Yu Zou (left) and Mingqiang Li (right) hold an atomic structure model of a crystal. Photo: Adrian So/University of Toronto Engineering.
Yu Zou (left) and Mingqiang Li (right) hold an atomic structure model of a crystal. Photo: Adrian So/University of Toronto Engineering.

An international team of researchers, led by Yu Zou, an engineering professor at the University of Toronto in Canada, has shown that electric fields can be used to control the motion of material defects in crystalline materials. This work has important implications for improving the properties and manufacturing processes of typically brittle ionic and covalent crystals, including semiconductors — crystalline materials that are a central component of the chips used in computers and other electronic devices.

In a paper in Nature Materials, researchers from University of Toronto Engineering, Dalhousie University in Canada, Iowa State University and Peking University in China present real-time observations of dislocation motion in single-crystalline zinc sulfide that was controlled with an external electric field.

“This research opens the possibility of regulating dislocation-related properties, such as mechanical, electrical, thermal and phase-transition properties, through using an electric field, rather than conventional methods,” says Mingqiang Li, a PhD candidate at University of Toronto Engineering and first author of the paper.

In materials science, a dislocation refers to a linear crystallographic defect within a crystal structure that contains an abrupt change in the arrangement of atoms. It is the most important defect in crystalline materials, says Zou, since it can affect the strength, ductility, toughness, and thermal and electrical conductivities of crystalline materials, from the steel used in airplanes to the silicon used in chips.

In crystalline solids, good ductility and formability are generally achieved by dislocation movement. As such, metals with highly mobile dislocations can be deformed into final products through compression, tension, rolling and forging.

In contrast, ionic and covalent crystals generally suffer from poor dislocation mobility that make them too brittle to process using mechanical methods, meaning they are unsuited to a broad range of manufacturing techniques. For example, semiconductors are typically too brittle to be rolled and forged.

“The primary driving force of dislocation motion has been commonly limited to mechanical stress, restricting the processing routes and engineering applications of many brittle crystalline materials,” says Zou.

“Our study provides direct evidence of dislocation dynamics controlled by a non-mechanical stimulus, which has been an open question since the 1960s. We also rule out other effects on the dislocation motion, including Joule heating, electron wind force and electron beam irradiation.”

The researchers used in-situ transmission electron microscopy to observe dislocation motion in zinc sulfide that was driven solely by an applied electric field in the absence of mechanical loading. Dislocations that carried negative or positive charges were both triggered by the electric field. 

The researchers observed these dislocations moving back and forth while they changed the direction of the electric field. They also found that the mobilities of the dislocations in an electric field depended on their dislocation type.

Since most semiconductors are brittle due to their poor dislocation mobility, the electric-field-controlled dislocation motion in this new study could be used to enhance their mechanical reliability and formability, says Li.

“In addition, our work offers an alternative method to reduce defect density in semiconductors, insulators and aged devices that doesn’t require traditional tedious thermal annealing, which uses temperature over time to reduce defects of a material,” he adds.

While this initial study focused on zinc sulfide, the team is planning to explore a wide range of materials, from covalent crystals to ionic crystals.

“As we work towards the application of this technology, our aim is to collaborate with the materials and manufacturing industries, particularly semiconductor companies, to develop a new manufacturing process to reduce defect density and improve the properties and performance of semiconductors,” says Zou.

This story is adapted from material from the University of Toronto Engineering, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.