14 research outputs found
Vacancy clustering and diffusion in silicon: Kinetic lattice Monte Carlo simulations
Diffusion and clustering of lattice vacancies in silicon as a function of
temperature, concentration, and interaction range are investigated by Kinetic
Lattice Monte Carlo simulations. It is found that higher temperatures lead to
larger clusters with shorter lifetimes on average, which grow by attracting
free vacancies, while clusters at lower temperatures grow by aggregation of
smaller clusters. Long interaction ranges produce enhanced diffusivity and
fewer clusters. Greater vacancy concentrations lead to more clusters, with
fewer free vacancies, but the size of the clusters is largely independent of
concentration. Vacancy diffusivity is shown to obey power law behavior over
time, and the exponent of this law is shown to increase with concentration, at
fixed temperature, and decrease with temperature, at fixed concentration.Comment: 14 pages, 12 figures. To appear in Physical Review
Simulation of phosphorus implantation into silicon with a single-parameter electronic stopping power model
We simulate dopant profiles for phosphorus implantation into silicon using a
new model for electronic stopping power. In this model, the electronic stopping
power is factorized into a globally averaged effective charge Z1*, and a local
charge density dependent electronic stopping power for a proton. There is only
a single adjustable parameter in the model, namely the one electron radius rs0
which controls Z1*. By fine tuning this parameter, we obtain excellent
agreement between simulated dopant profiles and the SIMS data over a wide range
of energies for the channeling case. Our work provides a further example of
implant species, in addition to boron and arsenic, to verify the validity of
the electronic stopping power model and to illustrate its generality for
studies of physical processes involving electronic stopping.Comment: 11 pages, 7 figures. See http://bifrost.lanl.gov/~reed
An Efficient Molecular Dynamics Scheme for Predicting Dopant Implant Profiles in Semiconductors
We present a highly efficient molecular dynamics scheme for calculating the
concentration profile of dopants implanted in group-IV alloy, and III-V zinc
blende structure materials. Our program incorporates methods for reducing
computational overhead, plus a rare event algorithm to give statistical
accuracy over several orders of magnitude change in the dopant concentration.
The code uses a molecular dynamics (MD) model, instead of the binary
collision approximation (BCA) used in implant simulators such as TRIM and
Marlowe, to describe ion-target interactions. Atomic interactions are described
by a combination of `many-body' and screened Coulomb potentials. Inelastic
energy loss is accounted for using a Firsov model, and electronic stopping is
described by a Brandt-Kitagawa model which contains the single adjustable
parameter for the entire scheme. Thus, the program is easily extensible to new
ion-target combinations with the minimum of tuning, and is predictive over a
wide range of implant energies and angles.
The scheme is especially suited for calculating profiles due to low energy,
large angle implants, and for situations where a predictive capability is
required with the minimum of experimental validation. We give examples of using
our code to calculate concentration profiles and 2D `point response' profiles
of dopants in crystalline silicon, silicon-germanium blends, and
gallium-arsenide. We can predict the experimental profile over five orders of
magnitude for and channeling and for non-channeling implants at
energies up to hundreds of keV.Comment: 10 pages, 7 figures. Proceedings of COSIRES98. Accepted for
publication in Nucl. Instrum. and Meth. B. See http://bifrost.lanl.gov/~reed
Direct simulation of ion beam induced stressing and amorphization of silicon
Using molecular dynamics (MD) simulation, we investigate the mechanical
response of silicon to high dose ion-irradiation. We employ a realistic and
efficient model to directly simulate ion beam induced amorphization. Structural
properties of the amorphized sample are compared with experimental data and
results of other simulation studies. We find the behavior of the irradiated
material is related to the rate at which it can relax. Depending upon the
ability to deform, we observe either the generation of a high compressive
stress and subsequent expansion of the material, or generation of tensile
stress and densification. We note that statistical material properties, such as
radial distribution functions are not sufficient to differentiate between
different densities of amorphous samples. For any reasonable deformation rate,
we observe an expansion of the target upon amorphization in agreement with
experimental observations. This is in contrast to simulations of quenching
which usually result in denser structures relative to crystalline Si. We
conclude that although there is substantial agreement between experimental
measurements and most simulation results, the amorphous structures being
investigated may have fundamental differences; the difference in density can be
attributed to local defects within the amorphous network. Finally we show that
annealing simulations of our amorphized samples can lead to a reduction of high
energy local defects without a large scale rearrangement of the amorphous
network. This supports the proposal that defects in amorphous silicon are
analogous to those in crystalline silicon.Comment: 13 pages, 12 figure
An Efficient Molecular Dynamics Scheme for the Calculation of Dopant Profiles due to Ion Implantation
We present a highly efficient molecular dynamics scheme for calculating the
concentration depth profile of dopants in ion irradiated materials. The scheme
incorporates several methods for reducing the computational overhead, plus a
rare event algorithm that allows statistically reliable results to be obtained
over a range of several orders of magnitude in the dopant concentration.
We give examples of using this scheme for calculating concentration profiles
of dopants in crystalline silicon. Here we can predict the experimental profile
over five orders of magnitude for both channeling and non-channeling implants
at energies up to 100s of keV.
The scheme has advantages over binary collision approximation (BCA)
simulations, in that it does not rely on a large set of empirically fitted
parameters. Although our scheme has a greater computational overhead than the
BCA, it is far superior in the low ion energy regime, where the BCA scheme
becomes invalid.Comment: 17 pages, 21 figures, 2 tables. See: http://bifrost.lanl.gov/~reed
Dynamical simulation of multicomponent carbon based materials
This thesis describes the simulation of important dynamical processes
involving carbon based materials. Much of the research has been aimed at
examining the properties of C6o (buckminsterfullerene), the recently discovered
third allotrope of carbon. Classical Molecular Dynamics (MD) simulation has
been applied to study such diverse processes as fullerene film growth, the
interaction of fullerenes with graphite and bare and hydrogen terminated crystal
surfaces, and the implantation of atoms within C6o. We have also studied
radiation damage to polymers and graphite. Collaboration with experimentalists
has resulted in realistic simulations being conducted to examine physical
processes. The results of simulations have been able to explain experimental
results and suggest alternative methods of achieving the goals of the experiment.
Several algorithms designed to improve the efficiency of simulations have
been programmed and tested. Timing results for these various algorithms are
presented and the most successful have been incorporated into a new MD
simulation code. This has enabled systems of up to 100,000 atoms to be studied in
a realistic time using single workstations (e.g. IBM RS6000 and SUN Sparc-10).
The interaction of atoms is modelled by many-body potential functions. Several
potential fuctions that describe covalent systems have been programmed. New
· potential functions have been produced to model the long-range interactions that
occur in graphite, fullelite and polymer systems, and a three-component, manybody
potential has been developed for the accurate and efficient simulation of
carbon-silicon-hydrogen systems. Computer visualisation and animation
techniques have been applied to the interpretation and display of simulation
results