## Introduction to Te Inclusions in CZT Crystals
Tellurium (Te) inclusions are common defects in
Cadmium Zinc Telluride (CZT) crystals, arising from compositional segregation during crystal growth. These inclusions are small, Te-rich regions that can range from sub-micron to tens of microns in size. In coplanar grid (CPG) detectors, Te inclusions significantly influence charge transport, signal uniformity, and spectral performance, particularly by introducing low-energy tailing in the gamma spectrum.
## Impact on Charge Transport
Te inclusions act as localized trapping centers for charge carriers:
* Electrons drifting toward the anode grids can be temporarily trapped or scattered by Te-rich regions, slowing their transit and reducing collected charge.
* Holes, which already have lower mobility-lifetime (μτ) products, may be trapped more readily near Te inclusions, further contributing to incomplete charge collection.
* The effect is strongly dependent on inclusion size, density, and spatial distribution within the crystal. High-density inclusions increase the probability that a drifting electron encounters multiple traps before reaching the anode.
These localized charge disturbances introduce variation in the differential signal between the collecting and non-collecting grids, resulting in fluctuations in measured pulse height.
## Spectral Tailing Effects
The trapping and delayed release of carriers near Te inclusions manifest as low-energy tailing in the gamma-ray spectrum:
* Interactions occurring near inclusions produce pulses with smaller amplitudes than ideal events, shifting their apparent energy downward.
* The tailing is asymmetric, extending from the full-energy peak toward lower energies.
* The severity of tailing scales with inclusion density and size; higher densities lead to broader and more pronounced tails.
Tailing reduces energy resolution and complicates peak fitting in high-precision spectroscopy.
## Dependence on Detector Geometry and Bias
In CPG detectors, the coplanar grid differential signal partially mitigates hole-induced variations, but Te inclusions still perturb electron collection:
* Thicker crystals exacerbate the impact of Te inclusions because electrons traverse longer paths and have a higher probability of encountering multiple inclusions.
* Higher cathode bias can reduce the dwell time of electrons near Te inclusions, partially mitigating trapping, but excessive voltage risks increased leakage and breakdown.
* The grid design (pitch, width, and inter-grid bias) also influences how effectively differential signals cancel inclusion-related variations.
Optimizing these parameters can reduce—but not entirely eliminate—tailing caused by Te inclusions.
## Spatial Nonuniformity and Production Variability
Te inclusions are rarely uniformly distributed:
* Localized clusters can create “hot spots” of trapping, producing position-dependent variations in energy resolution.
* Batch-to-batch variations in crystal growth can change inclusion density and size distribution, affecting reproducibility of spectral performance across detectors.
Advanced quality control and careful selection of low-inclusion crystals are essential for high-performance CPG detectors.
## Mitigation Strategies
Several strategies are employed to minimize the impact of Te inclusions:
*
Crystal Selection: High-quality CZT crystals with low Te inclusion density are chosen for detector fabrication.
*
Pulse Shape Analysis: Temporal characteristics of pulses can identify inclusion-affected events for correction or rejection.
*
Depth-of-Interaction Corrections: DOI information can partially compensate for charge loss near inclusions by adjusting pulse heights based on electron transit distance.
*
Optimized Biasing: Appropriate cathode and inter-grid voltages reduce electron dwell time in inclusion-rich regions, decreasing trapping probability.
These techniques improve spectral fidelity but cannot fully compensate for high inclusion densities.
## Conclusion
Te inclusion density in CZT coplanar grid detectors directly affects charge transport by introducing localized trapping sites, which delay or reduce electron collection. This leads to pronounced low-energy tailing in the gamma spectrum and degrades energy resolution. Mitigation requires a combination of high-quality crystal selection, pulse shape or depth-of-interaction corrections, and optimized bias and grid design. Minimizing Te inclusion density is critical for achieving the intrinsic spectroscopic performance of high-resolution CZT detectors.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/blog/how-does-te-inclusion-density-affect-charge-transport-and-tailing-in-the-spectrum-of-a-czt-coplanar-grid-detector.html
