Researchers demonstrate ultra-flexible heterogeneous electronics
Remote epitaxy uses a thick wafer substrate as the template for growing thin films, albeit separated by an intermediate layer of graphene which makes it easy to peel off the newly grown epitaxial layer. It also keeps the original substrate wafer intact and re-usable for another remote epitaxy, drastically saving on materials and chip costs compared to traditional epitaxial processes where the substrate remains as part of the chip.
The novel idea demonstrated in a Nature paper titled “Heterogeneous integration of single-crystalline complex-oxide membranes” is to use remote epitaxy to produce free-standing films of any functional material which can be stacked closely into heterogeneous multifunctional electronic devices otherwise impossible to achieve due to lattice mismatches and inherent epitaxial challenges.
Stacking the layers also hybridizes their physical properties, the researchers found, and because the films are less than a micrometre thick, the whole stacks remain highly flexible, which could make them good candidates for numerous freeform applications including solar-powered skins or conformable wearables.
“You can use this technique to mix and match any semiconducting material to have new device functionality, in one flexible chip,” explains Jeehwan Kim, an associate professor of mechanical engineering at MIT. “You can make electronics in any shape.”
In 2018, the team showed that they could use remote epitaxy to make semiconducting materials from metals in groups 3 and 5 of the periodic table. Since then, they experimented with a number of increasingly exotic semiconducting combinations, including complex oxides with a wide range of electrical and magnetic properties. Some combinations can generate a current when physically stretched (piezoelectricity) or exposed to a magnetic field (magnetostriction).
Kim says the ability to manufacture flexible films of complex oxides could open the door to new energy-havesting devices, such as sheets or coverings that stretch in response to vibrations and produce electricity as a result. Until now, complex oxide materials have only been manufactured on rigid, millimetre-thick wafers, with limited flexibility and therefore limited energy-generating potential.
In their paper, the authors report the remote epitaxial growth of multiple complex oxide materials, peeling off each 100-nanometer-thin layer as it was made before stacking them together and bonding them through mild heating (Van der Waals forces ensuring atomic-level bonding).
“This is the first demonstration of stacking multiple nanometres-thin membranes like LEGO blocks, which has been impossible because all functional electronic materials exist in a thick wafer form,” Kim notes.
In one experiment, the team stacked together films of two different complex oxides: cobalt ferrite, known to expand in the presence of a magnetic field, and PMN-PT, a material that generates voltage when stretched. When the researchers exposed the multilayer film to a magnetic field, the two layers worked together to both expand and produce a small electric current. In the case of cobalt ferrite and PMN-PT, each material has a different crystalline pattern.
“The big picture of this work is, you can combine totally different materials in one place together,” Kim explains. “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 various combinations of semiconducting films and is working on developing prototype devices, such as something Kim is calling an “electronic tattoo” — a flexible, transparent chip that can attach and conform to a person’s body to sense and wirelessly relay vital signs such as temperature and pulse.
The research was the outcome of close collaboration between the researchers at MIT and at the University of Wisconsin at Madison, which was supported by the Defense Advanced Research Projects Agency.
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