Materials science and engineering researchers at the University of Wisconsin-Madison have developed a new synthesis technique for creating extremely clean single crystalline thin films. Called hybrid pulsed-laser deposition, the method allows them to combine elements with large mismatches in their vapor pressures, an extremely challenging synthesis problem using previous techniques. This produces very high-quality materials, some that enable superconductivity and others that could be used in new battery materials, among other applications.
Led by Chang-Beom Eom, a professor of materials science and engineering at UW-Madison, the research appears in the May 24, 2024, issue of the journal Science Advances.
When certain crystalline materials are grown as extremely thin films—just a fraction of a nanometer thick—they can take on special magnetic and electrical properties, including superconducting states. One of these promising “quantum materials” is potassium tantalate, a metal-oxide crystal. When potassium tantalate is grown in a special direction called the 111 orientation and then layered with another oxide material, their interface displays impressive superconducting abilities.
Producing this type of interface requires a potassium tantalate thin film that is extremely uniform and defect free, or “clean.” However, potassium and tantalum have different vapor pressures—the temperature at which a material transitions into a gas. As a result, synthesizing a high-quality thin film that marries those two elements is challenging.
To get around the vapor pressure mismatch, Eom and his team combined two of the most common techniques for producing crystalline thin films: pulsed laser deposition and molecular beam epitaxy. By combining these disparate approaches, each optimized for one of the mismatched components, in the same vacuum chamber, the researchers could synthesize extremely high-quality potassium tantalate.
In pulsed laser deposition, a high-powered laser evaporates a target material, turning it into a mix of atoms and molecules. That “atomic flux” coats a substrate and creates a thin film of the material. This system is best for processing tantalum and other materials with low vapor pressures. In molecular beam epitaxy, the elements are evaporated in separate effusion cells, then deposited on a substrate using a low-energy beam of the evaporated atoms. This works best for high-vapor-pressure materials, including potassium.
Eom and his team combined the two systems, adding an effusion cell they constructed to a custom-designed pulsed laser deposition system. Their hybrid system allowed them to ablate the tantalum and evaporate the potassium simultaneously to combine them onto the same substrate in the vacuum chamber.
The team grew thin films of potassium tantalate on several different substrates, then used conventional pulsed laser deposition to deposit another oxide layer, lanthanum aluminate, on top of the potassium tantalate.
Microscopic analysis showed that the potassium tantalate did indeed include many fewer defects than films synthesized by other conventional methods. Further analysis showed that a two-dimensional “electron gas” created at the interface of the two thin films produced enhanced superconducting properties.
While Eom and his group are primarily interested in exploring superconducting materials using the new method, he says the synthesis technique could be important in other areas as well. “This method can used to combine other elements with large mismatched vapor pressures,” he says. “For example, it could be used to create compounds of lithium or sodium with other elements for solid state battery materials. We can apply variations of our approach to important energy materials or materials that have a large mismatch of vapor pressures.”
Chang-Beom Eom is the Raymond R. Holton Chair for Engineering and Theodore H. Geballe Professor in the Department of Materials Science and Engineering.
Other UW-Madison authors include Jieun Kim, Jung-Woo Lee, Pratap Pal, Kitae Eom, Neil Campbell and Mark Rzchowski.
Other authors include Muqing Yu, Ranjani Ramachandran and Jeremy Levy of the University of Pittsburgh and the Pittsburgh Quantum Institute; Shun-Li Shang and Zi-Kui Liu of Pennsylvania State University; Gi-Yeop Kim and Si-Young Choi of Pohang University of Science and Technology; and Jinsol Seo and Sang Ho Oh of Sungkyunkwan University, Suwon, South Korea and the Korea Institute of Energy Technology, Naju, South Korea.
The UW-Madison authors acknowledge support from the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) under award number DE-FG02-06ER46327.