+High resolution transmission electron microscopy (HREM) investigations of C-implanted Si at room temperature followed by rapid thermal annealing (RTA) indicate the formation of C-Si dumbbell agglomerates, which are stable up to annealing temperatures of about \unit[700--800]{$^{\circ}$C}, and a transformation into 3C-SiC precipitates at higher temperatures~\cite{werner96,werner97}.
+The precipitates with diameters between \unit[2]{nm} and \unit[5]{nm} are incorporated in the Si matrix without any remarkable strain fields, which is explained by the nearly equal atomic density of C-Si agglomerates and the SiC unit cell.
+Implantations at \unit[500]{$^{\circ}$C} likewise suggest an initial formation of C-Si dumbbells on regular Si lattice sites, which agglomerate into large clusters~\cite{lindner99_2}.
+The agglomerates of such dimers, which do not generate lattice strain but lead to a local increase of the lattice potential~\cite{werner96,werner97}, are indicated by dark contrasts and otherwise undisturbed Si lattice fringes in HREM, as can be seen in Fig.~\ref{fig:sic:hrem:c-si}.
+\begin{figure}[t]
+\begin{center}
+\subfigure[]{\label{fig:sic:hrem:c-si}\includegraphics[width=0.25\columnwidth]{tem_c-si-db.eps}}
+\subfigure[]{\label{fig:sic:hrem:sic}\includegraphics[width=0.25\columnwidth]{tem_3c-sic.eps}}
+\end{center}
+\caption[High resolution transmission electron microscopy (HREM) micrographs of agglomerates of C-Si dimers showing dark contrasts and otherwise undisturbed Si lattice fringes and equally sized Moir\'e patterns indicating 3C-SiC precipitates.]{High resolution transmission electron microscopy (HREM) micrographs~\cite{lindner99_2} of agglomerates of C-Si dimers showing dark contrasts and otherwise undisturbed Si lattice fringes (a) and equally sized Moir\'e patterns indicating 3C-SiC precipitates (b).}
+\label{fig:sic:hrem}
+\end{figure}
+A topotactic transformation into a 3C-SiC precipitate occurs once a critical radius of \unit[2]{nm} to \unit[4]{nm} is reached.
+The precipitation is manifested by the disappearance of the dark contrasts in favor of Moir\'e patterns (Fig.~\ref{fig:sic:hrem:sic}) due to the lattice mismatch of \unit[20]{\%} of the 3C-SiC precipitate and the Si host.
+The insignificantly lower Si density of SiC of approximately \unit[3]{\%} compared to c-Si results in the emission of only a few excess Si atoms.
+The same mechanism was identified by high resolution x-ray diffraction~\cite{eichhorn99}.
+For implantation temperatures of \unit[500]{$^{\circ}$C} C-Si dumbbells agglomerate in an initial stage followed by the additional appearance of aligned SiC precipitates in a slightly expanded Si region with increasing dose.
+The precipitation mechanism based on a preceding dumbbell agglomeration as indicated by the above-mentioned experiments is schematically displayed in Fig.~\ref{fig:sic:db_agglom}.
+\begin{figure}[t]
+\begin{center}
+\subfigure[]{\label{fig:sic:db_agglom:seq01}\includegraphics[width=0.30\columnwidth]{sic_prec_seq_01.eps}}
+%C-Si dumbbell formation
+\hspace*{0.2cm}
+\subfigure[]{\label{fig:sic:db_agglom:seq02}\includegraphics[width=0.30\columnwidth]{sic_prec_seq_02.eps}}
+%Dumbbell agglomeration
+\hspace*{0.2cm}
+\subfigure[]{\label{fig:sic:db_agglom:seq03}\includegraphics[width=0.30\columnwidth]{sic_prec_seq_03.eps}}
+%SiC formation and release of excess Si atoms
+\end{center}
+\caption[Two dimensional schematic of the assumed SiC precipitation mechanism based on an initial C-Si dumbbell agglomeration.]{Two dimensional schematic of the assumed SiC precipitation mechanism based on an initial C-Si dumbbell agglomeration. C atoms (red dots) incorporated into the Si (black dots) host form C-Si dumbbells (a), which agglomerate into clusters (b) followed by the precipitation of SiC and the emission of a few excess Si atoms (black circles), which are located in the interstitial Si lattice (c). The dotted lines mark the atomic spacing of c-Si in \hkl[1 0 0] direction indicating the $4/5$ ratio of the lattice constants of c-Si and 3C-SiC.}
+\label{fig:sic:db_agglom}
+\end{figure}
+The incorporated C atoms form C-Si dumbbells on regular Si lattice sites.
+With increasing dose and proceeding time the highly mobile dumbbells agglomerate into large clusters.
+Finally, when the cluster size reaches a critical radius, the high interfacial energy due to the 3C-SiC/c-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 c-Si matrix since there is more space than in 3C-SiC.
+
+In contrast, IR spectroscopy and HREM investigations on the thermal stability of strained Si$_{1-y}$C$_y$/Si heterostructures formed by solid-phase epitaxy (SPE)~\cite{strane94} and MBE~\cite{guedj98}, which finally involve the incidental formation of SiC nanocrystallites, suggest a coherent initiation of precipitation by agglomeration of substitutional instead of interstitial C.
+These experiments show that the C atoms, which are initially incorporated substitutionally at regular lattice sites, form C-rich clusters maintaining coherency with the Si lattice during annealing above a critical temperature prior to the transition into incoherent 3C-SiC precipitates.
+Increased temperatures in the annealing process enable the diffusion and agglomeration of C atoms.
+Coherency is lost once the increasing strain energy of the stretched SiC structure surpasses the interfacial energy of the incoherent 3C-SiC precipitate and the Si substrate.
+Estimates of the SiC/Si interfacial energy~\cite{taylor93} and the consequent critical size correspond well with the experimentally observed precipitate radii within these studies.
+
+This different mechanism of precipitation might be attributed to the respective method of fabrication.
+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.
+However, in another IBS study Nejim et~al.~\cite{nejim95} propose a
+% topotactic
+transformation that is likewise based on substitutional C, which replaces four of the eight Si atoms in the Si unit cell accompanied by the generation of four Si interstitials.
+The replacement of a Si unit cell by a 3C-SiC unit cell is accompanied by a volume reduction of \unit[48]{\%} due to the \unit[20]{\%} lower lattice constant.
+Since the emerging strain caused by the expected volume reduction would result in the formation of dislocations, which, however, are not observed, the interstitial Si is assumed to react with further implanted C atoms in the released volume.
+The resulting strain due to the slightly lower Si density of SiC compared to Si of about \unit[3]{\%} is sufficiently small to legitimate the absence of dislocations.
+However, the exact atomic rearrangement involved within this topotactic transformation is not identified.
+Furthermore, IBS studies of Reeson~et~al.~\cite{reeson87}, in which implantation temperatures of \unit[500]{$^{\circ}$C} were employed, revealed the necessity of extreme annealing temperatures for C redistribution, which is assumed to result from the stability of substitutional C and consequently high activation energies required for precipitate dissolution.
+The results support a mechanism of an initial coherent precipitation based on substitutional C that is likewise valid for the IBS of 3C-SiC by C implantation into Si at elevated temperatures.
+The fact that the metastable cubic phase instead of the thermodynamically more favorable hexagonal $\alpha$-SiC structure is formed and the alignment of these cubic precipitates within the Si matrix, which can be explained by considering a topotactic transformation by C atoms occupying substitutionally Si lattice sites of one of the two fcc lattices that make up the Si crystal, reinforce the proposed mechanism.
+
+To conclude, a controversy with respect to the precipitation of SiC in Si exists in literature.
+Next to the pure scientific interest, solving this controversy and gaining new insight in the SiC conversion mechanism might enable significant progress in the heteroepitaxial growth of thin films featuring non-coherent interfaces in the C/Si system.
+On the other hand, processes relying upon prevention of precipitation in order to produce strained heterostructures will likewise benefit.
+
+% remember
+% werner96/7: rt implants followed by rta < 800: C-Si db aggloms | > 800: 3C-SiC
+% taylor93: si_i reduces interfacial energy (explains metastability) of sic/si
+% eichhorn02: high imp temp more efficient than postimp treatment
+% eichhorn99: same as 02 + c-si agglomerates at low concentrations
+% strane94/guedj98: my model - c redist by si int (spe) and surface diff (mbe)
+% serre95: low/high t implants -> mobile c_i / non-mobile sic precipitates