NCSU, RTI Joint Research Solves Riddle of What Has Been Holding Two Unlikely Materials Together

RESEARCH TRIANGLE PARK, N.C. – For 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 state-of-the-art cooling devices for electronics. But while they knew it could be done, it was not clear how – because the atomic structures of those unlikely 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.

The findings were released in a paper, “Atomic scale structure and chemistry of Bi2Te3/GaAs interfaces grown by metallorganic van der Waals epitaxy,” published online in Applied Physics Letters. 

“We’ve used state-of-the-art technology to solve a mystery that has been around for years,” says James LeBeau, Ph.D., an assistant professor of materials science and engineering at North Carolina State University and co-author of the paper. “And 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. Ultimately, this could have applications in a wide range of electronic applications.”

To study the phenomenon, the researchers had to create the nanometer-scale thin films on a GaAs substrate, or foundation. The GaAs is first placed in a vapor deposition chamber. Molecules containing bismuth and tellurium are then introduced into the chamber, where they react with each other and ‘grow’ into a crystalline Bi2Te3 structure on the surface of the GaAs

“While these materials have been investigated previously by RTI and N.C. State, the state-of-the-art techniques applied by LeBeau and his team have revealed significant new insights into how the film grows,” notes Rama Venkatasubramanian, Ph.D., senior research director of the Center for Solid State Energetics at RTI and a co-author of the paper.

Using advanced ‘Super-X’ x-ray spectroscopy technology in conjunction with an aberration-corrected scanning transmission electron microscope, the researchers were able to determine what was binding the Bi2Te3 to the GaAs – and it was not what they were expecting.

What they determined was that when the tellurium species were introduced to the vapor deposition chamber, the tellurium reacts with the GaAs substrate to create a new surface layer of gallium telluride, which is only one molecule thick. The Bi2Te3 then forms 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 – meaning the materials are held together by weak electrical forces.

The research was supported by the Defense Advanced Research Projects Agency.