Exceptional ballistic transport in epitaxial graphene nanoribbons

Abstract: Graphene electronics has motivated much of graphene science for the past decade. A primary goal was to develop high mobility semiconducting graphene with a band gap that is large enough for high performance applications. Graphene ribbons were thought to be semiconductors with these properties, however efforts to produce ribbons with useful bandgaps and high mobility has had limited success. We show here that high quality epitaxial graphene nanoribbons 40 nm in width, with annealed edges, grown on sidewall SiC are not semiconductors, but single channel room temperature ballistic conductors for lengths up to at least 16 micrometers. Mobilities exceeding one million corresponding to a sheet resistance below 1 Ohm have been observed, thereby surpassing two dimensional graphene by 3 orders of magnitude and theoretical predictions for perfect graphene by more than a factor of 10. The graphene ribbons behave as electronic waveguides or quantum dots. We show that transport in these ribbons is dominated by two components of the ground state transverse waveguide mode, one that is ballistic and temperature independent, and a second thermally activated component that appears to be ballistic at room temperature and insulating at cryogenic temperatures. At room temperature the resistance of both components abruptly increases with increasing length, one at a length of 160 nm and the other at 16 micrometers. These properties appear to be related to the lowest energy quantum states in the charge neutral ribbons. Since epitaxial graphene nanoribbons are readily produced by the thousands, their room temperature ballistic transport properties can be used in advanced nanoelectronics as well.

• The paper demonstrates room-temperature ballistic transport in 40 nm epitaxial graphene nanoribbons over lengths up to 16 µm with sheet resistance below 1 Ω.
• The paper reveals a dual-channel mechanism where one mode remains ballistic across temperatures while a thermally activated channel becomes insulating at cryogenic conditions.
• The paper confirms high-quality monolayer formation via ARPES, indicating charge neutrality and scalable production potential for advanced nanoelectronic devices.

Overview of Epitaxial Graphene Nanoribbons and Ballistic Transport

This paper presents a comprehensive study on the transport properties of epitaxial graphene nanoribbons (GNRs) grown on silicon carbide (SiC) substrates. Over the last decade, the exploration of graphene's electronic properties has been motivated by the potential to develop high-mobility, semiconducting graphene for advanced electronic applications. While graphene nanoribbons have been postulated as potential semiconductors, practical achievements in producing ribbons with beneficial bandgaps and mobility have been limited. This work significantly advances the field by demonstrating room-temperature ballistic transport in high-quality graphene nanoribbons.

Key Findings

1. Ballistic Conductors: The research reveals that 40 nm wide epitaxial graphene nanoribbons on SiC exhibit single-channel ballistic transport at room temperature for lengths up to at least 16 µm. This large-scale ballistic conduction surpasses conventional two-dimensional graphene and even theoretical predictions for perfect graphene, highlighting exceptional electron mobility exceeding one million, with sheet resistance below 1 Ω.
2. Waveguide and Quantum Dot Behavior: The study identifies that the electronic transport in these ribbons is dominated by two components of the ground state transverse waveguide mode. One component is temperature-independent and ballistic, while the other is a thermally activated channel that behaves as a ballistic conductor at room temperature but becomes insulating at cryogenic temperatures.
3. Structural and Electronic Properties: The epitaxial nature of the nanoribbons ensures high-quality monolayers with minimal disorder, enabling the observation of intrinsic graphene properties. The band structure analysis via ARPES confirms charge neutrality and aligns with the Dirac cone idealizations.
4. Implications for Nanoelectronics: The findings suggest that these graphene nanoribbons can be reliably produced in large quantities, potentially integrating their ballistic transport properties into advanced nanoelectronic devices. Their scalability and room temperature capabilities make them promising candidates for future electronics.

Implications for Future Research

The discovery of room-temperature ballistic transport in GNRs could reshape both theoretical and practical applications in electronics. The implications extend beyond current semiconductor technologies to enable new design paradigms in nanoelectronics based on ballistic transport. Furthermore, understanding the underlying mechanisms of the observed phenomena in epitaxial GNRs may lead to the development of more efficient graphene-based transistors with minimal energy dissipation.

Speculation and Future Directions

Continued investigation into the coupling of electronic states and longitudinal modes in graphene is crucial. The paper suggests potential alignment with non-Fermi liquid behaviors and symmetry-breaking dynamics in graphene's ground state. Future research could explore these concepts more deeply, particularly in the context of ultrahigh vacuum and pristine substrate configurations to minimize external perturbations.

Given the practical possibilities, future work should aim to integrate ballistic GNRs into operational electronic circuits to evaluate their performance and stability in real-world applications. Additionally, the research raises intriguing questions about the potential for ballistic transport in other low-dimensional materials, which could guide the next decade of materials science and electronic engineering advancements.