Smaller, better semiconductors have consistently allowed computers to become faster and more energy-efficient than ever before.
But the 18-month cycle of exponential increases in computing power that has held since the mid 1960s now has leveled off. That’s because there are fundamental limits to integrated circuits made strictly from silicon—the material that forms the backbone of our modern computer infrastructure.
As they look to the future, however, engineers at the University of Wisconsin-Madison are turning to new materials to lay down the foundations for more powerful computers.
They have devised a method to grow tiny ribbons of graphene—the single-atom-thick carbon compound—directly on top of silicon wafers.
Graphene ribbons have a special advantage over the material when it’s in its more common form of a broad, flat sheet; namely, thin strips of graphene become excellent semiconductors.
“Compared to current technology, this could enable faster, low power devices,” says Vivek Saraswat, a PhD student in materials science and engineering at UW-Madison. “It could help you pack in more transistors onto chips and continue Moore’s law into the future.”
Saraswat and his colleagues published details of their work July 9, 2019, in the Journal of Physical Chemistry.
The advance could enable graphene-based integrated circuits, with much improved performance over today’s silicon chips.
“The main advantage of graphene nanoribbons is that electrons can travel faster through them, compared to silicon so you can make faster chips that use less energy,” says Mike Arnold, a professor of materials science and engineering at UW-Madison and a world expert in graphene growth.
Arnold is pioneer of a strategy to lay down long, thin strips of graphene—structures known as nanoribbons—on top a material called germanium.
That’s useful in many ways. However, since germanium isn’t a widely used semiconductor, it can’t form the basis for computer chips.
Meanwhile, other researchers have not been able to overcome a major barrier in layering graphene nanoribbons onto silicon. Graphene reacts with silicon to form an inert and less useful compound called silicon carbide.
Arnold’s group has developed an ingenious method to avoid that obstacle.
By laying down a thin protective layer of germanium before applying graphene, the researchers could successfully grow graphene nanoribbons on top of silicon wafers. The thin germanium layers protected graphene from reacting with silicon, yet didn’t interfere with the nanoribbons’ semiconducting capabilities.
It’s an important first step toward creating graphene-based integrated circuits. And because the base layer is composed of silicon, the graphene nanoribbon technology can be easily integrated into existing electronic/computing components.
“Our vision is to integrate graphene with existing devices,” says Arnold.
The scientists have patented their technology through the Wisconsin Alumni Research Foundation. One advantage of their synthesis approach is that it takes advantage of a scalable, industry-compatible chemical vapor deposition technique. Now, they’re working to improve the precision with which they lay down their nanoribbons so that they can achieve the complex patterns found in modern computer chips.
“We are using a few strategies to control the thickness and the orientation for the nanoribbons,” says Arnold. “We have a few really cool ideas.”