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

Why another conference on personalized medicine?

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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

Summaries

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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