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Flexible Avenue for Semiconductors: Staying in the Dark

17 May 2018
The brittle nature of zinc sulfide crystals (A) causes them to fracture after mechanical tests in ordinary light exposure (B). But in complete darkness, plastic deformation is possible (C). Source: Atsutomo Nakamura.

Here's a little Semiconductor 101: Inorganic materials such as silicon and gallium arsenide are considered semiconductors because they possess a tunable electrical conductivity in between that of a metal and that of an insulator. That conductivity is controlled by the energy difference between the material’s valance and conduction bands, also known as its band gap. A narrower gap makes it easier for electrons to move across, resulting in an increased conductivity.

That's all well and good for yesterday’s electronics, but what about the flexible electronics of tomorrow? The brittle nature of inorganic semiconductors limits their application range. Yet, a group of researchers at Nagoya University recently discovered that a representative inorganic semiconductor — crystals of zinc sulfide, or ZnS — is brittle when exposed to light but flexible when kept in the dark.

Room-temperature deformation tests performed under varying light conditions revealed that the crystals immediately fractured under light irradiation. But in complete darkness, they could be plastically deformed. "The influence of complete darkness on the mechanical properties of inorganic semiconductors had not previously been investigated," said study coauthor Atsutomo Nakamura. "We found that ZnS crystals in complete darkness displayed much higher plasticity than those under light exposure."

The team attributed the increased plasticity to the high mobility of dislocations — a type of defect found in crystals, known to influence their properties — in complete darkness. In addition, the deformation of the crystals in darkness appears to lead to a decrease in band gap — and, as a result, an increase in conductivity.

"This study reveals the sensitivity of the mechanical properties of inorganic semiconductors to light," said study coauthor Katsuyuki Matsunaga. "Our findings may allow development of technology to engineer crystals through controlled light exposure."

Indeed, the researchers' results suggest that the strength, brittleness and conductivity of inorganic semiconductors may be regulated by light exposure — opening an interesting avenue to optimize their performance. The researchers concluded that "inorganic semiconductors are not necessarily intrinsically brittle."

The research appears in the May 18, 2018, issue of Science.

To contact the author of this article, email [email protected]


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