99 research outputs found
Origin of the Mosaicity in Graphene Grown on Cu(111)
We use low-energy electron microscopy to investigate how graphene grows on
Cu(111). Graphene islands first nucleate at substrate defects such as step
bunches and impurities. A considerable fraction of these islands can be
rotationally misaligned with the substrate, generating grain boundaries upon
interisland impingement. New rotational boundaries are also generated as
graphene grows across substrate step bunches. Thus, rougher substrates lead to
higher degrees of mosaicity than do flatter substrates. Increasing the growth
temperature improves crystallographic alignment. We demonstrate that graphene
growth on Cu(111) is surface diffusion limited by comparing simulations of the
time evolution of island shapes with experiments. Islands are dendritic with
distinct lobes, but unlike the polycrystalline, four-lobed islands observed on
(100)-textured Cu foils, each island can be a single crystal. Thus, epitaxial
graphene on smooth, clean Cu(111) has fewer structural defects than it does on
Cu(100).Comment: Article revised following reviewer comment
Real-time observation of epitaxial graphene domain reorientation.
Graphene films grown by vapour deposition tend to be polycrystalline due to the nucleation and growth of islands with different in-plane orientations. Here, using low-energy electron microscopy, we find that micron-sized graphene islands on Ir(111) rotate to a preferred orientation during thermal annealing. We observe three alignment mechanisms: the simultaneous growth of aligned domains and dissolution of rotated domains, that is, 'ripening'; domain boundary motion within islands; and continuous lattice rotation of entire domains. By measuring the relative growth velocity of domains during ripening, we estimate that the driving force for alignment is on the order of 0.1âmeV per C atom and increases with rotation angle. A simple model of the orientation-dependent energy associated with the moirĂ© corrugation of the graphene sheet due to local variations in the graphene-substrate interaction reproduces the results. This work suggests new strategies for improving the van der Waals epitaxy of 2D materials
Cathodoluminescence Spectroscopy: An Accurate Technique for the Characterization of the Fabrication Technology of GaAlAs/GaAs Heterojunction Bipolar Transistors
Cathodoluminescence (CL) spectroscopy and imaging performed at low temperature have been used to qualify the heterojunction bipolar transistor fabrication technology, particularly the etching and ion implantation steps. CL has been used to optimize low defect technological processes. The protection of the active region during the insulation process has been optimized. The best result is obtained when using a bilayer of silicon nitride and photoresist. In order to minimize it, the damage induced by the etching process has also been studied. The best result is obtained when combining Ar ion beam etching and chemical etching. The possibilities to perform localized spectroscopy, to visualize the different emitting regions and to achieve semiquantitative signal analysis, makes CL a powerful microcharacterization method
Ferromagnetic GaâËâ Mnâ As produced by ion implantation and pulsed-laser melting
We demonstrate the formation of ferromagneticGaâËâMnâAsfilms by Mn ion implantation into GaAs followed by pulsed-laser melting. Irradiation with a single excimer laser pulse results in the epitaxial regrowth of the implanted layer with Mn substitutional fraction up to 80% and effective Curie temperature up to 29 K for samples with a maximum Mn concentration of xâ0.03. A remanent magnetization persisting above 85 K has been observed for samples with xâ0.10, in which 40% of the Mn resides on substitutional lattice sites. We find that the ferromagnetism in GaâËâMnâAs is rather robust to the presence of structural defects.The work at the Lawrence Berkeley National Laboratory
was supported by the Director, Office of Science, Office of
Basic Energy Sciences, Division of Materials Sciences and
Engineering, of the U.S. Department of Energy under Contract
No. DE-AC03-76SF00098. The work at Harvard was
supported by NASA Grant No. NAG8-1680. One of
the authors ~M.A.S.! acknowledges support from an NSF
Graduate Research Fellowship
Compensation-dependent in-plane magnetization reversal processes in Ga1-xMnxP1-ySy
We report the effect of dilute alloying of the anion sublattice with S on the
in-plane uniaxial magnetic anisotropy and magnetization reversal process in
Ga1-xMnxP as measured by both ferromagnetic resonance (FMR) and superconducting
quantum interference device (SQUID) magnetometry. At T=5K, raising the S
concentration increases the uniaxial magnetic anisotropy between in-plane
directions while decreasing the magnitude of the (negative) cubic anisotropy
field. Simulation of the SQUID magnetometry indicates that the energy required
for the nucleation and growth of domain walls decreases with increasing y.
These combined effects have a marked influence on the shape of the
field-dependent magnetization curves; while the direction remains the easy axis
in the plane of the film, the field dependence of the magnetization develops
double hysteresis loops in the [011] direction as the S concentration increases
similar to those observed for perpendicular magnetization reversal in lightly
doped Ga1-xMnxAs. The incidence of double hysteresis loops is explained with a
simple model whereby magnetization reversal occurs by a combination of coherent
spin rotation and noncoherent spin switching, which is consistent with both FMR
and magnetometry experiments. The evolution of magnetic properties with S
concentration is attributed to compensation of Mn acceptors by S donors, which
results in a lowering of the concentration of holes that mediate
ferromagnetism.Comment: 37 pages, 9 figures, 3 table
Heat flow model for pulsed laser melting and rapid solidification of ion implanted GaAs
Some of the authors thank for the support of the Center for Nanoscale Systems (CNS) at Harvard University is acknowledged. Harvard-CNS is a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science Foundation under NSF award No. ECS-0335765. K. M. Yu and J. W. Beeman were supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.Some of the authors thank for the support of the Center for Nanoscale Systems (CNS) at Harvard University is acknowledged. Harvard-CNS is a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science Foundation under NSF award No. ECS-0335765. K. M. Yu and J. W. Beeman were supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231
Vapor-liquid-solid growth of highly-mismatched semiconductor nanowires with high-fidelity van der Waals layer stacking
Nanobelts, nanoribbons and other quasi-one-dimensional nanostructures formed
from layered, so-called, van der Waals semiconductors have garnered much
attention due to their high-performance, tunable optoelectronic properties. For
layered alloys made from the gallium monochalcogenides GaS, GaSe, and GaTe,
near-continuous tuning of the energy bandgap across the full composition range
has been achieved in GaSe1-xSx and GaSe1-xTex alloys. Gold-catalyzed
vapor-liquid-solid (VLS) growth of these alloys yields predominantly nanobelts,
nanoribbons and other nanostructures for which the fast crystal growth front
consists of layer edges in contact with the catalyst. We demonstrate that in
the S-rich, GaS1-xTex system, unlike GaSe1-xSx and GaSe1-xTex, the Au-catalyzed
VLS process yields van der Waals nanowires for which the fast growth direction
is normal to the layers. The high mismatch between S and Te leads to
extraordinary bowing of the GaS1-xTex alloy's energy bandgap, decreasing by at
least 0.6 eV for x as small as 0.03. Calculations using density functional
theory confirm the significant decrease in bandgap in S-rich GaS1-xTex. The
nanowires can exceed fifty micrometers in length, consisting of tens of
thousands of van der Waals-bonded layers with triangular or hexagonal
cross-sections of uniform dimensions along the length of the nanowire. We
propose that the low solubility of Te in GaS results in an enhancement in Te
coverage around the Au catalyst-nanowire interface, confining the catalyst to
the chalcogen-terminated basal plane (rather than the edges) and thereby
enabling layer-by-layer, c-axis growth
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