\vspace{-0.5cm}
-\begin{center}
-\color{red}
-\framebox{
-{\footnotesize\color{black}
- Mismatch in \underline{thermal expansion coeefficient}
- and \underline{lattice parameter} w.r.t. substrate
-}
-}
-\end{center}
+%\begin{center}
+%\color{red}
+%\framebox{
+%{\footnotesize\color{black}
+% Mismatch in \underline{thermal expansion coeefficient}
+% and \underline{lattice parameter} w.r.t. substrate
+%}
+%}
+%\end{center}
\vspace{0.1cm}
\end{minipage}
}
\begin{minipage}{5.5cm}
-\begin{center}
-{\footnotesize
-No surface bending effects\\
-High areal homogenity\\[0.1cm]
-$\Downarrow$\\[0.1cm]
-Synthesis of large area SiC films possible
-}
-\end{center}
+ \includegraphics[width=5.8cm]{ibs_3c-sic.eps}\\[-0.2cm]
+ \begin{center}
+ {\tiny
+ XTEM: single crystalline 3C-SiC in Si\hkl(1 0 0)
+ }
+ \end{center}
\end{minipage}
+%\begin{minipage}{5.5cm}
+%\begin{center}
+%{\footnotesize
+%No surface bending effects\\
+%High areal homogenity\\[0.1cm]
+%$\Downarrow$\\[0.1cm]
+%Synthesis of large area SiC films possible
+%}
+%\end{center}
+%\end{minipage}
+
\end{slide}
\begin{slide}
\end{slide}
-% contents
-
\begin{slide}
\headphd
\begin{slide}
+% contents
+
\headphd
{\large\bf
Outline
\headphd
{\large\bf
- Summary / Conclusions
+ Summary and Conclusions
}
-\scriptsize
+\footnotesize
+
+\vspace{0.1cm}
\framebox{
\begin{minipage}{12.3cm}
\begin{itemize}
\item Point defects excellently / fairly well described
by DFT / EA
- \item C$_{\text{sub}}$ drastically underestimated by EA
- \item EA predicts correct ground state:
- C$_{\text{sub}}$ \& \si{} $>$ \ci{}
- \item Identified migration path explaining
- diffusion and reorientation experiments by DFT
- \item EA fails to describe \ci{} migration:
- Wrong path \& overestimated barrier
+ \item Identified \ci{} migration path
+ \item EA drastically overestimates the diffusion barrier
\end{itemize}
\item Combinations of defects
\begin{itemize}
\item Agglomeration of point defects energetically favorable
- by compensation of stress
- \item Formation of C-C unlikely
\item C$_{\text{sub}}$ favored conditions (conceivable in IBS)
\item \ci{} \hkl<1 0 0> $\leftrightarrow$ \cs{} \& \si{} \hkl<1 1 0>\\
Low barrier (\unit[0.77]{eV}) \& low capture radius
\begin{minipage}[t]{12.3cm}
\underline{Pecipitation simulations}
\begin{itemize}
- \item High C concentration $\rightarrow$ amorphous SiC like phase
\item Problem of potential enhanced slow phase space propagation
\item Low T $\rightarrow$ C-Si \hkl<1 0 0> dumbbell dominated structure
\item High T $\rightarrow$ C$_{\text{sub}}$ dominated structure
\item High T necessary to simulate IBS conditions (far from equilibrium)
- \item Precipitation by successive agglomeration of \cs (epitaxy)
+ \item Increased participation of \cs{} in the precipitation process
\item \si{}: vehicle to form \cs{} \& supply of Si \& stress compensation
(stretched SiC, interface)
\end{itemize}
}
\begin{center}
-{\color{blue}
+{\color{blue}\bf
\framebox{Precipitation by successive agglomeration of \cs{}}
}
\end{center}
\begin{slide}
- {\large\bf
- Acknowledgements
- }
+\headphd
+{\large\bf
+ Acknowledgements
+}
\vspace{0.1cm}
\underline{Augsburg}
\begin{itemize}
- \item Prof. B. Stritzker (accomodation at EP \RM{4})
- \item Ralf Utermann (EDV)
+ \item Prof. B. Stritzker
+ \item Prof. F. Haider
+ \item Ralf Utermann
\end{itemize}
\underline{Berlin/Brandenburg}
\begin{itemize}
- \item PD Volker Eyert (Ref)
+ \item PD V. Eyert
\end{itemize}
\underline{Helsinki}
\begin{itemize}
- \item Prof. K. Nordlund (MD)
+ \item Prof. K. Nordlund
\end{itemize}
\underline{Munich}
\begin{itemize}
- \item Bayerische Forschungsstiftung (financial support)
+ \item Bayerische Forschungsstiftung
\end{itemize}
\underline{Paderborn}
\begin{itemize}
- \item Prof. J. Lindner (SiC)
- \item Prof. G. Schmidt (DFT + financial support)
- \item Dr. E. Rauls (DFT + SiC)
+ \item Prof. J. Lindner
+ \item Prof. G. Schmidt
+ \item Dr. E. Rauls
\end{itemize}
+\vspace{0.1cm}
+
\begin{center}
\framebox{
\bf Thank you for your attention!
the wide band gap and high breakdown field
as well as the high electron mobility and saturation drift velocity
in conjunction with its unique thermal stability and conductivity
-unveil SiC as the ideal candidate for
+unveil SiC as the ideal candidate for highly efficient
high-temperature, high-power and high-frequency electronic
-and opto-electronic devices.
-
-in fact light emission from SiC crystal rectifiers was observed
-already in the very beginning of the 20th century
-constituting the brirth of solid state optoelectronics.
-and indeed, the first blue light emitting diodes in 1990 were based on SiC.
-(nowadays superceded by direct band gap materials like GaN).
-
-the focus of SiC based applications, however,
-is in the area of solid state electronic devices
-experiencing revolutionary performance improvements enabled by its capabilities.
-devices can be designed much thinner with increased dopant concentrations
-resulting in highly efficient rectifier diodes and switching transistors.
-one example is displayed: a SiC based inverter with an efficiency of 98.5%
-designed by the frauenhofer institute for solar energy systems.
+and opto-electronic devices, which can operate in harsh environments.
+
+#in fact light emission from SiC crystal rectifiers was observed
+#already in the very beginning of the 20th century
+#constituting the brirth of solid state optoelectronics.
+#and indeed, the first blue light emitting diodes in 1990 were based on SiC.
+#(nowadays superceded by direct band gap materials like GaN).
+
+as an example a SiC based inverter with an efficiency of 98.5%
+designed by the frauenhofer institute for solar energy systems is displayed.
therefore, SiC constitutes a promising candidate to become the key technology
towards an extensive development and use of regenerative energies and emobility.
-moreover, due to the large bonding energy,
-SiC is a hard and chemical inert material
-suitable for applications under extreme conditions.
-its radiation hardness allows the operation as a first wall reactor material
-and as electronic devices in space.
-
slide 3
-the stoichiometric composition of silicon and carbon
-is the only stable compound in the C/Si system.
-SiC is a mainly covalent material in which both,
-the Si and C atom are sp3 hybridized.
-the local order of the silicon and carbon atoms
-characterized by the tetrahedral bond is always the same.
-however, more than 250 different polytypes exist,
+although the local order of the silicon and carbon atoms
+characterized by the tetrahedral bond is always the same,
+more than 250 different polytypes exist,
which differ in the one-dimensional stacking sequence of
identical, close-packed SiC bilayers,
-which can be situated on one of three possible positions (abbreviated a,b,c).
the stacking sequence of the most important polytypes is displayed here.
the 3c polytype is the only cubic polytype.
different polytypes exhibit different properties,
which are listed in the table
and compared to other technologically relevant semiconductor materials.
-despite the lower charge carrier mobilities for low electric fields,
SiC clearly outperforms silicon.
among the different polytypes, the cubic phase shows the highest
break down field and saturation drift velocity.
thus, the cubic polytype is considered most effective for highly efficient
high-performance electronic devices.
-slide 4
+slide [4] OR 5
SiC is rarely found in nature and, thus, must be synthesized.
-it was first observed by moissan from a meteor crater in arizona.
-the fact that natural SiC is almost only observed
-as individual presolar SiC stardust grains near craters of meteorite impacts
-already indicates the complexity involved in the synthesis process.
-however, nowadays, much progress has been achieved in thin film growth
+nowadays, much progress has been achieved in SiC thin film growth
by molecular beam epitaxy and chemical vapor deposition.
indeed, commerically available semiconductor devices based on alpha SiC exist,
-although these are still extremely expensive.
however, production of the advantageous cubic type is less advanced,
-mainly due to the
-mismatches in the thermal expansion coefficient and the lattice parameter
-(with respect to the substrate)
-which cause a considerable amount of defects,
-that is responsible for structural and electrical qualities
-that are not yet satisfactory.
+structural and electrical qualities are not yet satisfactory.
next to CVD and MBE, the ion beam synthesis technique, which consists of
high dose ion implantation followed by a high-temperature annealing step
-turned out to constitute a promising method to form buried layers of SiC in Si
-as indicated in this sketch.
-due to the high areal homogenity achieved in ibs
-the size is only limited by the beam scanning equipment
-and sythesized films do not exhibit surface bending effects
-in contrast these formed by cvd and mbe.
-this enables the synthesis of large are SiC films.
-
-slide 5
-
-the ibs synthesis of SiC was extensively investigated and optimized
-here in augsburg in the group of joerg lindner.
+turned out to constitute a promising method to form buried layers of SiC in Si.
+this was extensively investigated and optimized here in augsburg
+in the group of joerg lindner to obtain homogeneous SiC layers
+with sharp interfaces to the Si host, as can bee seen in the hrtem image.
+
+slide 4 or [5]
+
+one method to fabricate the adavntageous cubic polytiype is ibs,
+i.e. high dose ion implantation followed by a high-temperature annealing step,
+as extensively investigated and optimzed here in augsburg
+in the group of joerg lindner.
a two-step implantation process was suggested.
-the trick is to destroy stable precipitates at the layer interface
+the trick is to destroy stable precipitates that form at the layer interface
by implanting a remaining low amount of the dose at lower temperatures
to enable redistribution of the C profile during annealing,
-which results in a homogeneous SiC layers with a sharp interface
+which results in a homogeneous SiC layer with a sharp interface
as you can see in this cross section tem image.
+slide 4/5
+
however, the precipitation itself is not yet fully understood.
understanding the effective underlying processes of precipitation
will enable significant progress in thin film formation of cubic SiC
a pair of black dots represent two atoms of the two fcc lattices.
the incorporated carbon atoms form C-Si dumbbells
situated on regular silicon lattice sites.
-with increasing doese these dumbbells agglomerate into large clusters,
+with increasing dose and time these dumbbells agglomerate into large clusters,
indicated by dark contrasts and an otherwise undisturbed lattice in hrtem.
once a critical radius of 2-4 nm is reached,
the interfacial energy due to the lattice mismatch is overcome
simulations are performed in the isothermal-isobaric ensemble
realized by the berendsen thermostat and barostat.
-furthermore, highly accurate quantum mechanical calculations
-based on dft are used.
the basic concept of dft is the hohenberg kohn (hk) theorem, which states that
the ground-state wavefunction is a unique functional of the ground-state
electron density, which minimizes the energy,
i.e. it has the variational property.
-in that way, the many body problem can be described by the electron density,
-which depends only on the 3 spatial coordinates.
now, the kohn sham (ks) approach constitutes a hartree-like formulation
of the hk minimal principle, which maps the system of interacting particles to
an auxillary system of non-interacting electrons in an effective potential.
however formally exact by introducing an energy functional,
which accounts for the exchange and correlation energy.
-the effective potential yields a ground-state density
-for non-interacting electrons, which is equal to that for interacting electrons
-in the external potential.
+#the effective potential yields a ground-state density
+#for non-interacting electrons, which is equal to that for interacting electrons
+#in the external potential.
the kohn sham equations need to be solved in a self consistency loop.
the vasp code was used for this purpose.
this artificial configuration, however, turns out to have negligible influence
in finite temperature simulations due to a low migration barrier into the
tetrahedral configuration.
-nevertheless, these artificats have to be taken into account
+nevertheless, all these discrepancies have to be taken into account
in the following investigations of defect combinations.
slide 12
next to the fact, that this is a different pathway,
the barrier is 2.4 times higher than the experimental and ab inito results.
-moreover, the ea description predicts the bc configuration to be unstable
+however, the ea description predicts the bc configuration to be unstable
relaxing into the 110 db configuration.
-indeed, the observed minima in the 00-1 to 0-10 transition,
+additionally, the observed minimum in the classical 00-1 to 0-10 transition,
is close to the 110 db structure.
this suggests to investigate the transition involving the 110 configuration.
as can be seen, the agglomeration of interstitial carbon is energetically
favorable.
-indeed, the most favorable configuration shows a strong C-C bond.
+the most favorable configuration shows a strong C-C bond.
however, due to high migration barriers or energetically unfavorable
intermediate configurations to obtain this configuration,
only a low probability is assumed for C-C clustering.
the interaction is found to be proportional to the reciprocal cube
of the distance for extended separations and saturates for the smallest
possible distance, i.e. the ground state.
-a capture radius clearly extending 1 nm is observed.
+a capture radius clearly exceeding 1 nm is observed.
the interpolated graph suggests the disappearance of attractive forces
between the two lowest separation distances of the defects.
if the vacancy is created at position 1, the Cs configuration is directly
obtained in the relaxation process.
if it is created at other positions, e.g. 2 and 3,
-only low barriers into the Cs configuration exist
-and high barriers are necessary for the reverse process.
+only low barriers are necessary for a transition into the Cs configuration
+whereas high barriers are necessary for the reverse process.
based on this, a high probability for the formation of Cs,
which is found to be extremely stable, must be concluded.
even for defects located within the capture radius.
to conclude, the results of the investigations of defect combinations
-suggest an increased participation of Cs in the precipitation process.
+suggest an increased participation of Cs already in the initial stage
+of precipitation due to its high probability of incidence.
slide 21
slide 27
-to summarize and conclude
+to summarize and conclude ...
slide 28
-in the end, I would like to say thank you.
+finally, I would like to say thank you.
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