- The paper reveals a ~100 meV gap near zero bias and distinct spatial electronic inhomogeneities in epitaxially grown graphene layers on SiC.
- It employs cryogenic scanning tunneling spectroscopy to differentiate atomic-scale carbon networks from larger-scale voltage-induced reconstructions at the graphene/SiC interface.
- The study highlights the role of local density of states mapping in guiding the integration of graphene into nanoelectronic applications.
Scanning Tunneling Spectroscopy of Inhomogeneous Electronic Structure in Monolayer and Bilayer Graphene on SiC
The paper presents a detailed examination of the local electronic structure of epitaxially grown monolayer and bilayer graphene on a SiC(0001) substrate using scanning tunneling spectroscopy (STS). Graphene's electronic properties, particularly when grown on silicon carbide (SiC), have garnered significant interest due to their potential for integration in nanoelectronic devices.
This paper harnesses STS to unveil both the atomic-scale and larger-scale inhomogeneous structures in graphene films. The low-voltage STM images reveal the atomic-scale arrangements of carbon, showcasing fine networks of carbon atoms, while high-bias images reveal more extensive spatial inhomogeneities resembling reconstructions rich in carbon content on the SiC surfaces. Such bimodal imaging benefits from voltage-dependent variations, offering insights into the inhomogeneous electronic landscapes that pervade graphene layers on these substrates.
A key observation from the STS measurements is the existence of a ~100 meV gap-like feature around zero bias in both monolayer and bilayer graphene on SiC, with varying degrees of spatial inhomogeneity present above the gap. Notably, the paper correlates the inhomogeneity in electronic structure with nanoscale variations at the SiC/graphene interface, which are discernible in topographic images. This is a critical realization for the technological utilization of graphene/SiC systems in electronic transport or tunneling applications.
Experimentation was conducted under cryogenic conditions (4.8K) with a highly controlled environment, using an Omicron LT-STM with distinctly prepared STM tips. dI/dV spectra reveal the local density of states (LDOS), underscoring pronounced spatial disparities, especially at higher energies, and a notable gap at zero bias voltage (E_F).
A comparative analysis with angle-resolved photoemission spectroscopy (ARPES) underscores discrepancies, highlighting an unexpected gapped structure contrasting the predicted linear dispersion of Dirac Fermion-like bands, particularly in bilayer configurations where parabolic band dispersion and a gap are expected. The paper postulates several potential explanations for the gap-like features observed: influences of electronic states in the SiC layer, charging or band-bending effects, and inelastic comprehension of surface interactions. These hypotheses invite further exploration to assess their veracity.
The implications of this research extend to both practical and theoretical domains. Accurate mapping of electronic landscapes in graphene/SiC systems is imperative for advancing nanoelectronics, particularly where precise control over electronic properties is essential. The potential for further refinement in epitaxial growth techniques and improved understanding of interface phenomena may facilitate the development of devices with customized electronic characteristics.
Future investigations could further elucidate the role of SiC-induced reconstructions in electronic behavior and explore how substrate-induced variations could be tailored for designed functionalities. The acknowledgment of spatial inhomogeneity and unexplained gap-like features points to the necessity of comprehensive modeling and perhaps a reevaluation of interface phenomena's contributions to governing electronic structures in epitaxial graphene.