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September 20, 2006 Effect of Te precipitates on the performance of CdZnTe (CZT) detectorsG.A. Carini1, A.E. Bolotnikov1, G.S. Camarda1, G.W. Wright1, L. Li2, and R.B. James1 Measurements taken at the National Synchrotron Light Source allowed us to make detailed comparisons of the microscale responses and infrared (IR) microscopy images of CdZnTe Frisch-ring x-ray and gamma-detectors. The data conclusively showed that local deteriorations of electron-charge collection and the device’s response to x-rays fully correlate with the presence of Te precipitates as revealed in the IR images. These data offer the first experimental evidence of the material property limiting the energy resolution of CZT gamma-ray detectors.
Cd1-xZnxTe is an attractive material for fabricating crystals for x- and gamma-ray detectors. With its large band-gap i.e., the energy gap determines its leakage current), detectors produced from the material function well at room temperature, whilst the high atomic numbers of its elements impart high quantum efficiency (i.e., sensitivity). In principle, detectors are simple devices that directly convert the charge pairs generated by ionizing- and gamma-radiation into electrical signals. These advantages, combined with very good resolution, make CZT detectors suitable for a wide range of purposes, such as imaging devices for nuclear medical imaging, detectors for measuring the movement of radioactive materials, monitors to prevent the diversion of stored nuclear weapons and components, and applications in astronomy. Unfortunately, structural defects, such as twins, dislocations, inclusions, grain- and tilt-boundaries result in crystals of low quality, which severely limit the detectors’ performances. Several groups have characterized CZT detectors at the micro-scale, thereby allowing detailed investigations of their spatial responses. These measurements delineated the detrimental effects of grain boundaries and twins, but were unable to describe the physical mechanisms responsible for the carrier trapping. Further, the role of dispersed Te inclusions and precipitates within the single-crystal CZT volumes remained unclear. Recently, by developing a unique measurement facility affording an order-of-magnitude improvement in spatial resolution, we obtained unequivocal evidence correlating regions where the detector’s performance was degraded with the presence of dispersed Te-rich inclusions. This correlation between Te precipitates, visible in the IR images of crystals, and the deterioration of the devices’
responses was measured for several 1-mm thick planar CZT detectors. The original CZT crystals, grown by the Modified Vertical
Bridgman method, were supplied by Yinnel Tech., Inc. To obtain the measurements, the detectors were mounted inside a standard
eV-Products device holder and irradiated from the cathode side. The output signals were readout and processed using standard
spectroscopy electronics, including a charge-sensitive preamplifier, a shaper, and a MCA card to collect pulse-height spectra.
To study local variations in the device’s response, x-ray scans were performed at the NSLS X12A beamline employing beams of
different sizes; the smallest was slightly less than 10x10
Comparing the IR micrographs and x-ray scans (Fig.2), the Te precipitates in the IR images clearly correspond to the dark spots in the x-ray maps where the device’s response drops off by up to 50% of its average value. Furthermore, several similar measurements for different thin devices fabricated from CZT crystals grown by different techniques showed, for the first time, a 100% correlation between the locations of the precipitates and the areas of the detector with poor performance. In contrast to randomly distributed single-level traps, precipitates can be considered as extended local defects with a very high local concentration of trapping centers. In this case, an unpredictable number of charges will be trapped, and the amount of trapping cannot be corrected with current techniques. Here, the fluctuations in charge loss are proportional to the total number of such defects encountered by the electron cloud. Moreover, any electric-field distortions around these defects also can contribute to dispersing the collected charges and to degrading the spectroscopic performance.
In the past, CZT detectors underwent x-ray, gamma-ray, and alpha-particle scans to investigate the uniformity of their
responses; the sizes of the beams used in those measurements were 100 µm or larger. This investigation clarified that the
size of the x-ray beam used for the scans is an important factor in determining the ability to spatially resolve the effects
of isolated Te-rich precipitates. Fig. 3 presents four maps of the same area on a device evaluated with different spatial
resolutions to simulate different beam sizes. The original x-ray map, measured with 10- BEAMLINE FUNDING PUBLICATION FOR MORE INFORMATION |
The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
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m2. For
each X-Y position of the x-ray beam, pulse-height spectra were collected, and the peak positions, which are proportional to
the total collected charge, were evaluated using a Gaussian fit. We plotted the findings from the scans as two-dimensional
maps of the devices’ responses (Fig.1 shows an example).
