The emergence of quantum capacitance in epitaxial graphene

TL;DR

This study demonstrates the spontaneous formation of a quantum capacitance in epitaxial graphene on SiC, caused by charge redistribution at the interface, without external doping, revealing new electronic properties relevant for quantum device development.

Contribution

It provides the first direct evidence of a self-formed quantum capacitor in epitaxial graphene on SiC, highlighting intrinsic charge redistribution effects without external doping.

Findings

Charge redistribution creates a quantum capacitor at the graphene-SiC interface.

A minigap of approximately 4.3 meV was observed in epitaxial graphene.

Charge self-compensation occurs without initial doping, affecting electronic properties.

Abstract

We found an intrinsic redistribution of charge arises between epitaxial graphene, which has intrinsically n-type doping, and an undoped substrate. In particular, we studied in detail epitaxial graphene layers thermally elaborated on C-terminated $4H$-$SiC$ ($4H$-$SiC$ ($000{\bar{1}}$)). We have investigated the charge distribution in graphene-substrate systems using Raman spectroscopy. The influence of the substrate plasmons on the longitudinal optical phonons of the $SiC$ substrates has been detected. The associated charge redistribution reveals the formation of a capacitance between the graphene and the substrate. Thus, we give for the first time direct evidence that the excess negative charge in epitaxial monolayer graphene could be self-compensated by the $SiC$ substrate without initial doping. This induced a previously unseen redistribution of the charge-carrier density at the substrate-graphene interface. There a quantum capacitor appears, without resorting to any intentional external doping, as is fundamentally required for epitaxial graphene. Although we have determined the electric field existing inside the capacitor and revealed the presence of a minigap ($\approx 4.3meV$) for epitaxial graphene on $4H$-$SiC$ face terminated carbon, it remains small in comparison to that obtained for graphene on face terminated $Si$. The fundamental electronic properties found here in graphene on $SiC$ substrates may be important for developing the next generation of quantum technologies and electronic/plasmonic devices.