364 research outputs found
Wafer-scale Epitaxial Graphene Growth on the Si-face of Hexagonal SiC (0001) for High Frequency Transistors
Up to two layers of epitaxial graphene have been grown on the Si-face of
two-inch SiC wafers exhibiting room-temperature Hall mobilities up to 1800
cm^2/Vs, measured from ungated, large, 160 micron x 200 micron Hall bars, and
up to 4000 cm^2/Vs, from top-gated, small, 1 micron x 1.5 micron Hall bars. The
growth process involved a combination of a cleaning step of the SiC in a
Si-containing gas, followed by an annealing step in Argon for epitaxial
graphene formation. The structure and morphology of this graphene has been
characterized using AFM, HRTEM, and Raman spectroscopy. Furthermore, top-gated
radio frequency field effect transistors (RF-FETs) with a peak cutoff frequency
fT of 100 GHz for a gate length of 240 nm were fabricated using epitaxial
graphene grown on the Si face of SiC that exhibited Hall mobilities up to 1450
cm^2/Vs from ungated Hall bars and 1575 cm^2/Vs from top-gated ones. This is by
far the highest cut-off frequency measured from any kind of graphene.Comment: 30 pages (double line spacing). Submitte
High-order harmonic generation in Xe, Kr, and Ar driven by a 2.1-\mu m source: high-order harmonic spectroscopy under macroscopic effects
We experimentally and numerically study the atomic response and pulse
propagation effects of high-order harmonics generated in Xe, Kr, and Ar driven
by a 2.1-\mu m infrared femtosecond light source. The light source is an
optical parametric chirped-pulse amplifier, and a modified strong-field
approximation and 3-dimensional pulse propagation code are used for the
numerical simulations. The extended cutoff in the long-wavelength driven
high-harmonic generation has revealed the spectral shaping of high-order
harmonics due to the atomic structure (or photo-recombination cross-section)
and the macroscopic effects, which are the main factors of determining the
conversion efficiency besides the driving wavelength. Using precise numerical
simulations to determine the macroscopic electron wavepacket, we are able to
extract the photo-recombination cross-sections from experimental high-order
harmonic spectra in the presence of macroscopic effects. We have experimentally
observed that the macroscopic effects shift the observed Cooper minimum of Kr
from 80 eV to 60-70 eV and wash out the Cooper minimum of Ar. Measured
high-harmonic conversion efficiencies per harmonic near the cutoff are ~10^{-9}
for all three gases.Comment: 19 pages, 8 figure
Strong and broadly tunable plasmon resonances in thick films of aligned carbon nanotubes
Low-dimensional plasmonic materials can function as high quality terahertz
and infrared antennas at deep subwavelength scales. Despite these antennas'
strong coupling to electromagnetic fields, there is a pressing need to further
strengthen their absorption. We address this problem by fabricating thick films
of aligned, uniformly sized carbon nanotubes and showing that their plasmon
resonances are strong, narrow, and broadly tunable. With thicknesses ranging
from 25 to 250 nm, our films exhibit peak attenuation reaching 70%, quality
factors reaching 9, and electrostatically tunable peak frequencies by a factor
of 2.3x. Excellent nanotube alignment leads to the attenuation being 99%
linearly polarized along the nanotube axis. Increasing the film thickness
blueshifts the plasmon resonators down to peak wavelengths as low as 1.4
micrometers, promoting them to a new near-infrared regime in which they can
both overlap the S11 nanotube exciton energy and access the technologically
important infrared telecom band.Comment: 19 pages, 5 figures, main text followed by supporting informatio
Nanoscale control of exchange bias with BiFeO3 thin films
We demonstrate a direct correlation between the domain structure of
multiferroic BiFeO3 thin films and exchange bias of Co0.9Fe0.1/BiFeO3
heterostructures. Two distinct types of interactions, an enhancement of the
coercive field (exchange enhancement) and an enhancement of the coercive field
combined with large shifts of the hysteresis loop (exchange bias), have been
observed in these heterostructures, which depend directly on the type and
crystallography of the nanoscale (2 nm) domain walls in the BiFeO3 film. We
show that the magnitude of the exchange bias interaction scales with the length
of 109 degree ferroelectric domain walls in the BiFeO3 thin films which have
been probed via piezoresponse force microscopy and x-ray magnetic circular
dichroism.Comment: Accepted to Nano Letters May 200
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