Ultra-thin semiconducting films have been fabricated using exotic materials rather than silicone by MIT engineers for the manufacture of flexible electronics. They find that flexible electronics made from gallium nitride, gallium arsenide, and lithium fluoride exhibit better performance than if made from silicone, although costly. But now, thanks to a new technique called “remote epitaxy“, these exotic materials are becoming more affordable.
An extensive majority of today’s computing devices are made from silicone. Following oxygen, silicone is the second most abundant element on Earth, found in various forms in rocks, clay, sand, and soil, rendering it the most readily available component. That is why it is the dominant material for use in most electrical devices such as sensors, solar cells, and the integrated circuits within our computers and smartphones. Despite this fact, it is not the best semiconductor material existing on the planet.
Jeehwan Kim, Associate Professor at the Massachusetts Institute of Technology, and his colleagues devised a method to produce copies of expensive semiconducting materials by using graphene, an atomically thin sheet of carbon atoms arranged in a hexagonal, chicken-wire pattern.
Electrons flow through graphene with virtually no friction making it an extremely good conductor. On the other hand, it is very difficult to stop the flow of electrons making it a poor semiconductor. Because of this, Kim’s group focused on the materials mechanical features rather than its electrical properties and thus discovered a perfect use for it as a wafer “copy-machine”.
- Stacking graphene on top of gallium arsenic (or any pure, expensive wafer of semiconducting material of the like)
- Flowing atoms of gallium and arsenide over the stack to cause some sort of interaction with the underlying atomic layer through the intermediate graphene layer that becomes seemingly transparent
- Separate the copied film from the wafer once the crystalline pattern is transferred. In conclusion, an exact replica results from the atoms assembling into precisely the same single-crystalline pattern of the underlying semiconducting wafer.
MIT researchers have devised a way to grow a single crystalline compound semiconductor on its substrate through two-dimensional materials. The compound semiconductor thin film is then exfoliated by a flexible substrate, showing the rainbow color that comes from thin film interference.
Photo credits: Wei Kong and Kuan Qiao
What makes it different?
In conventional semiconductor manufacturing, the wafer is so strongly bonded to the semiconductor that it is almost impossible to separate without damaging both layers. Kim states, “you end up having to sacrifice the wafer – it becomes part of the device.” But now this duplicate can be peeled away from the graphene layer all thanks to its “slippery” nature. Graphene does not stick to other materials easily because it has very weak van der Waals forces. In other words, it doesn’t react to anything vertically. With this new technique, the wafer can be reused.
“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…We have paved the way for the manufacture of flexible electronic systems with so many different materials, other than silicon.” Plans to use this technique to make low-cost, high-performance devices such as flexible solar cells, computers, and portable sensors are in store for the future.
What could this mean for the future?
This manufacturing technique allows for the mass production of flexible electronic circuits. This technique could lead the way in creating flexible circuits for use within the body or on the skin as electronic tattoos and sensors that monitor our vital signs. The limits are almost endless.