Transport Studies of the Electronic Properties of Graphene on Hexagonal Boron Nitride
Graphene's planar structure and unique low energy spectrum make it an intriguing material to study its electronic properties. Recent progresses in stacking graphene (G) on high quality hexagonal boron nitride (hBN) greatly advanced the electronic performance of graphene devices, pproaching the intrinsic properties of graphene. This thesis reports transport studies of graphene on hBN, including graphene/hBN moiré superlattice at small rotation angle and ballistic transport in short/wide encapsulated BN/G/BN structures.
Chapter 1 will introduce the basic properties of graphene, including the unique low energy electronic spectrum and the unconventional integer quantum Hall effect. The concept of Berry's phase and pseudospin winding number and their connection to the quantum Hall effect are also discussed. Chapter 2 reviews the properties of graphene on hBN, especially the long wavelength moiré superlattice at small rotation angle which modulates graphene's low energy spectrum. It also discusses the Hofstadter's butterfly and its realization in the graphene/hBN heterostructure.
Chapter 3 addresses some of the essential techniques used to fabricate graphene/hBN devices measured in this thesis, including the layer stacking techniques and fabrication of graphene field effect transistors.
Chapter 4 reports the measurements of Hofstadter's butterfly spectrum focusing at the large doping region where the Fermi level is above the secondary Dirac points generated by the moiré superlattice. At large electron doping, we observed a novel π phase shift in the magneto-oscillations. At large hole doping, inversion symmetry breaking generates a distinct hexagonal pattern.
Chapter 5 discusses measurements of short BN/G/BN cavities. The high quality BN/G/BN devices exhibit ballistic transport behavior - Fabry-Pérot oscillations.The effects of magnetic field on the system are also investigated, showing signatures of "pseudodiffussive" transport at the charge neutrality point for finite fields.