Uses an electric field to control the movement of defects in the crystal
(Nanowerk News) An international research team, led by Professor Yu Zou (MSE), uses electric fields to control the movement of material defects. This work has important implications for improving the properties and processes for manufacturing brittle ionic and covalent crystals, including semiconductors – crystalline materials that are key components of electronic chips used for computers and other modern devices.
In a study published in Natural Ingredients (“Utilizing dislocation motion using an electric field”), researchers from U of T Engineering, Dalhousie University, Iowa State University and Peking University, presented real-time observations of dislocation motions in single crystalline zinc sulfide controlled using an external electric field.
“This research opens up the possibility to regulate dislocation-related properties, such as mechanical, electrical, thermal, and phase transition properties, through the use of electric fields, rather than conventional methods” says Mingqiang Li (PhD candidate MSE), first author of the new paper. .
In materials science, a dislocation is a linear crystallographic defect in a crystal structure that contains sudden changes in the arrangement of atoms.
This is the most important defect in crystalline materials, said Zou, because it can affect the strength, ductility, toughness, thermal and electrical conductivity of crystalline materials, such as the steel used in airplanes and the silicon used in chips.
In crystalline solids, good ductility and formability are generally achieved by dislocation motion. Thus, metals that have highly mobile dislocations can be deformed into final products by compression, tension, rolling, and forging — for example, aluminum cans are punched to shape.
In contrast, ionic and covalent crystals generally suffer from poor dislocation mobility which makes them too brittle to be processed using mechanical methods, and consequently, unsuitable for various manufacturing techniques. In the case of semiconductors, they are usually too brittle to be rolled and hammered.
“The main driving force for dislocation motion is generally limited to mechanical stress, limiting the processing routes and engineering applications of many brittle crystalline materials,” said Zou.
“Our study provides direct evidence of dislocation dynamics controlled by non-mechanical stimuli, which has been an open question since the 1960s. We also ruled out other effects on dislocation motion, including Joule heating, electron wind forces, and electron beam irradiation.”
This study used an in-situ transmission electron microscope to observe dislocation motion in zinc sulfide, which is driven only by an applied electric field in the absence of any mechanical load. Dislocations that carry a negative or positive charge are both triggered by an electric field.
The researchers observed the dislocations moving back and forth while changing the direction of the electric field. They also found that the mobility of dislocations in an electric field also depends on the type of dislocation.
Since most semiconductors are brittle due to poor dislocation mobility, the electric field-controlled dislocation motion in this new study can be used to improve their reliability and mechanical formability, Li said.
“In addition, our work offers an alternative method of reducing defect density in semiconductors, insulators, and legacy devices that do not require traditional tedious thermal annealing, which uses temperature over time to reduce material defects,” he added.
While this initial study focused on zinc sulfide, the team plans to explore a wide range of materials, from covalent crystals to ionic crystals.
“As we seek to implement this technology, our goal is to collaborate with the materials and manufacturing industries, especially semiconductor companies, to develop new manufacturing processes to reduce defect density and improve semiconductor properties and performance,” said Zou.
