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Extraordinary plasticity of an inorganic semiconductor in darkness

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Science  18 May 2018:
Vol. 360, Issue 6390, pp. 772-774
DOI: 10.1126/science.aar6035

Plastic in the dark

Inorganic semiconductors, such as silicon and gallium arsenide, are brittle materials. This property means that large single crystals are cleaved into thin sheets. Oshima et al. show that zinc sulfide is, in contrast, a plastic material if deformed in total darkness. Plastic deformation is likely inhibited when light is present because photoexcited charge carriers become trapped at these sites and pin them through electrostatic effects.

Science, this issue p. 772

Abstract

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 εt = 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.

Development of shapeable high-strength materials has been essential to the improvement of advanced civilizations. Therefore, from an historical perspective, we have a broad interest in how materials deform and why materials exhibit failure. Inorganic semiconducting materials tend to fail in a brittle manner when subjected to an external force exceeding their fracture strength (1, 2). The brittleness is generally thought to originate from strong ionic and/or directional covalent bonds of inorganic semiconductors. However, easily shapeable strong and tough inorganic semiconductors are required as smart electronic components in a variety of electronic applications are becoming increasingly important (24). Because the poor mechanical properties of inorganic semiconductors limit their application range, it is of interest to understand how such materials deform and why they exhibit brittle failure. The plastic deformation properties of crystalline inorganic materials are controlled by motion of dislocations, which are topological line defects in crystals (5). Recently, dislocations have become of interest because they can be used to induce characteristic functional properties that are not found in bulk (610). On the other hand, fracture properties of crystalline materials are controlled by nucleation and extension of microcracks.

Electrons and holes can be excited in semiconducting materials when irradiated using light having the appropriate wavelength (11). These photoexcited electrons and holes will in turn affect the electrical properties of semiconducting materials, which then can exhibit electric conductivity. However, little is known about the influence of light irradiation on the brittle character of semiconducting materials. In particular, plasticity (12, 13) in complete darkness without light exposure has not been taken into consideration so far. This paper focused on the mechanical strength and fracture properties in complete darkness concerning the cubic form of sphalerite ZnS, a representative II-VI semiconductor. This is in part because large-size single crystals suitable for deformation tests are readily available for ZnS. ZnS is used as a luminescent material (14), an infrared optical material (15), and a photocatalyst (16) due to its electric and optical properties having a bandgap, Eg, in the range 3.6 to 3.7 eV (11, 17). We performed room-temperature deformation tests on single-crystal ZnS samples (fig. S1) under controlled light conditions, after which the deformation substructure was characterized using a combination of optical microscopy, conventional transmission electron microscopy (CTEM), and scanning transmission electron microscopy (STEM). The light-absorption properties were also evaluated using a spectrophotometer. (18) We found extraordinary plasticity of ZnS when deformed in complete darkness, as well as a drastic decrease in the optical bandgap.

Figure 1A shows stress-strain curves obtained during deformation tests under different conditions: white light (sample C), ultraviolet (UV) light (sample D), and in complete darkness (samples E, F, and G after varying deformation strain). Specimens fractured immediately after yielding during plastic deformation under white and UV light conditions. This behavior is expected because inorganic semiconductors are brittle. Conversely, it was found that specimens undergoing plastic deformation in complete darkness result in stable plastic deformation up to a deformation strain of εt = 45%, with limited work-hardening rates. In addition, the flow stress was smaller in samples deformed in complete darkness compared with samples deformed under white and UV light. (See supplementary text 1 and 2 with figs. S2 to S4 for other characteristics of deformation in complete darkness.) Thus, specimens exhibited extraordinary plasticity without fracture when deformed in complete darkness, despite the fact that inorganic semiconductors are brittle when deformed under regular light conditions. Figure 1, B to G, shows the shapes, surface morphologies, and colors of an undeformed specimen and the deformed specimens. It can be seen that distinct slip lines and an evidence of deformation twinning appear on the surfaces of the specimens deformed under white and UV light conditions (Fig. 1, C and D), whereas faint fine slip lines appear in samples deformed in complete darkness (Fig. 1, E to G). (See supplementary text 3 with fig. S5 for the detailed topographic structures of the surfaces.) The difference in the surface morphologies suggests that the deformation mechanism is different depending on the light condition. It was also found that the color of samples deformed in complete darkness gradually changed from colorless to orange as a function of the deformation strain, suggesting that the optical bandgap of the specimens depends on the deformation strain. The effect of light exposure on the fracture property is also demonstrated in Fig. 1H, showing a sample first deformed in complete darkness and, after about 10% deformation strain, being exposed to UV light, leading to brittle failure. It can be seen that the enhanced plasticity in complete darkness is easily reversed to the ordinary brittle behavior by deformation under UV light conditions.

Fig. 1 Characterizations of plastic deformation.

(A) Stress-strain curves of ZnS single crystals under white or UV light (365 nm) or in complete darkness. (B) An undeformed specimen. (C and D) The specimens deformed under (C) white light-emitting diode (LED) light and (D) UV LED light (365 nm). (E to G) The specimens deformed up to (E) εt = 11%, (F) εt = 25%, and (G) εt = 35% in complete darkness. (H) A stress-strain curve obtained by a deformation in complete darkness up to εt = 10% and the subsequent deformation under UV light.

Figure 2A shows the light-absorption characteristics of an undeformed specimen and specimens deformed to εt = 11, 25, and 35% plastic strain in complete darkness. Given that ZnS is a direct transition semiconductor (1, 17, 19), the optical bandgaps of the undeformed specimen and the specimen deformed up to εt = 35% plastic strain in complete darkness are estimated to be 3.52 eV and 2.92 eV, respectively. Figure 2B shows a shift in optical bandgap as a function of deformation strain. It is notable that the optical bandgap was lowered by 0.6 eV due to plastic deformation of εt = 35% in complete darkness. This shift could be due to dislocations introduced in the deformed specimens because the dislocation cores could have different band structures (2022) with respect to the dislocation-free region. In fact, density functional theory (DFT) calculations (Fig. 3, A and B) [see materials and methods 2 with figs. S6 and S7 (18)] showed that calculated bandgaps of the perfect crystal of ZnS and the dislocation-core region are 2.72 eV and 1.88 eV, respectively. The bandgap of the dislocation-core region was lower by 0.84 eV than that in the perfect crystal. This trend corresponds well to the observed shift of optical bandgap by plastic deformation, although underestimation of the bandgaps is a general feature of standard DFT calculations. The bandgap narrowing can be explained by formation of extra energy levels at the bandgap edge in the presence of dislocations. The further bandgap shift and the shape change in the bandgap edge with rising strain may arise from dislocation multiplication.

Fig. 2 Light-absorption characteristics.

(A) An undeformed specimen and the specimens deformed up to εt = 11%, 25%, and 35% in complete darkness. Here the α and E in the vertical axis of (αE)2 represent the absorption coefficient and the photon energy, respectively. (B) A shift in optical bandgap as a function of deformation strain.

Fig. 3 Characteristic band structure at the dislocation core.

(A) Density of states (DOS) around the valence band obtained by DFT calculations for the perfect bulk. (B) DOS for the dislocation-core region.

Figure 4, A and B, shows bright-field STEM images for the undeformed specimen and the specimen deformed in complete darkness up to a plastic strain of εt = 25%, respectively. It can be seen that the dislocation density before plastic deformation is considerably lower (less than 1 × 107 cm−2), whereas that of the deformed specimen at εt = 25% increased up to as large as 5 × 108 cm−2. Conventional electron diffraction contrast imaging (see supplementary text 4 with fig. S8) indicated that dislocations in the specimens have the Burgers vector of Embedded Image on the {111} primary slip plane. Because the dislocation density was further raised up to a plastic strain of εt = 35%, the plastic deformation in complete darkness is caused by glide and multiplication of dislocations belonging to the primary slip system. Also, since the grown-in dislocation substructure in undeformed specimens has a low density of dislocations, the shift in the optical bandgap results from the multiplied dislocations. In contrast, it was found that the specimens deformed under light irradiation accompany a number of twins (Fig. 4, C and D, and fig. S5, A to C), suggesting that the plastic deformation under light irradiation involves deformation twinning.

Fig. 4 Characterizations of microstructures in undeformed and deformed specimens.

(A and B) Typical bright-field STEM images of an undeformed specimen and the specimen deformed in complete darkness up to εt = 25%, respectively. The images in (A) and (B) were obtained using UHVEM (JEOL JEM-1000KRS, 1000 kV), which makes it possible to observe large areas of dislocation substructure at low magnification. (C) A bright-field TEM image of a twinning region in the specimen deformed up to εt = 2.0% under UV light. (D) A typical high-angle annular dark field STEM (HAADF-STEM) image of a crystal twin in the same specimen as in (C). The image in (D) was obtained using an atomic resolution electron microscope equipped with a spherical aberration corrector (JEOL JEM-ARM200F, 200 kV).

The different plastic deformation behaviors with or without light irradiation should be closely related to dislocation characters. Dislocations induced in ZnS in darkness dissociate into two partial dislocations (see supplementary text 4). Glide motion of a set of the two partials can bring about large slip deformations, as observed in darkness. In contrast, when one of the partials is much more mobile than the other, ZnS can undergo deformation twinning (23) (see supplementary text 5 with fig. S9).

As stated above, dislocation cores in ZnS have a smaller bandgap than the dislocation-free region, and accordingly electrons or holes excited by light irradiation can be trapped at extra energy levels around the bandgap edge of the dislocation cores. Consequently, the partial dislocations can be negatively or positively charged by electrons or holes. A charge state of each partial likely depends on its detailed atomic structure at the core (see supplementary text 5). Additionally, since motion of a charged dislocation corresponds to local charge transportation (5), the dislocation mobility may be limited by dragging of the surrounding charge cloud compensating the dislocation charge (24). Therefore, different charge states of the two partial dislocations in ZnS can cause a large difference in their mobilities, resulting in the observed deformation twinning. In some inorganic semiconductors, in fact, the hardness and flow deformation stress were reported to be influenced by photons (2531), the so-called photoplastic effect, but the previous studies were not aware of the extraordinary plasticity under the nonlighting condition.

In conclusion, it has been shown that cubic ZnS crystals deformed in complete darkness exhibit extraordinary plasticity even at room temperature. The reduced optical bandgap of the deformed crystals by 0.6 eV is thought to arise from the smaller bandgap at the dislocation core. Under light irradiation, on the other hand, the crystals immediately fractured after yielding. It is interesting to find out that the inorganic semiconductor can exhibit extraordinary plasticity when it deforms in complete darkness. This suggests that the mechanical strength and fracture properties in inorganic semiconductors may be controlled by exposure to light. Additionally, the behavior of dislocations plays a critical role in most synthesis and processing of crystalline materials—e.g., film synthesis and epitaxial crystal growth—and that such processes also could be affected by light exposure.

Supplementary Materials

www.sciencemag.org/content/360/6390/772/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S9

References (3248)

References and Notes

  1. Materials and methods are available as supplementary materials.
Acknowledgments: The authors thank Y. Kurokawa at Nagoya University for lending his expertise on the application of spectrophotometry. The authors acknowledge R. Nagahara and T. Yokoi for technical assistance with theoretical calculations. The authors also express their deep appreciation to K. P. D. Lagerlöf, K. Toyoura, and E. Tochigi for fruitful discussion on the effect of photons on the plastic properties of ZnS. STEM observations in this work were conducted at Nagoya University, supported by Nanotechnology Platform Program of MEXT, Japan. We are grateful to S. Arai for technical assistance with the ultra-high voltage electron microscopy (UHVEM) experiments. Funding: This work was mainly supported by Japan Society for the Promotion of Science (JSPS) KAKENHI grant numbers JP16K14414 and JP17H06094. A part of this study was supported by JSPS KAKENHI grant numbers JP15H04145, JP15K14122, and JP17K18983. A.N. also thanks Iketani Science and Technology Foundation for financial support (0281050-A). Author contributions: K.M. and A.N. conceived the research idea, and A.N. designed the experiments. Y.O. and A.N. performed the experiments and analyzed the data. K.M. gave advice about the experiments and performed DFT calculations. All the authors discussed the results and wrote the paper. Competing interests: Authors declare no competing interests. Data and materials availability: All data are available in the main text or the supplementary materials.
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