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.
We studied the influence of prolonged thermal treatment on the concentration and the acceptor energy level positions in p-CdTe samples. We found that heating them at 720 K entails a decrease in the concentration of electrically active centers, i.e., a "self-cleaning" of the adverse effects of some contaminants. In samples wherein the conductivity was determined by the concentration of acceptors of the A1 type (EV + 0.03-0.05) eV, after heating it becomes controlled by a deeper acceptor of the A2 type (EV + 0.13-0.14) eV, and both the charge-carrier’s mobility and the ratio μр80/μр300 increase. This effect reflects the fact that during thermal treatment, the A1 acceptors and the compensating donors are removed from their electrically active positions, most likely due to their diffusion and trapping within the inclusions in the CdTe bulk, where they have little or no influence on carrier scattering and trapping.
CdTexSe1-x, with its several advantages over the conventional CdZnTe (CZT) material, offers potential as a roomtemperature radiation detector. Its main advantage is the near-unity segregation coefficient of Se in the CdTe matrix that results in higher compositional homogeneity of the grown ingot. In this paper, we discussed the growth of CdTeSe crystals by various techniques, such as the Traveling Heater method and the Vertical Bridgman technique. We analyzed the different defects in the grown ingots, including Te inclusions/precipitations, sub-grain boundaries and dislocation networks, and studied their effects on the materials’ charge-transport characteristics. Our experimental findings demonstrated several advantages of CdTeSe over CZT, in addition to the near-unity segregation coefficient of Se, including lower concentrations of Te-inclusions/precipitations and sub-grain boundaries and a higher degree of uniformity. Our findings on its charge-transport characteristics also are very encouraging.
Having the ability to take an accurate 3D image of a tumor greatly helps doctors diagnose it and then create a treatment
plan for a patient. One way to accomplish molecular imaging is to inject a radioactive tracer into a patient and then
measure the gamma rays emitted from regions with high-uptake of the tracer, viz., the cancerous tissues. In large,
expensive PET- or SPECT-imaging systems, the 3D imaging easily is accomplished by rotating the gamma-ray detectors
and then employing software to reconstruct the 3D images from the multiple 2D projections at different angles of view.
However, this method is impractical in a very compact imaging system due to anatomical considerations, e.g., the transrectal
gamma camera under development at Brookhaven National Laboratory (BNL) for detection of intra-prostatic
tumors. The camera uses pixilated cadmium zinc telluride (CdZnTe or CZT) detectors with matched parallel-hole
collimator. Our research investigated the possibility of using a collimator with slanted holes to create 3D pictures of a
radioactive source. The underlying concept is to take 2D projection images at different angles of view by adjusting the
slant angle of the collimator, then using the 2D projection images to reconstruct the 3D image. To do this, we first
simulated the response of a pixilated CZT detector to radiation sources placed in the field of view of the camera. Then,
we formulated an algorithm to use the simulation results as prior knowledge and estimate the distribution of a shaped
source from its 2D projection images. From the results of the simulation, we measured the spatial resolution of the
camera as ~7-mm at a depth of 13.85-mm when using a detector with 2.46-mm pixel pitch and a collimator with 60°
The near-unity segregation coefficient of Se in a CdTe matrix ensures the compositional homogeneity, both axial- and
radial, of the CdTeSe ternary compound, so making it a material of choice for room- temperature radiation detectors. In
this study, we grew CdTeSe crystals by the Traveling Heater Method (THM), using Te as the solvent, and characterized
the crystals by IR transmission microscopy, white-beam X-ray diffraction topography, and low-temperature
photoluminescence. The total average concentration of the secondary phases obtained for the CdTeSe sample was about
7x104 cm-3 for crystals grown at two different laboratories. The best resistivity registered was 5x109 ohm-cm, and the estimated μτ product for the electrons was 3-4x10-3 cm2/V.
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.
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.
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
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
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.
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.
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.
CdZnTe (CZT) is the most promising semiconductor for room-temperature nuclear radiation detectors. At Brookhaven's
National Synchrotron Light Source (NSLS), we used a highly collimated synchrotron X-ray radiation to map different
CZT detectors. In this paper, the latest results from high spatial resolution X-ray mapping of CZT detectors are reported.
Effects of different internal defects on the performance of CZT detectors are discussed.
CdMnTe (CMT) can be a good candidate for producing gamma-ray detectors because of its wide band-gap, high
resistivity, and good electron transport properties. Further, the ability to grow CMT crystals at relatively low
temperatures ensures a high yield for manufacturing detectors with good compositional uniformity and few impurities.
Groups at Brookhaven National Laboratory and Institute of Physics are investigating several CMT crystals, selecting a
few of them to make detectors. In this paper, we discuss our initial characterization of these crystals and describe our
preliminary results with a gamma-ray source.
Virtual Frisch-grid CdZnTe detectors potentially can provide energy resolution close to the statistical limit. However, in real detectors, the quality of the crystals used to fabricate the devices primarily determines energy resolution. In this paper, we report our findings on the spectral response of devices and their relation to material-characterization data obtained using IR microscopy and X-ray diffraction topography.
Generally, mechanical polishing is performed to diminish the cutting damage followed by chemical etching to
remove the remaining damage on crystal surfaces. In this paper, we detail the findings from our study of the effects of
various chemical treatments on the roughness of crystal surfaces. We prepared several CdZnTe (CZT) and CdMnTe
(CMT) crystals by mechanical polishing with 5 μm and/or lower grits of Al2O3 abrasive papers including final polishing
with 0.05-μm particle size alumina powder and then etched them for different periods with a 2%, 5% Bromine-Methanol
(B-M) solution, and also with an E-solution (HNO3:H20:K2Cr2O7). The material removal rate (etching rate) from the
crystals was found to be 10 μm, 30 μm, and 15 μm per minute, respectively. The roughness of the resulting surfaces was
determined by the Atomic Force Microscopy (AFM) to identify the most efficient surface processing method by
combining mechanical and chemical polishing.