Epitaxial graphene on SiC(0001): More than just honeycombs

TL;DR

This study reveals that the interface of epitaxial graphene on SiC(0001) involves defect structures that induce a semiconducting gap, significantly influencing the electronic properties of the graphene layers.

Contribution

The paper provides detailed atomic-scale insights into the graphene-SiC interface, showing how defects induce a band gap and affect electronic behavior, advancing understanding of epitaxial graphene.

Findings

The interface contains hexagon-pentagon-heptagon defect complexes.

Defects break symmetry and induce a semiconducting gap.

The next graphene layer exhibits nonlinear dispersion and a 33 meV gap.

Abstract

The potential of graphene to impact the development of the next generation of electronics has renewed interest in its growth and structure. The graphitization of hexagonal SiC surfaces provides a viable alternative for the synthesis of graphene, with wafer-size epitaxial graphene on SiC(0001) now possible. Despite this recent progress, the exact nature of the graphene-SiC interface and whether the graphene even has a semiconducting gap remain controversial. Using scanning tunneling microscopy with functionalized tips and density functional theory calculations, here we show that the interface is a warped carbon sheet consisting of three-fold hexagon-pentagon-heptagon complexes periodically inserted into the honeycomb lattice. These defects relieve the strain between the graphene layer and the SiC substrate, while still retaining the three-fold coordination for each carbon atom. Moreover, these defects break the six-fold symmetry of the honeycomb, thereby naturally inducing a gap: the calculated band structure of the interface is semiconducting and there are two localized states near K below the Fermi level, explaining the photoemission and carbon core-level data. Nonlinear dispersion and a 33 meV gap are found at the Dirac point for the next layer of graphene, providing insights into the debate over the origin of the gap in epitaxial graphene on SiC(0001). These results indicate that the interface of the epitaxial graphene on SiC(0001) is more than a dead buffer layer, but actively impacts the physical and electronic properties of the subsequent graphene layers.