Ultrathin Complex Oxide Single-Crystal Layers Can Produce New Materials for Devices

The properties of complex oxides – magnetism, conductivity, and optics – make them the key to unlocking components that will be used in next-generation electronics for data storage, biomedical devices, sensing, energy, and other applications. And, by being able to stack ultrathin complex oxide single-crystal layers – ones that are composed of geometrically arranged atoms – researchers may be able to create new multifunctional structures with hybrid properties. 

Now, using a brand-new platform developed by engineers at the University of Wisconsin–Madison and the Massachusetts Institute of Technology, researchers will be able to make these stacked-crystal materials in limitless combinations. 

 

Thin films of metal oxides films that are peeled and stacked and designed to have unique magnetic and electronic properties. Image used courtesy of MIT.

 

MIT’s New Process: Remote Epitaxy

MIT’s new process, “remote epitaxy”, involves growing thin films of semiconducting material on a large, thick wafer of the same material, which itself is covered in an intermediate layer of graphene. Epitaxy is the process for depositing one material atop another in an orderly way. The researchers’ new layering method overcomes a challenge faced in conventional epitaxy – that each new oxide layer must be closely compatible with the atomic structure of the underlying layer. 

“The advantage of the conventional method is that you can grow a perfect single crystal on top of a substrate, but you have a limitation,” says Chang-Beom Eom, a UW–Madison professor of materials science and engineering and physics. “When you grow the next material, your structure has to be the same and your atomic spacing must be similar. That’s a constraint, and beyond that constraint, it doesn’t grow well.”

A few years ago, MIT researchers developed an alternative approach to the conventional method which was used as part of the new remote epitaxy process – the adding of an intermediate layer made of an ultrathin carbon material, graphene. This layer acts as a peel-away backing. 

When a semiconducting film has been grown, it can be peeled away from the wafer which can then be reused to grow another film. Using this method, an unlimited number of thin, flexible semiconducting films can be grown. 

As highlighted in their research, the scientists demonstrate that they are able to use remote epitaxy to produce freestanding films of any functional material. They can also stack films made from these different materials to produce flexible and multifunctional electronic devices. 

 

MIT researchers involved with the "peel and stack" method for stretchy electronics. From left to right: Kuan Qiao, Jeehwan Kim, Hyun S. Kum, Wei Kong, Sang-Hoon Bae, Jaewoo Shim, Sangho Lee, Chanyeol Choi. Image used courtesy of MIT.

 

Stretchy Electronic Films for VR and More

The MIT researchers believe that their process could eventually be used to produce stretchy electronic films for use in a variety of technologies, such as virtual reality-enabled contact lenses, solar-powered skins that contour to EVs and charge their batteries, and electronic fabrics that respond to changes in weather conditions. 

“You can use this technique to mix and match any semiconducting material to have new device functionality, in one flexible chip,” says Jeehwan Kim, an associate professor of mechanical engineering at MIT. “You can make electronics in any shape.”

“This advance, which would have been impossible using conventional thin film growth techniques, clears the way for nearly limitless possibilities in materials design,” says Evan Runnerstrom, program manager in materials design in the Army Research Office, which funded part of the research. “The ability to create perfect interfaces while coupling disparate classes of complex materials may enable entirely new behaviors and tunable properties, which could potentially be leveraged for new Army capabilities in communications, reconfigurable sensors, low power electronics, and quantum information science.

Kim and his colleagues’ results show that remote epitaxy can be used to make flexible electronics from a combination of different materials with different functionalities, which were previously difficult to combine into one device. 

“The big picture of this work is, you can combine totally different materials in one place together,” Kim says. “Now you can imagine a thin, flexible device made from layers that include a sensor, computing system, a battery, a solar cell, so you could have a flexible, self-powering, internet-of-things stacked chip.”

The team is exploring different combinations of semiconducting films and is working on developing prototype devices. One of these devices is what Kim refers to as an “electronic tattoo” – a flexible, transparent chip that can attach and conform to a person’s body and relay vital signs to an external device. 

“We can now make thin, flexible, wearable electronics with the highest functionality,” Kim added. “Just peel off and stack up.”