## Introduction
CdZnTe (CZT) detectors come in various electrode geometries, including planar, bar, and quasi-hemispherical designs, each offering distinct charge collection characteristics and device architectures. Surface passivation plays a critical role in enhancing the electrical performance and long-term stability of these detectors. However, the effects of passivation on device behavior and durability vary significantly depending on the detector geometry. Understanding these differences is essential for tailoring passivation strategies that maximize the longevity and performance of each detector type during extended operation or storage.
## Influence of Detector Geometry on Surface Area and Electric Field Distribution
The planar, bar, and quasi-hemispherical CZT detectors differ primarily in their electrode layout, which affects the exposed surface area and the internal electric field profile:
* Planar detectors feature a large, uniform electrode surface on the detector face opposite the cathode, resulting in a relatively large exposed surface area with a more uniform but weaker lateral electric field near surfaces.
* Bar detectors use narrow strip or bar electrodes on one or more surfaces, reducing the effective electrode coverage and concentrating electric fields near the electrode edges.
* Quasi-hemispherical detectors employ a small, point-like anode opposite a large cathode, producing highly non-uniform electric fields that converge towards the anode and create strong surface fields near the anode region.
These geometric differences influence how surface states, leakage currents, and trap densities impact detector performance and the efficacy of passivation treatments.
## Passivation Effects on Planar CZT Detectors
Planar detectors’ large exposed surfaces tend to accumulate surface defects and adsorb contaminants more readily, which can lead to elevated surface leakage currents if not properly passivated:
* Effective passivation reduces surface trap densities uniformly across the large planar area, significantly suppressing surface leakage current.
* This leads to improved charge collection efficiency and energy resolution that remain stable over time.
* The relatively homogeneous field distribution means surface-related effects influence the entire detector volume more evenly, so passivation that provides comprehensive surface coverage is critical for long-term performance retention.
* Due to the large planar area, passivation layer uniformity and chemical stability strongly affect long-term aging behavior.
Thus, in planar detectors, passivation is pivotal to preventing widespread surface degradation and preserving device stability over extended periods.
## Passivation Effects on Bar CZT Detectors
Bar detectors have reduced electrode coverage and increased edge effects due to their elongated electrode geometry:
* Surface leakage currents tend to concentrate near electrode edges and corners, where electric fields are locally enhanced.
* Passivation layers in bar detectors must therefore provide robust coverage and electrical isolation particularly near these high-field edge regions to suppress leakage currents and surface recombination.
* Over time, if passivation at the edges is incomplete or degrades, localized increases in leakage current and trap formation may cause gradual deterioration in charge collection near the electrode interfaces.
* However, the smaller overall surface area compared to planar detectors can make maintaining uniform passivation somewhat easier, potentially improving long-term stability if optimized properly.
Therefore, passivation in bar detectors critically influences edge-related leakage and localized surface state stability, which in turn governs their long-term performance.
## Passivation Effects on Quasi-Hemispherical CZT Detectors
Quasi-hemispherical detectors feature a small anode electrode and a large cathode, creating strong non-uniform electric fields with high intensities near the anode surface:
* This geometry inherently reduces bulk trapping effects by shortening the charge carrier transit path, but it also intensifies surface electric fields near the anode region.
* Such high surface fields can exacerbate the impact of surface traps and leakage currents, making effective passivation near the anode region especially crucial.
* Passivation layers must be highly stable and defect-free in this localized region to prevent degradation in energy resolution and charge collection over time.
* Due to the small anode area, localized passivation failure or chemical instability can have a disproportionately large effect on detector performance.
* In contrast, the large cathode area tends to experience less field concentration, and passivation there has a different, often less critical role in long-term performance.
Hence, for quasi-hemispherical detectors, targeted passivation near the anode is essential for long-term stability, demanding precise process control.
## Long-Term Stability and Aging Considerations Across Geometries
All detector types benefit from passivation that reduces surface oxidation, contamination, and trap state formation; however, the geometry-dependent factors influence aging behaviors differently:
* Planar detectors may experience relatively uniform performance degradation if passivation deteriorates, affecting broad areas and causing general leakage current increases.
* Bar detectors tend to show localized aging near electrode edges if passivation fails in these critical regions, possibly causing spatially non-uniform response degradation.
* Quasi-hemispherical detectors are most sensitive to localized passivation breakdown near the anode, where performance drops can be abrupt and severe due to the concentrated electric fields.
The difference in surface area, electric field strength, and electrode configuration determines the spatial pattern and severity of aging, making tailored passivation approaches vital for each geometry.
## Summary of Passivation Optimization Strategies
* Planar detectors require uniform, chemically stable passivation layers over large areas to maintain consistent low leakage and stable trap densities.
* Bar detectors benefit from reinforced passivation near edges and electrode interfaces to mitigate high-field induced surface degradation.
* Quasi-hemispherical detectors demand precise, localized passivation at the anode site with materials resistant to high-field stress and environmental exposure.
Each geometry’s unique electric field profile and surface exposure must guide the choice of passivation chemistry, layer thickness, and processing conditions to maximize long-term detector stability and performance retention.
## Conclusion
The effect of passivation on long-term performance varies significantly among planar, bar, and quasi-hemispherical CZT detectors due to differences in surface area, electrode geometry, and electric field distribution. Planar detectors require broad-area uniform passivation to prevent widespread surface degradation. Bar detectors need focused protection of electrode edges to suppress localized leakage current growth over time. Quasi-hemispherical detectors demand highly stable and localized passivation near the anode to withstand intense electric fields and preserve detector sensitivity. Recognizing these distinctions allows for optimized passivation strategies that enhance the durability and operational stability of each CZT detector type during extended storage and use.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/blog/how-does-the-passivation-effect-differ-between-planar-bar-and-quasi-hemispherical-czt-detectors-in-terms-of-long-term-performance.html
