50 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
Main Belt Asteroids with WISE/NEOWISE I: Preliminary Albedos and Diameters
We present initial results from the Wide-field Infrared Survey Explorer
(WISE), a four-band all-sky thermal infrared survey that produces data well
suited to measuring the physical properties of asteroids, and the NEOWISE
enhancement to the WISE mission allowing for detailed study of Solar system
objects. Using a NEATM thermal model fitting routine we compute diameters for
over 100,000 Main Belt asteroids from their IR thermal flux, with errors better
than 10%. We then incorporate literature values of visible measurements (in the
form of the H absolute magnitude) to determine albedos. Using these data we
investigate the albedo and diameter distributions of the Main Belt. As observed
previously, we find a change in the average albedo when comparing the inner,
middle, and outer portions of the Main Belt. We also confirm that the albedo
distribution of each region is strongly bimodal. We observe groupings of
objects with similar albedos in regions of the Main Belt associated with
dynamical breakup families. Asteroid families typically show a characteristic
albedo for all members, but there are notable exceptions to this. This paper is
the first look at the Main Belt asteroids in the WISE data, and only represents
the preliminary, observed raw size and albedo distributions for the populations
considered. These distributions are subject to survey biases inherent to the
NEOWISE dataset and cannot yet be interpreted as describing the true
populations; the debiased size and albedo distributions will be the subject of
the next paper in this series.Comment: Accepted to ApJ. Online table to also appear on the publisher's
websit
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
WISE/NEOWISE observations of Active Bodies in the Main Belt
We report results based on mid-infrared photometry of 5 active main belt
objects (AMBOs) detected by the Wide-field Infrared Survey Explorer (WISE)
spacecraft. Four of these bodies, P/2010 R2 (La Sagra), 133P/Elst-Pizarro,
(596) Scheila, and 176P/LINEAR, showed no signs of activity at the time of the
observations, allowing the WISE detections to place firm constraints on their
diameters and albedos. Geometric albedos were in the range of a few percent,
and on the order of other measured comet nuclei. P/2010 A2 was observed on
April 2-3, 2010, three months after its peak activity. Photometry of the coma
at 12 and 22 {\mu}m combined with ground-based visible-wavelength measurements
provides constraints on the dust particle mass distribution (PMD), dlogn/dlogm,
yielding power-law slope values of {\alpha} = -0.5 +/- 0.1. This PMD is
considerably more shallow than that found for other comets, in particular
inbound particle fluence during the Stardust encounter of comet 81P/Wild 2. It
is similar to the PMD seen for 9P/Tempel 1 in the immediate aftermath of the
Deep Impact experiment. Upper limits for CO2 & CO production are also provided
for each AMBO and compared with revised production numbers for WISE
observations of 103P/Hartley 2.Comment: 32 Pages, including 5 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
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
Charge transfer equilibria in ambient-exposed epitaxial graphene on (0001¯) 6 H-SiC
Article discussing research on charge transfer equilibria in ambient-exposed epitaxial graphene on (0001) 6 H-SiC