Atmospheric doping effects in epitaxial graphene: correlation of local and global electrical measurements

TL;DR

This study correlates local and global electrical measurements to understand how atmospheric gases like O2, water vapour, and NO2 influence doping in epitaxial graphene, revealing that thicker layers are less affected and ambient effects are complex.

Contribution

It provides a detailed correlation between local and global electrical properties of epitaxial graphene under controlled atmospheric conditions, clarifying misconceptions about ambient doping sources.

Findings

Oxygen, water vapour, and NO2 cause p-doping in all graphene layers.

Thicker graphene layers are less affected by atmospheric dopants.

Ambient air effects cannot be fully replicated by higher concentrations of individual gases.

Abstract

We directly correlate the local (20-nm scale) and global electronic properties of a device containing mono-, bi- and tri-layer epitaxial graphene (EG) domains on 6H-SiC(0001) by simultaneously performing local surface potential measurements using Kelvin probe force microscopy and global transport measurements. Using well-controlled environmental conditions, where the starting state of the surface can be reproducibly defined, we investigate the doping effects of N2, O2, water vapour and NO2 at concentrations representative of the ambient air. We show that presence of O2, water vapour and NO2 leads to p-doping of all EG domains. However, the thicker layers of EG are significantly less affected by the atmospheric dopants. Furthermore, we demonstrate that the general consensus of O2 and water vapour present in ambient air providing majority of the p-doping to graphene is a common misconception. We experimentally show that even the combined effect of O2, water vapour, and NO2 at concentrations higher than typically present in the atmosphere does not fully replicate the state of the EG surface in ambient air. All doping effects can be reproducibly reversed by vacuum annealing. Thus, for EG gas sensors it is essential to consider naturally occurring environmental effects and properly separate them from those coming from targeted species.