From aed690758bc0c3ec2c6c4e058597935f213e6bfd Mon Sep 17 00:00:00 2001 From: hackbard Date: Mon, 27 Sep 2010 14:54:55 +0200 Subject: [PATCH] heubuisch besuch --- posic/publications/sic_prec.tex | 38 +++++++++++++++++++++------------ 1 file changed, 24 insertions(+), 14 deletions(-) diff --git a/posic/publications/sic_prec.tex b/posic/publications/sic_prec.tex index 7d481b5..614be92 100644 --- a/posic/publications/sic_prec.tex +++ b/posic/publications/sic_prec.tex @@ -353,22 +353,32 @@ It turned out that the EA potential still favors a C$_{\text{i}}$ \hkl<1 0 0> DB MD simulations at temperatures used in IBS resulted in structures that were dominated by the C$_{\text{i}}$ \hkl<1 0 0> DB and its combinations if C is inserted into the total volume. Incoorporation into volmes $V_2$ and $V_3$ led to an amorphous SiC-like structure within the respective volume. -To compensate overestimated diffusion barriers we performed simulations at elevated temperatures. -TOOD: High C conc simulations ... then low: -Time scales are still too low to observe C agglomeration sufficient for SiC precipitation, which is attributed to the slow phase space propagation inherent to MD in general. +To compensate overestimated diffusion barriers we performed simulations at accordingly increased temperatures. +No significant change was observed for high C concentrations. +The amorphous phase is maintained. +Due to the incoorparation of a huge amount of C into a small volume within a short period of time damage is produced, which obviously decelerates strcutural evolution. +For the low C concentrations, time scales are still too low to observe C agglomeration sufficient for SiC precipitation, which is attributed to the slow phase space propagation inherent to MD in general. However, we observed a phase tranisiton of the C$_{\text{i}}$-dominated into a clearly C$_{\text{s}}$-dominated structure. The amount of substitutionally occupied C atoms increases with increasing temperature. -Entropic contributions are assumed to be responsible for these structures that deviate from the ground state at 0 K. -Indeed, in a previous ab initio MD simulation\cite{zirkelbach10b} performed at \unit[900]{$^{\text{C}}$} we observed the departing of a Si$_{\text{i}}$ \hkl<1 1 0> DB located next to a C$_{\text{s}}$ atom instead of a recombination into the ground state configuration, i.e. a C$_{\text{i}}$ \hkl<1 0 0> DB. - -Thus, we prpopose (support) the follwing model ... -Sii stress compensation and vehicle - -Concluded that C sub is very probable ... -Alignment lost, successive substitution more probable to end up with topotactic 3C-SiC. - -Both, low and high, acceleration not enough to either observe C agglomeration or amorphous to crystalline transition ... - +Entropic contributions are assumed to be responsible for these structures at eleveated temperatures that deviate from the ground state at 0 K. +Indeed, in a previous ab initio MD simulation\cite{zirkelbach10b} performed at \unit[900]{$^{\circ}$C} we observed the departing of a Si$_{\text{i}}$ \hkl<1 1 0> DB located next to a C$_{\text{s}}$ atom instead of a recombination into the ground state configuration, i.e. a C$_{\text{i}}$ \hkl<1 0 0> DB. +Ci to Cs by increased temperatures ...\cite{eichhorn99} +Increased temperatures during implantation more efficient than postannealing methods, which reflects the present problems of low temperature and low time strcutural evolution ...\cite{eichhorn02} +C-C for low temperatures, postannealing no that efficient as for high C implantations ...\cite{deguchi92} + +Thus, we propose an increased participation of C$_{\text{s}}$ already in the initial stages of the precipitation process. +Thermally activated, C$_{\text{i}}$ is enabled to turn into C$_{\text{s}}$ accompanied by Si$_{\text{i}}$. +The associated emission of Si$_{\text{i}}$ is needed for several reasons. +For the agglomeration and rearrangement of C Si$_{\text{i}}$ is needed to turn C$_{\text{s}}$ into highly mobile C$_{\text{i}}$ again. +Since the conversion of a coherent SiC structure, i.e. C$_{\text{s}}$ occupying the Si lattice sites of one of the two fcc lattices that build up the c-Si diamond lattice, into incoherent SiC is accompanied by a reduction in volume, large amount of strain is assumed to reside in the coherent as well as incoherent structure. +Si$_{\text{i}}$ serves either as supply of Si atoms needed in the surrounding of the contracted precipitates or as interstitial defect minimizing the emerging strain energy of a coherent precipitate. +The latter has been directly identified in the present simulation study, i.e. structures of two C$_{\text{s}}$ atoms with one being slightly displaced by a next neighbored Si$_{\text{i}}$ atom. + +It is, thus, concluded that precipitation occurs by successive agglomeration of C$_{\text{s}}$ as already proposed by Nejim et~al.\cite{nejim95}. +This agrees well with a previous ab inito study on defects in C implanted Si\cite{zirkelbach10b}, which showed C$_{\text{s}}$ to occur in all probability. +However, agglomeration and rearrangement is enabled by mobile C$_{\text{i}}$, which has to be present at the same time and is formed by recombination of C$_{\text{s}}$ and Si$_{\text{i}}$. +In contrast to assumptions of an abrupt precipitation of an agglomerate of C$_{\text{i}}$\cite{werner96,werner97,eichhorn99,lindner99_2,koegler03}, however, structural evolution is believed to occur by a successive occupation of usual Si lattice sites with substitutional C. +This mechanism satisfies the experimentally observed alignment of the \hkl(h k l) planes of the precipitate and the substrate, whereas there is no obvious reason for the topotactic orientation of an agglomerate consisting exclusively of C-Si dimers, which would necessarily involve a much more profound change in structure for the transition into SiC. \section{Summary} -- 2.39.2