## The Role of Deep Levels in the Bandgap of CdZnTe
Deep-level defects in CdZnTe, such as deep trap states, play a critical role in influencing the material's photo-plastic behavior under various external stimuli, including light and applied electric fields. These deep levels, typically located far from the conduction or valence bands, act as localized energy states within the bandgap. These states can trap charge carriers (electrons and holes), and their behavior influences the mechanical properties of the material by interacting with dislocations and defects in the crystal lattice.
The key feature of deep levels is their ability to significantly affect charge carrier dynamics, such as recombination, trapping, and transport, through their interaction with dislocation cores. These effects are pivotal in determining whether the photo-plastic effect will be positive (hardening) or negative (softening). The sign of the photo-plastic effect is ultimately governed by the interplay between the trap characteristics of deep levels, dislocation motion, and the way trapped carriers interact with dislocation cores under external stimuli.
## Deep-Level Trapping and its Influence on Dislocation Pinning
In CdZnTe, deep levels usually exist due to defects such as vacancies, antisites, or impurities, which introduce localized states within the bandgap. These levels act as traps for free carriers (electrons or holes) generated by illumination or applied electric fields. When a crystal is exposed to light, especially at wavelengths near the bandgap (e.g., 870 nm for CdZnTe), photogenerated charge carriers (electron-hole pairs) can be captured by these deep traps, thereby altering the electrostatic environment around dislocations.
The nature of the trapping states—whether they are electron or hole traps, and whether they are shallow or deep—determines how effectively they influence dislocation motion. If these traps are located deep within the bandgap, they typically provide strong electrostatic pinning forces on dislocations, reducing dislocation mobility. This results in an increase in the material's resistance to plastic deformation, which is generally associated with a positive photo-plastic effect (hardening).
## Recombination of Trapped Carriers and Softening
However, the effect of deep levels on the photo-plastic response can be more complex. In some cases, the trapped carriers may recombine with their counterparts (holes with electrons) when the light source is removed or when the applied bias is adjusted. This recombination process can relax the dislocation core, reducing the electrostatic pinning force that was previously stabilizing the dislocation. When this occurs, dislocations may become more mobile, leading to a negative photo-plastic effect (softening).
In this scenario, the material may experience a decrease in hardness, as recombination of trapped carriers at deep levels results in reduced dislocation resistance to motion. The material's plasticity is enhanced, and the system moves toward a more easily deformable state. This phenomenon is more pronounced when the deep levels are shallow enough to allow for fast carrier recombination but still capable of trapping carriers long enough to influence dislocation dynamics.
## The Competing Effects of Deep Levels: Hardening vs. Softening
The overall sign of the photo-plastic effect in CdZnTe is determined by a balance between the two competing phenomena: carrier trapping and recombination.
1. Hardening (Positive Photo-Plastic Effect): If the deep-level traps effectively immobilize charge carriers at dislocation cores, increasing dislocation pinning, the material becomes more resistant to plastic deformation, leading to hardening. This typically occurs when the traps are deep enough that carriers are trapped for extended periods, and the trapping mechanism dominates.
2. Softening (Negative Photo-Plastic Effect): If the trapped carriers recombine after the light source is turned off or during the application of an external bias, the dislocations may become less pinned and more mobile. This reduced pinning leads to softening, which is more likely when the traps are shallow and the recombination processes are fast, allowing dislocations to move more freely after carrier release.
## The Effect of Trap Density and Trap Distribution
The density and distribution of deep levels also influence the overall photo-plastic response. A high concentration of deep-level traps generally enhances the likelihood of hardening, as more traps lead to more carrier interactions with dislocation cores. Conversely, a lower density of deep traps may result in a weaker photo-plastic effect, and the material could experience more pronounced softening due to less effective carrier trapping.
Additionally, the spatial distribution of deep levels across the bandgap plays a role. Deep traps located closer to the middle of the bandgap tend to have a more pronounced impact on carrier trapping, while traps closer to the conduction or valence bands may have less influence on dislocation dynamics, leading to less significant changes in hardness.
## Impact of External Bias and Illumination on Trap Behavior
The response of deep-level traps can be modulated by the applied electric bias and illumination conditions. For example, under an external electric field, the trapped carriers can be released from their trapping sites and drift toward the electrodes, altering the material's mechanical properties. If the illumination is continuous and the bias is applied, the combination of trapped carriers and the electrostatic field can dynamically change the dislocation behavior, either enhancing or reducing dislocation pinning.
In certain scenarios, under bias and illumination, the deep levels may act as dynamic pinning sites, where the effect of trapping is intensified, leading to hardening. Alternatively, if the carriers are sufficiently mobile and can recombine quickly, dislocations may be more mobile, resulting in softening.
## Summary
Deep levels in the bandgap of CdZnTe play a pivotal role in determining the sign of the photo-plastic effect by interacting with dislocation cores and influencing carrier dynamics. Trapped carriers at deep levels can either enhance dislocation pinning, leading to hardening, or recombine, allowing dislocations to become more mobile, resulting in softening. The balance between these two effects depends on the trap characteristics (depth, density, and distribution) and the external conditions such as bias and illumination. Thus, deep-level traps in CdZnTe act as key factors that govern the photo-plastic behavior of the material, either promoting hardness (positive photo-plastic effect) or leading to softening (negative photo-plastic effect), depending on the nature of carrier trapping and recombination processes.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/blog/what-role-do-deep-levels-in-the-bandgap-of-cdznte-play-in-determining-the-sign-of-the-photo-plastic-effect.html
