface and bulk, are the leading source of electronic noise in X-ray and gamma
detectors, and are responsible for degrading a detector's energy resolution. The detector material itself determines the
bulk leakage current; however, the surface leakage current is controllable by depositing appropriate passivation layers. In
previous research, we demonstrated the effectiveness of surface passivation in CZT (CdZnTe) and CMT (CdMnTe)
materials using ammonium sulfide and ammonium fluoride. In this research, we measured the effect of such passivation
on the surface states of these materials, and on the performances of detectors made from them.
We detail our new results from testing an array of 15-mm long virtual Frisch-grid CdZnTe detectors with a cathode
signal readout-scheme intended to improve spectral response by correcting for electron trapping. We designed a novel
electrode configuration for these long-drift detectors that ensures an energy resolution close to the statistical limit, and
high detection efficiency. However, in reality, the quality of the crystals limits the performance of this type of device.
Here, we describe the characterization of the array, show our preliminary results obtained with gamma-ray sources, and
expound on their relation to our material-characterization data.
Cadmium Zinc Telluride (CZT) is attracting increasing interest with its promise as a room-temperature nuclear-radiationdetector
material. The distribution of the electric field in CZT detectors substantially affects their detection performance.
At Brookhaven National Laboratory (BNL), we employed a synchrotron X-Ray mapping technique and a Pockels-effect
measurement system to investigate this distribution in different detectors. Here, we report our latest experimental results
with three detectors of different width/height ratios. A decrease in this ratio aggravates the non-uniform distribution of
electric field, and focuses it on the central volume. Raising the bias voltage effectively can minimize such nonuniformity
of the electric field distribution. The position of the maximum electric field is independent of the bias voltage;
the difference between its maximum- and minimum-intensity of electric field increases with the applied bias voltage.
KEYWORDS: Video, Cameras, Digital cameras, Video surveillance, Imaging systems, Feature extraction, Computer programming, Data acquisition, Analytical research, Digital filtering
The increased availability and usage of digital video lead to a need for automated video content analysis techniques. Most research on digital video content analysis includes automatic detection of the shot boundaries. However, those methods are not efficient in terms of computational time. In this paper, we propose the digital video camera system that can provide real-time shot boundary detection using the MPEG-7 descriptor. The video camera system is built so that MPEG-7 descriptors are extracted from frames of video. In this paper, the shot boundaries are achieved by measuring a distance of MPEG-7 descriptors for consecutive frames in real-time. Experimental results showed that the proposed video camera system provides fast and effective real-time shot boundary detection.
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