From: hackbard Date: Mon, 6 Jun 2011 15:04:10 +0000 (+0200) Subject: essen security checkin X-Git-Url: https://hackdaworld.org/gitweb/?p=lectures%2Flatex.git;a=commitdiff_plain;h=12c64d4502d806d2847788c2403cdae155f435cd essen security checkin --- diff --git a/posic/thesis/defects.tex b/posic/thesis/defects.tex index 222e4e5..ac35f7c 100644 --- a/posic/thesis/defects.tex +++ b/posic/thesis/defects.tex @@ -1590,6 +1590,8 @@ Thus, a proper description with respect to the relative energies of formation is \section{Conclusions concerning the SiC conversion mechanism} +\ifnum1=0 + Obtained results for separated point defects in Si are in good agreement to previous theoretical work on this subject, both for intrinsic defects \cite{leung99,al-mushadani03} as well as for C point defects \cite{dal_pino93,capaz94}. The ground-state configurations of these defects, i.e. the Si$_{\text{i}}$ \hkl<1 1 0> and C$_{\text{i}}$ \hkl<1 0 0> DB, are reproduced and compare well to previous findings of theoretical investigations on Si$_{\text{i}}$ \cite{leung99,al-mushadani03} as well as theoretical \cite{dal_pino93,capaz94,burnard93,leary97,jones04} and experimental \cite{watkins76,song90} studies on C$_{\text{i}}$. A quantitatively improved activation energy of \unit[0.9]{eV} for a qualitatively equal migration path based on studies by Capaz et.~al.~\cite{capaz94} to experimental values \cite{song90,lindner06,tipping87} ranging from \unit[0.70-0.87]{eV} reinforce their derived mechanism of diffusion for C$_{\text{i}}$ in Si @@ -1638,6 +1640,8 @@ In addition, a rather small activation energy of \unit[0.77]{eV} allows for the Thus, elevated temperatures might lead to thermodynamically unstable configurations of C$_{\text{s}}$ and a remaining Si atom in the near interstitial lattice, which is supported by the result of an {\em ab initio} molecular dynamics run. %Thus, due to missing attractive interaction forces driving the system to form C-Si \hkl<1 0 0> dumbbell interstitial complexes substitutional C, while thermodynamically not stable, constitutes a most likely configuration occuring in IBS, a process far from equlibrium. +\fi + % todo % maybe move above stuff to conclusion chapter, at least shorten! % see remember in sic chapter diff --git a/posic/thesis/summary_outlook.tex b/posic/thesis/summary_outlook.tex index 09321a7..560854f 100644 --- a/posic/thesis/summary_outlook.tex +++ b/posic/thesis/summary_outlook.tex @@ -1,7 +1,8 @@ \chapter{Summary and conclusions} \label{chapter:summary} -In a short review of the C/Si compound and the fabrication of the technologically promising semiconductor SiC by IBS, two controversial assumptions of the precipitation mechanism of 3C-SiC in c-Si are elaborated. +\paragraph{To summarize,} +in a short review of the C/Si compound and the fabrication of the technologically promising semiconductor SiC by IBS, two controversial assumptions of the precipitation mechanism of 3C-SiC in c-Si are elaborated. These propose the precipitation of SiC by agglomeration of \ci{} DBs followed by a sudden formation of SiC and otherwise a formation by successive accumulation of \cs{} via intermediate stretched SiC structures, which are coherent to the Si lattice. To solve this controversy and contribute to the understanding of SiC precipitation in c-Si, a series of atomistic simulations is carried out. In the first part, intrinsic and C related point defects in c-Si as well as some selected diffusion processes of the C defect are investigated by means of first-principles quatum-mechanical calculations based on DFT and classical potential calculations employing a Tersoff-like analytical bond order potential. @@ -60,7 +61,8 @@ Subsequent investigations focus on defect combinations exclusively by the first- These configurations are constructed in such a way as to allow for a quantum-mechanical treatment. Investigations of two \ci{} defects of the \hkl<1 0 0>-type for varying separations and orientations state a rather attractive interaction between these interstitials. -Primiraly, energetically favorable configurations of two interstitials are found. +The capture radius is found to be rather large compared to other defect combinations. +Mostly energetically favorable configurations of two interstitials are found. This is due to strain compensation enabled by the combination of such defects in certain orientations. An interaction energy proportional to the reciprocal cube of the distance in the far field regime is found supporting the assumption of \ci{} DB agglomeration. The energetically most favorable configuration consists of a C-C bond. @@ -120,13 +122,52 @@ Diamond and graphite like bonds as well as the artificial bonds due to the cut-o Loose structures of stretched SiC, which are adjusted to the Si lattice with respect to the lattice constant and alignment, are identified. \si{} is often found in the direct surrounding. Entropic contributions are assumed to be responsible for these structures at elevated temperatures that deviate from the ground state at 0 K. +Indeed, utilizing increased temperatures is assumed to constitute a necessary condition to simulate IBS of 3C-SiC in c-Si. % conclusions 2nd part +\paragraph{Conclusions} +concerning the SiC conversion mechanism are derived from results of both, first-principles and classical potential calculations. +Although classical potential MD calculations fail to directly simulate the precipitation of SiC, obtained results, on the one hand, reinforce previous findings of the first-principles investigations and, on the other hand, allow further conclusions on the SiC precipitation in Si. + +Initially, quantum-mechanical investigations suggest agglomeration of \ci{} defects that form energetically favorable configurations by an effective stress compensation. +Low barriers of migration are found except for transitions into the ground-state configuration, which is composed of a strong C-C bond. +Thus, agglomeration of \ci{} in the absence of C clsutering is expected. +These initial results suggest a conversion mechansim based on the agglomeration of \ci{} defects followed by a sudden precipitation once a critical size is reached. +However, subsequent investigations of structures that are particularly conceivable under conditions prevalant in IBS and at elevated temperatures show \cs{} to occur in all probability. +The transition from the ground state of a single C atom incorporated into otherwise perfect c-Si, i.e. the \ci{} \hkl<1 0 0> DB, into a configuration of \cs{} next to a \si{} atom exhibits an activation energy lower than the one for the diffusion of the highly mobile \ci{} defect. +Considering additionally the likewise lower diffusion barrier of \si{}, configurations of separated \cs{} and \si{} will occur in all probability. +This is reinforced by the {\em ab initio} MD run at non-zero temperature, which shows structures of separating instead of recombining \cs{} and \si{} defetcs. +This suggests increased participation of \cs{} already in the initial stages of the implantation process. +The highly mobile \si{} is assumed to constitute a vehicle for the rearrangement of other \cs{} atoms onto proper lattice sites, i.e. lattice sites of one of the the two fcc lattices composing the diamond structure. +This way, stretched SiC strcutures, which are coherently aligned to the c-Si host, are formed by agglomeration of \cs. +Precipitation into an incoherent and partially strain-compensated SiC nucleus occurs once the increasing strain energy surpasses the interfacial energy of the incoherent 3C-SiC precipitate and the c-Si substrate. +As already assumed by Nejim~et~al.~\cite{nejim95}, \si{} serves as supply for subsequently implanted C atoms to form further SiC in the resulting free space due to the accompanied volume reduction. + +Several conclusions based on results obtained by classical potential MD simulations are drawn. +First of all, increased temperatures are considered a necessary condition to simulate the IBS of epitaxially aligned 3C-SiC in Si, which constitutes a process far from thermodynamic equilibrium. +The strong deviation from equilibrium by elevated temperatures enables the formation of \cs{}-\si{} structures as observed in the quantum-mechanical calculations. +In contrast, structures of \ci{} \hkl<1 0 0> DBs, which constitute the thermodynamic ground state, appear at low temperatures. +% +Secondly, in configurations of stretched SiC composed by \cs, the accompanied \si{} defect may be assigned further functionality. +Next to that as a vehicle that is able to rearrange \cs{} and a building block for the surrounding Si host or further SiC, the analyzed configurations suggest \si{} to be required for stress compensation. +As evidently observed in these structures, \si{} reduces tensile strain by capturing a position near one of the C atoms within a configuration of two C atoms that basically reside on Si lattice sites. +Furthermore, \si{} might compensate strain in the interface region of an incoherent, nucleated SiC precipitate and the c-Si matrix. +This could be achieved by \ci{} \hkl<1 0 0> DBs in the Si region slightly contracting the Si atoms next to the C atom to better match the spacing of Si atoms present in 3C-SiC. +Indeed, combinations of \cs{} and \ci{} \hkl<1 0 0> DBs are observed. +% +Further conclusions are derived from results of the high C concentration simulations, in which a large amount of C atoms is incorporated into a small volume within a short period of time, which results in essentially no time for the system to rearrange. +Due to this, the formation of strong C-C bonds and the production of a vast amount of damage is observed, which finally results in the formation of an amorphous phase. +The strong bonds and damage obviously decelerate structural evolution. +The short time, which is not sufficient for structural evolution, can be mapped to a system of low temperature, which lacks the kinetic energy required for the restructuring process. +HIER WEITER ... -HIER WEITER ... +To conclude, results of the present study indicate a precipitation of SiC in Si by successive agglomeration of \cs. +\si{}, which is likewise existent, serves several needs: +... Incoherent but epitaxially aligned SiC precipitates are ... + Experimental studies revealed increased implantation temperatures to be more efficient than postannealing methods for the formation of topotactically aligned precipitates \cite{kimura82,eichhorn02}. In particular, restructuring of strong C-C bonds is affected \cite{deguchi92}, which preferentially arise if additional kinetic energy provided by an increase of the implantation temperature is missing to accelerate or even enable atomic rearrangements.