## Mechanism of Optical Transitions Affecting Mechanical Hardness in CdZnTe
Optical transitions between deep levels within the bandgap and the conduction or valence bands in CdZnTe fundamentally alter the electronic and electrostatic landscape surrounding crystal defects, especially dislocations. These transitions involve the absorption or emission of photons that promote electrons either from deep trap states to the conduction band (photoionization) or from the valence band to deep acceptor states, changing the charge state of defects. Such changes in charge state directly influence the pinning forces acting on dislocations and, consequently, the mechanical hardness of the material.
## Charge State Modulation of Dislocation Cores Through Optical Excitation
Dislocation cores in CdZnTe often contain or are surrounded by deep-level defects, which can trap carriers and carry localized charges. When optical transitions occur due to illumination with photons matching the energy difference between a deep level and a band edge, trapped electrons or holes can be excited out of these levels. This process changes the charge occupancy of the defect states:
* Photoionization of Deep Levels: Electrons bound in deep donor states may be excited to the conduction band, leaving behind positively charged centers. Conversely, electrons can be excited from the valence band to deep acceptor states, creating negatively charged centers.
* Recharging of Dislocation Cores: Such changes in defect charge states modify the local electrostatic potential around dislocations. For example, an increase in positively charged traps near a dislocation core enhances the Coulombic pinning of the dislocation, making it harder for it to move, thus increasing mechanical hardness.
* Conversely, if optical transitions neutralize charged traps (e.g., by filling acceptor states with electrons), the pinning force may decrease, making dislocations more mobile and softening the material.
## Impact on Dislocation Mobility and Plasticity
Dislocation motion is the main mechanism behind plastic deformation and mechanical softness in semiconductors. The electrostatic interaction between charged defects at dislocation cores and the surrounding lattice determines the energy barrier for dislocation glide. Optical excitation that alters the occupancy of deep levels modulates these barriers:
* Increased Pinning from Charged Defects: When optical transitions increase the number of charged traps, dislocations experience stronger pinning, raising the critical resolved shear stress required for their motion. This results in an increase in Vickers microhardness.
* Reduced Pinning from Neutralized Defects: If optical transitions lead to neutral defect states, the electrostatic pinning weakens, allowing easier dislocation glide and thus reducing hardness.
## Dynamic and Reversible Modulation by Illumination
The optical transitions between deep levels and bands enable a dynamic and reversible control of mechanical hardness. Upon illumination, carriers are excited, changing defect charge states and modulating hardness. When the illumination is removed, carriers recombine or re-trap, restoring original charge distributions and mechanical properties. This reversibility underpins the photo-plastic effect observed in CdZnTe, where hardness can be tuned by light.
## Spectral Dependence of Optical Transition Effects
Because deep levels are distributed at specific energies within the bandgap, only photons with energies matching the transition energies can induce these charge state changes. Illumination wavelengths in the near-infrared to visible range (e.g., around 870 nm for CdZnTe) efficiently promote transitions, causing significant modulation of hardness. Photons with energies below these thresholds fail to excite carriers from deep levels, producing negligible mechanical changes.
## Summary
Optical transitions between deep levels and the conduction or valence bands in CdZnTe modulate mechanical hardness by changing the charge states of defect centers near dislocation cores. These charge state changes alter the electrostatic pinning forces acting on dislocations, controlling their mobility and the material’s resistance to plastic deformation. Illumination induces reversible, wavelength-dependent hardness modulation through this mechanism, enabling dynamic tuning of mechanical properties via optical excitation.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/blog/how-can-optical-transitions-between-deep-levels-and-the-conduction-or-valence-bands-modulate-mechanical-hardness-in-cdznte.html
