New study shows how electric fields can be used to improve the properties of crystalline materials

An international team of scientists, led by Professor Yu Zou from the University of Toronto Engineering, has made significant progress in controlling material defects using electric fields. This breakthrough has far-reaching implications for enhancing the properties and manufacturing processes of brittle ionic and covalent crystals, including semiconductors, which are vital components of electronic chips used in various modern devices.

Their groundbreaking research, published in the prestigious journal Nature Materials, showcases real-time observations of dislocation motion within a single-crystalline zinc sulfide. The team achieved precise control over these dislocations by applying an external electric field.

Ph.D. candidate Mingqiang Li, the lead author of the study, explains the significance of their findings, stating that this research opens up avenues for regulating various dislocation-related properties such as mechanical strength, electrical conductivity, thermal conductivity, and phase transitions. This can be accomplished by employing electric fields instead of conventional methods.

In the field of materials science, dislocation refers to a linear defect within the crystal structure of a material, characterized by an abrupt change in the arrangement of atoms. Zou emphasizes that dislocations are the most crucial defects in crystalline materials since they can significantly impact properties such as strength, ductility, toughness, thermal conductivity, and electrical conductivity. For instance, dislocations can affect the performance of steel used in aircraft or silicon used in computer chips.

A real-time observation of an individual dislocation moving under an electric field using transmission electron microscopy. The dislocation moves back and forth depending on the magnitude and direction of the electric field. Credit: Zou Research Group / University of Toronto Engineering

Dislocation movement plays a crucial role in achieving ductility and formability in crystalline solids, particularly metals. Metals with highly mobile dislocations can be easily deformed through processes like compression, tension, rolling, and forging. This flexibility allows for the production of various shapes, such as aluminum cans formed through punching.

On the other hand, ionic and covalent crystals tend to have poor dislocation mobility, making them brittle and unsuitable for mechanical processing methods. This limitation restricts their application in a wide range of manufacturing techniques. Semiconductors, for example, are often too brittle to undergo rolling and forging processes.

Professor Zou highlights that the primary driving force for dislocation motion has traditionally been limited to mechanical stress, which imposes constraints on the processing and engineering applications of many brittle crystalline materials. However, their study presents groundbreaking evidence of dislocation dynamics controlled by a non-mechanical stimulus, addressing a question that has remained unanswered since the 1960s. The researchers also ruled out other factors affecting dislocation motion, such as Joule heating, electron wind force, and electron beam irradiation.

To observe dislocation motion, the study employed in-situ transmission electron microscopy on zinc sulfide crystals. The researchers successfully demonstrated that dislocations could be induced solely by an applied electric field, without mechanical loading. Dislocations carrying negative or positive charges were both observed to respond to the electric field.

The team observed dislocations moving back and forth while altering the direction of the electric field. Furthermore, they discovered that the mobility of dislocations in an electric field depends on their specific dislocation types.

Given that most semiconductors exhibit poor dislocation mobility, the electric-field-controlled dislocation motion described in this study has the potential to enhance their mechanical reliability and formability. Additionally, the research offers an alternative method to reduce defect density in semiconductors, insulators, and aged devices, eliminating the need for traditional thermal annealing processes that rely on temperature and time to reduce defects.

While the study primarily focused on zinc sulfide, the research team plans to extend their investigations to a wide range of materials, including covalent and ionic crystals.

Professor Zou emphasizes the importance of collaboration with the materials and manufacturing industries, particularly semiconductor companies, to develop new manufacturing processes that reduce defect density and improve the properties and performance of semiconductors, as they continue to advance this technology.

Source: University of Toronto

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