94 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
Unconventional MBE Strategies from Computer Simulations for Optimized Growth Conditions
We investigate the influence of step edge diffusion (SED) and desorption on
Molecular Beam Epitaxy (MBE) using kinetic Monte-Carlo simulations of the
solid-on-solid (SOS) model. 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 exploit the presence of
desorption in order to achieve smooth surfaces. By additional short high flux
pulses of particles one can increase the growth rate and assist layer-by-layer
growth. If, however, mounds are formed (non-layer-by-layer growth) the SED can
be used to control size and shape of the three-dimensional structures. By
controlled reduction of the flux with time we achieve a fast coarsening
together with smooth step edges.Comment: 19 pages, 7 figures, submitted to Phys. Rev.
Human mass balance study of the novel anticancer agent ixabepilone using accelerator mass spectrometry
Ixabepilone (BMS-247550) is a semi-synthetic, microtubule stabilizing epothilone B analogue which is more potent than taxanes and has displayed activity in taxane-resistant patients. The human plasma pharmacokinetics of ixabepilone have been described. However, the excretory pathways and contribution of metabolism to ixabepilone elimination have not been determined. To investigate the elimination pathways of ixabepilone we initiated a mass balance study in cancer patients. Due to autoradiolysis, ixabepilone proved to be very unstable when labeled with conventional [14C]-levels (100 μCi in a typical human radio-tracer study). This necessitated the use of much lower levels of [14C]-labeling and an ultra-sensitive detection method, Accelerator Mass Spectrometry (AMS). Eight patients with advanced cancer (3 males, 5 females; median age 54.5 y; performance status 0–2) received an intravenous dose of 70 mg, 80 nCi of [14C]ixabepilone over 3 h. Plasma, urine and faeces were collected up to 7 days after administration and total radioactivity (TRA) was determined using AMS. Ixabepilone in plasma and urine was quantitated using a validated LC-MS/MS method. Mean recovery of ixabepilone-derived radioactivity was 77.3% of dose. Fecal excretion was 52.2% and urinary excretion was 25.1%. Only a minor part of TRA is accounted for by unchanged ixabepilone in both plasma and urine, which indicates that metabolism is a major elimination mechanism for this drug. Future studies should focus on structural elucidation of ixabepilone metabolites and characterization of their activities
Quantum Dimensional Zeeman Effect in the Magneto-optical Absorption Spectrum for Quantum Dot - Impurity Center Systems
Magneto-optical properties of the quantum dot - impurity center (QD-IC)
systems synthesized in a transparent dielectric matrix are considered. For the
QD one-electron state description the parabolic model of the confinement
potential is used. Within the framework of zero-range potential model and the
effective mass approach, the light impurity absorption coefficient for the case
of transversal polarization with respect to the applied magnetic field
direction, with consideration of the QD size dispersion, has been analytically
calculated. It is shown that for the case of transversal polarization the light
impurity absorption spectrum is characterized by the quantum dimensional Zeeman
effect.Comment: 18 pages, 1 figure, PDF fil
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