23 research outputs found
Graphene formation on SiC substrates
Graphene layers were created on both C and Si faces of semi-insulating,
on-axis, 4H- and 6H-SiC substrates. The process was performed under high vacuum
(<10-4 mbar) in a commercial chemical vapor deposition SiC reactor. A method
for H2 etching the on-axis sub-strates was developed to produce surface steps
with heights of 0.5 nm on the Si-face and 1.0 to 1.5 nm on the C-face for each
polytype. A process was developed to form graphene on the substrates
immediately after H2 etching and Raman spectroscopy of these samples confirmed
the formation of graphene. The morphology of the graphene is described. For
both faces, the underlying substrate morphology was significantly modified
during graphene formation; sur-face steps were up to 15 nm high and the uniform
step morphology was sometimes lost. Mo-bilities and sheet carrier
concentrations derived from Hall Effect measurements on large area (16 mm
square) and small area (2 and 10 um square) samples are presented and shown to
compare favorably to recent reports.Comment: European Conference on Silicon Carbide and Related Materials 2008
(ECSCRM '08), 4 pages, 4 figure
Improvement of Morphology and Free Carrier Mobility through Argon-Assisted Growth of Epitaxial Graphene on Silicon Carbide
Graphene was epitaxially grown on both the C- and Si-faces of 4H- and
6H-SiC(0001) under an argon atmosphere and under high vacuum conditions.
Following growth, samples were imaged with Nomarski interference contrast and
atomic force microscopies and it was found that growth under argon led to
improved morphologies on the C-face films but the Si-face films were not
significantly affected. Free carrier transport studies were conducted through
Hall effect measurements, and carrier mobilities were found to increase and
sheet carrier densities were found to decrease for those films grown under
argon as compared to high vacuum conditions. The improved mobilities and
concurrent decreases in sheet carrier densities suggest a decrease in
scattering in the films grown under argon.Comment: 215th Meeting of the Electrochemical Society (ECS 215), 14 pages, 6
figure
Hall Effect Mobility of Epitaxial Graphene Grown on Silicon Carbide
Epitaxial graphene films were grown in vacuo by silicon sublimation from the
(0001) and (000-1) faces of 4H- and 6H-SiC. Hall effect mobilities and sheet
carrier densities of the films were measured at 300 K and 77 K and the data
depended on the growth face. About 40% of the samples exhibited holes as the
dominant carrier, independent of face. Generally, mobilities increased with
decreasing carrier density, independent of carrier type and substrate polytype.
The contributions of scattering mechanisms to the conductivities of the films
are discussed. The results suggest that for near-intrinsic carrier densities at
300 K epitaxial graphene mobilities will be ~150,000 cm2V-1s-1 on the (000-1)
face and ~5,800 cm2V-1s-1 on the (0001) face.Comment: Accepted for publication in Applied Physics Letters, 10 pages, 2
figure
Morphology Characterization of Argon-Mediated Epitaxial Graphene on C-face SiC
Epitaxial graphene layers were grown on the C-face of 4H- and 6H-SiC using an
argon-mediated growth process. Variations in growth temperature and pressure
were found to dramatically affect the morphological properties of the layers.
The presence of argon during growth slowed the rate of graphene formation on
the C-face and led to the observation of islanding. The similarity in the
morphology of the islands and continuous films indicated that island nucleation
and coalescence is the growth mechanism for C-face graphene.Comment: 12 pages, 4 figure
Epitaxial Graphene Growth on SiC Wafers
An in vacuo thermal desorption process has been accomplished to form
epitaxial graphene (EG) on 4H- and 6H-SiC substrates using a commercial
chemical vapor deposition reactor. Correlation of growth conditions and the
morphology and electrical properties of EG are described. Raman spectra of EG
on Si-face samples were dominated by monolayer thickness. This approach was
used to grow EG on 50 mm SiC wafers that were subsequently fabricated into
field effect transistors with fmax of 14 GHz.Comment: 215th Meeting of the Electrochemical Society, 8 pages, 8 figure
LSST: from Science Drivers to Reference Design and Anticipated Data Products
(Abridged) We describe here the most ambitious survey currently planned in
the optical, the Large Synoptic Survey Telescope (LSST). A vast array of
science will be enabled by a single wide-deep-fast sky survey, and LSST will
have unique survey capability in the faint time domain. The LSST design is
driven by four main science themes: probing dark energy and dark matter, taking
an inventory of the Solar System, exploring the transient optical sky, and
mapping the Milky Way. LSST will be a wide-field ground-based system sited at
Cerro Pach\'{o}n in northern Chile. The telescope will have an 8.4 m (6.5 m
effective) primary mirror, a 9.6 deg field of view, and a 3.2 Gigapixel
camera. The standard observing sequence will consist of pairs of 15-second
exposures in a given field, with two such visits in each pointing in a given
night. With these repeats, the LSST system is capable of imaging about 10,000
square degrees of sky in a single filter in three nights. The typical 5
point-source depth in a single visit in will be (AB). The
project is in the construction phase and will begin regular survey operations
by 2022. The survey area will be contained within 30,000 deg with
, and will be imaged multiple times in six bands, ,
covering the wavelength range 320--1050 nm. About 90\% of the observing time
will be devoted to a deep-wide-fast survey mode which will uniformly observe a
18,000 deg region about 800 times (summed over all six bands) during the
anticipated 10 years of operations, and yield a coadded map to . The
remaining 10\% of the observing time will be allocated to projects such as a
Very Deep and Fast time domain survey. The goal is to make LSST data products,
including a relational database of about 32 trillion observations of 40 billion
objects, available to the public and scientists around the world.Comment: 57 pages, 32 color figures, version with high-resolution figures
available from https://www.lsst.org/overvie
The Effect Of Preparation Conditions On Raman And Photoluminescence Of Monolayer Ws2
We report on preparation dependent properties observed in monolayer WS2 samples synthesized via chemical vapor deposition (CVD) on a variety of common substrates (Si/SiO2, sapphire, fused silica) as well as samples that were transferred from the growth substrate onto a new substrate. The as-grown CVD materials (as-WS2) exhibit distinctly different optical properties than transferred WS2 (x-WS2). In the case of CVD growth on Si/SiO2, following transfer to fresh Si/SiO2 there is a ∼50 meV shift of the ground state exciton to higher emission energy in both photoluminescence emission and optical reflection. This shift is indicative of a reduction in tensile strain by ∼0.25%. Additionally, the excitonic state in x-WS2 is easily modulated between neutral and charged exciton by exposure to moderate laser power, while such optical control is absent in as-WS2 for all growth substrates investigated. Finally, we observe dramatically different laser power-dependent behavior for as-grown and transferred WS2. These results demonstrate a strong sensitivity to sample preparation that is important for both a fundamental understanding of these novel materials as well as reliable reproduction of device properties
Field emission energy distribution and three-terminal current-voltage characteristics from planar graphene edges
We demonstrate field emission from an integrated three-terminal device using a suspended planar graphene edge as the source of vacuum electrons. Energy spectra of the emitted electrons confirm the field-emission mechanism. The energy spectra produced by graphene grown by chemical vapor deposition and reduced graphene oxide are compared. The drain source voltage required to produce a given drain current increases when negative voltages are applied to the gate, confirming field-effect transistor operation. The emission current rises exponentially with inverse voltage over the measured current range from 1 pA to 10 nA. The current-voltage characteristics are consistent with tunneling through barrier potentials calculated numerically from the device geometry.11Nsciescopu