Direct evidence and atomic-scale mechanisms of reduced dislocation mobility in an inorganic semiconductor under illumination

Photo-plasticity in semiconductors, wherein their mechanical properties such as strength, hardness and ductility are influenced by light exposure, has been reported for several decades. Although such phenomena have drawn significant attention for the manufacturability and usage of deformable semiconductor devices, their underlying mechanisms are not well understood due to the lack of direct evidence. Here we provide experimental observation and atomic insights into the reduced mobility of dislocations in zinc sulfide, as a model material, under light. Using photo-nanoindentation and transmission electron microscopy, we observe that dislocations glide shorter distances under light than those in darkness and there are no apparent deformation twins in both conditions. By atomic-scale simulations, we demonstrate that the decreased dislocation mobility is attributed to the increased Peierls stress for dislocation motion and enhanced stress fields around dislocation cores due to photoexcitation. This study improves the understanding of photo-plastic effects in inorganic semiconductors, offering the opportunities for modulating their mechanical properties using light.

💡 Research Summary

This paper investigates the phenomenon of photo‑plasticity in the inorganic semiconductor zinc sulfide (ZnS) by directly observing and modeling the reduced mobility of dislocations under illumination. Using a custom photo‑nanoindentation setup, the authors performed indentations on single‑crystal ZnS both in darkness and under UV illumination (365 nm, corresponding to the material’s band‑gap). Load‑depth curves reveal a clear hardening effect: hardness increases from 2.49 GPa in the dark to 2.99 GPa under light, and the elastic modulus rises from 87.6 GPa to 97.3 GPa. Multiple wavelengths and intensities were tested, confirming that the maximum effect occurs at 365 nm where absorption is strongest.

Transmission electron microscopy (TEM) of focused‑ion‑beam (FIB) cross‑sections shows that dislocation density beneath the indents is markedly lower under illumination—approximately 59 % less at the deepest region—indicating that dislocations glide shorter distances when the sample is illuminated. Selected‑area electron diffraction (SAED) patterns demonstrate that dark‑indented regions retain a perfect single‑crystal pattern, whereas light‑indented regions exhibit spot splitting, reflecting lattice rotation caused by a high density of geometrically necessary dislocations (GNDs). The authors focus on the 30° S partial dislocation, which is the most mobile component in ZnS, while the 90° Zn partial remains essentially immobile due to a higher glide barrier.

First‑principles density functional theory (DFT) calculations reveal that photo‑excited electron‑hole pairs modify the atomic configuration of the 30° S core, notably altering Zn‑S bond angles. This structural change raises the Peierls stress—the intrinsic lattice resistance to dislocation glide—from 1.38 GPa (ground state) to 1.64 GPa (excited state), a ~19 % increase. To bridge experiment and theory, large‑scale molecular dynamics (MD) simulations were performed on a half‑million‑atom model containing both 30° S and 30° Zn partials. Under identical shear strain rates, the stress‑time curves show that dislocation motion initiates at higher shear stress in the excited state, consistent with the measured hardening. The glide distance at 400 ps is reduced from 2687 Å (dark) to 2579 Å (light), matching the TEM observation of shorter dislocation tracks.

Beyond single‑dislocation behavior, the authors examine the stress fields surrounding both partials. The absolute value of the xz component of the stress tensor is larger around the cores in the excited state, indicating stronger elastic interactions between neighboring dislocations. Enhanced interactions promote the formation of dislocation junctions and increase line roughness, further elevating the critical resolved shear stress and contributing to work‑hardening. This explains why, despite a lower overall dislocation density under light, the material exhibits higher hardness.

The paper also discusses a reversible “after‑effect”: after switching off the light, hardness and internal stress gradually relax back to dark‑state values over several hundred seconds, reflecting the recombination lifetime of photo‑generated carriers. The authors model this by explicitly inserting electron‑hole pairs into the semiconductor lattice, noting that the 30° S core carries a net negative charge while the 30° Zn core is positively charged; thus, photo‑generated holes preferentially trap at the S‑partial, further stabilizing it and raising the glide barrier.

In summary, the study provides the first direct experimental evidence that illumination reduces dislocation mobility in an inorganic semiconductor, and it elucidates two atomic‑scale mechanisms: (1) an increase in Peierls stress due to photo‑induced core reconstruction, and (2) amplified stress fields around dislocation cores that strengthen dislocation‑dislocation interactions. These findings open avenues for tailoring mechanical properties of semiconductor devices with light, suggesting potential applications in stress‑controlled processing, light‑tunable hard coatings, and opto‑mechanically responsive electronic components.