## Accumulation of Space Charges Near the Cathode and Its Contribution to Polarization in CdZnTe Detectors
The polarization behavior in CdZnTe detectors is closely linked to the formation and accumulation of space charges, especially near the cathode. This process significantly affects the internal electric field distribution, carrier transport, and ultimately the detector’s performance.
## Formation of Space Charges Near the Cathode
When a CdZnTe detector is exposed to ionizing radiation such as X-rays or gamma rays, electron-hole pairs are generated throughout the crystal. Under the influence of the applied bias voltage, electrons drift toward the anode, while holes move toward the cathode. However, due to the presence of traps—particularly deep-level defects—in the CdZnTe material, some of these charge carriers become immobilized.
* Electron Trapping: Electrons moving toward the anode can get trapped within deep-level defects distributed inside the bulk.
* Hole Trapping Near the Cathode: Holes traveling toward the cathode tend to be trapped near the cathode interface or in defect-rich regions close to it. The cathode region often has a higher density of defect states or impurities that can capture holes.
As holes accumulate near the cathode due to trapping and slow recombination, a positive space charge layer builds up at or near the cathode interface.
## Impact of Space Charge Accumulation on Electric Field Distribution
The buildup of this localized positive space charge near the cathode modifies the electric field profile inside the detector in several ways:
* Electric Field Distortion: The positive space charge creates an internal electric field opposing the externally applied bias field. This distorts the original uniform electric field distribution, particularly weakening the electric field strength near the cathode side.
* Field Screening: The opposing internal field partially cancels the applied bias field in the cathode vicinity, effectively “screening” or reducing the net electric field experienced by carriers in that region.
* Non-Uniform Field Profile: The electric field shifts from a uniform distribution to a profile where the field near the cathode is significantly diminished, while the field closer to the anode may remain stronger.
## Consequences for Carrier Transport and Detector Response
The modified electric field caused by space charge accumulation near the cathode has several consequences that define the polarization behavior:
* Reduced Hole Drift Velocity: With a weaker local field near the cathode, holes experience reduced drift velocity, causing them to accumulate even more and reinforcing the space charge effect.
* Increased Carrier Trapping: The slowed movement of holes and distorted field increase the probability that additional carriers get trapped, worsening the space charge buildup.
* Decreased Charge Collection Efficiency: Because the effective field guiding charge carriers to the electrodes is diminished, fewer carriers are successfully collected, reducing the measured signal from the detector.
* Signal Instability: As the trapped space charge grows, the detector exhibits polarization, where its response degrades over time under continuous radiation exposure. The count rate or signal amplitude drops and only recovers slowly when the radiation is stopped or the bias is removed.
## Feedback Loop and Polarization Stabilization
This process often creates a feedback loop:
1. Initial trapping near the cathode leads to space charge buildup.
2. The altered electric field slows carrier transport.
3. Slower transport leads to more trapping.
4. Increased space charge further distorts the field.
Eventually, an equilibrium state is reached where the space charge and field distortion stabilize, defining the polarized condition of the detector. The detector’s output remains suppressed until charge detrapping, recombination, or neutralization processes allow the internal field to recover, which is the relaxation phase.
## Summary
The accumulation of space charges—primarily positive holes trapped near the cathode—plays a central role in the polarization behavior of CdZnTe detectors by creating an internal electric field that opposes the applied bias. This leads to distortion of the electric field, reduced carrier drift velocity, enhanced trapping, and decreased charge collection efficiency. The result is a gradual degradation of detector response under continuous radiation, known as polarization, which significantly impacts detector stability and performance. Understanding and mitigating space charge accumulation near the cathode is essential for improving the reliability of CdZnTe radiation detectors.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/blog/how-does-the-accumulation-of-space-charges-near-the-cathode-contribute-to-the-polarization-behavior-in-cdznte-detectors.html
