STEM captures elusive atomic motion
Image: Using STEM to capture diffusion in thin films [Linköping Universitet]
Researchers at Sweden-based Linköping University and the University of California in Berkeley, US, have observed the migration of atoms between the layers of a thin film.
Using high resolution scanning transmission electron microscopy, they imaged the positions of individual atoms in the material.
The specimen they studied was a thin film in which layers of a metal, hafnium nitride (HfN), around 5 nm thick, alternate with layers of a semiconductor, scandium nitride (ScN).
According to the researchers, the properties of the HfN/ScN layers make this material a suitable candidate for use in, for example, coating technology and microelectronics.
As they point out, it is very important that the layers of metal and semiconductor do not mix, as problems arise if the atoms diffuse across an interlayer forming a closed bridge between the layers in the film, similar to an electric short circuit.
"The material we have studied acts as a perfect model system, but this type of diffusion occurs in nearly all materials, which is why it is important that materials scientists understand this type of diffusion," says Professor Magnus Garbrecht from the Department of Physics, Chemistry and Biology at Linköping University.
The discovery came about when Garbrecht heated HfN/ScN to 950°C, and he noticed that the hafnium was diffusing down into the underlying layers.
Further analysis revealed that a defect was present in the material where this phenomenon arose.
The researchers heated the material several times and subsequently examined it using STEM and measured how far individual atoms moved.
"The values we measured agree well with those from previous experiments using indirect methods and with the theoretical models, and this makes us confident that what we are seeing really is dislocation-pipe diffusion," says Garbrecht.
The researchers believe the atoms diffuse when the material is heated as individual atoms are slightly displaced relative to each other in the regions around the linear defects.
The atoms tend to arrange themselves in a perfect cubic symmetry, and strain builds up within the lattice when this arrangement is disturbed.
The researchers show in the study that this strain relaxes as the atoms diffuse.
"The diffusion reduces the strain in the material and this is why it only occurs along the linear defects threading through the material," says Garbrecht.
Research is published in Scientific Reports.