World’s 1st Graphene Chip!

Graphene’s lack of an intrinsic band gap has posed a challenge for its use in nanoelectronics, but the development of semiconducting graphene is a critical advancement in this field. Despite numerous attempts over the last two decades to engineer a band gap in graphene via quantum confinement or chemical functionalization, viable semiconducting graphene has eluded researchers until now.

A groundbreaking discovery by Professor Walter de Heer of the Georgia Institute of Technology and Professor Marek Marek from Tianjin University has revealed that semiconducting epitaxial graphene (SEG) grown on a single-crystal silicon carbide (SiC) substrate exhibits a 0.6 eV band gap and an impressive room-temperature mobility exceeding 5,000 cm² V⁻¹ s⁻¹ — ten times higher than that of silicon, and twenty times more than other two-dimensional semiconductors. This means that electrons can move with exceptionally low resistance, enhancing efficiency. This remarkable achievement represents the world’s first functional semiconductor made from graphene, breaking through a major obstacle in electronics and paving the way for novel electronic devices.

Graphene multilayer films are crystallized on the carbon-rich surface created when silicon evaporates from the SiC crystal surface. The first graphene layer on the silicon face of SiC acts as an insulating outer layer of graphene that is partially covalently bound to the SiC surface. Spectral measurements of this buffer layer display semiconducting characteristics but its mobility is limited due to the presence of impurities.

Researchers developed a near-equilibrium annealing technique capable of creating SEG with an orderly buffer layer on the macroscopic atomically flat terraces aligned with the SiC substrate. This structure offers chemical, mechanical, and thermal stability, and can be patterned using conventional semiconductor fabrication technologies, seamlessly interfacing with semi-metallic epitaxial graphene. These fundamental properties make SEG applicable for nanoelectronics. The research, titled “Ultrahigh-mobility semiconducting epitaxial graphene on silicon carbide,” was published in the journal Nature on January 3rd.

In its natural form, graphene is neither a semiconductor nor a metal; it is a semimetal. The band gap is a feature of materials that can be switched on and off with an applied electric field, which is the principle behind all transistors and silicon electronic devices. A primary issue in graphene electronics research has been figuring out how to switch graphene on and off so that it can function like silicon.

However, producing functional transistors often requires extensive manipulation of semiconductor materials, which can degrade their performance. To demonstrate their platform could indeed be a viable semiconductor, the team needed to measure its electronic properties without damaging it. They placed atoms on graphene, “donating” electrons — a technique known as doping — to see if the material would conduct well.

SEG Production

The conventional epitaxial graphene and buffer layer are grown inside a sealed controlled sublimation (CCS) furnace by placing a 3.5 mm × 4.5 mm semi-insulating SiC chip into a cylindrical graphite crucible, annealing it at temperatures ranging from 1300 °C to 1600 °C in an argon atmosphere at 1 bar. The rate of silicon escape from the crucible determines the rate of graphene formation on the surface, so both growth temperature and the rate of graphene formation are controllable.

In the configuration with the C-face towards the Si-face, a thin silicon film forms on the hotter C-face, while the larger mesa of SEG coating (0001) grows on the Si-face. Silicon absent from the Si-face likely condenses on the C-face to maintain stoichiometry. When the temperature gradient is inverted, the Si-face has a higher temperature than the C-face, and mass transfer occurs from the Si-face to the C-face, also forming a large SEG coating (0001) mesa. It’s clear from this inverted crystal growth process that the substrate steps evaporate from the source, leaving behind large (0001) mesas on the Si-face. The conclusion is that the SEG coating (0001) face is more stable than any other SiC face, especially more so than the bare (0001) face, suggesting that wafer-scale single-crystal SEG could, in principle, be produced.

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