This image shows LEDs grown on graphene and then peeled. Photo courtesy of the researchers.In 2016, annual global semiconductor sales reached their highest-ever point: $339 billion. In that same year, the semiconductor industry spent about $7.2 billion worldwide on wafers that serve as the substrates for microelectronics components that can be turned into transistors, light-emitting diodes, and other electronic and photonic devices.
A new technique developed by engineers at Massachusetts Institute of Technology (MIT) could vastly reduce the overall cost of wafer technology and permit devices made from more exotic, higher-performing semiconductor materials than conventional silicon. The new method, reported in a paper in Nature, uses graphene – single-atom-thin sheets of graphite – as a sort of ‘copy machine’ to transfer intricate crystalline patterns from an underlying semiconductor wafer to a top layer of identical material.
The engineers worked out carefully controlled procedures for placing single sheets of graphene onto an expensive wafer, and then grew semiconducting material over the graphene layer. They found that graphene is thin enough to appear electrically invisible. This allows the top layer to see through the graphene to the underlying crystalline wafer, which can imprint its patterns into the top layer without being influenced by the graphene. Graphene is also rather ‘slippery’ and does not tend to stick to other materials easily, allowing the engineers to simply peel the top semiconducting layer from the wafer after its structures have been imprinted.
In conventional semiconductor manufacturing, the wafer, once its crystalline pattern is transferred, is so strongly bonded to the semiconductor that it is almost impossible to separate without damaging both layers. "You end up having to sacrifice the wafer – it becomes part of the device," says Jeehwan Kim, assistant professor in the departments of Mechanical Engineering and Materials Science and Engineering at MIT.
With the group's new technique, Kim says manufacturers can now use graphene as an intermediate layer, allowing them to copy and paste the wafer, separate the copied film from the wafer, and reuse the wafer many times over. In addition to saving on the cost of wafers, this opens opportunities for exploring more exotic semiconductor materials.
"The industry has been stuck on silicon, and even though we've known about better performing semiconductors, we haven't been able to use them, because of their cost," Kim says. "This gives the industry freedom in choosing semiconductor materials by performance and not cost."
Since graphene's discovery in 2004, researchers have been investigating its exceptional electrical properties in hopes of improving the performance and cost of electronic devices. Graphene is an extremely good conductor of electricity, as electrons flow through it with virtually no friction. Researchers, therefore, have been intent on finding ways to adapt graphene as a cheap, high-performance semiconducting material.
"People were so hopeful that we might make really fast electronic devices from graphene," Kim says. "But it turns out it's really hard to make a good graphene transistor."
In order for a transistor to work, it must be able to turn a flow of electrons on and off, to generate a pattern of ones and zeros that instruct a device in how to carry out a set of computations. As it happens, it is very hard to stop the flow of electrons through graphene, making it an excellent conductor but a poor semiconductor.
Kim's group took an entirely new approach to using graphene in semiconductors. Instead of focusing on graphene's electrical properties, the researchers looked at the material's mechanical features.
"We've had a strong belief in graphene, because it is a very robust, ultrathin material and forms very strong covalent bonding between its atoms in the horizontal direction," Kim says. "Interestingly, it has very weak Van der Waals forces, meaning it doesn't react with anything vertically, which makes graphene's surface very slippery."
The team now reports that graphene, with its ultrathin, Teflon-like properties, can be sandwiched between a wafer and its semiconducting layer, providing a barely perceptible, non-stick surface through which the semiconducting material's atoms can still rearrange in the pattern of the wafer's crystals. The material, once imprinted, can simply be peeled off the graphene surface, allowing manufacturers to reuse the original wafer.
The team found that its technique, which they term ‘remote epitaxy’, was successful in copying and peeling off layers of semiconductors from the same semiconductor wafers. The researchers had success in applying their technique to some exotic wafer and semiconducting materials, including indium phosphide, gallium arsenenide and gallium phosphide – materials that are 50 to 100 times more expensive than silicon.
Kim says that this new technique makes it possible for manufacturers to reuse wafers – of silicon and higher-performing materials – "conceptually, ad infinitum".
The group's graphene-based peel-off technique may also advance the field of flexible electronics. In general, wafers are very rigid, making the devices they are fused to similarly inflexible. Kim says that with their new technique semiconductor devices such as LEDs and solar cells can be made to bend and twist. In fact, the group demonstrated this possibility by fabricating a flexible LED display, patterned in the MIT logo, using their technique.
"Let's say you want to install solar cells on your car, which is not completely flat – the body has curves," Kim says. "Can you coat your semiconductor on top of it? It's impossible now, because it sticks to the thick wafer. Now, we can peel off, bend, and you can do conformal coating on cars, and even clothing."
Going forward, the researchers plan to design a reusable ‘mother wafer’ with regions made from different exotic materials. Using graphene as an intermediary, they hope to create multifunctional, high-performance devices. They are also experimenting with mixing and matching various semiconductors and stacking them up as a multi-material structure.
"Now, exotic materials can be popular to use," Kim says. "You don't have to worry about the cost of the wafer. Let us give you the copy machine. You can grow your semiconductor device, peel it off, and reuse the wafer."
This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.