349 research outputs found
Ehrlich-Schwoebel barrier controlled slope selection in epitaxial growth
We examine the step dynamics in a 1+1 dimensional model of epitaxial growth
based on the BCF-theory. The model takes analytically into account the
diffusion of adatoms, an incorporation mechanism and an Ehrlich-Schwoebel
barrier at step edges. We find that the formation of mounds with a stable slope
is closely related to the presence of an incorporation mechanism. We confirm
this finding using a Solid-On-Solid model in 2+1 dimensions. In the case of an
infinite step edge barrier we are able to calculate the saturation profile
analytically. Without incorporation but with inclusion of desorption and
detachment we find a critical flux for instable growth but no slope selection.
In particular, we show that the temperature dependence of the selected slope is
solely determined by the Ehrlich-Schwoebel barrier which opens a new
possibility in order to measure this fundamental barrier in experiments.Comment: 17 pages, 4 figure
Evaporation and Step Edge Diffusion in MBE
Using kinetic Monte-Carlo simulations of a Solid-on-Solid model we
investigate the influence of step edge diffusion (SED) and evaporation on
Molecular Beam Epitaxy (MBE). Based on these investigations we propose two
strategies to optimize MBE-growth. The strategies are applicable in different
growth regimes: during layer-by-layer growth one can reduce the desorption rate
using a pulsed flux. In three-dimensional (3D) growth the SED can help to grow
large, smooth structures. For this purpose the flux has to be reduced with time
according to a power law.Comment: 5 pages, 2 figures, latex2e (packages: elsevier,psfig,latexsym
A simple model of epitaxial growth
A discrete solid-on-solid model of epitaxial growth is introduced which, in a
simple manner, takes into account the effect of an Ehrlich-Schwoebel barrier at
step edges as well as the local relaxation of incoming particles. Furthermore a
fast step edge diffusion is included in 2+1 dimensions. The model exhibits the
formation of pyramid-like structures with a well-defined constant inclination
angle. Two regimes can be distinguished clearly: in an initial phase (I) a
definite slope is selected while the number of pyramids remains unchanged. Then
a coarsening process (II) is observed which decreases the number of islands
according to a power law in time. Simulations support self-affine scaling of
the growing surface in both regimes. The roughness exponent is alpha =1 in all
cases. For growth in 1+1 dimensions we obtain dynamic exponents z = 2 (I) and z
= 3 (II). Simulations for d=2+1 seem to be consistent with z= 2 (I) and z= 2.3
(II) respectively.Comment: 8 pages Latex2e, 4 Postscript figures included, uses packages
a4wide,epsfig,psfig,amsfonts,latexsy
Particle currents and the distribution of terrace sizes in unstable epitaxial growth
A solid-on-solid model of epitaxial growth in 1+1 dimensions is investigated
in which slope dependent upward and downward particle currents compete on the
surface. The microscopic mechanisms which give rise to these currents are the
smoothening incorporation of particles upon deposition and an Ehrlich-Schwoebel
barrier which hinders inter-layer transport at step edges. We calculate the
distribution of terrace sizes and the resulting currents on a stepped surface
with a given inclination angle. The cancellation of the competing effects leads
to the selection of a stable magic slope. Simulation results are in very good
agreement with the theoretical findings.Comment: 4 pages, including 3 figure
The role of step edge diffusion in epitaxial crystal growth
The role of step edge diffusion (SED) in epitaxial growth is investigated. To
this end we revisit and extend a recently introduced simple cubic
solid-on-solid model, which exhibits the formation and coarsening of pyramid or
mound like structures. By comparing the limiting cases of absent, very fast
(significant), and slow SED we demonstrate how the details of this process
control both the shape of the emerging structures as well as the scaling
behavior. We find a sharp transition from significant SED to intermediate
values of SED, and a continuous one for vanishing SED. We argue that one should
be able to control these features of the surface in experiments by variation of
the flux and substrate temperature.Comment: revised and enlarged version 12 pages, 5 figures, to appear in
Surface Scienc
Modeling sublimation by computer simulation: morphology dependent effective energies
Solid-On-Solid (SOS) computer simulations are employed to investigate the
sublimation of surfaces. We distinguish three sublimation regimes:
layer-by-layer sublimation, free step flow and hindered step flow. The
sublimation regime is selected by the morphology i.e. the terrace width. To
each regime corresponds another effective energy. We propose a systematic way
to derive microscopic parameters from effective energies and apply this
microscopical analysis to the layer-by-layer and the free step flow regime. We
adopt analytical calculations from Pimpinelli and Villain and apply them to our
model. Key-Words: Computer simulations; Models of surface kinetics; Evaporation
and Sublimation; Growth; Surface Diffusion; Surface structure, morphology,
roughness, and topography; Cadmium tellurideComment: 12 pages, 6 Postscript figures, uses psfig.st
EPOTHILONE: NEUE WIRKSTOFFE GEGEN KREBS
Myxobakterien produzieren einen Naturstoff, das Epothilon, der auf dieselbe Weise gegen Tumorzellen wirkt wie das aus der pazifischen Eibe isolierte Taxol. Letzteres wird bereits erfolgreich in der Krebstherapie eingesetzt.Unser Forscherteam arbeitet derzeit mit einem Schweizer Pharmaunternehmen an der Weiterentwicklung und Testung von Epothilon in der Tumortherapie. Jedoch werden noch langjährige Studien bis zur medikamentösen Anwendung und Behandlung folgen müssen. Erste klinische Studien laufen bereits in den USA
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