Electronic transport in graphene
Subtitle: p-n junctions, shot noise, and nanoribbons
By: by James Ryan Williams
Published: 2009
Language: eng
Number of pages: 170
Description: Novel, two-dimensional materials have allowed for the inception and elucidation of a plethora of physical phenomena. On such material, a hexagonal lattice of carbon atoms called graphene, is a unique, truly two-dimensional molecular conductor. This thesis describes six experiments that elucidate some interesting physical properties and technological applications of graphene, with an emphasis on graphene-based p-n junctions. A technique for the creation of high-quality p-n junctions of graphene is described. Transport measurements at zero magnetic field demonstrate local control of the carrier type and density bipolar graphene-based junctions. In the quantum Hall regime, new plateaus in the conductance are observed and explained in terms of mode mixing at the p-n interface. Shot noise in unipolar and bipolar graphene devices is measured. A density-independent Fano factor is observed, contrary to theoretical expectations. Further, an independence on device geometry is also observed. The role of disorder on the measured Fano factor is discussed, and comparison to recent theory for disordered graphene is made. The effect of a two-terminal geometry, where the device aspect ratio is different from unity, is measured experimentally and analyzed theoretically. A method for extracting layer number from the conductance extrema is proposed. A method for a conformal mapping of a device with asymmetric contacts to a rectangle is demonstrated. Finally, possible origins of discrepancies between theory and experiment are discussed.

Transport along p-n junctions in graphene is reported. Enhanced transport along the junction is observed and attributed to states that exist at the p-n interface. A correspondence between the observed phenomena at low-field and in the quantum Hall regime is observed. An electric field perpendicular to the junction is found to reduce the enhanced conductance at the p-n junction. A corollary between the p-n interface states and "snake states" in an inhomogeneous magnetic field is proposed and its relationship to the minimum conductivity in graphene is discussed. A final pair of experiments demonstrate how a helium ion microscope can be used to reduce the dimensionality of graphene one further, producing graphene nanoribbons. The effect of etching on transport and doping level of the graphene nanoribbons is discussed.
Notes: "May 2009."

Thesis (Ph.D., School of Engineering and Applied Sciences (Applied Physics))--Harvard University, 2009.

Includes bibliographical references.