Growth of Zn1−xNixO Thin Films and Their Structural, Optical and Magneto-Optical Properties
, Oleh Kuzyk 1, Olesya Dan’kiv 1, Andrzej Dziedzic 3, Gennadiy Kleto 2, Andriy Stolyarchuk 1, Andriy Popovych 1 and Ivan Hadzaman 1Round 1
Reviewer 1 Report
In this paper, Zn1-xNixO thin films with x < 0.08 deposited by RF reactive sputtering technique on sapphire, glass and quartz substrates were presented. XRD, AFM, PL, and magnetic field dependence of Faraday rotation were studied. This work might be useful in the traditional semiconductor devices. It could be considered for publishing in the journal after some revisions.
1. Figure 1: X-ray diffraction spectra should be drawn in a 2D axis, it would be more convinent for comparison.
2. There are too many exceptions in the sample of x=0.02, such as the "lattice parameter c", bandgap, transmittance spectrum, photoluminescence spectrum, please consider it overall. More samples would be preferred for comparison.
3. Figure 10: all samples should be measured.
4. Introduction should be improved, especially the literature review part and the purpose of this work.
5. The numbers of the subtitles are wrong, and many mistakes in the manuscript, revise and polish the manuscript carefully.
Author Response
Point 1: Figure 1: X-ray diffraction spectra should be drawn in a 2D axis, it would be more convinent for comparison.
Response 1: We agree with the reviewer's comment about a more convenient comparison of research results for thin films with different nickel contents in 2D format. At the same time, the format chosen by us for presenting the research results allows us to more clearly demonstrate a significant increase in the intensity of the main peak (002) and other peaks corresponding to the directions (100), (101), and (102) of ZnO with increasing of Ni content (x) in studied films. Also, with the selected option, the absence of any peaks that would correspond to the presence of nickel or nickel oxide clusters in the investigated thin films is clearly visible.
Point 2: There are too many exceptions in the sample of x=0.02, such as the "lattice parameter c", bandgap, transmittance spectrum, photoluminescence spectrum, please consider it overall. More samples would be preferred for comparison.
Response 2: The attention in our research to Zn1-xNixO films with a nickel content of x=0.02 is due to the differences for this concentration in the behavior of the band gap (it increases from x=0 to 0.02) in contrast to higher concentrations of the x>0.02, where it decreases. This is directly represented by the dependence of Eg versus x (Fig. 6). In other figures, such as transmission, photoluminescence, and Faraday rotation spectra, we have presented the results for all investigated samples.
Point 3: Figure 10: all samples should be measured.
Response 3: The main purpose of presenting the results in Fig. 10 was to demonstrate the change in the magnetic properties of Zn1-xNixO thin films, that is, the transition from paramagnetic to ferromagnetic behavior with increasing of Ni content. And this is clearly demonstrated by experimental data for Zn0.98Ni0.02O and Zn0.96Ni0.04O films. As can be seen from Figure 9, where the spectral dependence of the Faraday rotation angle is presented for all samples, an increase in the nickel content leads to an increase in the absolute value of the angle of rotation and an increase in the oscillatory nature of this dependence. The magnetic field dependence of Faraday rotation and ferromagnetic behavior for other compositions of Zn1-xNixO films with x>0.04 is similar as the composition with x=0.04 and therefore we do not show it on this Figure.
Point 4: Introduction should be improved, especially the literature review part and the purpose of this work.
Response 4: The Introduction has been partially corrected, in particular in the literature review.
Point 5: The numbers of the subtitles are wrong, and many mistakes in the manuscript, revise and polish the manuscript carefully.
Response 5: Thank you for your careful review of our manuscript. Corresponding spelling and stylistic corrections have been made.
Reviewer 2 Report
I have reviewed the submitted paper to Coatings entitled GROWTH OF Zn1-xNixO THIN FILMS AND THEIR STRUCTURAL, OPTICAL AND MAGNETO-OPTICAL PROPERTIES” by Ivan Hadzaman et al. Nanosized transition metal oxide based coatings and other systems demonstrate a good combination of novel and unique chemical, electrical, magnetic, and physical properties which make them ideal candidates to be used as various functional and smart materials. The reported material has excellent industrial applications. Therefore, the reported subject content is a significant area of research and suitable to the chosen journal addressing the materials for coatings. Transition metal oxides doped ZnO semiconducting structures have been gaining interest for their optical, photoluminescence, magnetic, and semiconducting properties due to their potential applications in magneto-optic, electronics, optoelectronics, microwave, and spintronic devices. The work is well presented, and the manuscript is OK with reasonable corresponding physical properties. The present work is publishable but needs some revisions before making a final decision.
The following points need to be considered.
· The level of originality must be emphasized strongly in the last few lines of the introduction. RF reactive sputtering could be new but must be clearly mentioned with the outcomes achieved in this work.
· Do the material Zn1-xNixO exist as a solid solution? How the Co composition has been varied? Please provide the x value range.
· Please mention the sputtering sources for ZnO and Co.
· The various doping level indicated in the legend in Figure 1 is not visible to the reader.
· From the XRD analysis, the ionic radius of Co2+ is about 90% of that of Zn2+, the in-plane lattice constant of relaxed Zn1−x CoxO film is expected to decrease when Zn atoms are replaced by Co atoms, leading to an increase of out-of-plane lattice? Is that what has been observed in peak positioning?
· Co and CoO nanoparticles may exist in both cubic and hexagonal structures.
· Is it possible to differentiate CoO from ZnO by XRD or S/TEM observation?
· Please provide the surface roughness parameters observed from AFM in the table.
· Which "X" dopant value lead to the formation of the ferromagnetic phase among the samples studied? Does the hole doping promote ferromagnetism in Zn1−x Co x O samples?
· Please compare the results reported for similar materials in the literature doi.org/10.1016/j.ceramint.2019.02.167; and doi.org/10.1016/j.tsf.2019.137671.
· How do the ternary ceramic coatings compare with microstructure and mechanical properties to that of the material Zn1-xNixO?
· The resolution of the figures must be improved.
· The role of doping and defects on the structural, and optical. and magnetic properties must be summarized qualitatively. The long-range of ferromagnetism on dopant samples must be addressed.
Author Response
Thank you for your careful study of our manuscript and your remarks and wishes as to improve its quality.
Point 1: Do the material Zn1-xNixO exist as a solid solution? How the Co composition has been varied? Please provide the x value range.
Response 1: As is known, the properties of thin semiconductor films strongly depend on the conditions and methods of their synthesis. The RF reactive sputtering method is one of those that have been intensively developed in recent decades. It is certain that some corrections regarding the originality of the obtained results have been added in the Introduction, although we would not like it to echo with the abstract.
The Zn1-xNixO films obtained by us exist in the form of a solid substitution solution, which is confirm by the results of energy dispersive X-ray detection (EDX), shown in Figure 3, c (in revised manuscript).
Both in the abstract and in the text of the manuscript, we indicate the range of change of nickel content in Zn1-xNixO films from x=0.0 to x=0.08 with a step of 0.02. That is, films with a nickel content of 2%, 4%, 6% and 8% were studied.
We hope that the esteemed reviewer made a technical error by specifying the transition metal impurity cobalt instead of the impurity nickel in further questions or comments.
Point 2: Please mention the sputtering sources for ZnO and Co.
Response 2: The composite targets with a diameter R of 70 mm were formed by mixing and pressing of ZnO and NiO powders with appropriate rations of components. The distance from target to substrate was taken as 35 mm. The base vacuum in the chamber was 2×10-4 Pa and the working gas pressure – 0.2 Pa of oxygen and 0.8 Pa of argon. Sputtering was carried out with input RF power 300 W and deposition rate of 10 nm/min. The temperature of substrates was (350-400)oC during deposition. Postdeposition annealing in oxygen atmosphere at (500-550)o C was performed.
Point 3: The various doping level indicated in the legend in Figure 1 is not visible to the reader.
Response 3: This is taken into account in the corrected version of the manuscript.
Point 4: From the XRD analysis, the ionic radius of Co2+ is about 90% of that of Zn2+, the in-plane lattice constant of relaxed Zn1−x CoxO film is expected to decrease when Zn atoms are replaced by Co atoms, leading to an increase of out-of-plane lattice? Is that what has been observed in peak positioning?
Response 4: Analyzing the results of XRD studies of Zn1-xNixO thin films, in particular of the main peak (002), we really got the results of the increase of a lattice constant in all the studied samples with nickel content in comparison with pure ZnO. Moreover, nickel doping increased the lattice parameters of Zn1-xNixO films for x = 0.02, and for a further increase in x leads to a decrease in constant lattices. Obtained results related to the differences in the ionic radii of Ni2+ in tetrahedral configuration (0.055 nm) and Zn2+ in tetrahedral coordination (0.06 nm) and systematic replacement of Zn2+ ions by Ni2+ ions without changing the crystal structure of the films.
Point 5: Co and CoO nanoparticles may exist in both cubic and hexagonal structures.
Response 5: The results of our researchs shows that there is no nickel metal clusters or nickel oxides are observed which indicates that Ni2+ has entered the ZnO films lattice without changing the wurtzite structures and systematically substituted the Zn2+ ions in the lattice.
Point 6: Is it possible to differentiate CoO from ZnO by XRD or S/TEM observation?
Response 6: The results of X-ray diffraction analysis clearly allows us to establish the presence of any NiO inclusions in the ZnO matrix.
Point 7: Which "X" dopant value lead to the formation of the ferromagnetic phase among the samples studied? Does the hole doping promote ferromagnetism in Zn1−x Co xO samples?
Response 7: As shown by the results of studies of Faraday's magneto-optical effect, ferromagnetic behavior is observed for samples with a nickel content of 4% and higher.
We suggest that such magnetic behavior can be explained in terms of magnetic polaron (BMP) model and that it is the high density of oxygen vacancies that helps provide the BMP to enhance ferromagnetism in ZnO.
Point 8: Please compare the results reported for similar materials in the literature doi.org/10.1016/j.ceramint.2019.02.167; and doi.org/10.1016/j.tsf.2019.137671.
Response 8: The first of the publications (doi.org/10.1016/j.ceramint.2019.02.167) proposed for comparison concerns thin films of CoZn oxide obtained by the sol-gel spin-coating method. In contrast to our Zn1-xNixO thin films, which according to research are solid replacement solid solutions without inclusions of secondary phases, in this work, as evidenced by the results of high-resolution XPS analysis, the formation of various cobalt oxides, cobalt hydroxides, and zinc is observed. oxides, Co metal, Zn metal, surface oxygen and/or OH-like forms. This, of course, affects the properties of such structures.
At the same time, the results of research on the surface of synthesized films deserve attention. As in our results for ZnNiO films, AFM analysis showed that an increase in the content of transition element ions leads to an increase in the mean and root mean square (RMS) values of the surface roughness. It is also important that both in our studies of the films obtained by the physical method and in the films studied in the work, the growth of granuls occurred perpendicular to the surface along the z-direction.
The second publication (doi.org/10.1016/j.tsf.2019.137671) proposed by the reviewer is devoted to Cr-Mo-N thin films with different Mo content, synthesized by magnetron sputtering. The results of these studies are rather difficult to compare with our data for zinc oxide-based diluted magnetic semiconductor films.
If we talk about the synthesis technology, then an extremely large number of factors affect the resulting structures, ranging from the formation and composition of targets, the environment in which the process takes place, the substrate temperature, etc. Therefore, in our opinion, it is incorrect to compare the results in this particular case.
Point 9: How do the ternary ceramic coatings compare with microstructure and mechanical properties to that of the material Zn1-xNixO?
Response 9: At this stage, such studies were not the purpose of the presented manuscript and were not conducted by us.
Point 10: The resolution of the figures must be improved.
Response 10: This was taken into account in the revised version of the manuscript.
Point 11: The role of doping and defects on the structural, and optical. and magnetic properties must be summarized qualitatively. The long-range of ferromagnetism on dopant samples must be addressed.
Response 11: We agree with the reviewer regarding the effect of doping and defects on the properties of our obtained films, including structural, optical and magnetic properties. Our results of experimental studies of ZnO thin films doped with Ni indicate the manifestation of intrinsic ferromagnetism with increasing nickel concentration x ≥ 4%. To explain this, we use the BMP model, which we believe is suitable for explaining the origin of intrinsic ferromagnetism. According to this model, the detected ferromagnetism is caused by electrons captured by donor defects (for example, oxygen vacancies), which occupy extended orbits and, overlapping, form a spin-split impurity band with localized spins of transient Ni2+ ions (carrier-mediated long-range exchange interactions of localized d-electrons of Ni2+ ions )
Author Response File: 
Author Response.pdf
Reviewer 3 Report
The paper should be proceed further, but before publication it should be improved. Some remarks are presented below:
The information given in line 187 should be explained in more details.
line 214 - The auhors have written: "The f(hν) function has a positive sign, whereas the g(hν) function is negative" - please provide additional explanations regarding this sentence. The reader should have clearer information on this issue.
Figure 3 - the quality of the images is poor, even the information which is placed on them
Figure 6 - what it the value of x in the presented case?
Figure 10 - there is the wrong description "Magnetic Field [T]" on the OX axis. If the unit is correct, there should be "Magnetic induction [T]".
The conclusions should contain more specific information related to optical and magneto-optical properties depending on content of nickel
The literature review should be wider. In references there are only 1 item from the last two years.
Author Response
Thank you for your careful study of our manuscript and your remarks and wishes as to improve its quality.
Point 1: The information given in line 187 should be explained in more details.
Response 1: From our point of view, there are quite a lot of literary sources that give a detailed description of the energy level scheme and selection rules for Ni2+ (3d8) in the tetrahedral field. We have added references to some of them in the revised manuscript.
Point 2: line 214 - The auhors have written: "The f(hν) function has a positive sign, whereas the g(hν) function is negative" - please provide additional explanations regarding this sentence. The reader should have clearer information on this issue.
Response 2: Appropriate additions have been made to the revised version of the manuscript.
Point 3: Figure 3 - the quality of the images is poor, even the information which is placed on them
Response 3: Figure 3 corrected and presented in the corrected version of revised manuscript
Point 4: Figure 6 - what it the value of x in the presented case?
Response 4: This figure shows the dependence of the band gap Eg on the Ni content (x) in thin films Zn1-xNixO, i.e., this is the dependence of Eg(x).
Point 5: Figure 10 - there is the wrong description "Magnetic Field [T]" on the OX axis. If the unit is correct, there should be "Magnetic induction [T]".
Response 5: Indeed, the value of magnetic induction, measured in Tesla, is presented along the OX axis in this Figure. However, the axis signature presented by us is more often used in the scientific literature when it comes to studying the dependence of some quantity on the magnetic field induction.
Point 6: The conclusions should contain more specific information related to optical and magneto-optical properties depending on content of nickel
Point 7: The literature review should be wider. In references there are only 1 item from the last two years.
Response 6,7: This remarks are taken into account in the revised manuscript.
Round 2
Reviewer 1 Report
All the comments have been explained, however, the revisions should be further improved.
1. In my opinion, the XRD patterns should be drawn in 2D because the lower or higher diffraction shifts of the (100), (110), (111) peaks should be noted in the figure. Also, the (100), (110) and (111) peaks should be marked in the figure.
2. Figure 10: all samples should be measured. If the results are similar, put it into the supplementary information.
3. The introduction part should be improved, the purpose of this work is not clear. Is it improved compared with other works? What is the innovation points of this work?
4. Many mistakes still exist. For example:
1) Figure 2: x-axis: 0.00 is not "0,00".
2) "In works [19-22] authors have demonstrated paramagnetic behavior of ZnMnO thin films and ferromagnetic ordering in co-doped quaternary ZnMnFeO oxides prepared by RF sputtering."
Before it can be recommended for publication, all these points should be revised carefully.
Author Response
Thank you for your careful study of our manuscript and your remarks and wishes as to improve its quality.
Point 1: In my opinion, the XRD patterns should be drawn in 2D because the lower or higher diffraction shifts of the (100), (110), (111) peaks should be noted in the figure. Also, the (100), (110) and (111) peaks should be marked in the figure.
Response 1: This reviewer's wish is reflected in the corrected version of the manuscript.
Point 2: Figure 10: all samples should be measured. If the results are similar, put it into the supplementary information.
Response 2: The corrected version of manuscrit in Figure 10 shows the results of studies of the magnetic field dependence of the Faraday rotation angle for all samples.
Point 3: The introduction part should be improved, the purpose of this work is not clear. Is it improved compared with other works? What is the innovation points of this work?
Response 3: Corresponding corrections were made in the corrected version of the manuscript.
Point 4: Many mistakes still exist. For example:
1) Figure 2: x-axis: 0.00 is not "0,00".
2) "In works [19-22] authors have demonstrated paramagnetic behavior of ZnMnO thin films and ferromagnetic ordering in co-doped quaternary ZnMnFeO oxides prepared by RF sputtering."
Before it can be recommended for publication, all these points should be revised carefully.
Response 4: We hope that the revised version of the manuscript meets the corresponding all requirements
Reviewer 2 Report
I have read the revised version of the manuscript. The authors have considered my queries and addressed them OK except for a few, which are itemized below.
1. There are lots of syntax errors, the revised version of the manuscript must be proof read carefully.
2. In Figure 1, x-axis, please mention the target used for measurement. Is that Cu / Co or anything else?
3. Please provide a unit in the x-axis for Figure 3 & 6; Ni content ?
4. With reference to my list, point 8 (in my previous list of comments), the comparison has been made for similar materials but this must be reflected in the manuscript while including the two stated relevant references.
5. Overall, the next version must be polished well to meet the requirements for a high-standard outlet.
Author Response
Thank you for your careful study of our manuscript and your remarks and wishes as to improve its quality.
Point 1: There are lots of syntax errors, the revised version of the manuscript must be proof read carefully.
Response 1: Corresponding corrections were made in the corrected version of the manuscript.
Point 2: In Figure 1, x-axis, please mention the target used for measurement. Is that Cu / Co or anything else?
Response 2: At the suggestion of another reviewer, figure 1 is presented in version 2D in a revised version of the manuscript.
Point 3: Please provide a unit in the x-axis for Figure 3 & 6; Ni content ?
Response 3: Figure 3 presents the results of HR TEM and EDX studies for thin films of Zn1-xNixO with a nickel content of 4%, which is indicated in the caption.
Figure 6 shows the results of dependence of the band gap Zn1-xNixO thin films on the nickel content of 0, 2, 4, 6, and 8%.
Point 4: With reference to my list, point 8 (in my previous list of comments), the comparison has been made for similar materials but this must be reflected in the manuscript while including the two stated relevant references.
Response 4: This wish of the reviewer was partially taken into account in relation to the article on CoZn-oxide films (doi.org/10.1016/j.ceramint.2019.02.167), which is related to oxide DMS thin films, like our samples. Regarding the proposal to make a reference to the study of CrMoN films with different Mo contents (doi.org/10.1016/j.tsf.2019.137671), we consider it not entirely appropriate to include this excellent publication in our manuscript in order to avoid doubts that other reviewers might to be have.
Point 5: Overall, the next version must be polished well to meet the requirements for a high-standard outlet.
Response 5: We hope that the revised version of the manuscript meets the corresponding high requirements.
Reviewer 3 Report
I'm sorry but I cannot agree with authors about the point 5 from the previous review. The term magnetic field is a very broad term and should not be used to describe an axis on a graph.
"Point 5: Figure 10 - there is the wrong description "Magnetic Field [T]" on the OX axis. If the unit is correct, there should be "Magnetic induction [T]".
Response 5: Indeed, the value of magnetic induction, measured in Tesla, is presented along the OX axis in this Figure. However, the axis signature presented by us is more often used in the scientific literature when it comes to studying the dependence of some quantity on the magnetic field induction."
Author Response
Thank you for your careful study of our manuscript and your remarks and wishes as to improve its quality.
I'm sorry but I cannot agree with authors about the point 5 from the previous review. The term magnetic field is a very broad term and should not be used to describe an axis on a graph.
"Point 5 (previous review): Figure 10 - there is the wrong description "Magnetic Field [T]" on the OX axis. If the unit is correct, there should be "Magnetic induction [T]".
Response: Corresponding changes to the OX axis signature in Figure 10 have been made in the new revised version of the manuscript. In addition, at the urging of another reviewer, the results for all studied Zn1-xNixO thin films have been added to the graph.
Round 3
Reviewer 1 Report
This work has been improved, and it is recommended for publication as it is.
Reviewer 2 Report
The revised version is suitable to publish.
Reviewer 3 Report
The paper is ready to be published
