Inorganic semiconductors generally tend to fail in a brittle manner. Here, we report that extraordinary "plasticity" can take place in an inorganic semiconductor if the deformation is carried out "in complete darkness." Room-temperature deformation tests of zinc sulfide (ZnS) were performed under varying light conditions. ZnS crystals immediately fractured when they deformed under light irradiation. In contrast, it was found that ZnS crystals can be plastically deformed up to a deformation strain of ε = 45% in complete darkness. In addition, the optical bandgap of the deformed ZnS crystals was distinctly decreased after deformation. These results suggest that dislocations in ZnS become mobile in complete darkness and that multiplied dislocations can affect the optical bandgap over the whole crystal. Inorganic semiconductors are not necessarily intrinsically brittle.

It was recently found that extremely large plasticity is exhibited in bulk compression of single-crystal ZnS in complete darkness. Such effects are believed to be caused by the interactions between dislocations and photoexcited electrons and/or holes. However, methods for evaluating dislocation behavior in such semiconductors with small dimensions under a particular light condition had not been well established. Here, we propose the “photoindentation” technique to solve this issue by combining nanoscale indentation tests with a fully controlled lighting system. The quantitative data analyses based on this photoindentation approach successfully demonstrate that the first pop-in stress indicating dislocation nucleation near the surface of ZnS clearly increases by light irradiation. Additionally, the room-temperature indentation creep tests show a drastic reduction of the dislocation mobility under light. Our approach demonstrates great potential in understanding the light effects on dislocation nucleation and mobility at the nanoscale, as most advanced technology-related semiconductors are limited in dimensions.

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An enormous change in the dislocation-mediated plasticity has been found in a bulk semiconductor that exhibits the photoplastic effect. Herein, we report that UV (365 nm) light irradiation during mechanical testing dramatically decreases the fracture toughness of ZnS. The crack tip toughness on a (001) single-crystal ZnS, as measured by the near-tip crack opening displacement method, is increased by ∼45% in complete darkness compared to that in UV light. The increase in fracture toughness is attributed to a significant increase in the dislocation mobility in darkness, as explained by the crack tip dislocation shielding model. Our finding suggests a route toward controlling the fracture toughness of photoplastic semiconductors by tuning the light irradiation.