## Effect of Deep Levels in CZT Crystal on Spectral Tailing
The presence of
deep levels (or deep trap states) in
Cadmium Zinc Telluride (CZT) crystals has a significant impact on the performance of CZT detectors, especially in terms of
spectral tailing observed in the energy spectrum. Spectral tailing is the phenomenon where the recorded energy of an event is lower than the true energy of the incident radiation, resulting in an extended "tail" on the low-energy side of the energy peaks. This issue can degrade the energy resolution of the detector and reduce its accuracy in identifying and quantifying radiation.
## 1. Deep Trap States and Carrier Recombination
Deep levels in CZT are typically
trap states located within the
bandgap of the material. These traps can capture charge carriers (electrons or holes) generated by incident radiation before they can be fully collected by the electrodes. The trapped carriers may recombine with opposite charge carriers or be thermally released back into the conduction band, depending on the depth and energy of the traps.
*
Carrier Trapping: Deep levels act as traps for charge carriers, especially for
electrons in CZT, which are typically the dominant carriers in detectors. When carriers are trapped by deep levels, they are temporarily immobilized at these sites rather than moving toward the collection electrodes. This can delay the charge collection process and lead to
charge loss or incomplete charge collection.
*
Recombination and Energy Loss: Some of the trapped carriers may recombine, releasing energy that is not detected by the electrodes, resulting in a lower-than-expected signal. This manifests as spectral
tailing, where events that should have been recorded at a higher energy appear as though they have a lower energy.
## 2. Impact of Deep Levels on Charge Collection Efficiency
Charge collection efficiency (CCE) is critical for determining the accuracy of energy measurement in a detector. The presence of deep levels directly affects the CCE in several ways:
*
Increased Charge Loss: As the charge carriers are trapped and recombine, the overall CCE decreases, leading to fewer carriers being collected at the electrodes and hence a lower signal output. This results in
poor energy resolution and
spectral distortion.
*
Time Delays: When charge carriers are trapped, their collection is delayed, which can lead to
time-dependent signal distortions. This delay can create
artificially low signals or extend the response of the detector, contributing to the
low-energy tail.
## 3. Charge Transport and Electrical Field Effects
The
electric field distribution in CZT crystals plays a key role in charge transport. Deep levels in the crystal can alter the
local electric field and influence the way charge carriers are transported within the crystal.
*
Field Distortion: The presence of deep trap states can locally distort the electric field, leading to
inhomogeneous charge collection across the detector. This distortion can result in
non-uniform drift of carriers, causing incomplete or delayed collection of charge, further contributing to spectral tailing.
*
Localized Charge Recombination: In regions with high densities of deep trap states, charge carriers may recombine near the electrodes before they reach the collection points, leading to
distorted signals. This recombination effect enhances spectral tailing, as the signal is weakened or lost entirely.
## 4. Effect on the Energy Spectrum and Low-Energy Tailing
Spectral tailing in CZT detectors is especially prominent at lower energies. The influence of deep levels on the energy spectrum is a result of the incomplete collection of charge and the additional recombination processes taking place.
*
Extended Low-Energy Tail: Deep levels tend to cause an
extended low-energy tail in the energy spectrum, which is especially noticeable in gamma-ray and X-ray spectroscopic measurements. The tail represents events that are detected with lower energy than the actual photon energy due to recombination or incomplete charge collection.
*
Impact on Energy Resolution: The low-energy tail reduces the sharpness of the energy peaks, leading to a decrease in
energy resolution. This degradation in resolution makes it more difficult to distinguish between events that are close in energy, further reducing the accuracy of the detector.
## 5. Temperature Dependence of Deep Level Effects
The effect of deep levels on spectral tailing can be temperature-dependent. At higher temperatures, carriers have more thermal energy, which can influence the behavior of traps:
*
Thermal Activation of Deep Levels: At elevated temperatures, deep levels become more active. This can increase the rate at which carriers are trapped and released, potentially exacerbating spectral tailing due to increased charge recombination and delayed charge collection.
*
Reduced Charge Transport: At lower temperatures, the mobility of charge carriers in CZT decreases, which can increase the likelihood of trapping. In this case, deep levels may still contribute to spectral tailing, but the overall effect may be less pronounced due to slower carrier motion.
## 6. Mitigating Deep-Level Induced Tailing
Several strategies can be employed to reduce the impact of deep levels and mitigate spectral tailing:
*
Material Purification: Reducing impurities and defects during crystal growth can decrease the density of deep trap states, improving charge collection efficiency and reducing spectral tailing.
*
Post-Growth Annealing: Heat treatments and
annealing processes can be used to reduce the concentration of deep-level traps by promoting the migration of defects and allowing charge carriers to recombine in a more controlled manner.
*
Passivation: Surface passivation and
coating techniques can help reduce the effects of surface states and prevent additional deep-level traps from forming at the crystal surface.
*
Biasing and Electric Field Optimization: Optimizing the biasing conditions and improving the electric field distribution can help minimize the impact of deep levels on charge transport, leading to more uniform charge collection and reducing spectral tailing.
## 7. Conclusion
The presence of deep levels in CZT crystals leads to increased charge trapping, recombination, and incomplete charge collection, all of which contribute to
spectral tailing. This manifests as a
low-energy tail in the energy spectrum, which reduces energy resolution and accuracy in radiation measurements. The impact of deep levels can be mitigated through material processing, annealing, and optimization of biasing conditions, but deep-level-induced spectral tailing remains a fundamental challenge in enhancing the performance of CZT detectors for precise radiation spectroscopy.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/blog/how-does-the-presence-of-deep-levels-in-czt-crystal-affect-spectral-tailing.html
