1,250 research outputs found
Fast pick up technique for high quality heterostructures of bilayer graphene and hexagonal boron nitride
We present a fast method to fabricate high quality heterostructure devices by
picking up crystals of arbitrary sizes. Bilayer graphene is encapsulated with
hexagonal boron nitride to demonstrate this approach, showing good electronic
quality with mobilities ranging from 17 000 cm^2/V/s at room temperature to 49
000 cm^2/V/s at 4.2 K, and entering the quantum Hall regime below 0.5 T. This
method provides a strong and useful tool for the fabrication of future high
quality layered crystal devices.Comment: 5 pages, 3 figure
Controlling spin relaxation in hexagonal BN-encapsulated graphene with a transverse electric field
We experimentally study the electronic spin transport in hBN encapsulated
single layer graphene nonlocal spin valves. The use of top and bottom gates
allows us to control the carrier density and the electric field independently.
The spin relaxation times in our devices range up to 2 ns with spin relaxation
lengths exceeding 12 m even at room temperature. We obtain that the ratio
of the spin relaxation time for spins pointing out-of-plane to spins in-plane
is 0.75 for zero applied perpendicular
electric field. By tuning the electric field this anisotropy changes to
0.65 at 0.7 V/nm, in agreement with an electric field tunable in-plane
Rashba spin-orbit coupling
Linear scaling between momentum and spin scattering in graphene
Spin transport in graphene carries the potential of a long spin diffusion
length at room temperature. However, extrinsic relaxation processes limit the
current experimental values to 1-2 um. We present Hanle spin precession
measurements in gated lateral spin valve devices in the low to high (up to
10^13 cm^-2) carrier density range of graphene. A linear scaling between the
spin diffusion length and the diffusion coefficient is observed. We measure
nearly identical spin- and charge diffusion coefficients indicating that
electron-electron interactions are relatively weak and transport is limited by
impurity potential scattering. When extrapolated to the maximum carrier
mobilities of 2x10^5 cm^2/Vs, our results predict that a considerable increase
in the spin diffusion length should be possible
A transfer technique for high mobility graphene devices on commercially available hexagonal boron nitride
We present electronic transport measurements of single- and bilayer graphene
on commercially available hexagonal boron nitride. We extract mobilities as
high as 125 000 cm^2/V/s at room temperature and 275 000 cm^2/V/s at 4.2 K. The
excellent quality is supported by the early development of the nu = 1 quantum
Hall plateau at a magnetic field of 5 T and temperature of 4.2 K. We also
present a new and accurate transfer technique of graphene to hexagonal boron
nitride crystals. This technique is simple, fast and yields atomically flat
graphene on boron nitride which is almost completely free of bubbles or
wrinkles. The potential of commercially available boron nitride combined with
our transfer technique makes high mobility graphene devices more accessible.Comment: 3 pages, 3 figure
Spin transport in graphene nanostructures
Graphene is an interesting material for spintronics, showing long spin
relaxation lengths even at room temperature. For future spintronic devices it
is important to understand the behavior of the spins and the limitations for
spin transport in structures where the dimensions are smaller than the spin
relaxation length. However, the study of spin injection and transport in
graphene nanostructures is highly unexplored. Here we study the spin injection
and relaxation in nanostructured graphene with dimensions smaller than the spin
relaxation length. For graphene nanoislands, where the edge length to area
ratio is much higher than for standard devices, we show that enhanced spin-flip
processes at the edges do not seem to play a major role in the spin relaxation.
On the other hand, contact induced spin relaxation has a much more dramatic
effect for these low dimensional structures. By studying the nonlocal spin
transport through a graphene quantum dot we observe that the obtained values
for spin relaxation are dominated by the connecting graphene islands and not by
the quantum dot itself. Using a simple model we argue that future nonlocal
Hanle precession measurements can obtain a more significant value for the spin
relaxation time for the quantum dot by using high spin polarization contacts in
combination with low tunneling rates
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Temperature dependent transport characteristics of graphene/n-Si diodes
Realizing an optimal Schottky interface of graphene on Si is challenging, as
the electrical transport strongly depends on the graphene quality and the
fabrication processes. Such interfaces are of increasing research interest for
integration in diverse electronic devices as they are thermally and chemically
stable in all environments, unlike standard metal/semiconductor interfaces. We
fabricate such interfaces with n-type Si at ambient conditions and find their
electrical characteristics to be highly rectifying, with minimal reverse
leakage current (10 A) and rectification of more than . We
extract Schottky barrier height of 0.69 eV for the exfoliated graphene and 0.83
eV for the CVD graphene devices at room temperature. The temperature dependent
electrical characteristics suggest the influence of inhomogeneities at the
graphene/n-Si interface. A quantitative analysis of the inhomogeneity in
Schottky barrier heights is presented using the potential fluctuation model
proposed by Werner and G\"{u}ttler.Comment: 5 pages, 5 figure
Large yield production of high mobility freely suspended graphene electronic devices on a PMGI based organic polymer
The recent observation of fractional quantum Hall effect in high mobility
suspended graphene devices introduced a new direction in graphene physics, the
field of electron-electron interaction dynamics. However, the technique used
currently for the fabrication of such high mobility devices has several
drawbacks. The most important is that the contact materials available for
electronic devices are limited to only a few metals (Au, Pd, Pt, Cr and Nb)
since only those are not attacked by the reactive acid (BHF) etching
fabrication step. Here we show a new technique which leads to mechanically
stable suspended high mobility graphene devices which is compatible with almost
any type of contact material. The graphene devices prepared on a
polydimethylglutarimide based organic resist show mobilities as high as 600.000
cm^2/Vs at an electron carrier density n = 5.0 10^9 cm^-2 at 77K. This
technique paves the way towards complex suspended graphene based spintronic,
superconducting and other types of devices.Comment: 14 pages, 4 figure
Spin transport in high quality suspended graphene devices
We measure spin transport in high mobility suspended graphene (\mu ~ 10^5
cm^2/Vs), obtaining a (spin) diffusion coefficient of 0.1 m^2/s and giving a
lower bound on the spin relaxation time (\tau_s ~ 150 ps) and spin relaxation
length (\lambda_s=4.7 \mu m) for intrinsic graphene. We develop a theoretical
model considering the different graphene regions of our devices that explains
our experimental data.Comment: 22 pages, 6 figures; Nano Letters, Article ASAP (2012)
(http://pubs.acs.org/doi/abs/10.1021/nl301050a
Electronic spin transport in graphene field effect transistors
Spin transport experiments in graphene, a single layer of carbon atoms,
indicate spin relaxation times that are significantly shorter than the
theoretical predictions. We investigate experimentally whether these short spin
relaxation times are due to extrinsic factors, such as spin relaxation caused
by low impedance contacts, enhanced spin flip processes at the device edges or
the presence of an aluminium oxide layer on top of graphene in some samples.
Lateral spin valve devices using a field effect transistor geometry allowed for
the investigation of the spin relaxation as a function of the charge density,
going continuously from metallic hole to electron conduction (charge densities
of cm) via the Dirac charge neutrality point (). The results are quantitatively described by a one dimensional spin
diffusion model where the spin relaxation via the contacts is taken into
account. Spin valve experiments for various injector/detector separations and
spin precession experiments reveal that the longitudinal (T) and the
transversal (T) relaxation times are similar. The anisotropy of the spin
relaxation times and , when the spins are injected
parallel or perpendicular to the graphene plane, indicates that the effective
spin orbit fields do not lie exclusively in the two dimensional graphene plane.
Furthermore, the proportionality between the spin relaxation time and the
momentum relaxation time indicates that the spin relaxation mechanism is of the
Elliott-Yafet type. For carrier mobilities of 2-5 cm2^/Vs and
for graphene flakes of 0.1-2 m in width, we found spin relaxation times of
the order of 50-200 ps, times which appear not to be determined by the
extrinsic factors mentioned above.Comment: 11 pages, 13 figure
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