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[lectures/latex.git] / posic / publications / emrs2008_full.tex

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\begin{center}
 {\LARGE {\bf Molecular dynamics simulation
              of defect formation and precipitation
              in heavily carbon doped silicon
              }\\}
 \vspace{16pt}
 \textsc{\Large F. Zirkelbach$^1$, J. K. N. Lindner$^1$,
         K. Nordlund$^2$, B. Stritzker$^1$}\\
 \vspace{16pt}
 $^1$ Experimentalphysik IV, Institut f\"ur Physik, Universit\"at Augsburg,\\
 Universit\"atsstr. 1, D-86135 Augsburg, Germany\\
 \vspace{16pt}
 $^2$ Accelerator Laboratory, Department of Physical Sciences,
 University of Helsinki,\\
 Pietari Kalmink. 2, 00014 Helsinki, Finland\\
 \vspace{16pt}
 {\scriptsize Corresponding author: Frank Zirkelbach
                              <frank.zirkelbach@physik.uni-augsburg.de>}
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%\vspace{24pt}

\section*{Abstract}
The precipitation process of silicon carbide in heavily carbon doped silicon is not yet fully understood.
High resolution transmission electron microscopy observations suggest that in a first step carbon atoms form C-Si dumbbells on regular Si lattice sites which agglomerate into large clusters.
In a second step, when the cluster size reaches a radius of a few $nm$, the high interfacial energy due to the SiC/Si lattice misfit of almost 20\% is overcome and the precipitation occurs.
By simulation, details of the precipitation process can be obtained on the atomic level.
A newly parametrized Tersoff-like bond order potential is used to model the system appropriately.
First results gained by molecular dynamics simulations using this potential are presented.
\\\\
{\bf Keywords:} Silicon, carbon, silicon carbide, nucleation, defect formation,
                molecular dynamics simulations

\section*{Introduction}
Understanding the precipitation process of cubic silicon carbide (3C-SiC) in heavily carbon doped silicon will enable significant technological progress in thin film formation of the important wide band gap semiconductor material SiC \cite{edgar92}.
On the other hand it will likewise offer perspectives for processes which rely upon prevention of precipitation events, e.g. the fabrication of strained, pseudomorphic $\text{Si}_{1-y}\text{C}_y$ heterostructures \cite{}.

Epitaxial growth of 3C-SiC films is achieved either by ion beam synthesis (IBS) \cite{lindner02} and chemical vapour deposition (CVD) or molecular beam epitaxy (MBE) techniques.
While in CVD and MBE surface effects need to be taken into account, SiC formation during IBS takes place in the bulk of the Si crystal.
In the present work the simulation tries to realize conditions which hold for the ion implantation process.

First of all a picture of the supposed precipitation event is presented.
Afterwards the applied simulation sequences are discussed.
Finally first results gained by simulation are presented.

\section*{Supposed conversion mechanism}
Silicon has diamond structure and thus is composed of two fcc lattices which are displaced by one quarter of the volume diagonal.
3C-SiC grows in zincblende structure, i.e. is also composed of two fcc lattices out of which one is occupied by Si the other by C atoms.
The length of four lattice constants of Si is approximately equal to the length of five 3C-SiC lattice constants ($4a_{\text{Si}}\approx 5a_{\text{3C-SiC}}$) resulting in a lattice misfit of almost 20\%.
Due to this the silicon atomic density of 3C-SiC is slightly lower than the one of pure Si.

%\begin{figure}[!h]
% \begin{center}
% \begin{minipage}{5.5cm}
% \includegraphics[width=5cm]{sic_prec_seq_01_s.eps}
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% \includegraphics[width=5cm]{sic_prec_seq_02_s.eps}
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% \begin{minipage}{5.5cm}
% \includegraphics[width=5cm]{sic_prec_seq_03_s.eps}
% \end{minipage}
% \caption{Schematic of the supposed conversion mechanism of highly C (${\color{red}\Box}$) doped Si (${\color{black}\bullet}$) into SiC ($_{\color{black}\bullet}^{{\color{red}\Box}}$) and residual Si atoms ($\circ$). The figure shows the dumbbell formation (left), the agglomeration into clusters (middle) and the situation after precipitation (right).}
% \end{center}
%\end{figure}
There is a supposed conversion mechanism of heavily carbon doped Si into SiC \cite{werner97}.
As concluded by high resolution transmission electron microscopy introduced carbon atoms form C-Si dumbbells on regular Si lattice sites.
The dumbbells agglomerate into large clusters, called embryos.
Finally, when the cluster size reaches a critical radius of 2 to 4 nm, the high interfacial energy due to the 3C-SiC/Si lattice misfit is overcome and precipitation occurs.
Due to the slightly lower silicon density of 3C-SiC excessive silicon atoms exist which will most probably end up as self-interstitials in the silicon matrix since there is more space than in 3C-SiC.

Thus, in addition to the precipitation event itself, knowledge of C and Si interstitials in Si are of great interest in order to investigate the precipitation of heavily C doped Si into SiC.
%Additionaly the influence of interstitials on atomic diffusion is investigated.

\section*{Simulation sequences}
A molecular dynamics simulation approach is used to examine the steps involved in the precipitation process.
For integrating the equations of motion the velocity verlet algorithm \cite{verlet67} with a timestep of $1\, fs$ is adopted.
The interaction of the silicon and carbon atoms is realized by a newly parametrized Tersoff-like bond order potential \cite{albe_sic_pot}.
Since temperature and pressure of the system is kept constant in experiment the isothermal-isobaric NPT ensemble is chosen for the simulation.
Coupling to the heat bath is achieved by the Berendsen thermostat \cite{berendsen84} with a time constant $\tau_T=100\, fs$.
The pressure is scaled by the Berendsen barostat \cite{berendsen84} again using a timeconstant of $\tau_P=100\, fs$ and a bulk modulus of $100\, GPa$ for silicon.
To exclude surface effects periodic boundary conditions are applied.

\begin{figure}[!h]
 \begin{center}
 \includegraphics[width=8cm]{unit_cell_s.eps}
 \caption{Insertion positions for the tetrahedral (${\color{red}\triangleleft}$), hexagonal (${\color{green}\triangleright}$) and <110> dumbbell (${\color{magenta}\Box}$) interstitial configuration.}
 \end{center}
\end{figure}
To investigate the interstitial configurations of C and Si in Si, a simulation volume of 9 silicon unit cells in each direction is used.
The temperature is set to $T=0\, K$.
The insertion positions are illustrated in Fig. 2.
In separated simulation runs a carbon and a silicon atom respectively is inserted at the tetrahedral $(0,0,0)$ (${\color{red}\triangleleft}$), hexagonal $(-1/8,-1/8,1/8)$ (${\color{green}\triangleright}$), nearby dumbbell $(-1/8,-1/8,-1/4)$ (${\color{magenta}\Box}$) and at random positions (in units of the silicon lattice constant) where the origin is located in the centre of the unit cell.
In order to avoid too high potential energies in the case of the dumbbell configuration the nearest silicon neighbour atom is shifted to $(-3/8,-3/8,-1/4)$ ($\circ$).
The energy introduced into the system is scaled out within a relaxation phase of $2\, ps$.

%The same volume is used to investigate diffusion.
%Different amounts of silicon atoms are inserted at random positions within a centered region of $11 \,\textrm{\AA}$ in each direction.
%Insertion events are carried out step by step maintaining a constant system temperature of $450\, ^{\circ} \textrm{C}$.
%Finally a single carbon atom is inserted at a random position within the unit cell located in the middle of the simulation volume.
%The simulation is proceeded for another $30\, ps$.

For the simulations aiming to reproduce a precipitation process the volume is 31 silicon lattice constants in each direction.
The system temperature is set to $450\, ^{\circ} \textrm{C}$.
$6000$ carbon atoms (the amount necessary to form a 3C-SiC precipitate with a radius of 3 nm) are consecutively inserted in a way to keep constant the system temperature.
Precipitation is examined for three insertion volumes which differ in size.
The whole simulation volume $V_1$, the volume corresponding to the size of a minimal SiC precipitate $V_2$ and the volume containing the amount of silicon necessary for the formation of such a minimal precipitate $V_3$ are examined.
The two latter ones are accomplished since no long range diffusion of the carbon atoms is expected at this temperature.
Following the insertion procedure the system is cooled down to $20\, ^{\circ} \textrm{C}$.

\section*{Results}

The tetrahedral and the <110> dumbbell self-interstitial configurations can be reproduced as observed in \cite{albe_sic_pot}.
The formation energies are $3.4\, eV$ and $4.4\, eV$ respectively.
However the hexagonal one is not stable opposed to what is presented in \cite{albe_sic_pot}.
The atom moves towards an energetically more favorable position very close to the tetrahedral one but slightly displaced along the three coordinate axes.
The formation energy of $4.0\, eV$ of this type of interstitial equals the result obtained in the reference for the hexagonal one.
The same type of interstitial may arise using random insertions.
In addition variations exist in which the displacement is only along two axes ($E_f=3.8\, eV$) or along a single axis ($E_f=3.6\, eV$) succesively approximating the tetrahedral configuration and formation energy.

The tetrahedral and <110> dumbbel carbon interstitial configurations are stable.
The formation energies are $2.7\, eV$ and $1.8\, eV$ respectively.
Again the hexagonal one is found to be unstable.
The interstitial atom moves to the more favorable <100> dumbbell position which has a formation energy of $0.5\, eV$.
The interstitial configuration is shown in Fig. 2.
There is experimental evidence \cite{watkins76} of the existence of this configuration.
It is frequently generated and has the lowest formation energy of all the defects observed in all the simulation runs in which carbon is inserted at random positions.

\begin{figure}[!h]
 \begin{center}
 \includegraphics[width=8cm]{c_in_si_int_001db_0.eps}
 \caption{Position of a <100> dumbbell carbon interstitial in silicon.
          Only bonds of the carbon interstitial atom are shown.}
 \end{center}
\end{figure}

%\begin{figure}[!h]
% \begin{center}
% \includegraphics[width=12cm]{../plot/diff_dep.ps}
% \caption{Diffusion coefficients of a single carbon atom for different amount of Si selft interstitials}
% \end{center}
%\end{figure}
%The influence of Si self-interstitials on the diffusion of a single carbon atom is displayed in Fig. 3.
%Diffusion coefficients for different amount of Si self-interstitials are shown.
%A slight increase is first observed in the case of 30 interstitial atoms.
%Further increasing the amount of interstitials leads to a tremendous decay of the diffusion coeeficient.
%Generally there is no long range diffusion of the carbon atom for a temperature of $450\, ^{\circ} \textrm{C}$.
%The maximal displacement of the carbon atom relativ to its insertion position is between 0.5 and 0.7 \AA.

\begin{figure}[!h]
 \begin{center}
 \includegraphics[width=12cm]{pc_si-c_c-c.ps}
 \caption{Pair correlation functions for Si-C and C-C bonds.
          Carbon atoms are introduced into the whole simulation volume $V_1$, the region which corresponds to the size of a minimal SiC precipitate $V_2$ and the volume which contains the necessary amount of silicon for such a minimal precipitate $V_2$ respectively.}
 \end{center}
\end{figure}
\begin{figure}[!h]
 \begin{center}
 \includegraphics[width=12cm]{pc_si-si.ps}
 \caption{Si-Si pair correlation function for pure Si and Si with 3000 inserted C atoms.
          The inset shows a magnified region between 0.28 and 0.36 nm.}
 \end{center}
\end{figure}
Fig. 3 shows resulting pair correlation functions of the simulation runs targeting the observation of precipitation events.
The contributions of Si-C and C-C pairs are presented separately each of them displaying the pair correlation for the three different volumes $V_1$, $V_2$ and $V_3$ (as explained above) exposed to carbon insertion.
Results show no signigicant difference between $V_1$ and $V_2$.
Si-Si pairs for the case of 3000 inserted C atoms inserted into $V_2$ and a reference function for pure Si are displayed in Fig. 4.

The amount of C-C bonds for $V_1$ are much smaller than for $V_2$ and $V_3$ since carbon atoms are spread over the total simulation volume which means that there are only 0.2 carbon atoms per silicon unit cell on average.
The first C-C peak appears at about 0.15 nm.
This is comparable to the nearest neighbour distance for graphite or diamond.
It is assumed that these carbon atoms form strong C-C bonds, which is supported by a decrease of the total energy during carbon insertion for the $V_2$ and $V_3$ in contrast to the $V_3$ simulation.

The peak at 0.31 nm perfectly matches the distance of two carbon atoms in the SiC lattice which in SiC is also expected for the Si-Si bonds.
After insertion of carbon atoms the Si-Si pair correlation function in fact shows non-zero values in the range of the C-C peak width while the amount of Si pairs at the regular distances at 0.24 and 0.38 nm decreases.
However no clear peak is observed and random analyses of configurations in which distances around 0.3 nm appear, i.e. visualization of such atom pairs, identify <100> C-Si dumbbells to be responsible for stretching the Si-Si next neighbour distance for low concentrations of carbon, i.e. for the $V_1$ and early stages of $V_2$ and $V_3$ simulation runs.
For higher carbon concentrations the defect concentration is likewise increased and a considerable amount of damage is introduced into the inserted volume.
Damage and superposition of defects generate new displacement arrangements which become hard to categorize and trace.
The slightly higher amount and intense increase of Si-Si pairs at distances smaller 0.31 nm is probably due to the Si-Si cutoff radius of 0.296 nm.
The cutoff function causes artificial forces pushing the Si atoms out of the cutoff region.

Wieder durch visuelle untersuchungen -> c-c 0.31 paare durch aufeinandertreffen unterschiedlich orientierter 100 dumbbells bzw mit 110.
C fuer sic schon besser arrangiert. Vorstellung, dass diese zuerst anordnen und spaeter dann evtl am Si ziehen ...
Nevertheless this might indicate that carbon arranges first, then 'pulls' the Si ...
\\\\
On the other hand the Si-C pair correlation function indicates formation of SiC bonds with an increased crystallinity for the simulation in which carbon is inserted into the whole simulation volume.
There is more carbon forming Si-C bonds than C-C bonds.
This gives suspect to the competition of Si-C and C-C bond formation in which the predominance of either of them depends on the method handling carbon insertion.

\section*{Summary}
The supposed conversion mechanism of heavily carbon doped silicon into silicon carbide is presented.
Molecular dynamics simulation sequences to investigate interstitial configurations
%, the influence of interstitials on the atomic diffusion
and the precipitation of SiC are explained.
The <100> C-Si dumbbel is reproduced and is the energetically most favorable configuration observed by simulation.
%The influence of silicon self-interstitials on the diffusion of a single carbon atom is demonstrated.
Two competing bond formations, either Si-C or C-C, seem to coexist, where the strength of either of them depends on the size of the region in which carbon is introduced.

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