Lead-Free BiFeO3 Thin Film: Ferroelectric and Pyroelectric Properties
, Iuliana Pasuk 1, Lucian Trupina 1, Ioana Pintilie 1, Luminiţa Mirela Hrib 1, Becherescu Nicu 2 and Lucian Pintilie 1Round 1
Reviewer 1 Report
The quality of the article has impoved significantly after the reviewing process. I suggest the publication of the article after a brief language check.
Author Response
Response letter
Reviewer 1
The quality of the article has improved significantly after the reviewing process. I suggest the publication of the article after a brief language check.
Answer: We thank for the kind appreciation of our manuscript. We have revised the language.
Reviewer 2 Report
The paper looks interesting. I suggest that it could be accepted after some revision, and here are my suggestions for the authors.
The Authors wrote about terraces, but from the Figure 2 it doesn’t look like terraces. I propose to supplement the figure with a profile. There is probably a mistake in the value of the width of the terraces – 800 nm?
There is no description of how to measure to obtain the results presented in the Figure 4b. There is also no comment on these results.
In Figure 4c the Authors present the temperature dependences of polarization for positive and negative voltages. The dependences show different slopes but there is only one value of pyroelectric coefficient. Which case does it correspond to? What is the uncertainty?
Where is Figure 5c?
The Authors write „the electrical conductance estimated to about 1.8x10-6 S from the current-voltage characteristics shown in figure 4b) at voltages close to the origin, where the characteristic is almost linear.” – I think that it should be shown in the figure.
The authors compare the values of pyroelectric coefficients obtained from the polarization measurements performed using triangular pulses of 1 kHz frequency and based on capacitance measurements performed at 100 kHz. Whether the frequency does not matter? What is the uncertainty of pyroelectric coefficient obtained from pyroelectric measurement?
Are these two obtained values of pyroelectric coefficient close to each other if there is a difference is greater than 40%?
Author Response
Response letter
Reviewer 2
The paper looks interesting. I suggest that it could be accepted after some revision, and here are my suggestions for the authors.
The Authors wrote about terraces, but from the Figure 2 it doesn’t look like terraces. I propose to supplement the figure with a profile. There is probably a mistake in the value of the width of the terraces – 800 nm?
Answer: Figure 2 was complemented with a line scan across several terraces. It is confirmed that the height of the terraces is of about one-unit cell and the width is in between 400 and 600 nm.
Corresponding text was added at page 6 lines 163-166.
There is no description of how to measure to obtain the results presented in the Figure 4b. There is also no comment on these results.
Answer: The ferroelectric analyzer TF2000 from AixACCT allows simultaneous recording of polarization and current hysteresis loops. Therefore, Figure 4a) shows polarization hysteresis while Figure 4b) shows current hysteresis recorded at different temperatures and using a triangular voltage wave of 1 kHz, as described in the section 2. Materials and Methods. Explanatory text was added at page 3 lines 98-100 and page 6 lines 192-195.
In Figure 4c the Authors present the temperature dependences of polarization for positive and negative voltages. The dependences show different slopes but there is only one value of pyroelectric coefficient. Which case does it correspond to? What is the uncertainty?
Answer: The value given in the text, of 4.8*10-4 C/m2K, is for positive values of remnant polarization. For negative values the estimated pyroelectric coefficient is of about 4.4*10-4 C/m2K. A correction was now made, considering a pyroelectric coefficient that is an average between the two values, with corresponding error, as given in the text, page 6 line 212.
See also the answer to issue 4 expressed by the Reviewer 4.
Where is Figure 5c?
Answer: This was a mistake.
The Authors write „the electrical conductance estimated to about 1.8x10-6 S from the current-voltage characteristics shown in figure 4b) at voltages close to the origin, where the characteristic is almost linear.” – I think that it should be shown in the figure.
Answer: It was added as an inset in Figure 4b).
The authors compare the values of pyroelectric coefficients obtained from the polarization measurements performed using triangular pulses of 1 kHz frequency and based on capacitance measurements performed at 100 kHz. Whether the frequency does not matter? What is the uncertainty of pyroelectric coefficient obtained from pyroelectric measurement?
Answer: The dielectric constant extracted from capacitance measurements does not play any role in the estimation of the pyroelectric coefficient. One method is based on the temperature dependence of the remnant polarization, from where the pyroelectric coefficient is directly estimated as dP/dT, while the other method is based on the frequency dependence of the pyroelectric signal in correlation with equation (3), valid for the frequency range where the pyroelectric signal is approximately constant. The equation (3) does not involve dielectric constant.
The capacitance is used only to estimate the electrical time constant. Even there is a variation of the capacitance value from about 60 pF at 1kHz to about 42 pF at 100 kHz, the condition that remains fulfilled.
However, we have considered the point and we have replaced the capacitance used to calculate the electrical time constant with the one measured at 1 kHz. See the modified text at page 8 starting with line 263.
The errors for the pyroelectric coefficient estimated using the second method were given in the revised manuscript, see page 9 starting with line 285.
Are these two obtained values of pyroelectric coefficient close to each other if there is a difference is greater than 40%?
Answer: Indeed, the statement was not properly formulated. We have changed the text to “of the same range of magnitude”.
Reviewer 3 Report
See the attachment
Comments for author File: 
Comments.pdf
Author Response
Response letter
Reviewer 3
The authors report on the ferroelectric and pyroelectric properties of an epitaxial BiFeO3 (BFO) thin
film. The structural investigation reveal that the BFO film is fully is fully strained to the strontium
titanate (STO) (001) substrate. Although the value of spontaneous polarization, measured at room
temperature, is lower than published previously, the pyroelectric coefficient is significantly larger than
other values reported in the literature. I find the result interesting and would like to see it published.
However, the present manuscript calls for some minor revisions:
- All symbols of physical units must be edited in normal fonts, not the italic one.
Answer: Corrected in the text.
- All numbers of references should be edited in normal/bold not italic/bold like in line 123.
Answer: Corrected.
- All results must be completed with their uncertainties. Results without uncertainties have not
any scientific meaning.
Answer: Uncertainties were added for some quantities such as the pyroelectric coefficient or the electrical conductance, which were not directly measured but estimated using other quantities. For quantities extracted from experimental graphs like hysteresis loops or C-V measurements we have considered that the uncertainties are negligible, being directly measured and not estimated.
- The manuscript calls for professional linguistic correction in order to remove some bugs and
improper phrases. For example: "…properties were obtained…" – line 95. What does it mean?
One can obtain sample, results, but properties?
Answer: We did our best to correct the language.
Regarding the improper phrase mentioned by the Reviewer, the complete phrase is “Thus, information at the micrometre scale about BFO thin film surface morphology and its ferroelectric properties were obtained simultaneously. “The sense is that information about ferroelectric properties is obtained by the previously mention technique, that it is AFM/PFM. We believe that the formulation is not wrong, since information about properties is obtained, not properties. Anyway, we have re-phrased as “Thus, information at the micrometre scale about BFO thin film surface morphology and about its ferroelectric properties were obtained simultaneously.” We hope this is acceptable for the Reviewer.
- The notion "lattice constant" is no longer preferred. This quantity is not constant. Better use
"lattice parameter"
Answer: Corrected.
Reviewer 4 Report
The manuscript cannot be recommended for publication at the current stage. The authors may consider the following issues and modify their work before submitting elsewhere.
- The introduction is not focused well on the topic. Pyroelectric effect, BFO, and PLD are mixed. These could be organized and more focused towards the target of the work.
- The discussion is not written/organized properly.
- Symbols and punctuations should be corrected.
- The pyroelectric study is not sufficient and scientific. Polarization vs. temperature is not a linear relation.
- There are plenty of studies on pyroelectric studies and lots of studies on BFO itself. There is nothing new in this work.
Author Response
Response letter
Reviewer 4
The manuscript cannot be recommended for publication at the current stage. The authors may consider the following issues and modify their work before submitting elsewhere.
- The introduction is not focused well on the topic. Pyroelectric effect, BFO, and PLD are mixed. These could be organized and more focused towards the target of the work.
Answer: Our manuscript is focused on the ferroelectric and pyroelectric properties of epitaxial BFO. Therefore, we have organized the introduction so that first we introduce the pyroelectric effect and related materials, including BFO, then we discuss the existing results and shortcomings of BFO, then introduce PLD as deposition technique allowing to obtain good epitaxial films, thus suitable to grow epitaxial BFO films and to eliminate some of the shortcomings of previous studies. We believe that there is logic in the way we have written the introduction, and our results support the claim that epitaxial BFO films grown by PLD can have enhanced pyroelectric properties, comparable to those of PZT.
With all due respect we believe that there is no need to re-organize the introduction.
- The discussion is not written/organized properly.
Answer: We do not understand this comment. What is wrong with the discussion section? We have followed the model of many previous articles, discussing first the structural properties then the physical properties of interest for the manuscript.
- Symbols and punctuations should be corrected.
Answer: We have done our best to eliminate typing errors and to polish the language.
- The pyroelectric study is not sufficient and scientific. Polarization vs. temperature is not a linear relation.
Answer: We agree with the reviewer that the polarization dependence is not linear but quadratic as predicted by thermodynamic theories. However, we have found evidences in the literature that linear dependence is also possible in the case of ferroelectrics with perovskite structure. These references were introduced in the revised manuscript. We have also detailed the procedures used to estimate the pyroelectric coefficient from the temperature dependence of remnant polarization. First procedure was to estimate the pyroelectric coefficient from the slope of the apparently linear temperature dependence of polarization as shown in Figure 4c). The second procedure was to calculate the pyroelectric coefficient as p=ΔP/ΔT for each interval of 50 K in the graph shown in Figure 4c) and then to extract an average value for the temperature range between 100 K and 400 K. The results are similar. New text was added at page 6, lines 198-216.
- There are plenty of studies on pyroelectric studies and lots of studies on BFO itself. There is nothing new in this work.
Answer: We all due respect, we do not agree with the reviewer. Indeed, there are plenty of studies on the pyroelectric properties of various materials, in bulk or thin film forms. There are also plenty of studies about the properties of BFO single crystal, thin films or ceramics. However, regarding the pyroelectric properties of BFO, the main topic of our study, are not so many previous reports. We have performed a thorough search of the existing literature, at least in the journals that are accessible through our institutional subscription, and all the articles we have found in the literature are given as references in the manuscript. All the previously reported values for the pyroelectric coefficient are summarized in Table 2, together with our results. It is evident that our results are at least 2 to 4 times larger than the previously reported values. We believe that this is a novel result, that can be of interest for the community.
Round 2
Reviewer 2 Report
In the previous review I mention: The Authors write „the electrical conductance estimated to about 1.8x10-6 S from the current-voltage characteristics shown in figure 4b) at voltages close to the origin, where the characteristic is almost linear.” – I think that it should be shown in the figure.
The Authors added the inset in Fig. 4b in the revised version. However it is unclear what this inset presents. The results in the inset cross the origin of coordinate system while the results in Fig 4b don’t do it!
Author Response
Response letter
Reviewer 2
In the previous review I mention: The Authors write „the electrical conductance estimated to about 1.8x10-6 S from the current-voltage characteristics shown in figure 4b) at voltages close to the origin, where the characteristic is almost linear.” – I think that it should be shown in the figure.
The Authors added the inset in Fig. 4b in the revised version. However, it is unclear what this inset presents. The results in the inset cross the origin of coordinate system while the results in Fig 4b don’t do it!
Answer: We apologize for the omission of details how the I-V characteristic at low voltages was obtained. In the revised version we give details in the “Materials and methods” section. The measurements had to be performed in different conditions than the hysteresis measurements, meaning significantly lower frequencies to avoid parasitic contributions from transient currents that can affect the current reading and the electrical conductance extracted from the I-V characteristic. The frequency or, in other words, the measurement time for the current was set in such a way to obtain quasi-steady state values, similar to a dc measurement.
Reviewer 4
Following other reviewers' comments, the authors have made an overall significant change. This is commendable.
However, I am not convinced about the introduction and organization of the discussion.
- A) Introduction
1) The introduction is not focused. I quote from the introduction: “Thus, even if BFO has one of the largest remnant polarization between ferroelectric materials there are many difficulties obtaining good samples for the investigation of the pyroelectric properties [32]. Here we report on the ferroelectric and pyroelectric properties of an epitaxial BFO thin film. The structural investigation revealed that the BFO film is fully strained to the STO (001) substrate. Although the polarization value at room temperature is of ?? ????
?? , lower than previous reports [8,11,12], the pyroelectric coefficient evaluated from two methods is significantly larger than other values reported in the literature.”
How does a “good sample” (with ?? ?????2, according to authors) have lower remnant polarization from the previously reported bad samples (with ??− ??? ?????2, according to authors)?
2) Though, the authors have emphasized the facts (“the microstructure and thus the functionality of the BFO films obtained by PLD depends on many factors, including: substrate quality, gas pressure, deposition temperature, laser frequency and laser energy density, etc”) controls the microstructure and functionalities.
Does good microstructure produce low remnant polarization?
If not, did the author deliberately makes the microstructure bad with the aim that this could make a very good pyroelectric effect? What is the goal described in the introduction?
3) Again, I quote from the introduction:
“One way to grow good quality epitaxial BFO films is to use pulsed laser deposition (PLD), which is also known for the good transfer of the target stoichiometry into the deposited layer [27]. However, the microstructure and thus the functionality of the BFO films obtained by PLD depends on many factors, including: substrate quality, gas pressure, deposition temperature, laser frequency and laser energy density, etc. The substrate plays a critical role in the epitaxial growth of thin films, the key element being the lattice mismatch between the film and the substrates [18,28–31].”
I see this as a general introduction to PLD. Nothing is specific here about the topic of the research presented in the manuscript. I can simply replace the term BFO with any other materials and still the part will express the same. We can talk about such in an introductory class lecture on PLD.
4) Again, I quote from the introduction:
“Thus, even if BFO has one of the largest remnant polarization between ferroelectric materials there are many difficulties obtaining good samples for the investigation of the pyroelectric properties [32].”
This expresses that the authors are going to address the difficulties in obtaining good samples. However, they neither said what are the difficulties they are going to address, nor what is a “good sample” according to them. If they intend to say the samples with ??− ??? ?????2, are not good enough or they should have low remnant and large pyroelectric effect to be called good, then the authors must state that first. Then, they should mention their aim/target/goal.
Overall, I see the introduction has: a) definition of pyroelectric effect, b) application of pyroelectric effect, c) introduction to BFO (d) difficulties of making BFO by PLD. (e) summary of their work in 2 sentences. I don’t think this is an introduction of a scientific research paper titled “Lead-free BiFeO3 thin film: ferroelectric and pyroelectric properties”. I suggest to revise the introduction and making it more precise on the topic.
- B) Discussion
By saying “(2) The discussion is not written/organized properly” I mean (a) to structure properly to improve the quality of the manuscript. The most recent published one on Electronic Materials can be found here https://www.mdpi.com/journal/electronicmat . The author may follow check the “Latest Articles” section. For examples: [Latest] https://www.mdpi.com/2673-3978/3/1/10/htm [second latest] https://www.mdpi.com/2673-
3978/3/1/9/htm
(b) The discussion (writing) had to be improved which has been considered by the authors after the comments of other reviewers. I see a commendable improvement of the discussion in the revised manuscript.
Answer: We have revised the Introduction section, we hope that it is acceptable in the present form. The largest values of polarization are obtained when BFO is grown on substrates with (111) orientation, considering that polarization in BFO is oriented along this direction. Larger strain imposed by substrate can also enhance polarization, as is the case for DyScO3. However, for applications is preferable to grow the films on more common and less expensive substrates, such as SrTiO3 with (001) orientation. The polarization is lower in this case, because the BFO film is constrained to grow on (001) direction also, thus only a projection of polarization along this direction will contribute to the hysteresis loop. The structural investigation of our sample has revealed a dispersion of the c/a ratio in the volume. Polarization is proportional with c/a, thus it is possible to have regions with different polarization values, with complicated domain structure in the volume. Some domains may not switch, leading to lower polarization value than in other cases, but the domain walls may respond to small temperature variations, leading to enhanced pyroelectric coefficient. We tried to emphasis these aspects in the revised Introduction.
We have also re-organized the Results and Discussion section, introducing subtitles for the main parts.
Reviewer 4 Report
Attached
Comments for author File: Comments.pdf
Author Response
Response letter
Reviewer 2
In the previous review I mention: The Authors write „the electrical conductance estimated to about 1.8x10-6 S from the current-voltage characteristics shown in figure 4b) at voltages close to the origin, where the characteristic is almost linear.” – I think that it should be shown in the figure.
The Authors added the inset in Fig. 4b in the revised version. However, it is unclear what this inset presents. The results in the inset cross the origin of coordinate system while the results in Fig 4b don’t do it!
Answer: We apologize for the omission of details how the I-V characteristic at low voltages was obtained. In the revised version we give details in the “Materials and methods” section. The measurements had to be performed in different conditions than the hysteresis measurements, meaning significantly lower frequencies to avoid parasitic contributions from transient currents that can affect the current reading and the electrical conductance extracted from the I-V characteristic. The frequency or, in other words, the measurement time for the current was set in such a way to obtain quasi-steady state values, similar to a dc measurement.
Reviewer 4
Following other reviewers' comments, the authors have made an overall significant change. This is commendable.
However, I am not convinced about the introduction and organization of the discussion.
- A) Introduction
1) The introduction is not focused. I quote from the introduction: “Thus, even if BFO has one of the largest remnant polarization between ferroelectric materials there are many difficulties obtaining good samples for the investigation of the pyroelectric properties [32]. Here we report on the ferroelectric and pyroelectric properties of an epitaxial BFO thin film. The structural investigation revealed that the BFO film is fully strained to the STO (001) substrate. Although the polarization value at room temperature is of ?? ????
?? , lower than previous reports [8,11,12], the pyroelectric coefficient evaluated from two methods is significantly larger than other values reported in the literature.”
How does a “good sample” (with ?? ?????2, according to authors) have lower remnant polarization from the previously reported bad samples (with ??− ??? ?????2, according to authors)?
2) Though, the authors have emphasized the facts (“the microstructure and thus the functionality of the BFO films obtained by PLD depends on many factors, including: substrate quality, gas pressure, deposition temperature, laser frequency and laser energy density, etc”) controls the microstructure and functionalities.
Does good microstructure produce low remnant polarization?
If not, did the author deliberately makes the microstructure bad with the aim that this could make a very good pyroelectric effect? What is the goal described in the introduction?
3) Again, I quote from the introduction:
“One way to grow good quality epitaxial BFO films is to use pulsed laser deposition (PLD), which is also known for the good transfer of the target stoichiometry into the deposited layer [27]. However, the microstructure and thus the functionality of the BFO films obtained by PLD depends on many factors, including: substrate quality, gas pressure, deposition temperature, laser frequency and laser energy density, etc. The substrate plays a critical role in the epitaxial growth of thin films, the key element being the lattice mismatch between the film and the substrates [18,28–31].”
I see this as a general introduction to PLD. Nothing is specific here about the topic of the research presented in the manuscript. I can simply replace the term BFO with any other materials and still the part will express the same. We can talk about such in an introductory class lecture on PLD.
4) Again, I quote from the introduction:
“Thus, even if BFO has one of the largest remnant polarization between ferroelectric materials there are many difficulties obtaining good samples for the investigation of the pyroelectric properties [32].”
This expresses that the authors are going to address the difficulties in obtaining good samples. However, they neither said what are the difficulties they are going to address, nor what is a “good sample” according to them. If they intend to say the samples with ??− ??? ?????2, are not good enough or they should have low remnant and large pyroelectric effect to be called good, then the authors must state that first. Then, they should mention their aim/target/goal.
Overall, I see the introduction has: a) definition of pyroelectric effect, b) application of pyroelectric effect, c) introduction to BFO (d) difficulties of making BFO by PLD. (e) summary of their work in 2 sentences. I don’t think this is an introduction of a scientific research paper titled “Lead-free BiFeO3 thin film: ferroelectric and pyroelectric properties”. I suggest to revise the introduction and making it more precise on the topic.
- B) Discussion
By saying “(2) The discussion is not written/organized properly” I mean (a) to structure properly to improve the quality of the manuscript. The most recent published one on Electronic Materials can be found here https://www.mdpi.com/journal/electronicmat . The author may follow check the “Latest Articles” section. For examples: [Latest] https://www.mdpi.com/2673-3978/3/1/10/htm [second latest] https://www.mdpi.com/2673-
3978/3/1/9/htm
(b) The discussion (writing) had to be improved which has been considered by the authors after the comments of other reviewers. I see a commendable improvement of the discussion in the revised manuscript.
Answer: We have revised the Introduction section, we hope that it is acceptable in the present form. The largest values of polarization are obtained when BFO is grown on substrates with (111) orientation, considering that polarization in BFO is oriented along this direction. Larger strain imposed by substrate can also enhance polarization, as is the case for DyScO3. However, for applications is preferable to grow the films on more common and less expensive substrates, such as SrTiO3 with (001) orientation. The polarization is lower in this case, because the BFO film is constrained to grow on (001) direction also, thus only a projection of polarization along this direction will contribute to the hysteresis loop. The structural investigation of our sample has revealed a dispersion of the c/a ratio in the volume. Polarization is proportional with c/a, thus it is possible to have regions with different polarization values, with complicated domain structure in the volume. Some domains may not switch, leading to lower polarization value than in other cases, but the domain walls may respond to small temperature variations, leading to enhanced pyroelectric coefficient. We tried to emphasis these aspects in the revised Introduction.
We have also re-organized the Results and Discussion section, introducing subtitles for the main parts.
