ZnO Deposition on Silicon and Porous Silicon Substrate via Radio Frequency Magnetron Sputtering
, MarÃa R. Jiménez-Vivanco 3 and Heberto Gómez-Pozos 4Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsIn this manuscript, the authors have deposited ZnO thin films on silicon and porous silicon substrates using RF magnetron sputtering and investigated the variation in the properties of the deposited films at different powers and temperatures. The article is well organized, logical and well argued.
However, there are still some problems in the article that need to be improved, and the main comments are as follows,
(1) The authors discuss the effects of power and temperature on the properties of ZnO thin films, but in the case of the RF magnetron sputtering process itself, factors such as pressure, gas type, and gas flow rate also have a significant effect on the quality of the formed film. If the authors are investigating the technique of "deposition of thin films by RF magnetron sputtering", they should consider these factors and discuss them accordingly.
(2) In Section 2 (Materials and Methods) of the article, the model number of the equipment used for magnetron sputtering should be labeled, and the model number of the other characterization equipment used should be labeled in a consistent way, e.g., with the country of manufacture of the equipment, as in the case of the "Varian fluorescence spectrometer (Cary Eclipse, Varian Inc., Palo Alto, CA, USA)".
(3) After the SEM characterization of the deposited ZnO films, the addition of EDS analysis can better demonstrate the elemental distribution and the localization of ZnO, which will make it easier for the reader to understand.
(4) The figure notes in Figure 2 are not standardized, please check all the figure notes to ensure that the meaning of all sub-figures is explained in the figure notes. It better be like this. X-Ray diffraction spectra for the ZnO deposited on (a) mPS and (b)cSi.
(5) The authors claim that the residual stress on the ZnO lattice increases with increasing RF power, but A8-cSi and B8-cSi are clearly inconsistent with this phenomenon, so please provide a reasonable explanation.
(6) The authors compare the grain size calculated by Scheele's formula with the particle size in the SEM image which is obviously not reasonable (the grains are not visible in the SEM image), if a comparison is to be made TEM characterization of the deposited ZnO films can be performed.
Comments for author File: 
Comments.pdf
In this manuscript, the authors have deposited ZnO thin films on silicon and porous silicon substrates using RF magnetron sputtering and investigated the variation in the properties of the deposited films at different powers and temperatures. The article is well organized, logical and well argued.
However, there are still some problems in the article that need to be improved, and the main comments are as follows,
(1) The authors discuss the effects of power and temperature on the properties of ZnO thin films, but in the case of the RF magnetron sputtering process itself, factors such as pressure, gas type, and gas flow rate also have a significant effect on the quality of the formed film. If the authors are investigating the technique of "deposition of thin films by RF magnetron sputtering", they should consider these factors and discuss them accordingly.
(2) In Section 2 (Materials and Methods) of the article, the model number of the equipment used for magnetron sputtering should be labeled, and the model number of the other characterization equipment used should be labeled in a consistent way, e.g., with the country of manufacture of the equipment, as in the case of the "Varian fluorescence spectrometer (Cary Eclipse, Varian Inc., Palo Alto, CA, USA)".
(3) After the SEM characterization of the deposited ZnO films, the addition of EDS analysis can better demonstrate the elemental distribution and the localization of ZnO, which will make it easier for the reader to understand.
(4) The figure notes in Figure 2 are not standardized, please check all the figure notes to ensure that the meaning of all sub-figures is explained in the figure notes. It better be like this. X-Ray diffraction spectra for the ZnO deposited on (a) mPS and (b)cSi.
(5) The authors claim that the residual stress on the ZnO lattice increases with increasing RF power, but A8-cSi and B8-cSi are clearly inconsistent with this phenomenon, so please provide a reasonable explanation.
(6) The authors compare the grain size calculated by Scheele's formula with the particle size in the SEM image which is obviously not reasonable (the grains are not visible in the SEM image), if a comparison is to be made TEM characterization of the deposited ZnO films can be performed.
Author Response
Manuscript 2612929
Dear Doctor
Thank you for giving me the opportunity to submit a revised draft to manuscript title “ZnO deposition on silicon and porous silicon substrate by RF magnetron sputtering”. I appreciate the time and effort that you have dedicated to providing your valuable feedback on my manuscript. I grateful for their insightful comments on my paper. I have been able to incorporated changes to reflect most of the suggestions provided by you, and I have highlighted the changes within manuscript.
The points you requested and the responses to each of them are listed below.
Comments for Reviewer
In this manuscript, the authors have deposited ZnO thin films on silicon and porous silicon substrates using RF magnetron sputtering and investigated the variation in the properties of the deposited films at different powers and temperatures. The article is well organized, logical and well argued.
However, there are still some problems in the article that need to be improved, and the main comments are as follows,
- The authors discuss the effects of power and temperature on the properties of ZnO thin films, but in the case of the RF magnetron sputtering process itself, factors such as pressure, gas type, and gas flow rate also have a significant effect on the quality of the formed film. If the authors are investigating the technique of "deposition of thin films by RF magnetron sputtering", they should consider these factors and discuss them accordingly.
Response. The reviewer has raised an important point, so we added the discussion of significant factors in deposition films by RF magnetron sputtering.
Page 2…” In the present work we applied magnetron sputtering technique because is the most used and studied for its efficiency, high interfacial adhesion, and high denser deposited films, and it also allows films to be deposited with excellent uniformity and quality crystalline on different types of substrates. When magnetron sputtering technique is adopted, the pressure, gas type, gas flow, temperature, and power deposition have significant effect on the quality of the formed films. For example, the working pressure can change the grain size and crystal structure ZnO deposited, allowing the films of ZnO deposited to be oriented in different crystalline planes. Likewise, the deposits made with low power density show a very smooth surface and preferential orientation of the grains [26]. Furthermore, the increase of oxygen content in argon environment results in a decrease in the deposition rate of the films [26]. Husam S. Al-Salam and M. J. Abdullah deposited ZnO on PS maintaining a RF power deposition of 150W with its posterior annealing at 500°C during 2 h. The results revealed a high and deep porosity with a roughness of 178 nm [27]. K. Cicek et al. formed ZnO on PS and silicon utilizing RF&DC magnetron sputtering technique with flow rate of Ar and O2 at 120W power. They found that a pyroelectric coefficient of 8.2 can be achieved for deposits on PS, which is more than ~40 times higher than the one on Si substrate [28]”…
- In Section 2 (Materials and Methods) of the article, the model number of the equipment used for magnetron sputtering should be labeled, and the model number of the other characterization equipment used should be labeled in a consistent way, e.g., with the country of manufacture of the equipment, as in the case of the "Varian fluorescence spectrometer (Cary Eclipse, Varian Inc., Palo Alto, CA, USA)".
Response. Thank you for pointing this out. As suggested by the reviewer, we have added more details on the equipment of characterization.
Page 2, 3…“ ZnO film deposition on m-PS was made by magnetron sputtering (ATC Orion 8 Cluster Flange, Aja International) using two radio frequencies (RF) power of 60W (A) and 80W (B) respectively. It is important to note that in each deposition of ZnO, the temperature of the substrates was kept at 500°C (A5/m-PS, B5/m-PS) and 800°C (A8/m-PS, B8/m-PS) for 1 hour. The deposition was achieved using a 2-inch ZnO target with 99.99% purity, the base pressure of the system was 2 x 10-6 Torr, and the Ar flow was 30 sccm, to obtain a work pressure of 5 x 10-3 Torr. The ZnO films were deposited on c-Si to obtain the reference samples: A5/c-Si, A8/c-Si, B5/c-Si and B8/c-Si. Table 1 shows the summary of the prepared samples. The surface morphology of the fabricated structures was characterized using a Scanning Electron Microscope (SEM, JEOL JSM-7800F, Tokyo, Japan). The ZnO crystal structures were studied by an X-ray diffractometer (XRD, ORION, D2 PHASER Bruker, Baden-Wurtemberg, Karlsrueh, Alemania) using the CuKα radiation and λ=0.15406 nm, X-ray photoelectron spectroscopy (XPS, Thermo K-Alpha, Waltham, Massachusetts, USA) equipped with Al Kα monochromatic x-ray source (hv = 1486.6 eV) in analysis chamber at a base pressure of 10-7 mbar was applied to investigate the chemical state of the elements in the prepared ZnO, and Photoluminescence studies were carried out using a Varian fluorescence spectrometer (Cary Eclipse, Varian Inc., Palo Alto, CA, USA) under 325 nm excitation”…
- After the SEM characterization of the deposited ZnO films, the addition of EDS analysis can better demonstrate the elemental distribution and the localization of ZnO, which will make it easier for the reader to understand.
Response. Thank you for this suggestion. It would have been interesting to analyze the EDS films, however we have not been able to perform the analysis because the equipment had technical problems and now, we are waiting for the piece to be repaired. Also, we tried to carried out the measurements with other SEM equipment from other research centers, but the list of date is extensive.
- The figure notes in Figure 2 are not standardized, please check all the figure notes to ensure that the meaning of all sub-figures is explained in the figure notes. It better be like this. X-Ray diffraction spectra for the ZnO deposited on (a) mPS and (b)cSi.
Response. Thank for your recommendation. We agree with this and we have incorporated your suggestion throughout the figure’s notes.
Figure 1. SEM image of top-view (a) and cross-section (b) of bare m-PS substrates.
Figure 2. SEM images of ZnO deposited on c-Si (left images) and m-PS (right images) at RF power of 60W (a, b, e, f) and 80 W (c, d, g, h) for different temperature deposition: 500°C (a-d) and 800°C (e-h).
Figure 3. X-Ray diffraction spectra for the ZnO deposited on m-PS (a) and c-Si (b) at different temperatures and power deposition by RF magnetron sputtering.
Figure 4. XPS spectra (a, b) and high-resolution spectra of Zn 2p (c, d) and O 1s (e, f) for the ZnO deposited on c-Si (left spectra) and m-PS (right spectra) by RF magnetron sputtering technique at different temperatures and power deposition.
Figure 5. High-resolution and deconvolution of the O1s scan spectra of ZnO deposited on c-Si (a) and m-PS (b) at different temperatures and power deposition by RF magnetron sputtering.
Figure 6. Photoluminescence spectra of ZnO/c-Si (a) and ZnO/m-PS (b) samples. The down Figure shows the schematic bandgap diagram (c).
- The authors claim that the residual stress on the ZnO lattice increases with increasing RF power, but A8-cSi and B8-cSi are clearly inconsistent with this phenomenon, so please provide a reasonable explanation.
Response. The reviewer has raised an important point, so we added the discussion to explain the increase of residual stress of samples A8-cSi and B8-cSi.
Page 6…”. From Table 2 can also be observed that residual stress decrease for samples A5/c-Si and B8/c-Si. This may be due to the mechanical instability of ZnO nanorods [40]. M. Riaz et al reported the relationship between the pore diameter of nanorods and residual stress [41]. They found that with the increase of pore diameter, the residual stress tends to increase. The above is reflected in the shift of angle towards higher angles”.
[40] P. O, Renault, C. Krauss, E. L. Bourhis, G. Geandier, A. Benedetto, S. Y. Grachev, E. Barthel, “In situ thermal stress evolution in ultrathin ZnO and Ag films studied by synchroton x-ray diffraction”, Thin Solid Films, 520 (2011) 1390-1394, doi: https://doi.org/10.1016/j.tsf.2011.07.060
[41] T. H. Vu, A. T. Pham, V. Q. Nguyen, A. D. Nguyen, T. N. T. Nguyen, M. H. Nguyen T, Y. S. Kim, V. T. Tran, S. Cho, “Growth and thermal stability studies of Layered GaTe single crystals in inert atmospheres”, J of Solid St. Chem. 296(2021)1219960, doi: https://doi.org/10.1016/j.jssc.2021.121996
- The authors compare the grain size calculated by Scheele's formula with the particle size in the SEM image which is obviously not reasonable (the grains are not visible in the SEM image), if a comparison is to be made TEM characterization of the deposited ZnO films can be performed.
Response. The reviewer is right, for that we have omitted the comparison of the crystal size obtained in XRD analysis with the particle size from SEM analysis.
In addition to above comments, all spelling and grammatical errors pointed out by the reviewer have been corrected.
We look forward to hearing from you due time regarding our submission and to respond to any further questions and comments.
Sincerely
Corresponding author
Author Response File: Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript presents experimental results involving the deposition of ZnO thin films on both c-Si and mPS substrates using the reactive RF magnetron sputtering method. The findings revealed the formation of ZnO nanorods on c-Si and star-shaped ZnO nanostructures composed of nanorod assemblies on mPS. It was noted that increasing the RF energy deposition led to a higher density of ZnO nanostructures on the substrates and an associated increase in residual stress. Furthermore, a consistent (002) preferred orientation, indicative of the wurtzite crystal structure, was observed in all ZnO nanostructures. This contribution is significant in the field of thin film coating. However, several points should be addressed during the revision process:
1. Previous research efforts have explored the utilization of ZnO nanowires or nanoparticles in applications such as gas sensors, catalysis, and energy harvesting (e.g., J. Gue et al., Sensors and Actuators B: Chemical, 199, 339-345 (2014); J. Huh et al., Nanotechnology 22, 085502 (2011); L. Gao et al., Catalysts 11, 1289 (2021); S. B. T. Tran et al., RSC Advances 8, 21528-21533 (2018)). These earlier studies are highly relevant to the current investigation, and it is suggested that these approaches be mentioned and properly cited in the manuscript.
2. It was observed that increasing the deposition temperature resulted in an improvement in crystal arrangement, attributed to reduced crystal defects. The hypothesis proposed is that the higher temperature promotes the diffusion of Zn and O atoms within the crystal lattice, enhancing crystallinity. While this assumption appears sound, it's worth considering whether changes in the oxidation states of the ZnO film occur with varying deposition temperatures. To address this, the authors could perform X-ray Photoelectron Spectroscopy (XPS) analysis on samples subjected to different deposition temperatures.
3. A minor point to address is the clarity of the figures. In Figure 1 and its inset figures, the scale bars are not clearly visible. Ensuring that all figures are presented with clear and visible scale bars will enhance the comprehensibility of the manuscript.
Comments on the Quality of English LanguageExtensive editing of English language is required
Author Response
Manuscript 2612929
Dear Doctor
Thank you for giving me the opportunity to submit a revised draft to manuscript title “ZnO deposition on silicon and porous silicon substrate by RF magnetron sputtering”. I appreciate the time and effort that you have dedicated to providing your valuable feedback on my manuscript. I grateful for their insightful comments on my paper. I have been able to incorporated changes to reflect most of the suggestions provided by you, and I have highlighted the changes within manuscript.
The points you requested and the responses to each of them are listed below.
Commentrs for Reviewer 2
The manuscript presents experimental results involving the deposition of ZnO thin films on both c-Si and mPS substrates using the reactive RF magnetron sputtering method. The findings revealed the formation of ZnO nanorods on c-Si and star-shaped ZnO nanostructures composed of nanorod assemblies on mPS. It was noted that increasing the RF energy deposition led to a higher density of ZnO nanostructures on the substrates and an associated increase in residual stress. Furthermore, a consistent (002) preferred orientation, indicative of the wurtzite crystal structure, was observed in all ZnO nanostructures. This contribution is significant in the field of thin film coating. However, several points should be addressed during the revision process:
- Previous research efforts have explored the utilization of ZnO nanowires or nanoparticles in applications such as gas sensors, catalysis, and energy harvesting (e.g., J. Gue et al., Sensors and Actuators B: Chemical, 199, 339-345 (2014); J. Huh et al., Nanotechnology 22, 085502 (2011); L. Gao et al., Catalysts 11, 1289 (2021); S. B. T. Tran et al., RSC Advances 8, 21528-21533 (2018)). These earlier studies are highly relevant to the current investigation, and it is suggested that these approaches be mentioned and properly cited in the manuscript.
Response. Thank you for your recommendations. We consider the proposed research to be relevant, so we have added it to the article.
Page 1…“ZnO is a semiconductor material with a wide bang gap of 3.37 eV and high excitation binding energy of 60 meV; compared with other semiconductor materials, ZnO has the main characteristic of presenting piezoelectricity, thermal and chemical stability, high stability against environmental corrosions, it is also non-toxic, and its fabrication is low cost [1]– [3]. Such properties have made ZnO an attractive material in technological applications, especially in ligth-emiting diodes [4], [5], solar cells [6], [7] , catalysis [8]– [10], gas sensors [11]– [13], and optoelectronic devices [14], [15]. ZnO applications become even more interesting when it is deposited on porous nanostructure substrates such as porous silicon (PS)”…
Page 9. References
(8) J. Huh, J. Park, G. T. Kim, and J. Y. Park, “Highly sensitive hydrogen detection of catalyst-free ZnO nanorod networks suspended by lithography-assisted growth,” Nanotechnology, vol. 22, no. 8, Feb. 2011, doi: 10.1088/0957-4484/22/8/085502.
(9) L. Gao et al., “Tio2-coated zno nanowire arrays: A photocatalyst with enhanced chemical corrosion resistance,” Catalysts, vol. 11, no. 11, Nov. 2021, doi: 10.3390/catal11111289.
(10) S. B. Trung Tran, H. S. Choi, S. Y. Oh, S. Y. Moon, and J. Y. Park, “Iron-doped ZnO as a support for Pt-based catalysts to improve activity and stability: Enhancement of metal-support interaction by the doping effect,” RSC Adv, vol. 8, no. 38, pp. 21528–21533, 2018, doi: 10.1039/c8ra03664k.
(13) J. Guo, J. Zhang, M. Zhu, D. Ju, H. Xu, and B. Cao, “High-performance gas sensor based on ZnO nanowires functionalized by Au nanoparticles,” Sens Actuators B Chem, vol. 199, pp. 339–345, 2014, doi: 10.1016/j.snb.2014.04.010.
- It was observed that increasing the deposition temperature resulted in an improvement in crystal arrangement, attributed to reduced crystal defects. The hypothesis proposed is that the higher temperature promotes the diffusion of Zn and O atoms within the crystal lattice, enhancing crystallinity. While this assumption appears sound, it's worth considering whether changes in the oxidation states of the ZnO film occur with varying deposition temperatures. To address this, the authors could perform X-ray Photoelectron Spectroscopy (XPS) analysis on samples subjected to different deposition temperatures.
Response. Our results are consistent with previous reports, such as Weihia Yang et al who demonstrated by XPS and photoluminescence analysis that with the increase of annealing temperature (from 600°C to 900°C) provides a great force for the O atoms to diffuse into the ZnO thin films, thus reducing the number of oxygen vacancies/defects and defects of Zn. In addition, we have added XPS analysis of our samples.
Page 6… “According to Table 2 (from 500 °C to 800 °C), the broadening of FWHM decreased due to the temperature increase. This could be attributed to the fact that with the rise of temperature, atoms diffusion (Zn and O) in the crystal´s arrangement increases, causing an enhanced its crystallinity. Haiyan Wang et al demonstrated by XPS and photoluminescence analysis that with the increase of annealing temperature (from 600°C to 900°C) provides a great force for the O atoms to diffuse into the ZnO thin films. This reduce the number of oxygen vacancies/defects and defects of Zn [37]”...
Page 7…” The oxidation state of ZnO on c-Si and m-PS by RF sputtered at different temperatures and power deposition were investigated by XPS (Figure 4). Figure 4 a, b shows the characteristics peaks assigned to zinc (Zn), oxygen (O) and carbon (C). Figure 4 c, d shows high-resolution spectra of the Zn 2p region for the samples. It can be observed that the Zn2p3/2 core level are located around 1022.00 eV and 1022.05 eV for ZnO deposited on c-Si and m-PS, respectively. While, the Zn2p1/2 core level are located around 1045.13 eV and 1045.05 eV for ZnO deposited on c-Si and m-PS, respectively. The core level from Zn2p3/2 has been assigned to the Zn2+ ions in the ZnO thin films [42]. It can be seen that the position of the peaks differs slightly, it is probably for the different surface morphologies of ZnO deposited [43].
The high-resolution spectra and deconvolution of the O1s peak was performed to further study the binding state of Zn and O. Deconvolution of the XPS peaks were performed using the Fityk software. Figure 5 shows the O1s scan spectra of ZnO deposited on c-Si and m-PS at different temperatures and power depositions. Three peaks were observed around 527.62 eV (1), 530.54 eV (2), and 533.34 eV (3). The peak (1) is characteristic of the metal oxides [44]. The peak (2) at a low binding energy is associated to O2- ions in the deficient regions within the ZnO array [45]. It can also be associated with hydrated oxides that could have been incorporated from deposition chamber or the presence of weakly bound oxygen on the surface of the films [45]. Weijia Yang et al. attributed that the binding energy around 530.75 eV corresponds to oxygen defects/vacancies. The central level of peak (2) shows asymmetry, indicating the presence of various forms of oxygen bonds in the near-surface region of ZnO [45]. The intensity is associated to the number of the oxygen vacancies [42][46]. The peak (3) is associated with hydroxyl groups (-OH), O2 or C-O bond chemisorbed or adsorbed species on the sample surface [43][46][47]. Previously, it has been reported that intrinsic defects, such as zinc interstitial atoms (Zni) and oxygen vacancies (Vo), are electrically active and can induce localized states near to the conduction band. These species can be act as donor [47]”...
Figure 4. XPS spectra (a, b) and high-resolution spectra of Zn 2p (c, d) and O 1s (e, f) for the ZnO deposited on c-Si (left spectra) and m-PS (right spectra) by RF magnetron sputtering technique at different temperatures and power deposition.
Figure 5. High-resolution and deconvolution of the O1s scan spectra of ZnO deposited on c-Si (a) and m-PS (b) at different temperatures and power deposition by RF magnetron sputtering.
- A minor point to address is the clarity of the figures. In Figure 1 and its inset figures, the scale bars are not clearly visible. Ensuring that all figures are presented with clear and visible scale bars will enhance the comprehensibility of the manuscript.
Response. Thank you for the observation. The scales have been added visibly on all figures to ensure equal scale and improve understanding of the manuscript.
Figure 1. SEM images of ZnO deposited on c-Si (left images) and m-PS (right images) at RF power of 60W (a, b, e, f) and 80 W (c, d, g, h) for different temperature deposition: 500°C (a-d) and 800°C (e-h).
In addition to above comments, all spelling and grammatical errors pointed out by the reviewer have been corrected.
We look forward to hearing from you due time regarding our submission and to respond to any further questions and comments.
Sincerely
Corresponding author
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors group eventually has some experience in ZnO deposition on Si substrates of different morphology using well known and actively used deposition technique as sol-gel, hydothermal and magnetron sputtering. No doubts, that ZnO/Si nanostructures are attractive for numerous applications as gas sensing and light-emitters. However, novelty of the proposed study is very suspected. Despite authors claimed that ~ "however there are few reports about ZnO deposition on porous substrate", one can easily find a lot devoting to this topic using Google Academy with "ZnO" AND "porous silicon" AND "magnetron sputtering" in the request field. In addition, it rised some ethical question, be cause authors do not cite any relevant articles dealing with RF magnetron sputtering of ZnO above different porous Si substrate. It seems that all the key deposition parameters have already been optimized conserning any relevant application fields of theresulted ZnO/Si heterostructures. That is why, submtted manuscript deals with (1) already studied ZnO/mPS heterostructure; (2) already used and optimized RF sputtering; (3) already defined correlation between RF deposition parameters and resulted morphology, crystal quality and materials properties of interest (luminescence in the current study).
Moreover, experiment design contains some issues to be adressed:
- "Finally, mPS were oxidized in air to stabilize its structure": niether FTIR or XPS data were presented to confirm this statement
- Why uncontrolled air oxidation was used?
- What about the both morphology and PL spectra of the bare mPS samples?
- Why authors claimed that obtained Si porous substrates are of mPS type? any proofs or references?
- The thickness of the ZnO deposits is unknown.
- Figure 1 e-g did not confirm nanorods formation due to a top-view image presented. To judge about nanorods, cross-sectional images are necessary instead.
- Figure 1 f-h did not demonstrate star-shape ZnO morphology. It is just few and of rare occurrence deposits, which can be only vaguely resemble as "stars"
- Tables 1 deals with some XRD-extracted parameters characterizing ZnO crystallinity, however niether error bars no extracting of both the apparatus error no thermal drift were taking into account. Otherwise, comparisson is invalid.
Comments on the Quality of English Language
Abstract, Introduction and Materials and Methods sections are fine with some English to be brushed. However, when we moving to Results and Discussion it is obvious that intensive English editting is required. In addition, authors use unappropriate terms very frequency.
Abstract:
cSi=>c-Si
mPS=>m-PS
diffractometer=>diffraction
Introduction:
higher band gap => ZnO is a wide band gap semiconductor
significant => high
non-toxicity=> non toxic
high electrical resistanse of 35% => high electrical resistanse rate of 35%
high density in the depositin films => denser deposited films?
Materials and Methods:
4 mA => current density should be provided
important to stress => important to note
PL studies were using => PL studdies were carried out usg
under 325 nm => under 325 nm exitation
Author Response
Manuscript 2612929
Dear Doctor
Thank you for giving me the opportunity to submit a revised draft to manuscript title “ZnO deposition on silicon and porous silicon substrate by RF magnetron sputtering”. I appreciate the time and effort that you have dedicated to providing your valuable feedback on my manuscript. I grateful for their insightful comments on my paper. I have been able to incorporated changes to reflect most of the suggestions provided by you, and I have highlighted the changes within manuscript.
The points you requested and the responses to each of them are listed below.
Comments for Reviewer 3
The authors group eventually has some experience in ZnO deposition on Si substrates of different morphology using well known and actively used deposition technique as sol-gel, hydothermal and magnetron sputtering. No doubts, that ZnO/Si nanostructures are attractive for numerous applications as gas sensing and light-emitters. However, novelty of the proposed study is very suspected. Despite authors claimed that ~ "however there are few reports about ZnO deposition on porous substrate", one can easily find a lot devoting to this topic using Google Academy with "ZnO" AND "porous silicon" AND "magnetron sputtering" in the request field. In addition, it rised some ethical question, because authors do not cite any relevant articles dealing with RF magnetron sputtering of ZnO above different porous Si substrate. It seems that all the key deposition parameters have already been optimized conserning any relevant application fields of theresulted ZnO/Si heterostructures. That is why, submtted manuscript deals with (1) already studied ZnO/mPS heterostructure; (2) already used and optimized RF sputtering; (3) already defined correlation between RF deposition parameters and resulted morphology, crystal quality and materials properties of interest (luminescence in the current study).
Response. Thank you for the observation. The reviewer has raised an important point, so we added the discussion considering the points mentioned by the reviewer.
…” 1. Introduction
ZnO is a semiconductor material with a wide bang gap of 3.37 eV and high excitation binding energy of 60 meV; compared with other semiconductor materials, ZnO has the main characteristic of presenting piezoelectricity, thermal and chemical stability, high stability against environmental corrosions, it is also non-toxic, and its fabrication is low cost [1]–[3]. Such properties have made ZnO an attractive material in technological applications, especially in ligth-emiting diodes [4], [5], solar cells [6], [7] , catalysis [8]–[10], gas sensors [11]–[13], and optoelectronic devices [14], [15]. ZnO applications become even more interesting when it is deposited on porous nanostructure substrates such as porous silicon (PS).
Recently, studies of ZnO deposited on PS by EBIC (electron-beam-induced current) demonstrated that the ZnO/PS is an anisotype heterojunction with the possibility of enhancing the charge carrier flow, which make it possible to obtain light-emitting diodes and solar cells [15]. Additionally, a sensor based on ZnO/PSNW (zinc oxide on porous silicon nanowires) showed excellent gas sensing performance for various NO2 concentrations (5-50ppm), reaching a high electrical resistance rate of 35% for 50 ppm of NO2 [16]. Furthermore, studies revealed a pyroelectric coefficient in ZnO/PS of 40 times higher than ZnO/c-Si and a pyroelectric voltage as high as 2.4 V [17].
Various techniques have been applied to deposit ZnO on PS, such as spray pyrolysis [18][19], chemical vapor deposition [20], hydrothermal [3][21], sol-gel [22], and magnetron sputtering [23]. Even, exist computational methods to describe and understand the formation of ZnO such as density functional theory (DFT) and ab initio molecular dynamics simulation (AIMD) [24], [25].
In the present work we applied magnetron sputtering technique because is the most used and studied for its efficiency, high interfacial adhesion, and for allowing the deposition of high-density films. Furthermore, magnetron sputtering allows thin films to be deposited on different type of substrates at high temperatures with excellent uniformity and quality crystalline. From a practical point of view, the study of the properties of thin films at higher temperatures makes it possible to ensure the durability, and repeatability of devices operated at high temperatures. Therefore, a material needs to exhibit good thermal stability at operating temperatures. In general, the thermal stability of a material depends of factors such as structural phases and degree crystallinity, which are correlated with the route of synthesis [26].
When magnetron sputtering technique is adopted, the pressure, gas type, gas flow, temperature, and power deposition have significant effect on the quality of the formed films. For example, the working pressure can change the grain size and crystal structure ZnO deposited, allowing the films of ZnO deposited to be oriented in different crystalline planes. Likewise, the deposits made with low power density show a very smooth surface and preferential orientation of the grains [26]. Furthermore, the increase of oxygen content in argon environment results in a decrease in the deposition rate of the films [26]. Husam S. Al-Salam and M. J. Abdullah deposited ZnO on PS maintaining a RF power deposition of 150W with its posterior annealing at 500°C during 2 h. The results revealed a high and deep porosity with a roughness of 178 nm [27]. K. Cicek et al. formed ZnO on PS and silicon utilizing RF&DC magnetron sputtering technique with flow rate of Ar and O2 at 120W power. They found that a pyroelectric coefficient of 8.2 can be achieved for deposits on PS, which is more than ~40 times higher than the one on Si substrate [28].
Although deposition of ZnO films on PS substrates using magnetron sputtering technique have been carried out, there are few reports detailing the study of higher temperatures and its comparison with power deposition on the properties of the ZnO on PS. Under this scenario, it is of vital importance to study the synthesis parameters for the design and development of new devices. In this work, we have deposited ZnO on macroporous silicon by magnetron sputtering technique, varying the RF power and the deposition temperature to study the effect caused by the porous substrate on the ZnO” ...
- "Finally, mPS were oxidized in air to stabilize its structure": niether FTIR or XPS data were presented to confirm this statement.
Response. With its high surface area and reactive Si – H and Si – Si moieties, porous silicon is particularly susceptible to air or water oxidation, these being the simplest oxidants. Previous studies have demonstrated that thermal oxidation is often used to stabilize the porous silicon surface and it has been found that this method is effectively useful for organic vapor sensing [1-3]. A. E. Pap et al. corroborated that passivation of the porous surface on uncontrolled air oxidation from 200°C to 800 °C can be possible [4]. In addition, N. Hadj et al. demonstrated that with a thermal annealing of porous silicon at 400°C or 450°C results in the desorption of hydrogen from the SiH-sites, which were corroborate with FTIR measurements [5].
According to the reviewer's assessment, we have decided to cite the previous studies in the methodology section.
[1] H. Saha, Porous silicon sensors- elusive and erudite. Int. J. Smart Sens. Intel. Syst. 1, 34–56 (2008)
[2] Sailor, M. J, Chemical reactivity and surface chemistry of porous silicon, In. Canham, L. (eds) Handbook of Porous Silicon. Springer, Cham. 2014, https://doi.org/10.1007/978-3-319-05744-6_37
[3] Ogata, Yukio H., Hiroyuki Niki, Tetsuo Sakka and Matae Iwasaki. “Hydrogen in Porous Silicon: Vibrational Analysis of SiH x Species.” Journal of The Electrochemical Society 142 (1995): 195-201.
[4] A. E. Pap, K. Kordás, G. Tóth, J. Levoska, A. Uusimäki, J. Vähäkangas, S. Leppävuori, T. F. George, Thermal oxidation of porous silicon: Study on structure, Appl. Phys. Lett. 86 (2005) 041501, https://doi.org/10.1063/1.1853519.
[5] N. H. Zoubir, M. Vergant, T. Delatour, A. Burneau, Ph. De Donato, O. Barres, Natural oxidation of annealed chemically etched porous silicon, Thin Solid Films, 255 (1995), pp 228-230, https://doi.org/10.1016/0040-6090(94)05659-2
- Why uncontrolled air oxidation was used?
Response. The uncontrolled air oxidation was because previous studies have shown that hydrogen desorption from SiH2 sites occurs at 400°C and 450°C, as demonstrated by FTIR results performed by N. Hadj Zoubir and co-workers [1]. In addition, A. E. Pap et al. also demonstrated that passivation of the porous surface on uncontrolled air oxidation at 400°C is possible [2]. Also, we decided to passivate the surface of the porous silicon because one of the problems of porous silicon is its initial reactivity of the hydride-terminated groups, which makes it a highly reactive and unstable structure and the deposition of the ZnO films was carried out in a research center located at a great distance (~350 km) from the place where the synthesis of porous silicon substrates was manufactured [3].
[1] N. H. Zoubir, M. Vergant, T. Delatour, A. Burneau, Ph. De Donato, O. Barres, Natural oxidation of annealed chemically etched porous silicon, Thin Solid Films, 255 (1995), pp 228-230, https://doi.org/10.1016/0040-6090(94)05659-2
[2] A. E. Pap, K. Kordás, G. Tóth, J. Levoska, A. Uusimäki, J. Vähäkangas, S. Leppävuori, T. F. George, Thermal oxidation of porous silicon: Study on structure, Appl. Phys. Lett. 86 (2005) 041501, https://doi.org/10.1063/1.1853519
[3] Sailor, M. J, Chemical reactivity and surface chemistry of porous silicon, In. Canham, L. (eds) Handbook of Porous Silicon. Springer, Cham. 2014, https://doi.org/10.1007/978-3-319-05744-6_37
- What about the morphology of the bare mPS samples?
Response. The reviewer has raised an important point, for that we added the top-view and cross-sectional SEM images of bare m-PS substrates to the article. From Figure 1 we can observe interconnected pores with a pore diameter between 100 nm – 200 nm and thickness around 200 nm.
Figure 1. SEM image of top-view (a) and cross-section (b) of bare m-PS substrates.
- Why authors claimed that obtained Si porous substrates are of mPS type? any proofs or references?
Response. We added the top-view and cross-sectional SEM images of m-PS substrates to prove the type of substrate. SEM images revealed interconnected pores with a pore diameter between 100 nm- 200 nm and thickness around 200 nm. According to the IUPAC classification, pore size is above about 50 nm, which is consistent with the naming of our porous silicon substrates [1].
Figure 1. SEM image of top-view (a) and cross-section (b) of bare m-PS substrates.
[1] K. S. W. Sing, Characterization of Porous Solids: An Introductory Survey, Stud. Surf. Sci. Catal., 62 (1991), pp 1-9, 10.1016/S0167-2991(99)80316-4
- The thickness of the ZnO deposits is unknown.
Response. Thank you for this suggestion. It would have been interesting to analyze the thickness films, however we have not been able to perform the cross-sectional analysis because the equipment had technical problems and now, we are waiting for the piece to be repaired. Also, we tried to carry out the measurements with other SEM equipment from other research centers, but the list of date is extensive.
- Figure 1 e-g did not confirm nanorods formation due to a top-view image presented. To judge about nanorods, cross-sectional images are necessary instead.
Response. Contemplating the answer from the previous point (No 5) and to confirm the nanorods formation, we have replaced the images inside Figure 1e-g that could confirm the formation of nanorods.
Figure 2. SEM images of ZnO deposited on c-Si (left images) and m-PS (right images) at RF power of 60W (a, b, e, f) and 80 W (c, d, g, h) for different temperature deposition: 500°C (a-d) and 800°C (e-h).
- Figure 1 f-h did not demonstrate star-shape ZnO morphology. It is just few and of rare occurrence deposits, which can be only vaguely resemble as "stars"
Response. Thank you for this point. Like you, we consider it important to adjust the form that we have named the ZnO nanostructures grown on m-PS. For the above we have changed the redaction for:
Page 5. “Figure 2e, g indicated the formation of ZnO nanorods on the c-Si substrates, while the samples with ZnO deposited on m-PS (Figure 2f, h) revealed the formation of ZnO nanostructures constituted by the assembly of nanorods. Likewise, it can be observed that such nanostructures were growth over smaller nanoparticles with diameter size around 29 nm and 35 nm for the samples A8/m-PS and B8/m-PS, respectively (inside Figure 2f, h)”.
- Tables 1 deals with some XRD-extracted parameters characterizing ZnO crystallinity, however niether error bars no extracting of both the apparatus error no thermal drift were taking into account. Otherwise, comparisson is invalid.
Response. To validate the information from XRD measurements of our samples, we have made a new table with the crystal size average, d-spacing, % error d-spacing. In Table 1 we compare the standard values with the experimental values.
Page 6 and 7…” All the diffraction peaks, crystallite size and respective d-spacing values are tabulated in Table 1. From Table 1 we can observe the good agreement with the standard values. It can also shows % d error and it has been observed that the difference between experimental and standard values are within the acceptable range. Table 2 shows the Full Width Half Maximum (FWHM), D, δ, and σ obtained for the (002) orientation in the samples.
Table 1. The X-ray diffraction peaks, 2ÆŸ, d-spacing, % d error, crystal size and average crystal size of ZnO deposited on c-Si and m-PS.
|
Sample |
(hkl) |
2ÆŸ JCPDS |
2ÆŸ experimental |
d-spacing JCPDS |
d-spacing experimental |
% d error |
D (nm) |
Average D (nm) |
|
A5/c-Si |
(002) |
34.4937 |
34.4044 |
2.60332 |
2.605 |
0.05 |
15.01 |
15 |
|
B5/c-Si |
(002) |
34.4937 |
34.2269 |
2.60332 |
2.618 |
0.55 |
11.72 |
12 |
|
A8/c-Si |
(002) |
34.4937 |
33.8357 |
2.60332 |
2.647 |
1.68 |
18.42 |
18 |
|
B8/c-Si |
(002) |
34.4937 |
33.7026 |
2.60332 |
2.657 |
2.07 |
22.24 |
22 |
|
A5/m-PS |
(002) |
34.4937 |
34.4700 |
2.60332 |
2.600 |
0.14 |
13.76 |
17 |
|
(101) |
36.4084 |
36.3904 |
2.47592 |
2.467 |
0.36 |
19.32 |
||
|
B5/m-PS |
(002) |
34.4937 |
34.4783 |
2.60332 |
2.599 |
0.16 |
11.19 |
15 |
|
(101) |
36.4084 |
36.2574 |
2.47592 |
2.476 |
0.01 |
17.89 |
||
|
A8/m-PS |
(100) |
31.8384 |
31.8459 |
2.81430 |
2.808 |
0.23 |
33.65 |
33 |
|
(002) |
34.4937 |
34.3530 |
2.60332 |
2.608 |
0.19 |
36.15 |
||
|
(101) |
36.4084 |
36.2755 |
2.47592 |
2.474 |
0.06 |
29.21 |
||
|
B8/m-PS |
(100) |
31.8384 |
31.8932 |
2.81430 |
2.804 |
0.38 |
25.04 |
24 |
|
(002) |
34.4937 |
34.5327 |
2.60332 |
2.595 |
0.31 |
23.63 |
||
|
(101) |
36.4084 |
36.3742 |
2.47592 |
2.468 |
0.32 |
22.55 |
According to Table 2 (from 500 °C to 800 °C), the broadening of FWHM decreased due to the temperature increase. This could be attributed to the fact that with the rise of temperature, atoms diffusion (Zn and O) in the crystal´s arrangement increases, causing an enhanced its crystallinity. Haiyan Wang et al demonstrated by XPS and photoluminescence analysis that with the increase of annealing temperature (from 600°C to 900°C) provides a great force for the O atoms to diffuse into the ZnO thin films. This reduce the number of oxygen vacancies/defects and defects of Zn [37]. The enhance of crystallinity can be corroborated with the diminution of δ values. The dislocation density represents the irregularities and the amount of crystalline defects in the crystal as oxygen and zinc interstitial [38], [39]. It is also observed that with the increased RF power deposition, the atoms receive more energy and have more driving force. This leads to an increase intrinsic stress as evidenced by the increase of residual stress on the ZnO lattice. Such intrinsic stress is due to the accumulating effect of the crystallographic flaws during deposition (increase of δ values)[38], [40]. From Table 2 can also be observed that residual stress decrease for samples A5/c-Si and B8/c-Si. This may be due to the mechanical instability of ZnO nanorods. M. Riaz et al reported the relationship between the pore diameter of nanorods and residual stress [41]. They found that with the increase of pore diameter, the residual stress tends to increase. The above is reflected in the shift of angle towards higher angles.
Table 2. FWHM, crystallite size, dislocation density, and biaxial stress of ZnO deposited on c-Si and m-PS by RF magnetron sputtering.
|
Sample |
2ÆŸ (°) |
FWHM |
D (nm) |
δ (1/nm2) |
σ (Gpa) |
|
A5/c-Si |
34.4044 |
0.5544 |
15 |
0.0044 |
-1.2631 |
|
B5/c-Si |
34.2269 |
0.7094 |
12 |
0.0073 |
-0.9764 |
|
A8/c-Si |
33.8357 |
0.4508 |
18 |
0.0029 |
-1.5607 |
|
B8/c-Si |
33.7026 |
0.3732 |
22 |
0.002 |
-1.8944 |
|
A5/m-PS |
34.4151 |
0.5572 |
15 |
0.0045 |
-1.2561 |
|
B5/m-PS |
34.3264 |
0.963 |
9 |
0.0134 |
-0.7075 |
|
A8/m-PS |
34.4946 |
0.2054 |
41 |
0.0006 |
-3.486 |
|
B8/m-PS |
34.5317 |
0.3015 |
28 |
0.0013 |
-2.3607 |
- Comments on the Quality of English Language:
Abstract, Introduction and Materials and Methods sections are fine with some English to be brushed. However, when we moving to Results and Discussion it is obvious that intensive English editting is required. In addition, authors use unappropriate terms very frequency.
Abstract:
cSi=>c-Si
mPS=>m-PS
diffractometer=>diffraction
Introduction:
higher band gap => ZnO is a wide band gap semiconductor
significant => high
non-toxicity=> non toxic
high electrical resistanse of 35% => high electrical resistanse rate of 35%
high density in the depositin films => denser deposited films?
Materials and Methods:
4 mA => current density should be provided
important to stress => important to note
PL studies were using => PL studdies were carried out usg
under 325 nm => under 325 nm excitation
Response. As suggested by the reviewer, we have corrected the terms suggested and verify the English along all the manuscript.
We look forward to hearing from you due time regarding our submission and to respond to any further questions and comments.
Sincerely
Corresponding author
Author Response File: Author Response.pdf
Reviewer 4 Report
Comments and Suggestions for AuthorsIn this manuscript, the authors present a detailed study ZnO deposition on silicon and porous silicon substrate by RF magnetron sputtering. A systematic investigation and deep understanding of the physics behind the deposition process is reported in detail. Because of the general principles of the investigation, and the high and adequate level of characterization efforts (SEM, XRD, PL) employed, the present work is quite original and comes very timely. The characterization as employed, and its concrete interpretation and realization are carefully discussed, explained, and presented in all the necessary detail. Consequently, the results are both original and realistically applicable to a wide range of problems that arise during ZnO deposition on silicon and porous silicon substrate by RF magnetron sputtering. Also importantly, the discussion of the results on deposition trends not only looks credible but inspires future work in the field.
The figures, their captions and their corresponding discussion in the main text are easy to understand and they are logically organized too.
This work certainly represents a much valuable contribution with possible wider impact to the field.
There are only minor concerns about textual details of this already excellent work that should be addressed before the manuscript becoming suitable for publication, i.e., it can be considered for publication after a minor revision:
1: The authors could provide a more detailed motivation and background as to why the temperature range of 500 °C and/to 800 °C (substrate temperature) was chosen.
2: The authors mention the elevated thermal stability of ZnO. The relationship between good crystal quality (supposed to be achieved by RF magnetron sputtering) and the elevated thermal stability of the material in general should be briefly but concretely discussed.
3: In the introduction, the authors miss that in previous works dedicated theoretical methods such as DFT and also ab initio molecular dynamics to guide the deposition of high-quality nanostructured crystals (including in the shape of nanorods) and the control of wide range of their thermal, mechanical and semiconductor properties, namely [ACS Nanosci. Au 2023, 3, 1, 84–93; and Nanotechnology 33 (2022) 335706]. Such works should be acknowledged.
Comments on the Quality of English LanguageSpell-check and stylistic revision of the paper are necessary. Some long sentences, as well as misspellings, etc., are noticeable throughout the text.
Author Response
Manuscript 2612929
Dear Doctor
Thank you for giving me the opportunity to submit a revised draft to manuscript title “ZnO deposition on silicon and porous silicon substrate by RF magnetron sputtering”. I appreciate the time and effort that you have dedicated to providing your valuable feedback on my manuscript. I grateful for their insightful comments on my paper. I have been able to incorporated changes to reflect most of the suggestions provided by you, and I have highlighted the changes within manuscript.
The points you requested and the responses to each of them are listed below.
Comments for Reviewer 4
In this manuscript, the authors present a detailed study ZnO deposition on silicon and porous silicon substrate by RF magnetron sputtering. A systematic investigation and deep understanding of the physics behind the deposition process is reported in detail. Because of the general principles of the investigation, and the high and adequate level of characterization efforts (SEM, XRD, PL) employed, the present work is quite original and comes very timely. The characterization as employed, and its concrete interpretation and realization are carefully discussed, explained, and presented in all the necessary detail. Consequently, the results are both original and realistically applicable to a wide range of problems that arise during ZnO deposition on silicon and porous silicon substrate by RF magnetron sputtering. Also importantly, the discussion of the results on deposition trends not only looks credible but inspires future work in the field.
The figures, their captions and their corresponding discussion in the main text are easy to understand and they are logically organized too.
This work certainly represents a much valuable contribution with possible wider impact to the field.
There are only minor concerns about textual details of this already excellent work that should be addressed before the manuscript becoming suitable for publication, i.e., it can be considered for publication after a minor revision:
1: The authors could provide a more detailed motivation and background as to why the temperature range of 500 °C and/to 800 °C (substrate temperature) was chosen.
Response. From a practical point of view, the study of the properties of thin films at higher temperatures makes it possible to ensure the durability, and repeatability of devices operated at high temperatures. Therefore, a material needs to exhibit good thermal stability at operating temperatures. In general, the thermal stability of a material depends of factors such as structural phases and degree crystallinity, which are correlated with the route of synthesis.
2: The authors mention the elevated thermal stability of ZnO. The relationship between good crystal quality (supposed to be achieved by RF magnetron sputtering) and the elevated thermal stability of the material in general should be briefly but concretely discussed.
Response. The reviewer has raised an important point, so we added the discussion of crystal quality and the high thermal stability in the Introduction section.
Page 2…” In the present work we applied magnetron sputtering technique because is the most used and studied for its efficiency, high interfacial adhesion, and for allowing the deposition of high-density films. Furthermore, magnetron sputtering allows thin films to be deposited on different type of substrates at high temperatures with excellent uniformity and quality crystalline. From a practical point of view, the study of the properties of thin films at higher temperatures makes it possible to ensure the durability, and repeatability of devices operated at high temperatures. Therefore, a material needs to exhibit good thermal stability at operating temperatures. In general, the thermal stability of a material depends of factors such as structural phases and degree crystallinity, which are correlated with the route of synthesis” …
3: In the introduction, the authors miss that in previous works dedicated theoretical methods such as DFT and also ab initio molecular dynamics to guide the deposition of high-quality nanostructured crystals (including in the shape of nanorods) and the control of wide range of their thermal, mechanical and semiconductor properties, namely [ACS Nanosci. Au 2023, 3, 1, 84–93; and Nanotechnology 33 (2022) 335706]. Such works should be acknowledged.
Response. Thank you for your recommendations. We consider the proposed research to be relevant, so we have added it to the article.
Page 2…” Various deposition techniques have been applied to deposit ZnO on PS, such as spray pyrolysis [18][19], chemical vapor deposition [20], hydrothermal [3][21], sol-gel [22], and magnetron sputtering [23]. Even, exist computational methods to describe and understand the formation of ZnO such as density functional theory (DFT) and ab initio molecular dynamics simulation (AIMD) [24], [25]”…
[24] M. Alves Machado Filho, C. L. Hsiao, R. B. dos Santos, L. Hultman, J. Birch, and G. K. Gueorguiev, “Self-Induced Core-Shell InAlN Nanorods: Formation and Stability Unraveled by Ab Initio Simulations,” ACS Nanoscience Au, vol. 3, no. 1, 2023, doi: 10.1021/acsnanoscienceau.2c00041.
[25] C. Lundgren, A. Kakanakova-Georgieva, and G. K. Gueorguiev, “A perspective on thermal stability and mechanical properties of 2D Indium Bismide from ab initio molecular dynamics,” Nanotechnology, vol. 33, no. 33, 2022, doi: 10.1088/1361-6528/ac6baf.
In addition to above comments, all spelling and grammatical errors pointed out by the reviewer have been corrected.
We look forward to hearing from you due time regarding our submission and to respond to any further questions and comments.
Sincerely
Corresponding author
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsThe authors' response to my comments is appropriate, therefore, I suggest the paper be accepted as it is.
Reviewer 3 Report
Comments and Suggestions for AuthorsThe authors adressed all the questions arised during the first round of review
