Researchers at Georgia Tech, in Atlanta, have developed what they are calling the world’s first functioning graphene-based semiconductor. This breakthrough holds the promise to revolutionize the landscape of electronics, enabling faster traditional computers and offering a new material for future quantum computers.
The research, published on 3 January in Natureand led by Walt de Heer, a professor of physics at Georgia Tech, focuses on leveraging epitaxial graphene, a crystal structure of carbon chemically bonded to silicon carbide (SiC). This novel semiconducting material, dubbed semiconducting epitaxial graphene (SEC)—or alternatively, epigraphene—boasts enhanced electron mobility compared with that of traditional silicon, allowing electrons to traverse with significantly less resistance. The outcome is transistors capable of operating at terahertz frequencies, offering speeds 10 times as fast as that of the silicon-based transistors used in current chips.
De Heer describes the method used as a modified version of an extremely simple technique that has been known for over 50 years. “When silicon carbide is heated to well over 1,000 °C, silicon evaporates from the surface, leaving a carbon-rich surface which then forms into graphene,” says de Heer.
Georgia Tech Researchers Create First Functional Graphene Semiconductor
This heating step is done with an argon quartz tube in which a stack of two SiC chips are placed in a graphite crucible, according to de Heer. Then a high-frequency current is run through a copper coil around the quartz tube, which heats the graphite crucible through induction. The process takes about an hour. De Heer added that the SEC produced this way is essentially charge neutral, and when exposed to air, it will spontaneously be doped by oxygen. This oxygen doping is easily removed by heating it at about 200 °C in vacuum.
“The chips we use cost about [US] $10, the crucible about $1, and the quartz tube about $10,” said de Heer.
While it has been known since 2008 that it’s possible to make graphene behave like a semiconductor by heating it in a vacuum with SiC, it’s the method developed by de Heer that makes the difference in the bandgap. “If it is done correctly, using the modified method described above, then the bonding is very regular and the mobility is very large, as we have shown in the paper,” says de Heer.
Semiconductors—critical components in any electronic device—exhibit properties of both conductors and insulators. However, silicon, the predominant material for semiconductors, is reaching its limits in terms of speed, heat generation, and miniaturization. De Heer underscores that the swift progress witnessed throughout the history of computing is decelerating due to these constraints on silicon.
“We have produced large areas of semiconducting SEC on defect-free, atomically flat SiC terraces.”
—Walt de Heer, Georgia Tech
Graphene, a single layer of carbon atoms arranged in a hexagonal lattice, is emerging as a superior conductor to silicon, facilitating more efficient electron movement through the material. Despite these advantages, previous endeavors to integrate graphene into electronics faced challenges due to the absence of a bandgap, a critical factor for transistors to switch on and off.
There has been a decade’s worth of work in developing functional opportunities with graphene, which involves chemically bonding atoms to the graphene so that it exhibits a bandgap. De Heer notes that previous methods resulted in low-mobility semiconducting graphene due to various issues in either its chemical or mechanical makeup.
For instance, graphene ribbons have been seen as promising, but they are only semiconducting with very specific widths and armchair edges that are inversely proportional to the ribbon width. These ribbons are best made by chemical means, and ultimately must be accurately deposited on substrate and then interconnected with metallic wires.
“There has been some success with graphene nanoribbons, but in principle this technology is very similar to semiconducting carbon-nanotube technology which has not been successful after 30 years of nanotube research,” says de Heer.
Another method that has been used to give graphene a bandgap is putting wrinkles into the material. Mechanical deformations will open a bandgap, and bandgaps up to 0.2 electron volts have been demonstrated. (For comparison, silicon has a bandgap of 1.12 eV, which is significantly larger.) The small bandgap makes it unclear how these materials could be used in applications, while the relative lack of information on their mobilities adds another complication.
“Our research is distinct from these other approaches because we have produced large areas of semiconducting SEC on defect-free, atomically flat SiC terraces,” says de Heer. “SiC is a highly developed, readily available electronic material that is fully compatible with conventional microelectronics processing methods.”
Elaborating on the potential applications of their breakthrough, the researchers noted that graphene-based semiconductors could play a pivotal role in quantum computing. This is due to the fact that when graphene is used in devices at very low temperatures, its electrons exhibit quantum-mechanical wavelike properties like those seen in light.
“One main aspect of graphene electronics is that we can utilize the quantum-mechanical wave properties of the electrons and [electron] holes which are not accessible in silicon electronics,” says de Heer. “If this is possible, then that constitutes a paradigm shift in electronics.”
“The chips we use cost about $10, the crucible about $1, and the quartz tube about $10.”
—Walt de Heer, Georgia Tech
De Heer and his research team concede, however, that further exploration is needed to determine whether graphene-based semiconductors can surpass the current superconducting technology used in advanced quantum computers.
The Georgia Tech team do not envision incorporating graphene-based semiconductors with standard silicon or compound semiconductor lines. Instead, they are aiming for a paradigm shift beyond silicon, utilizing silicon carbide. They are developing methods, such as coating SEC with boron nitride, to protect and enhance its compatibility with conventional semiconductor lines.
Comparing their work with commercially available graphene field-effect transistors (GFETs), de Heer explains that there is a crucial difference: “Conventional GFETs do not use semiconducting graphene, making them unsuitable for digital electronics requiring a complete transistor shutdown.” He says that the SEC developed by his team allows for a complete shutdown, meeting the stringent requirements of digital electronics.
De Heer says that it will take time to develop this technology. “I compare this work to the Wright brothers’ first 100-meter flight. It will mainly depend on how much work is done to develop it.”
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