171 research outputs found
Ambipolar Graphene Field Effect Transistors by Local Metal Side Gates
We demonstrate ambipolar graphene field effect transistors individually
controlled by local metal side gates. The side gated field effect can have
on/off ratio comparable with that of the global back gate, and can be tuned in
a large range by the back gate and/or a second side gate. We also find that the
side gated field effect is significantly stronger by electrically floating the
back gate compared to grounding the back gate, consistent with the finding from
electrostatic simulation.Comment: 4 pages, 3 figure
Observation of Quantized Hall Effect and Shubnikov-de Hass Oscillations in Highly Doped Bi2Se3: Evidence for Layered Transport of Bulk Carriers
Bi2Se3 is an important semiconductor thermoelectric material and a prototype
topological insulator. Here we report observation of Shubnikov-de Hass (SdH)
oscillations accompanied by quantized Hall resistances (Rxy) in highly-doped
n-type Bi2Se3 with bulk carrier concentrations of few 10^19 cm^-3. Measurements
under tilted magnetic fields show that the magnetotransport is 2D-like, where
only the c-axis component of the magnetic field controls the Landau level
formation. The quantized step size in 1/Rxy is found to scale with the sample
thickness, and average ~e2/h per quintuple layer (QL). We show that the
observed magnetotransport features do not come from the sample surface, but
arise from the bulk of the sample acting as many parallel 2D electron systems
to give a multilayered quantum Hall effect. Besides revealing a new electronic
property of Bi2Se3, our finding also has important implications for electronic
transport studies of topological insulator materials.Comment: accepted by Physical Review Letters (2012
Synthetic Graphene Grown by Chemical Vapor Deposition on Copper Foils
The discovery of graphene, a single layer of covalently bonded carbon atoms,
has attracted intense interests. Initial studies using mechanically exfoliated
graphene unveiled its remarkable electronic, mechanical and thermal properties.
There has been a growing need and rapid development in large-area deposition of
graphene film and its applications. Chemical vapour deposition on copper has
emerged as one of the most promising methods in obtaining large-scale graphene
films with quality comparable to exfoliated graphene. In this chapter, we
review the synthesis and characterizations of graphene grown on copper foil
substrates by atmospheric pressure chemical vapour deposition. We also discuss
potential applications of such large scale synthetic graphene.Comment: 23 pages, 4 figure
Topological insulator based spin valve devices: evidence for spin polarized transport of spin-momentum-locked topological surface states
Spin-momentum helical locking is one of the most important properties of the
nontrivial topological surface states (TSS) in 3D topological insulators (TI).
It underlies the iconic topological protection (suppressing elastic
backscattering) of TSS and is foundational to many exotic physics (eg.,
majorana fermions) and device applications (eg., spintronics) predicted for
TIs. Based on this spin-momentum locking, a current flowing on the surface of a
TI would be spin-polarized in a characteristic in-plane direction perpendicular
to the current, and the spin-polarization would reverse when the current
direction reverses. Observing such a spin-helical current in transport
measurements is a major goal in TI research and applications. We report
spin-dependent transport measurements in spin valve devices fabricated from
exfoliated thin flakes of Bi2Se3 (a prototype 3D TI) with ferromagnetic (FM) Ni
contacts. Applying an in-plane magnetic (B) field to polarize the Ni contacts
along their easy axis, we observe an asymmetry in the hysteretic
magnetoresistance (MR) between opposite B field directions. The polarity of the
asymmetry in MR can be reversed by reversing the direction of the DC current.
The observed asymmetric MR can be understood as a spin-valve effect between the
current-induced spin polarization on the TI surface (due to
spin-momentum-locking of TSS) and the spin-polarized ferromagnetic contacts.
Our results provide a direct transport evidence for the spin helical current in
TSS.Comment: 10 pages, 3 figure
Quantum and Classical Magnetoresistance in Ambipolar Topological Insulator Transistors with Gate-tunable Bulk and Surface Conduction
Weak antilocalization (WAL) and linear magnetoresistance (LMR) are two most commonly observed magnetoresistance (MR) phenomena in topological insulators (TIs) and often attributed to the Dirac topological surface states (TSS). However, ambiguities exist because these phenomena could also come from bulk states (often carrying significant conduction in many TIs) and are observable even in non-TI materials. Here, we demonstrate back-gated ambipolar TI field-effect transistors in (Bi0.04Sb0.96)(2)Te-3 thin films grown by molecular beam epitaxy on SrTiO3(111), exhibiting a large carrier density tunability (by nearly 2 orders of magnitude) and a metal-insulator transition in the bulk (allowing switching off the bulk conduction). Tuning the Fermi level from bulk band to TSS strongly enhances both the WAL (increasing the number of quantum coherent channels from one to peak around two) and LMR (increasing its slope by up to 10 times). The SS-enhanced LMR is accompanied by a strongly nonlinear Hall effect, suggesting important roles of charge inhomogeneity (and a related classical LMR), although existing models of LMR cannot capture all aspects of our data. Our systematic gate and temperature dependent magnetotransport studies provide deeper insights into the nature of both MR phenomena and reveal differences between bulk and TSS transport in TI related materials
Topological insulator Bi2Te3 films synthesized by metal organic chemical vapor deposition
Topological insulator (TI) materials such as Bi2Te3 and Bi2Se3 have attracted
strong recent interests. Large scale, high quality TI thin films are important
for developing TI-based device applications. In this work, structural and
electronic properties of Bi2Te3 thin films deposited by metal organic chemical
vapor deposition (MOCVD) on GaAs (001) substrates were characterized via X-ray
diffraction (XRD), Raman spectroscopy, angle-resolved photoemission
spectroscopy (ARPES), and electronic transport measurements. The characteristic
topological surface states (SS) with a single Dirac cone have been clearly
revealed in the electronic band structure measured by ARPES, confirming the TI
nature of the MOCVD Bi2Te3 films. Resistivity and Hall effect measurements have
demonstrated relatively high bulk carrier mobility of ~350 cm^2/Vs at 300K and
~7,400 cm^2/Vs at 15 K. We have also measured the Seebeck coefficient of the
films. Our demonstration of high quality topological insulator films grown by a
simple and scalable method is of interests for both fundamental research and
practical applications of thermoelectric and TI materials.Comment: 14 pages, 4 figure
Large-scale Graphitic Thin Films Synthesized on Ni and Transferred to Insulators: Structural and Electronic Properties
We present a comprehensive study of the structural and electronic properties
of ultrathin films containing graphene layers synthesized by chemical vapor
deposition (CVD) based surface segregation on polycrystalline Ni foils then
transferred onto insulating SiO2/Si substrates. Films of size up to several
mm's have been synthesized. Structural characterizations by atomic force
microscopy (AFM), scanning tunneling microscopy (STM), cross-sectional
transmission electron microscopy (XTEM) and Raman spectroscopy confirm that
such large scale graphitic thin films (GTF) contain both thick graphite regions
and thin regions of few layer graphene. The films also contain many wrinkles,
with sharply-bent tips and dislocations revealed by XTEM, yielding insights on
the growth and buckling processes of the GTF. Measurements on mm-scale
back-gated transistor devices fabricated from the transferred GTF show
ambipolar field effect with resistance modulation ~50% and carrier mobilities
reaching ~2000 cm^2/Vs. We also demonstrate quantum transport of carriers with
phase coherence length over 0.2 m from the observation of 2D weak
localization in low temperature magneto-transport measurements. Our results
show that despite the non-uniformity and surface roughness, such large-scale,
flexible thin films can have electronic properties promising for device
applications.Comment: This version (as published) contains additional data, such as cross
sectional TEM image
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