First observation of atomic diffusion inside bulk material

"This is the first time that anyone has directly imaged single dopant atoms moving around inside a material," said Rohan Mishra of Vanderbilt University who is also a visiting scientist in ORNL's Materials Science and Technology Division.

Semiconductors, which form the basis of modern electronics, are "doped" by adding a small number of impure atoms to tune their properties for specific applications. The study of the dopant atoms and how they move or "diffuse" inside a host lattice is a fundamental issue in materials research.

Traditionally, diffusion of atoms has been studied through indirect macroscopic methods or through theoretical calculations. Diffusion of single atoms has previously been directly observed only on the surface of materials.

The experiment also allowed the researchers to test a surprising prediction: Theory-based calculations for dopant motion in aluminum nitride predicted faster diffusion for cerium atoms than for manganese atoms. This prediction is surprising as cerium atoms are larger than manganese atoms.

"It's completely counterintuitive that a bigger, heavier atom would move faster than a smaller, lighter atom," said the Material Science and Technology Division's Andrew Lupini, a coauthor of the paper.

In the study, the researchers used a scanning transmission electron microscope to observe the diffusion processes of cerium and manganese dopant atoms. The images they captured showed that the larger cerium atoms readily diffused through the material, while the smaller manganese atoms remained fixed in place.

The team's work could be directly applied in basic material design and technologies such as energy-saving LED lights where dopants can affect color and atom movement can determine the failure modes.

"Diffusion governs how dopants get inside a material and how they move," said Lupini. "Our study gives a strategy for choosing which dopants will lead to a longer device lifetime."

The study was funded by the DOE Office of Science, the Australian Research Council, Vanderbilt University and the Japan Society for the Promotion of Science Postdoctoral Fellowship for research abroad.

The project's authors include Ryo Ishikawa of Oak Ridge National Laboratory and the University of Tokyo; Scott Findlay of Monash University; Takashi Taniguchi of the National Institute for Materials Science; Sokrates Pantelides of Oak Ridge National Laboratory and Vanderbilt University; and Stephen Pennycook of the University of Tennessee.