In fact the resulting Si-C and C-C radial distribution functions compare quite well with these obtained by cascade amorphized and melt-quenched amorphous SiC using a modified Tersoff potential \cite{gao02}.
\subsection{Limitations of conventional MD and short range potentials}
+\label{subsection:md:limit}
At first the formation of an amorphous SiC-like phase is unexpected since IBS experiments show crystalline 3C-SiC precipitates at prevailing temperatures.
On closer inspection, however, reasons become clear, which are discussed in the following.
On the other hand, during implantation, the actual temperature inside the implantation volume is definetly higher than the experimentally determined temperature tapped from the surface of the sample.
\subsection{Increased temperature simulations}
+\label{subsection:md:inct}
Due to the limitations of short range potentials and conventional MD as discussed above elevated temperatures are used in the following.
The simulation sequence and other parameters aside system temperature remain unchanged as in section \ref{subsection:initial_sims}.
{\color{red}Todo: If C sub and Si self-int is energetically more favorable, the migration towards the surface can be kicked out. Otherwise we should actually care about removal of Si! In any way these findings suggest a different prec model.}
\subsection{Valuation of a practicable temperature limit}
+\label{subsection:md:tval}
\begin{figure}[!ht]
\begin{center}
Thus, the interface tension, given by the energy of the interface devided by the surface area of the precipitate is $20.15\,\frac{\text{eV}}{\text{nm}^2}$ or $3.23\times 10^{-4}\,\frac{\text{J}}{\text{cm}^2}$.
This is located inside the eperimentally estimated range of $2-8\times 10^{-4}\,\frac{\text{J}}{\text{cm}^2}$ \cite{taylor93}.
-
-Since interface region is constructed and not neccesarily corresponds to the energetically most favorable layout we will now try hard to improve this ...
-Let's see, whether annealing will lead to some energetically more favorable configurations.
-
+Since the precipitate configuration is artificially constructed the resulting interface does not necessarily correspond to the energetically most favorable configuration or to the configuration that is expected for an actually grown precipitate.
+Thus annealing steps are appended to the gained structure in order to allow for a rearrangement of the atoms of the interface.
+The precipitate structure is rapidly heated up to $2050\,^{\circ}\mathrm{C}$ with a heating rate of approximately $75\,^{\circ}\mathrm{C}/\text{ps}$.
+From that point on the heating rate is reduced to $1\,^{\circ}\mathrm{C}/\text{ps}$ and heating is continued to 120 \% of the Si melting temperature, that is 2940 K.
+\begin{figure}[!ht]
+\begin{center}
+\includegraphics[width=12cm]{fe_and_t_sic.ps}
+\end{center}
+\caption{Free energy and temperature evolution of a constructed 3C-SiC precipitate embedded in c-Si at temperatures above the Si melting point.}
+\label{fig:md:fe_and_t_sic}
+\end{figure}
+Figure \ref{fig:md:fe_and_t_sic} shows the free energy and temperature evolution.
+The sudden increase of the free energy indicates possible melting occuring around 2840 K.
+\begin{figure}[!ht]
+\begin{center}
+\includegraphics[width=12cm]{pc_500-fin.ps}
+\end{center}
+\caption{Radial distribution of the constructed 3C-SiC precipitate embedded in c-Si at temperatures below and above the Si melting transition point.}
+\label{fig:md:pc_500-fin}
+\end{figure}
+Investigating the radial distribution function shown in figure \ref{fig:md:pc_500-fin}, which shows configurations below and above the temperature of the estimated transition, indeed supports the assumption of melting gained by the free energy plot.
+However the precipitate itself is not involved, as can be seen from the Si-C and C-C distribution, which essentially stays the same for both temperatures.
+Thus, it is only the c-Si surrounding undergoing a structural phase transition, which is very well reflected by the difference observed for the two Si-Si distributions.
+This is surprising since the melting transition of plain c-Si is expected at temperatures around 3125 K, as discussed in the last section.
+Obviously the precipitate lowers the transition point of the surrounding c-Si matrix.
+For the rearrangement simulations temperatures well below the transition point should be used since it is very unlikely to recrystallize the molten Si surrounding properly when cooling down.
+To play safe the precipitate configuration at 100 \% of the Si melting temperature is chosen and cooled down to $20\,^{\circ}\mathrm{C}$ with a cooling rate of $1\,^{\circ}\mathrm{C}/\text{ps}$.
+{\color{blue}TODO: Wait for results and then compare structure (PC) and interface energy, maybe a energetically more favorable configuration arises.}
+{\color{red}TODO: Mention the fact, that the precipitate is stable for eleveated temperatures, even for temperatures where the Si matrix is melting.}
\subsection{Simulations at temperatures exceeding the silicon melting point}
-LL Cool J is hot as hell!
-
-A different simulation volume and refined amount as well as shape of insertion volume for the C atoms, to stay compareable to the results gained in the latter section, is used throughout all following simulations.
+As discussed in section \ref{subsection:md:limit} and \ref{subsection:md:inct} a further increase of the system temperature might help to overcome limitations of the short range potential and accelerate the dynamics involved in structural evolution.
+A maximum temperature to avoid melting was determined in section \ref{subsection:md:tval}, which is 120 \% of the Si melting point.
+In the following simulations the system volume, the amount of C atoms inserted and the shape of the insertion volume are modified from the values used in the first MD simulations to now match the conditions given in the simulations of the self-constructed precipitate configuration for reasons of comparability.
+To quantify, the initial simulation volume now consists of 21 Si unit cells in each direction and 5500 C atoms are inserted in either the whole volume or in a sphere with a radius of 3 nm.
+Since the investigated temperatures exceed the Si melting point the initial Si bulk material is heated up slowly by $1\,^{\circ}\mathrm{C}/\text{ps}$ starting from $1650\,^{\circ}\mathrm{C}$.
-\subsection{Todo}
+\subsection{Further accelerated dynamics approaches}
{\color{red}TODO: self-guided MD!}
projects / lectures / latex.git / commitdiff