From 0bf034646e0a4cf69ba6235d16646758150c7252 Mon Sep 17 00:00:00 2001 From: hackbard Date: Mon, 20 Jun 2011 11:35:41 +0200 Subject: [PATCH 1/1] started reply of copmbined manuscript --- posic/publications/sic_prec_reply02.txt | 218 ++++++++++++++++++++++++ 1 file changed, 218 insertions(+) create mode 100644 posic/publications/sic_prec_reply02.txt diff --git a/posic/publications/sic_prec_reply02.txt b/posic/publications/sic_prec_reply02.txt new file mode 100644 index 0000000..01a13f6 --- /dev/null +++ b/posic/publications/sic_prec_reply02.txt @@ -0,0 +1,218 @@ + +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. + +and + +Re: BA11443 + First-principles study of defects in carbon-implanted silicon + by F. Zirkelbach, B. Stritzker, J. K. N. Lindner, et al. + + + +Dear Dr. Dahal, + +thank you for the feedback to our submission. + +> We look forward to receiving such a comprehensive manuscript. When you +> resubmit, please include a summary of the changes made, and a detailed +> response to all recommendations and criticisms. + +We decided to follow your's and the referee's suggestion to merge the +two manuscripts into a single comprehensive manuscript. + +Please find below the summary of changes and a detailed response to +the recommendations of the referee. + +Most of the criticism is pasted from the previous review justified by +the accusation that we did ignore or not adequatley answered them. +However, we did comment on every single issue and a more adequate +answer is hindered if the referee does not specify the respective +points of criticism. Thus, some responses are identical to these +included of our previous answer. + +Sincerely, + +Frank Zirkelbach + + +--------------- Response to recommendations ---------------- + +> I am not happy with these two papers for a multitude of reasons, +> and I recommend that the authors rewrite them as a single longer +> paper, to eliminate the criticism of serial publication. I do not +> accept the authors argument that they should be two papers ­ they +> address the same issues, using the same methods. If they were to +> be split into two papers, it would be one for the VASP +> calculations, and one for the MD ­ this is not how I suggest you +> do it, though. + +We now combined the two manuscripts into a single comprehensive one. + +> do it, though. First, though, the following issues should be +> addressed (some are simply pasted from my previous reviews, where +> I feel that the authors have ignored them, or not responded +> adequately). +> +> 1. I feel that the authors are a bit too convinced by their own +> calculations. They do not state the error bars that would be +> expected for calculations like this +/- 0.2 eV would be a very +> optimistic estimate, I suggest. That being so, many of their +> conclusions on which structure or migration routes are most +> likely start to look rather less certain. + +Although differences of 0.2 eV in DFT calculations would generally be +acknowledged to be insignificant when being compared to experimental +results or data of other ab initio studies, these differences are +considered to be reliable when comparing results, i.e. differences in +energy, of a systematic study among each other. This is commonly done +as can be seen in a great deal of literature, some of which is cited +in the section of the present manuscript that investigates defect +structures and formation energies. Very often differences less than +0.2 eV are obtained and conclusions on the stability of a particular +structure are derived. + +> 2. Why is 216 atoms a large enough supercell ­ many defect +> properties are known to converge very slowly with supercell size. +> They appear to be separating defects by as large a distance as +> can be accommodated in the supercell to approximate the isolated +> defects, but then they are only separated by a few lattice +> spacings from a whole array of real and image defects ­ how does +> that compare with taking the energies of each defect in a +> supercell. + +Choosing a 216 atom supercell constitutes a tradeoff, of course. +However, it is considered the optimal choice with respect to both, +computing time and accuracy of the results. + +The convergence of the formation energies of single defects with +respect to the size of the supercell is ensured. For this reason, they +are referred to as single isolated defects. + +It is not our purpose to separate defects by a large distance in order +to approximate the situation of isolated defects. However, we find +that for increasing defect distance configurations appear, which +converge to the energetics of two isolated defects. This is indicated +by the (absolute value of the) binding energy, which is approaching +zero with increasing distance. From this, we conclude a decrease in +interaction, which is already observable for defect separation +distances accessible in our simulations. This is stated now more +clearly in section II of the revised manuscript. (-> Change 6) + +Nevertheless, the focus is on closely neighbored, interacting defects +(for which an interaction with their own image is, therefore, supposed +to be negligible, too). At no time, our aim was to investigate single +isolated defect structures and their properties by increasing the +separation distance of two defects belonging to a a defect +combination. + +A note is added to let the reader know that convergence with respect +to the system size is ensured. (-> Change 2) + +> 3. Constant pressure solves some problems, but creates others ­ +> is it really a sensible model of implantation? What differences +> are seen for constant volume calculations (on a few simple +> examples, say)? + +In experiment substrate swelling is observed for high-dose carbon +implantation into silicon. Indeed, using the NpT ensemble for +calculations of a single (double) C defect in Si is questionable. +However, only small changes in volume were observed and, thus, it is +assumed that there is no fundamental difference between calculations +in the canonical and isothermal-isobaric ensemble. + +Constant volume calculations were not performed and, thus, we cannot +provide concrete differences. + +The fact that there are only small changes in volume is added to the +methodology section. (-> Change 3) + +> 4. What method do they use to determine migration paths? How can +> they convince us that the calculations cover all possible +> migrations paths ­ that is, the paths they calculate are really +> the lowest energy ones? This is a major issue ­ there are a +> number of methods used in the literature to address it ­ are the +> authors aware of them? Have they used one of them? + +A slightly modified version of the constrained conjugate gradient +relaxation method is used. It is named in the very beginning of the +second part of chapter II and a reference is given. Although, in +general, the method not necessarily unveils the lowest energy +migration path it gives reasonable results for the specific system. +This can be seen for the resulting pathway of C interstitial DB +migration, for which the activation energy perfectly matches +experimental data. + +For clarity we added a statement, however, that of course the true +minimum energy path may still be missed. (-> Change 4) + +> 5. I have some serious reservations about the methodology +> employed in the MD calculations. The values given for the basic +> stabilities and migration energies in some cases disagree +> radically with those calculated by VASP, which I would argue +> (despite 4 above) to be the more reliable values. The main +> problems is 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. 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. +> 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 demonstratably +> untrue in this case. For this reason, despite the reputation and +> previous work with Tersoff (and similar) potentials, the results +> need a critical scrutiny, which I am not very convinced by in +> this case. + +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 'accommodated' 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 equilibrium position +are well described, the above mentioned effects increase for +non-equilibrium structures and dynamics. 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 (to an even +larger extent) for Tersoff potentials, 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 extent 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 does not pose a problem in the present context. Since we +deal with a perfect Si crystal and the number of particles is +conserved, 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 initio calculations. + +The arguments discussed above are now explained in more detail in the +revised version of our work. (-> Change 1, Change 2) + -- 2.20.1