18 research outputs found
Detecting the local transport properties and the dimensionality of transport of epitaxial graphene by a multi-point probe approach
The electronic transport properties of epitaxial monolayer graphene (MLG) and
hydrogen-intercalated quasi free-standing bilayer graphene (QFBLG) on SiC(0001)
are investigated by micro multi-point probes. Using a probe with 12 contacts,
we perform four-point probe measurements with the possibility to effectively
vary the contact spacing over more than one order of magnitude, allowing us to
establish that the transport is purely two-dimensional. Combined with the
carrier density obtained by angle-resolved photoemission spectroscopy, we find
the room temperature mobility of MLG to be (870+-120)cm2/Vs. The transport in
QFBLG is also found to be two-dimensional with a mobility of (1600+-160)
cm2/Vs
Ramifications of Optical Pumping on the Interpretation of Time-Resolved Photoemission Experiments on Graphene
In pump-probe time and angle-resolved photoemission spectroscopy (TR-ARPES)
experiments the presence of the pump pulse adds a new level of complexity to
the photoemission process in comparison to conventional ARPES. This is
evidenced by pump-induced vacuum space-charge effects and surface
photovoltages, as well as multiple pump excitations due to internal reflections
in the sample-substrate system. These processes can severely affect a correct
interpretation of the data by masking the out-of-equilibrium electron dynamics
intrinsic to the sample. In this study, we show that such effects indeed
influence TR-ARPES data of graphene on a silicon carbide (SiC) substrate. In
particular, we find a time- and laser fluence-dependent spectral shift and
broadening of the acquired spectra, and unambiguously show the presence of a
double pump excitation. The dynamics of these effects is slower than the
electron dynamics in the graphene sample, thereby permitting us to deconvolve
the signals in the time domain. Our results demonstrate that complex
pump-related processes should always be considered in the experimental setup
and data analysis.Comment: 9 pages, 4 figure
Ultrafast Dynamics of Massive Dirac Fermions in Bilayer Graphene
Bilayer graphene is a highly promising material for electronic and
optoelectronic applications since it is supporting massive Dirac fermions with
a tuneable band gap. However, no consistent picture of the gap's effect on the
optical and transport behavior has emerged so far, and it has been proposed
that the insulating nature of the gap could be compromised by unavoidable
structural defects, by topological in-gap states, or that the electronic
structure could be altogether changed by many-body effects. Here we directly
follow the excited carriers in bilayer graphene on a femtosecond time scale,
using ultrafast time- and angle-resolved photoemission. We find a behavior
consistent with a single-particle band gap. Compared to monolayer graphene, the
existence of this band gap leads to an increased carrier lifetime in the
minimum of the lowest conduction band. This is in sharp contrast to the second
sub-state of the conduction band, in which the excited electrons decay through
fast, phonon-assisted inter-band transitions.Comment: 5 pages, 4 figure
Direct view on the ultrafast carrier dynamics in graphene
The ultrafast dynamics of excited carriers in graphene is closely linked to
the Dirac spectrum and plays a central role for many electronic and
optoelectronic applications. Harvesting energy from excited electron-hole
pairs, for instance, is only possible if these pairs can be separated before
they lose energy to vibrations, merely heating the lattice. While the hot
carrier dynamics in graphene could so far only be accessed indirectly, we here
present a direct time-resolved view on the Dirac cone by angle-resolved
photoemission (ARPES). This allows us to show the quasi-instant thermalisation
of the electron gas to a temperature of more than 2000 K; to determine the
time-resolved carrier density; to disentangle the subsequent decay into
excitations of optical phonons and acoustic phonons (directly and via
supercollisions); and to show how the presence of the hot carrier distribution
affects the lifetime of the states far below the Fermi energy.Comment: 15 pages, 4 figure
Detecting the local transport properties and the dimensionality of transport of epitaxial graphene by a multi-point probe approach
Polarization doping of graphene on silicon carbide
The doping of quasi-freestanding graphene (QFG) on H-terminated, Si-face 6H-, 4H-, and 3C-SiC is studied by angle-resolved photoelectron spectroscopy (ARPES) close to the Dirac point. Using semi-insulating as well as n-type doped substrates we shed light on the contributions to the charge carrier density in QFG caused by i) the spontaneous polarization of the substrate, and ii) the band alignment between the substrate and the graphene layer. In this way we provide quantitative support for the previously suggested model of polarization doping of graphene on SiC [Phys. Rev. Lett. 108, 246104 (2012)]
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