Researchers put materials in a bind

For a number of years, researchers have developed thin films of bismuth telluride (Bi2Te3) – which converts heat into electricity or electricity to cooling – on top of gallium arsenide (GaAs) to create cooling devices for electronics.

However, whilst they knew it could be done it was not clear how because the atomic structures of those pair of materials do not appear to be compatible.

Now researchers from North Carolina State University and RTI International have solved the mystery, opening the door to new research in the field.

‘Now that we know what is going on, we can pursue research to fine-tune the interface of these materials to develop more efficient mechanisms for converting electricity to cooling or heat into electricity,’ said Dr James LeBeau, an assistant professor of materials science and engineering at NC State and co-author of a paper on the research.

According to a statement, researchers have found that the two materials are separated by a thin layer of a third material.

To study the phenomenon, the researchers had to create the nanometre-scale thin films on a GaAs substrate.

The GaAs is first placed in a vapour deposition chamber. Molecules containing bismuth and tellurium are then introduced into the chamber, where they react with each other and develop into a crystalline Bi2Te3 structure on the surface of the GaAs.

Using advanced ‘Super-X’ X-ray spectroscopy technology in conjunction with an aberration-corrected scanning transmission electron microscope, the researchers were reportedly able to determine what was binding the Bi2Te3 to the GaAs.

They found that when the tellurium molecules were introduced to the vapour deposition chamber, the tellurium reacted with the GaAs substrate to create a new surface layer of gallium telluride, which was one molecule thick. The Bi2Te3 then formed a thin film on top of that new surface layer.

Because gallium telluride does not react with Bi2Te3, the research team knew chemical bonding could not be holding them together. Instead, the two layers are held together by the weaker force of van der Waals bonds.

‘While these materials have been investigated previously by RTI and NC State, the…techniques applied by LeBeau and his team have revealed significant new insights into how the film grows,’ said Dr Rama Venkatasubramanian of RTI International, who is also a co-author of the paper.

The paper, ‘Atomic scale structure and chemistry of Bi2Te3/GaAs interfaces grown by metallorganic van der Waals epitaxy,’ is published online in Applied Physics Letters.