54 research outputs found
A method to extract pure Raman spectrum of epitaxial graphene on SiC
A method is proposed to extract pure Raman spectrum of epitaxial graphene on
SiC by using a Non-negative Matrix Factorization. It overcomes problems of
negative spectral intensity and poorly resolved spectra resulting from a simple
subtraction of a SiC background from the experimental data. We also show that
the method is similar to deconvolution, for spectra composed of multiple sub-
micrometer areas, with the advantage that no prior information on the impulse
response functions is needed. We have used this property to characterize the
Raman laser beam. The method capability in efficient data smoothing is also
demonstrated.Comment: 3 figures, regular pape
Spectral Current Density and Responsivity Scaling for Fourier Transform Photocurrent Spectroscopy
We propose and experimentally verify two methods to scale arbitrary units to
photocurrent spectral density (A/eV) in Fourier Transform Photocurrent (FTPC)
spectroscopy. We also propose the FTPC scaling to responsivity (A/W), provided
a narrow-band optical power measurement is available. The constant background
of the interferogram provides a precise determination of the photocurrent
spectral density. The second method relies on the scaled amplitude of the
interferogram. Although the latter method leads to more significant errors, it
still provides good order of magnitude estimates of the total photocurrent. We
demonstrate the technique on a calibrated InGaAs diode and weak responsivity
SiC interdigital sensors. We identify a series of impurity-band and interband
transitions in the SiC sensors.Comment: 5 pages, 4 figure
Controlled epitaxial graphene growth within amorphous carbon corrals
Structured growth of high quality graphene is necessary for technological
development of carbon based electronics. Specifically, control of the bunching
and placement of surface steps under epitaxial graphene on SiC is an important
consideration for graphene device production. We demonstrate lithographically
patterned evaporated amorphous carbon corrals as a method to pin SiC surface
steps. Evaporated amorphous carbon is an ideal step-flow barrier on SiC due to
its chemical compatibility with graphene growth and its structural stability at
high temperatures, as well as its patternability. The amorphous carbon is
deposited in vacuum on SiC prior to graphene growth. In the graphene furnace at
temperatures above 1200C, mobile SiC steps accumulate at these
amorphous carbon barriers, forming an aligned step free region for graphene
growth at temperatures above 1330C. AFM imaging and Raman spectroscopy
support the formation of quality step-free graphene sheets grown on SiC with
the step morphology aligned to the carbon grid
Wafer bonding solution to epitaxial graphene - silicon integration
The development of graphene electronics requires the integration of graphene
devices with Si-CMOS technology. Most strategies involve the transfer of
graphene sheets onto silicon, with the inherent difficulties of clean transfer
and subsequent graphene nano-patterning that degrades considerably the
electronic mobility of nanopatterned graphene. Epitaxial graphene (EG) by
contrast is grown on an essentially perfect crystalline (semi-insulating)
surface, and graphene nanostructures with exceptional properties have been
realized by a selective growth process on tailored SiC surface that requires no
graphene patterning. However, the temperatures required in this structured
growth process are too high for silicon technology. Here we demonstrate a new
graphene to Si integration strategy, with a bonded and interconnected compact
double-wafer structure. Using silicon-on-insulator technology (SOI) a thin
monocrystalline silicon layer ready for CMOS processing is applied on top of
epitaxial graphene on SiC. The parallel Si and graphene platforms are
interconnected by metal vias. This method inspired by the industrial
development of 3d hyper-integration stacking thin-film electronic devices
preserves the advantages of epitaxial graphene and enables the full spectrum of
CMOS processing.Comment: 15 pages, 7 figure
Investigation of internal electric fields in graphene/6H-SiC under illumination by Pockels effect
In this paper, we introduce a method for mapping profiles of internal
electric fields in birefringent crystals based on the electro-optic Pockels
effect and measuring phase differences of low-intensity polarized light. In the
case of the studied 6H-SiC crystal with graphene electrodes, the experiment is
significantly affected by birefringence at zero bias voltage applied to the
crystal and a strong thermo-optical effect. We dealt with these phenomena by
adding a Soleil-Babinet compensator and using considerations based on
measurements of crystal heating under laser illumination. The method can be
generalized and adapted to any Pockels crystal that can withstand sufficiently
high voltages. We demonstrate the significant formation of space charge in
semi-insulating 6H-SiC under illumination by above-bandgap light
Local work function measurements of plasma-fluorinated epitaxial graphene
International audiencePlasma-fluorination is an attractive route toward the work function engineering of graphene. The effect of surface topography of epitaxial graphene grown on silicon carbide on the increase in work function after plasma-fluorination was investigated using scanning Kelvin probe microscopy. Results of these studies demonstrate the ability of plasma-treatments to functionalize epitaxial graphene without significant surface roughening. For few-layer epitaxial graphene on the Si-face, work function distribution corresponds to its surface topography. A bimodal distribution is observed before and after fluorination and the separation between the two modes widens after the fluorination. For multi-layer epitaxial graphene on the C-face, no correlation is observed between the work function distribution and the surface topography. After fluorination, the work function is fairly uniform except in few peeled off areas that show a stronger work function contrast
Plasmon-plasmon interaction and the role of buffer in epitaxial graphene micro-flakes
We investigate the origin of the translational symmetry breaking in
epitaxially grown single-layer graphene. Despite the surface morphology of
homogeneous graphene films influenced by the presence of mutually parallel SiC
surface terraces, the far-infrared magneto-plasmon absorption is almost
independent of the angle between the probing light polarization and the
orientation of terraces. Based on a detailed analysis of the plasmon absorption
lineshape and its behavior in the magnetic field, supported by confocal Raman
mapping and atomic force microscopy, we explain this discrepancy by
spontaneously formed graphene micro flakes. We further support our conclusions
using data collected on artificially created graphene nanoribbons: we recognize
similar plasmon origin in artificial ribbons and naturally formed grains. An
unexpectedly large plasmon resonance redshift was observed in nanoribbons. In a
hydrogen-intercalated sample (which does not contain the buffer), this redshift
is quantitatively taken into account by a plasmon-plasmon interaction. In
non-intercalated samples featuring a buffer layer, this redshift is due to an
interplay between the plasmon-plasmon coupling and Coulomb screening by the
buffer-induced interface states. This model determines the density of interface
states in good agreement with experimentally reported values.Comment: 17 pages, 10 figures, 4 table
Circular polarization in a non-magnetic resonant tunneling device
We have investigated the polarization-resolved photoluminescence (PL) in an asymmetric n-type GaAs/AlAs/GaAlAs resonant tunneling diode under magnetic field parallel to the tunnel current. The quantum well (QW) PL presents strong circular polarization (values up to -70% at 19 T). The optical emission from GaAs contact layers shows evidence of highly spin-polarized two-dimensional electron and hole gases which affects the spin polarization of carriers in the QW. However, the circular polarization degree in the QW also depends on various other parameters, including the g-factors of the different layers, the density of carriers along the structure, and the Zeeman and Rashba effects
Axion Search by Laser-based Experiment OSQAR
International audienceLaser-based experimentOSQAR in CERN is aimed to the search of the axions by twomethods. The photon regeneration experiment is using two LHC dipole magnets of the length 14.3 m and magnetic field 9.5 T equipped with an optical barrier at the end of the first magnet. It looks as light shining through the wall. No excess of events above the background was detected at this arrangement. Nevertheless, this result extends the exclusion region for the axion mass. The second method wants to measure the ultra-fine Vacuum Magnetic Birefringence for the first time. An optical scheme with electro-optical modulator has been proposed, validated and subsequently improved. Cotton-Mouton constant for air was determined in this experiment setup
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