Date of Award

August 2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Physics

First Advisor

Lian Li

Committee Members

Lian Li, Michael Weinert, Daniel Agterberg, Marija Gajdardziska-Josifovska, Jorg Woehl

Keywords

Graphene, Graphene Heterojunctions, Scanning Tunneling Microscopy, Scanning Tunneling Spectroscopy, Van der Waals Heterostructures

Abstract

This dissertation presents results of scanning tunneling microscopy/spectroscopy experiments performed on graphene, a two-dimensional membrane of carbon atoms arranged in a honeycomb lattice, where charge carriers behave like massless fermions described by the Dirac equation. Our findings demonstrate that interface engineering is a viable route to control and further enhance the electronic properties of graphene.

In the first experiment, by transferring chemical vapor deposited (CVD) graphene onto substrates of opposite polarization - H-terminated Si-face and C-faces of hexagonal silicon carbide (SiC), we show that the type of charge carrier in graphene can be controlled by substrate polarization. Furthermore, we find that the charge carrier in epitaxial graphene/Si-face SiC(0001) convert from n- to p-type upon H-intercalation at the interface. Finally, we observe the formation of ripples in the graphene H-terminated SiC heterojunctions, which causes atomic scale fluctuations in the Dirac point. Density functional theory calculations suggest the formation of a Schottky dipole just ~ 1 nm at the graphene/SiC interface, thus the Dirac point depends strongly on the spacing between graphene and SiC. As a result, ripples, i.e., atomic scale topographic fluctuations of graphene with respect to the substrate, lead to the variations in the Dirac point.

In the second experiment, we discover two types of intrinsic atomic-scale inhomogeneities that can cause fluctuations in the Schottky barrier height at graphene/semiconductor junctions: graphene ripples and/or trapped charge impurities and surface states of the semiconductor. These findings provide insight into the fundamental physics of nanoscale devices based on graphene - semiconductor junctions.

In the third experiment, we experimentally demonstrate proximity-induced spin-orbit coupling in graphene-topological insulator van der Waals (vdW) heterostructures fabricated by transferring CVD graphene onto Bi2Se3 grown by molecular beam epitaxy. We observe a spin-orbit splitting of up to 80 meV in the graphene Dirac states, an enhancement of several orders of magnitude compared to the intrinsic value. Moreover, the spin-orbit splitting exhibits spatial variations of ±20 meV, as a result of the lack of epitaxial relation between the graphene and Bi2Se3 layers. Density functional theory calculations further reveal that this giant spin-orbit splitting of the graphene bands is a consequence of the orthogonalization requirement on the overlapping wave functions, rather than arising from simple direct bonding at the interface. This revelation of the indirect bonding mechanism of the proximity effect is an enabling step towards more effective engineering of desired properties in vdW heterostructures.

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