Amorphous silicon is indeed a glass-until you add hydrogen

Using scanning tunneling microscopy, researchers at the University of Illinois at Urbana-Champaign (Urbana, IL) have observed the subnanometer structure of amorphous silicon (a-Si) and found that it is indeed a glass—until hydrogen is added. The finding expands knowledge of the material properties of silicon that could be important to future solar cell and optoelectronic component development.

Unlike the crystalline form of silicon, a-Si has a random, glassy structure well-suited to flexible thin-film solar cells; however, it cannot be manufactured like a glass by rapidly cooling it in its melted state and instead is fabricated through ion implantation or chemical-vapor deposition techniques. For the first time, the researchers were able to observe two-state dynamics in a-Si—a classic characteristic of glass in which clusters of silicon atoms “hop” between two positions forming a semistable structure. And although hydrogen is typically added to a-Si to increase stability and improve performance, it was long assumed that hydrogen had no effect on its random structure. But the imaging also showed that adding hydrogen robs a-Si of its two-state dynamics, inducing crystallization such that a-Si is not completely amorphous; essentially, a-Si shares some—but not all—features of a glass material.

Contact Martin Gruebele atmgruebel@illinois.edu.

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