-the second peak is an artifact due to the cut-off.
-the C-C peak at 0.31 nm, as expected in cubic SiC,
-is generated by concatenated, differently oriented Ci dbs.
-the same distance is also expected for the Si atoms, and, indeed,
-the db structure stretches the Si-Si next neighbor distance,
-which is represented by nonzero values in the correlation function.
-
-so, the formation of Ci dumbbells indeed occurs.
-even the C atoms are already found in a separation as expected in cubic SiC.
-
-turning to the high C concentration simulations,
-a high amount of strongly bound C-C bonds
-as in graphite or diamond is observed.
-due to increased defect and damage densities
-defect arrangemnets are hard to categorize and trace.
-only short range order is observed.
-and, indeed, by comparing to other distribution data,
-an amorphous SiC-like phase is identified.
-
-slide 19
-
-to summarize, the formation of cubic SiC fails to appear.
-neither agglomeration of C interstitials
-nor a transition into SiC can be identified.
-
-slide 20
-
-having a closer look, there are two obvious reasons for this obstacle.
-
-first of all, there is the time scale problem inherent to md in general,
-which results in a slow phase space propagation due to
-a large amount of local minima separated by large energy barriers.
-accelerated methods, like temperature accelerated MD and so on, exist
-to bypass the time scale problem while retaining proper thermodynamic sampling.
-
-however, in addition, the overestimated diffusion barriers,
-due to the short range character of the potential,
-intensify this problem, which I termed:
-potential enhanced slow phase space propagation.
-
-the approach used in this study is to simply increase the temperature, however,
-without possible corrections.
-accelerated methods or higher time scales applied exclusively
-are assumed to be not sufficient.
-anyways, in this case,
-structural evolution instead of equilibrium properties are matter of interest.
-
-slide 21
-
-and indeed, promising changes are observed by comparing,
-again the radial distribution data for temperatures up to 2050 dc.
-first of all, the cut-off artifact disappears.
-more important, a transition into a clearly Cs dominated structure takes place,
-as can be seen by direct comparison with the respective reference peaks of Cs.
-
-the rising Si-Si peak is due to stretched Si-C-Si structures
-along a 110 direction.
-
-the C-C next neighbor pairs are reduced,
-which is mandatory for SiC formation.
-the peak at roughly 0.3 nm gets slightly shifted to higher distances,
-due to a decrease of interstitial carbon combinations accompanied by an
-increase in interstitial and substitutional as well as pure substitutional
-combinations.
-increasing values in this range
-correspond to bonds of Cs and another Cs with a nearby Si_i atom.
-
-slide 22
-
-to conclude, stretched coherent structures are directly observed.
-therefore, it is expected that Cs is extensively involved
-in the precipitation process for implantations at elevated temperatures.
-
-the emission of Si_i serves several needs:
-as a vehicle to rearrange stable Cs,
-as a building block for the surrounding Si host or further SiC formation.
-and for strain compensation either at the Si/SiC interface
-or in the stretched SiC structure, which, again,
-was diretly observed in simulation.
-
-this perfectly explains the results of the annealing experiments
-stated in the beginning of this talk.
-at low temperatures highly mobile Ci
-whereas at high temperatures stable Cs configurations are formed.
-
-thus, it is further concluded that high temperatures are necessary to model
-ibs conditions, which are far from equilibrium.
-the high temperatures deviate the system from thermodynamic equilibrium
-enabling Ci to turn into Cs.
-
-slide 23
-
-to summarize and conclude ...
-point defects were investigated by both methods.
-the interstitial carbon mmigration path was identified.
-it turned out that the diffusion barrier is drastically overestimated
-within the ea description.
-
-combinations of defects were investigated by first principles methods.
-the agglomeration of point defects is energetically favorable.
-however, substitutional carbon arises in all probability.
-even transitions from the ground state are very likely to occur.
-
-concerning the precipitation simulations, the problem of
-potential enhanced slow phase space propagation was discussed.
-high temperatures are assumed necessary to simulate ibs conditions.
-at low temperatures a dumbbell dominated structure is obatined
-whereas
-it is expected that
-Stretched structures of SiC were observed at elevated temperatures.
-it is thus concluded that
-substitutional carbon is heavily involved in the precipitation process.
-the role of the Si_i was outlined.
-
-in total, these results suggest,
-that cubic SiC precipitation occurs by successive agglomeration of Cs.
-
-slide 24
-
-finally, I would like to thank all of the people listed on this slide,
-categorized by location.
-
-thank you for your attention!
-
-
-
-
-
-slide X polytypes
-
-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,
-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.
-SiC clearly outperforms silicon.
-among the different polytypes, the cubic phase shows the highest
-break down field and saturation drift velocity.
-additionally, these properties are isotropic.
-thus, the cubic polytype is considered most effective for highly efficient
-high-performance electronic devices.
-
-slide X silicon self interstitials
-
-in the following, structures and formation energies
-of silicon self-interstitial defects are shown.
-the classical potential and ab initio method predicts formation energies,
-which are within the same order of magnitude.
-however, discrepancies exist.
-quantum-mechanical results reveal the silicon 110 interstitial dumbbell (db)
-as the ground state closely followed by the hexagonal and tetrahedral
-configuration, which is the consensus view for silicon interstitials.
-in contrast, the ea potential favors the tetrahedral configuration,
-a known problem, which arises due to the cut-off
-underestimating the closely located second next neighbors.
-the hexagonal defect is not stable
-opposed to results of the authors of the potential.
-first, it seems to condense at the hexagonal site but suddenly
-begins to move towards a more favoarble position,
-close to the tetrahedral one but slightly displaced along all 3 coordinate axes.
-this energy is equal to the formation energy given in the original work.
-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, all these discrepancies have to be taken into account
-in the following investigations of defect combinations.
-
-slide X quantum mechanical details of 100 and bc
-
-it is worth to note that there are differences in the 100 defect geometries
-obtained by both methods.
-while the carbon-silicon distance of the db is equal,
-the db position inside the tetrahedron differs significantly.
-of course, the classical potential is not able to reproduce
-the clearly quantum mechanically dominated character of bonding.
-
-more important, the bc configuration is found to constitute
-a local minimum configuration and not a saddle point as found in another study.
-this is due to the neglection of spin in these calculations, which,
-however, is necessary as can already be seen from simple molecular orbital
-considerations, assuming a sp hybridized carbon atom due to the linear bond.
-this assumption turns to be right as indicated by the charge density isosurface
-which shows a net spin up density located in a torus around the C atom.
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