Researchers Create World’s First Functional Semiconductor Made From Graphene
The technology could lead to smaller and faster devices, as well as new breakthroughs in quantum computing.
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With an electrical conductivity somewhere in between that of conductors and insulators, semiconductors have become an indispensable part of modern electronics.
Nearly all electronics currently use silicon as their semiconducting component. However, in the face of increasing demand for faster computing and smaller electronic devices, silicon is approaching its physical limit.
In a breakthrough new paper published in Nature, an international research team has unveiled the world’s first functional semiconductor made from graphene. The research team, led by Professor Walter de Heer of the Georgia Institute of Technology, says that their discovery could open the door to a whole new way of building electronics.
The search for alternative semiconductors
To push past the limits of silicon and create the next generation of advanced electronics, scientists are on the hunt for alternative semiconducting materials.
More than two decades ago, de Heer began looking into whether carbon-based materials could provide this next breakthrough. Graphene, a single layer of carbon atoms tightly bound together in a hexagonal lattice, quickly emerged as a potentially interesting material for electronics.
"We were motivated by the hope of introducing three special properties of graphene into electronics," he said. "It's an extremely robust material, one that can handle very large currents, and can do so without heating up and falling apart."
However, graphene came with one big hurdle. It does not naturally have a band gap – the key electronic property for semiconductivity.
To function properly, electronics rely on special switching devices made from semiconducting materials, as these materials can precisely control the movement of electric current due to their unique electronic structure – their narrow band gap.
So, what is a band gap? According to the laws of quantum physics, electrons can only take on certain discrete values of energy, called energy levels. In a solid, which is made up of many billions of atoms, these energy levels smear out to become energy bands.
A solid’s “valence band” contains electrons that are tightly bound to their parent atoms and so cannot move freely. A solid’s “conduction band”, in contrast, contains electrons that have gained enough energy to break away from their parent atoms and flow more freely around the material.
A material’s band gap describes the energy difference between that material’s valence and conduction bands. An insulator will have a large band gap, making it impossible for electrons to circulate by being excited into the conduction band. In contrast, conductors have a band gap of almost zero and so will conduct heat and electricity freely.
Semiconductors are unique because their band gap lies somewhere between that of an insulator and a conductor. As a result, semiconducting materials can function ordinarily as insulators, but then “become” conductors when external factors – such as heat or an electric field – are applied and valence electrons begin to excite and jump into the conduction band.
The making of a graphene semiconductor
De Heer’s first breakthrough in creating a graphene semiconductor came when his team figured out how to grow graphene on silicon carbide wafers using special furnaces. This process resulted in epitaxial graphene – the name given to a single layer of graphene grown on a crystal face. The researchers found that, when the epitaxial graphene was properly bound to the silicon carbide surface, the material began to show semiconducting properties.
Following this discovery, de Heer founded the Tianjin International Center for Nanoparticles and Nanosystems at Tianjin University, China, alongside his long-term collaborator and current center director, Dr. Lei Ma. For the next decade, the team would work towards perfecting their new semiconducting graphene material.
"A long-standing problem in graphene electronics is that graphene didn’t have the right band gap and couldn’t switch on and off at the correct ratio,” said Ma. “Over the years, many have tried to address this with a variety of methods. Our technology achieves the band gap, and is a crucial step in realizing graphene-based electronics."
In their latest Nature publication, the researchers demonstrate a new quasi-equilibrium method of annealing that produces semiconducting epigraphene on silicon carbide. This semiconducting graphene is chemically, mechanically and thermally robust, and can be patterned.
Additionally, the researchers say that it can be seamlessly connected to semi-metallic epigraphene using conventional semiconductor fabrication techniques, meaning that the new semiconductor is fully compatible with conventional microelectronics processing methods.
The researchers’ measurements also show that their new graphene semiconductor has 10 times greater electron mobility than silicon, which means potentially a lot less resistance when it comes to building faster electronics.
"It's like driving on a gravel road versus driving on a freeway," de Heer said. "It's more efficient, it doesn't heat up as much, and it allows for higher speeds so that the electrons can move faster."
The next generation of powerful electronics
For years, attempts to synthesize such a layer of semiconducting graphene were rendered unusable due to disorder, scattering, or other issues that plagued the material. With this new method of producing semiconducting epigraphene, the researchers present the first functional piece of semiconductor technology based on graphene.
"Our motivation for doing graphene electronics has been there for a long time, and the rest was just making it happen," de Heer said. "We had to learn how to treat the material, how to make it better and better, and finally how to measure the properties. That took a very, very long time."
De Heer sees this new technology as the tip of the iceberg for new discoveries. Undoubtedly, there is still a long way to go before graphene semiconductor production rivals that of today’s silicon semiconductors. But that next generation of semiconductors could be closer than we think.
"To me, this is like a Wright brothers moment," de Heer said. "They built a plane that could fly 300 feet through the air. But the skeptics asked why the world would need flight when it already had fast trains and boats. But they persisted, and it was the beginning of a technology that can take people across oceans."
Reference: Zhao J, Ji P, Li Y, et al. Ultrahigh-mobility semiconducting epitaxial graphene on silicon carbide. Nature. 2024;625(7993):60-65. doi:10.1038/s41586-023-06811-0
This article is a rework of a press release issued by the Georgia Institute of Technology. Material has been edited for length and content.