Engineering Epitaxial Graphene for Quantum Metrology

Licentiatavhandling, 2018

Quantum resistance metrology deals both with the precise and accurate measurement of electrical resistance, by utilizing the quantum hall effect (QHE) in two-dimensional electron gases (2DEGs) such as those based on gallium arsenide (GaAs). Due to the unique properties of graphene, and specifically epitaxial graphene grown on silicon carbide (SiC/G), quantum Hall resistance (QHR) standards based on graphene perform better in a wider parameter space (temperature, current and magnetic field) than conventional semiconducting materials. To date, this is possibly the only electronic application of graphene that surpasses previous technologies. However, due to the nature of SiC/G there are still certain issues than remain unsolved, which stand in the way for widespread use of graphene QHR devices. This work aims to discuss, and suggest solutions to, one of the major problems: charge carrier density control.

Control over the charge carrier density is crucial in order to observe the QHE at sufficiently low magnetic fields. Since SiC/G is intrinsically n-doped due to interactions with the SiC substrate, external doping methods must be used in order to bring graphene closer to charge neutrality. Previous techniques such as photochemical gating, corona discharge of ions or simply electrostatic gating lack either potency, stability or tuneability. This thesis presents a new air-stable chemical gating method using the acceptor molecule 2,3,5,6-Tetrauoro-tetracyanoquinodimethane (F4TCNQ) mixed with a poly(methyl-methacrylate) (PMMA) polymer. This dopant blend can be applied to SiC/G using simple spin coating, forgoing the need for ultra-high vacuum (UHV) systems. It provides potent and homogeneous doping, with the ability to bring SiC/G close to charge neutrality, with measured mobilities reaching 70,000 cm2/Vs. Furthermore, the method is compatible with macroscopic devices with the doping being significantly homogeneous even on the millimeter scale. Interestingly, chemical analysis reveal that the doping effect is a consequence of F4TCNQ molecules diffusing through the PMMA matrix and preferentially assembling near the graphene surface. Charge transfer and doping is attributed to the formation of a charge-transfer complex between F4TCNQ and graphene. The low carrier densities and high carrier mobilities for chemically doped samples is the result of low charge disorder (± 9 meV), thus far only attainable in state-of-the-art exfoliated graphene flakes encapsulated by hexagonal boron nitride (hBN) or suspended graphene. Initial measurements performed at metrological institutes, comparing SiC/G to GaAs, suggest that the chemical dopant is compatible with precision measurements of quantized resistance with part-per-billion accuracy.