Engineering Epitaxial Graphene for Quantum Metrology
Licentiate thesis, 2018
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.
Epitaxial Graphene
Magnetotransport
Quantum Resistance Metrology
Chemical Doping
Author
Hans He
Chalmers, Microtechnology and Nanoscience (MC2), Quantum Device Physics
H. He, T. J.B. M. Janssen, S. Rozhko, A. Tzalenchuk, S. Lara-Avila, R. Yakimova, and S. Kubatkin. Fabrication of graphene quantum hall resistance standard in a cryogen-table-top system. 2016 Conference on Precision Electromagnetic Measurements (CPEM 2016) (2016)
H. He, K. H. Kim, A. Danilov, D. Montemurro, L. Yu, Y. W. Park, F. Lombardi, T. Bauch, K. Moth-poulsen, T. Iakimov, R. Yakimova, P. Malmberg, C. Muller, S. Kubaktkin, and S. Lara-Avila. Uniform doping of graphene close to the charge neutrality point by polymerassisted spontaneous assembly of molecular dopants. Manuscript under consideration in Nature Materials (2018)
T. J.B. M. Janssen, S Rozhko, I Antonov, A Tzalenchuk, J. M. Williams, Z Melhem, H He, S Lara-Avila, S Kubatkin, and R Yakimova. Operation of graphene quantum Hall resistance standard in a cryogen-free table-top system. 2D Materials 2.3 (2015), 035015
Areas of Advance
Nanoscience and Nanotechnology (SO 2010-2017, EI 2018-)
Materials Science
Roots
Basic sciences
Subject Categories
Materials Chemistry
Nano Technology
Condensed Matter Physics
Infrastructure
Nanofabrication Laboratory
Publisher
Chalmers
Fasrummet A820, MC2, Kemivägen 9
Opponent: Prof. Avgust Yurgens, Department of Microtechnology and Nanoscience, Chalmers University of Technology