SCANNING TUNNELING SPECTROSCOPY OF A GATED GRAPHENE DEVICE IN THE QUANTUM HALL REGIME
Gregory M. Rutter1, Suyong Jung1,2, Nikolai B. Zhitenev1, Joseph A. Stroscio1
1CNST, NIST, Gaithersburg, MD
2Maryland NanoCenter, UMD, College Park, MD
In this work, scanning tunneling spectroscopy
is used to investigate the magnetic quantization of a single-layer exfoliated
graphene device at a temperature of 4.3 K in magnetic fields up to 8 T. Exfoliated
graphene samples were fabricated by removing graphene layers from natural
graphite, then placing them onto a SiO2/Si substrate. A gold
electrode, used for the tunneling bias, was deposited using a stencil mask. Single-layer
graphene is then probed with both the application of a perpendicular magnetic
field and with an external gate voltage applied to the Si substrate below the
graphene. Under high magnetic field conditions, charge carriers in graphene are
condensed into well-defined degenerate states called Landau levels (LL), which
have an energy dependence of 
(where n is the
Landau index and B is the magnetic field).[1] The Landau level distribution is
centered around the n = 0 LL, which resides at the charge neutrality point in
graphene, called the Dirac point. By varying the charge density (ne)
with the externally applied gate potential, we can move the LL distribution
with respect to the Fermi energy (EF) allowing the filling of
different Landau levels. Detailed plots of the differential conductance, dI/dV(E,
ne), show a combination of plateaus followed by discrete steps at each
Landau level as the carrier density (gate potential) is varied. This behavior,
observed in GaAs heterostructures, has been attributed to chemical potential
jumps as each Landau level is pinned to EF.[2] Additional sharp
peaks observed in the dI/dV measurements, which originate from different types
of localized states, will be also discussed.
[1] D. L. Miller et al., Science 324, 924 (2009).
[2] O. E. Dial et al., Nature 448, 176 (2007).