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.
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.
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.
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
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.