This paper presented a new carrier board attachment method for pixellated CdZnTe (CZT) radiation detectors by using a
special type of anisotropic conductive film (ACF) based on micro-wires. This ACF has very small pitch, high vertical
electrical conductivity, and strong mechanical strength. It was found to be suitable for pixellated CZT detector assembly
by optimizing detector fabrication processes and attachment conditions. ACF attached detector modules showed
excellent spectra responses. Long-term stability and reliability tests on these detectors showed promising results. This
ACF attachment technology had been successfully used for pixellated CZT detectors with various physical dimensions
and anode pixel patterns.
A special CdZnTe (CZT) device on THM grown crystal has been developed. The device has different work function
metals on opposite electrodes yet operates at room temperature like a conventional back-to-back symmetric MSM
detector and not a one directional Schottky diode device. Aiming at creating a big breakthrough in CZT imaging device
technology, the special CZT device presented in this study is capable of increasing the photopeak count by up to 50%
compared to conventional CZT imaging device while maintaining good room temperature energy resolution by not
significantly trading off detector leakage current. Pixel pad size and interpixel gap on a 20x20x5 mm3, 8x8 pixel pattern
that result in optimum detector efficiency and interpixel resistance are presented. Sensitivity improvement impact on
other device configuration will also be discussed. The design is highly practical, reliable and suitable for mass
A 4.7×4.7×9.5 mm<sup>3</sup> Frisch collar device was fabricated from CdZnTe materials grown by the Traveling Heater
Method (THM). The device was then characterized through probing with a highly collimated 662 keV gammaray
source of <sup>137</sup>Cs. In a systematic series of experiments, the detector, at its best design, was probed using a
collimated <sup>137</sup>Cs source. The results were confirmed through simulating the charge collection efficiency (CCE)
maps of the device under the operated condition. It is proved that, unlike the planar configuration, the charge
collection efficiency profile along the length of Frisch collar device is considerably improved. It is also shown
that enhancement in spectral performance occurs due to the application of the Frisch collar to a planar device.
This enhancement is due to the fact that the Frisch collar alters the nonuniform CCE profile in a planar device
to a more uniform distribution in a Frisch collar device. Additionally, a technique to optimize this uniform
distribution is investigated for a 5.0 × 4.7 × 19.6 mm<sup>3</sup> Frisch collar device, while the experimental results
are confirmed though numerical simulation. Based on this technique, there exists an optimum dielectric layer
thickness for the CdZnTe Frisch collar device, for which the CCE profile has its best (most uniform) distribution
and shows its best spectroscopic performance. The CdZnTe materials for this study were grown by THM at
Redlen Technologies and the CdZnTe devices were fabricated and characterized at the S.M.A.R.T. Laboratory
at Kansas State University.
Long term reliability is critical for a detector module to be used in applications that can not afford failure and require high accuracy such as medical imaging and homeland security. In this study, we report the reliability of pixellated Cadmium Zinc Telluride (CZT) detector modules fabricated from crystals grown by the Traveling Heater Method (THM). The reliability of the module which consists of the pixellated detector assembled to a PCB carrier board via
conductive epoxy was studied with both a Quantitative Accelerated Life Test (QALT) as well as the Highly Accelerated Stress Test (HAST) which is a common form of a Qualitative Accelerated Life test. The robustness of the THM pixellated CZT detector modules is demonstrated via the pre- and post- accelerated life test comparison of the leakage
current and the spectral performance of the assembled module. A shear test was also used to ensure the adhesion strength of the epoxy bonded method. To our knowledge, this type of study on pixellated CZT detector module has been very rare if not the first of its kind.
Surface passivation and final surface treatment on the lateral sides of CdZnTe/CdTe gamma ray detectors have been studied by many research groups. However, systematic studies of spectroscopic performance and the current voltage (I-V) characteristic behavior of devices as a result of surface treatments have not been conducted. Additionally, few studies report results for high energy gamma ray detection, which requires different techniques and technologies. In this study, a variety of final surface treatments and oxidizing agents have been applied on different CdZnTe detectors, and the effects on the I-V characteristic behavior and spectral performance of Frisch collar devices at 662 keV are reported. Further, the possibility of an alternative method is investigated, in which ion milling is utilized to etch the lateral surfaces with energetic ions of Xenon. The process is described in detail and the challenges are presented. Electron Microprobe (EMP) technique was performed on the device sides to determine the surface elements using Energy Dispersive Spectroscopy (EDS) before and after each treatment. The CdZnTe materials for this study were acquired from Redlen Technologies, and the CdZnTe devices were fabricated and characterized for each treatment at the S.M.A.R.T. Laboratory at Kansas State University.
CdZnTe (or CZT) crystals can be used in a variety of detector-type applications. This large band gap material
shows great promise for use as a gamma radiation spectrometer. Historically, the performance of CZT has typically been
adversely affected by point defects, structural and compositional heterogeneities within the crystals, such as twinning,
pipes, grain boundaries (polycrystallinity) and secondary phases (SP). The synthesis of CZT material has improved
greatly with the primary performance limitation being attributed to mainly SP. In this presentation, we describe the
extensive characterization of detector grade material that has been treated with post growth annealing to remove the SPs.
Some of the analytical methods used in this study included polarized, cross polarized and transmission IR imaging, I-V
curves measurements, synchrotron X-ray topography and electron microscopy.
The excellent room temperature spectral performance of cadmium zinc telluride detectors grown via the Traveling
Heater Method (THM) makes this approach suitable for the mass deployment of radiation detectors for applications in
homeland security and medical imaging. This paper reports our progress in fabricating thicker and larger area detectors
from THM grown CZT. We discuss the performance of such 20x20x10 mm<sup>3</sup>, and 10x10x10 mm<sup>3</sup> monolithic pixellated
detectors and virtual Frisch-Grid 4x4x12 mm3 devices, and describe the various physical properties of the materials.
Thermoelectric Effect Spectroscopy and Thermally Stimulated Current measurements were used to investigate trapping levels in a semi-insulating CdTe and Cd<SUB>1-x</SUB>Zn<SUB>x</SUB>Te crystals from multiple ingots grown by vertical Bridgman with over pressure control and high-pressure Bridgman methods. The crystals from different growth methods have different dislocation densities as well as Zn concentrations. The thermal ionization energies of these levels were extracted using both the variable heating rate and initial rise methods; the trapping cross sections were then calculated using the temperature maximum method. We report here that the shallow levels observed at E<SUB>1</SUB>=0.11+/- 0.02 and E<SUB>2</SUB>=0.17+/- 0.02 eV are intrinsic and the latter level is most likely related to the dislocation density.
The effects of two intrinsic deep levels on electrical compensation and effects of dislocations on carrier mobility in semi-insulating CdTe and CdZnTe radiation detector crystals are reported here. These levels were found in samples grown by conventional Bridgman and high-pressure Bridgman techniques. The levels were observed with thermoelectric effect spectroscopy at distinct temperatures corresponding to thermal ionization energies of E<SUB>s1</SUB> equals 0.27 +/- 0.07 eV, E<SUB>d1</SUB> equals E<SUB>v</SUB> + 0.735 +/- 0.005 eV and E<SUB>d2</SUB> equals E<SUB>v</SUB> + 0.743 +/- 0.005 eV. The shallow level (E<SUB>s1</SUB>) is associated with dislocations. The first deep level (E<SUB>d1</SUB>) is associated with the doubly ionized Cd vacancy acceptor and the second deep level (E<SUB>d2</SUB>) is associated with the Te-antisite (Te<SUB>Cd</SUB>). The second deep level (Te<SUB>Cd</SUB>) was found to electrically compensate the material to produce high resistivity CdTe and CdZnTe, provided that the Cd vacancy concentration is sufficiently reduced during crystal growth or by post-growth thermal processing. The dislocations were found to affect the mobility of the carriers in the CdTe and CdZnTe crystals.