A new manufacturing process could produce flexible electronics for things like virtual reality-enabled contact lenses, solar-powered skins that mold to the contours of your car, and electronic fabrics that respond to the weather.
The process, called “remote epitaxy,” involves growing thin films of semiconducting material on a large, thick wafer of the same material, which is covered in an intermediate layer of graphene. Once the film is grown, it can be peeled away from the graphene-covered wafer. The wafer, which can be expensive, can then be reused. In this way, the team can copy and peel away any number of thin, flexible semiconducting films, using the same underlying wafer.
The researchers said they can use remote epitaxy to produce freestanding stretchable films of any functional material. More importantly, they can stack films made from these different materials, to produce flexible, multifunctional electronic devices.
“You can use this technique to mix and match any semiconducting material to have new device functionality, in one flexible chip,” said Jeehwan Kim, associate professor of mechanical engineering at MIT.
Kim and his colleagues reported their first results using remote epitaxy in 2017. Then, they were able to produce thin, flexible films of semiconducting material by first placing a layer of graphene on a thick, expensive wafer made from a combination of exotic metals. They flowed atoms of each metal over the graphene-covered wafer and found the atoms formed a film on top of the graphene, in the same crystal pattern as the underlying wafer. The graphene provided a nonstick surface from which the researchers could peel away the new film, leaving the reusable graphene-covered wafer.
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, but not from group 4. The reason, they found, boiled down to polarity, or the respective charges between the atoms flowing over graphene and the atoms in the underlying wafer.
Since this realization, Kim and his colleagues have tried a number of increasingly exotic semiconducting combinations. As reported in their new paper, the team used remote epitaxy to make flexible semiconducting films from complex oxides — chemical compounds made from oxygen and at least two other elements. These are known to have a wide range of electrical and magnetic properties, some combinations of which can generate a current when physically stretched or exposed to a magnetic field.
Kim says the ability to manufacture flexible films of complex oxides could open the door to new energy-harvesting 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, millimeter-thick wafers, with limited flexibility and therefore limited energy-generating potential.
The researchers did have to tweak their process to make complex oxide films. They initially found that when they tried to make a complex oxide such as strontium titanate (a compound of strontium, titanium, and three oxygen atoms), the oxygen atoms that they flowed over the graphene tended to bind with the graphene’s carbon atoms, etching away bits of graphene instead of following the underlying wafer’s pattern and binding with strontium and titanium. As a surprisingly simple fix, the researchers added a second layer of graphene.
“We saw that by the time the first layer of graphene is etched off, oxide compounds have already formed, so elemental oxygen, once it forms these desired compounds, does not interact as heavily with graphene,” Kim said. “So, two layers of graphene buys some time for this compound to form.”
The team used their newly tweaked process to make films from multiple complex oxide materials, peeling off each 100-nanometer-thin layer as it was made. They were also able to stack layers of different complex oxide materials and effectively glue them together by heating them slightly, producing a flexible, multifunctional device.
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.
The results demonstrate that remote epitaxy can be used to make flexible electronics from a combination of materials with different functionalities, which previously were difficult to combine into one device. In the case of cobalt ferrite and PMN-PT, each material has a different crystalline pattern. Kim said that traditional epitaxy techniques, which grow materials at high temperatures on one wafer, can only combine materials if their crystalline patterns match. He said that with remote epitaxy, researchers can make any number of different films, using different, reusable wafers, and then stack them together, regardless of their crystalline pattern. “Now you can imagine a thin, flexible device made from layers that include a sensor, computing system, a battery, and 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.