15,333 research outputs found
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
Power dissipation in nanoscale conductors: classical, semi-classical and quantum dynamics
Modelling Joule heating is a difficult problem because of the need to introduce correct correlations between the motions of the ions and the electrons. In this paper we analyse three different models of current induced heating (a purely classical model, a fully quantum model and a hybrid model in which the electrons are treated quantum mechanically and the atoms are treated classically). We find that all three models allow for both heating and cooling processes in the presence of a current, and furthermore the purely classical and purely quantum models show remarkable agreement in the limit of high biases. However, the hybrid model in the Ehrenfest approximation tends to suppress heating. Analysis of the equations of motion reveals that this is a consequence of two things: the electrons are being treated as a continuous fluid and the atoms cannot undergo quantum fluctuations. A means for correcting this is suggested
Observation of Interactions between Trapped Ions and Ultracold Rydberg Atoms
We report on the observation of interactions between ultracold Rydberg atoms
and ions in a Paul trap. The rate of observed inelastic collisions, which
manifest themselves as charge transfer between the Rydberg atoms and ions,
exceeds that of Langevin collisions for ground state atoms by about three
orders of magnitude. This indicates a huge increase in interaction strength. We
study the effect of the vacant Paul trap's electric fields on the Rydberg
excitation spectra. To quantitatively describe the exhibited shape of the ion
loss spectra, we need to include the ion-induced Stark shift on the Rydberg
atoms. Furthermore, we demonstrate Rydberg excitation on a dipole-forbidden
transition with the aid of the electric field of a single trapped ion. Our
results confirm that interactions between ultracold atoms and trapped ions can
be controlled by laser coupling to Rydberg states. Adding dynamic Rydberg
dressing may allow for the creation of spin-spin interactions between atoms and
ions, and the elimination of collisional heating due to ionic micromotion in
atom-ion mixtures.Comment: 7 pages, 5 figures, including appendices. Note that the title has
been changed in version
Simulation of ion track ranges in uranium oxide
Direct comparisons between statistically sound simulations of ion-tracks and
published experimental measurements of range densities of iodine implants in
uranium dioxide have been made with implant energies in the range of 100-800
keV. Our simulations are conducted with REED-MD (Rare Event Enhanced
Domain-following Molecular Dynamics) in order to account for the materials
structure in both single crystalline and polycrystalline experimental samples.
We find near-perfect agreement between REED-MD results and experiments for
polycrystalline target materials.Comment: Eleven pages, four figures
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
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