+
+Re: BC11912
+ Combined ab initio and classical potential simulation study on the
+ silicon carbide precipitation in silicon
+ by F. Zirkelbach, B. Stritzker, K. Nordlund, et al.
+
+
+Dear Dr. Dahal,
+
+> The comments of the referee suggest that the current manuscript on its
+> own does not meet our acceptance criteria. We feel that it will be in
+> your interest to combine your earlier submission BA11443 with the
+> current manuscript. We look forward to receiving such a modified
+> manuscript.
+
+BA11443 (Ref. 60) is a self contained and comprehensive manuscript,
+which already now has an appreciable length. Although dealing with the
+same material the present manuscript uses a different, continuative
+approach.
+
+While manuscript BA11443 constitutes a study on defects in
+carbon-implanted silicon exclusively investigated by first-principles
+methods, the present study picks up the limitation of the employed
+classical potential as revealed already in Ref. 42, investigates this
+limitation in more detail and proposes an approach to work around it
+enabling the application to classical potential molecular dynamics
+simulations. Although conclusions on the SiC precipitation in Si are
+already derived in manuscript BA11443, the present study is considered
+an interesting and self contained continuation since it allows for the
+description of larger systems and time scales than accessible by ab
+inito methods. Furthermore, a majority of the results of the classical
+potential as well as the ab initio results on defect structures are
+not presented in either of the two preceding publications.
+
+We would greatly appreciate if you could reconsider your decission and
+we look forward to receiving further comments. It would also be nice
+to know how the processing of manuscript BA11443 is now influenced by
+the present decission.
+
+Please find attached a reply to the comments of the referee, which we
+hope will clarify all your and the referee's concerns enabling a
+separated publication in the Physical Review B.
+
+
+Sincerely,
+
+Frank Zirkelbach
+
+
+
+Response to the comments of the referee
+---------------------------------------
+
+> It follows on naturally from a previous paper on the carbon
+> interstitial in silicon (their ref 42), but does not appear to be a
+> "serial publication". However, it also refers to an (as yet)
+> unpublished study (ref 60) of the same topic as the present paper with
+> almost the same authors, using ab initio MD. Perhaps the authors could
+> comment on how these two papers differ, and whether ref 60 improves on
+> the results of the present paper in such a way that makes present
+> paper superfluous.
+
+Manuscript BA11443 (Ref. 60) entitled 'First-principles study of
+defects in carbon-implanted silicon' investigates single native and C
+point defects as well as their combinations in Si by highly accurate
+ab initio methods. In that, it constitutes a self contained,
+substantial study.
+
+The present work studies in more detail the limitation of the employed
+classical potential by comparing it to the results of ab initio
+calculations. We would like to point out that a majority of the
+results of the ab initio as well as the classical potential
+investigations on defect structures, which are in the closer context
+of this manuscript, have not been presented in either of the two
+preceding documents.
+Additionally, a work around is proposed to overcome the limitation of
+the potential as well as the general problem inherent to MD describing
+phase transitions made up of a multiple of infrequent transition
+events. This enables the observation of a phase transition of the
+obtained structure with increasing temperature during C insertion.
+
+Although conclusions on the SiC precipitation in Si are already
+derived from the DFT study on single defects and some selected
+combinations, the classical potential MD simulations allow the
+investigation of larger and, thus, much more complex systems on a
+larger time scale, reinforcing conclusions concerning the SiC
+precipitation in Si.
+
+There are no contradictions or improvements to the current study in
+Ref. 60 that would make one of the manuscripts obsolteted by the
+other.
+
+Both manuscripts constitute self contained and comprehensive studies,
+which - in our opinion - should be presented separately.
+
+> I have some serious reservations about the methodology employed in
+> this paper, for reasons that are discussed at length in it. I am not
+> convinced that the measures they take to circumvent the problems in
+> the method do not introduce further uncertainties, and I would need a
+> bit more convincing that the results are actually valid. Actually, the
+> proof I would need is probably within the simulations of ref 60, hence
+> my question above! The problems I refer to are the huge over-estimate
+> of the C interstitial migration energy (a process which is at the
+> heart of the simulations) using the potential used in the paper,
+> probably due to the short cut-off of the interactions. The authors'
+> circumvention of this is to do the simulations at much heightened
+> temperatures. However, this only gives a good model of the system if
+> all cohesive and migration energies are over-estimated by a similar
+> factor, which is demonstrably untrue in this case, where the C_s
+> formation energy is actually underestimated. There are long
+> discussions of these points in the paper, which leads me to the
+> conclusion that the EA potential used is unreliable in these
+> simulations, possibly unless backed up by some ab initio work, which
+> the authors have done in ref 60.
+
+There is not necessarily a correlation of cohesive energies or defect
+formation energies with activation energies for migration. Cohesive
+energies are most often well described by the classical potentials
+since these are most often used to fit the potential parameters. The
+overestimated barriers, however, are due to the short range character
+of these potentials, which drop the interaction to zero within the
+first and next neighbor distance using a special cut-off function.
+Since the total binding energy is 'accomodated' within this short
+distance, which according to the universal energy relation would
+usually correspond to a much larger distance, unphysical high forces
+between two neighbored atoms arise. This is explained in detail in the
+study of Mattoni et. al. (Phys. Rev. B 76, 224103 (2007)).
+
+Since most of the defect structures show atomic distances below the
+critical distance, for which the cut-off function is taking effect,
+the respective formation energies are quite well described, too (at
+least they are not necessarily overestimated in the same way).
+
+While the properties of some structures near the equilibirum position
+are well described the above mentioned effects increase for
+structures/dynamics more distant from the balanced state. Thus, for
+instance, it is not surprising that short range potentials show
+overestimated melting temperatures. This is not only true for the EA
+but also (even to a greater extent) for the Tersoff potential, one of
+the most widely used classical potentials for the Si/C system. The
+fact that the melting temperature is drastically overestimated
+although the cohesive energies are nicely reproduced indicates that
+there is no reason why the cohesive and formational energies should be
+overestimated to the same extend in order to legitimate the increase
+in temperature to appropriately consider the overestimated barrier
+heights for diffusion.
+
+Indeed, a structural transformation with increasing temperature is
+observed, which can be very well explained and correlated to experimental
+findings.
+
+The underestimated energy of formation of substitutional C for the EA
+potential turns out to not constitute a real problem concerning the
+raised question (interstitial versus substitutional C). Since we deal
+with a perfect Si crystal and conservation of particles the creation
+of substitutional C is accompanied by the creation of a Si
+interstitial. The formation energies of the different structures of
+an additional C atom incorporated into otherwise perfect Si shows the
+same ground state, i.e. the C-Si 100 DB structure, for classical
+potential as well as ab inito calculations.
+
+> Therefore, I do not feel that this paper can stand alone - either its
+> conclusions are contradicted by those of ref 60 (in which case there's
+> no need to publish this paper), or supported by them (in which case
+> ref 60 supercedes this paper, and some brief account of this work
+> could be included in it).
+
+As mentioned above, there are no conclusions in Ref. 60 that
+contradict to the results of the present manuscript. Indeed, results
+of Ref. 60 are important for the current study and, therefore,
+supporting this work. However, the different approach, i.e. modelling
+thousands of C atoms incorporated into a large Si host matrix by
+molecular dynamics simulations on a large time scale opposed to highly
+accurate investigations of the structure of single and double defects
+in Si and some selected diffusion processes, militates the separate
+publication of the results presented in the two manuscripts.
+
+If considered helpful, we would be happy to send a copy of manuscript
+BA11443 to the referee.
+
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