## Impact of Small-Sized Te Inclusions on Internal Electric Fields in CdZnTe Detectors
In
CdZnTe (Cadmium Zinc Telluride) detectors,
Te inclusions are often considered as a form of
secondary phase defects that arise during the crystal growth process. These
Te inclusions, particularly those with
small sizes, can accumulate and significantly affect the
internal electric fields and
charge carrier transport.
## 1. Formation of Localized Electric Field Disruptions
*
Te inclusions are often
insulating in nature, meaning that they do not contribute to the conduction of charge carriers within the
CdZnTe matrix. However, their presence can create local disruptions in the
electric field within the detector material.
* These
small-sized inclusions act as
local charge traps, which influence the
distribution of the electric field inside the crystal. The
electric field that is crucial for
charge carrier drift can become
non-uniform, especially in
large-volume detectors.
* When these inclusions accumulate in certain regions, they can lead to
local field distortions, creating areas where the electric field may either be enhanced or weakened. This leads to
non-uniform charge collection, which can reduce the
overall efficiency of the detector in terms of
signal collection and
spectroscopic accuracy.
## 2. Accumulation of Electric Field Disturbances in Large Volumes
* In
large-volume CdZnTe detectors, the cumulative effect of
small Te inclusions becomes more pronounced. The larger the detector volume, the more likely these
small inclusions will
aggregate into clusters, further disrupting the electric field across the entire detector.
* This disruption in the electric field can lead to
charge carrier misdirection and
uneven drift paths, which degrade the detector’s
ability to efficiently collect generated charge carriers from ionizing radiation.
* The overall
electric field distribution is essential for guiding the generated
electrons and
holes towards the
electrodes. If these inclusions interfere with the field, it can cause
reduced charge collection efficiency and contribute to
signal losses.
## Effect on Electron Trapping and Charge Transport
## 3. Electron Trapping at Te Inclusion Sites
*
Te inclusions act as
trap centers for
charge carriers. When
electrons or
holes pass through regions near or within these inclusions, they are more likely to get
trapped at these sites due to the
local defect states introduced by the inclusions.
* The
trap states associated with Te inclusions have relatively deep
energy levels within the
bandgap of the CdZnTe material, which makes it difficult for the trapped
electrons to recombine quickly and be released. This results in
persistent trapping, which can significantly affect the
charge transport and
charge collection efficiency.
## 4. Reduced Carrier Mobility and Increased Recombination
* The presence of
Te inclusions and the subsequent
electron trapping lead to
reduced carrier mobility. The trapped
electrons cannot move efficiently to the
electrode, leading to
increased recombination and a decrease in the number of
collected charge carriers.
*
Electron trapping at
Te inclusions can also affect the
electron lifetime, which is critical in determining the
spectroscopic performance of the detector. A decrease in electron lifetime results in
poor energy resolution, as fewer carriers contribute to the final signal.
## 5. Impact on Spectroscopic Performance and Detector Efficiency
* The
accumulation of Te inclusions and the resulting
trap centers can reduce the
spatial uniformity of charge collection, leading to
energy resolution degradation. Since the trapped carriers are unable to contribute to the signal efficiently, the detector may show
non-linearities in response, especially in
high-energy gamma-ray detection.
* The
internal electric field distortions induced by these inclusions can also result in
pulse pile-up or
signal broadening, which further compromises the detector’s ability to differentiate between
gamma-ray events at different energies.
## 6. Larger Volume Detectors: More Pronounced Effects
* In
large-volume detectors, the
size and
distribution of
Te inclusions become critical. While small inclusions may have a negligible effect in
smaller devices, in large-volume
CdZnTe detectors, their cumulative effect can become a
significant factor in determining the
performance and
efficiency of the system.
* As the
volume increases, the probability of
trap centers increasing as well, leading to a
proportional decrease in
energy resolution and
overall detection efficiency.
## Mitigating the Effects of Te Inclusions
## 7. Optimizing Crystal Growth Conditions
* The
crystal growth process can be optimized to reduce the formation of
Te inclusions.
Slow cooling rates during the growth of
CdZnTe crystals can help mitigate the formation of these inclusions, as well as the use of a
Cd overpressure during the
Bridgman process.
*
Post-growth annealing methods, such as
two-step annealing, can also be employed to remove or reduce the size of Te inclusions, improving the
electrical properties of the material.
## 8. Increasing Bias Voltages
* Higher
bias voltages are sometimes applied to
thick CdZnTe detectors to overcome the effects of
Te inclusions. By increasing the bias, the
internal electric field can be enhanced, which helps
overcome the trapping effects and allows trapped
electrons to escape more readily and contribute to charge collection.
## Conclusion
Small-sized
Te inclusions in
CdZnTe detectors accumulate and form localized disturbances in
electric field distribution and
carrier transport. The effects of these inclusions become more pronounced in
large-volume detectors, leading to
non-uniform charge collection,
electron trapping,
reduced mobility, and
increased recombination. These factors significantly degrade the detector's
spectroscopic performance,
energy resolution, and
overall detection efficiency. Mitigating strategies such as optimizing the crystal growth process, post-growth annealing, and applying higher bias voltages are essential to improve the
charge collection efficiency and maintain the
performance of CdZnTe detectors in large-volume applications.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/blog/how-do-small-sized-te-inclusions-cumulatively-affect-the-internal-electric-fields-and-electron-trapping-in-large-volume-cdznte-detectors.html
