Comparison of RF and High Impulse Magnetron Sputtered Gallium-Doped Zinc Oxide Thin Films

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this work entitled “Comparison of RF and High Impulse Magnetron Sputtered Gallium Doped Zinc Oxide Thin Films” by J Phelps et al, the study is based on the evaluation of optoelectrical properties of RF versus HiPIMS sputtered gallium doped zinc oxide (GZO). The deposition technique was followed by characterization. Profilometer, XRD, SEM, UV-Vis, hot-probe and 4-point probe measurement are conducted to analyze the prepared samples, and the presented results are reasonable. Overall, this study is advantageous due to addressing the microstructural aspects of GZO films formed by different sputtering power impulses, such as grain compactness, and optoelectrical characteristics.

Therefore, I would recommend publication pending minor revisions:

1.      In Abstract, include tangible values for the transmittance bandgap and grain size improvement.

2.      In the introduction, include more literature on the studies that use the specified RF and DC sputtering to grow GZO and address the shortcoming in the outputs, why other techniques like HiPIMS might be helpful.

3.      In method section, last paragraph, please include the SEM system specifications used in this study.

4.      In XRD section, include the JCPDS card numbers derived from the output graphs.

5.      To make the comparison easier, provide an XRD result table with the highlighted outputs such as D-spacing, crystal size FWHM, relative lattice planes, etc.

6.      Figure 8, revise the x axis label to “power”

7.      In electrical properties section, include carrier concentration and mobility for a more detailed analysis.

 

Thanks.

Author Response

Dear Reviewer,

Thank you for your thoughtful comments and suggestions. I have made several revisions based on your feedback.

1. In Abstract, include tangible values for the transmittance bandgap and grain size improvement.

In the Abstract section, I have added tangible values for absorption, band gap, resistivity, and grain size.

2. In the introduction, include more literature on the studies that use the specified RF and DC sputtering to grow GZO and address the shortcoming in the outputs, why other techniques like HiPIMS might be helpful.

In my introduction, I have added another review paper reference detailing RF and DC sputtering techniques and the impacts they were found to have on GZO films. To illustrate the utility that might be expected from the deposition method, I cite references [11, 12] regarding the refractive index and film density that is expected. I have also added reference [13] regarding an improvement of film properties in reactively sputtered aluminum doped zinc oxide using HiPIMS.

3. In method section, last paragraph, please include the SEM system specifications used in this study.

I have included the SEM specifications for the study in the last paragraph of my Materials and Methods section.

4. In XRD section, include the JCPDS card numbers derived from the output graphs.

I have included the JCPDS card number from my output graph.

5. To make the comparison easier, provide an XRD result table with the highlighted outputs such as D-spacing, crystal size FWHM, relative lattice planes, etc.

I created a table containing the structural properties measured. Including FWHM, Bragg angle, c-lattice constant, calculated grain size, and stress.

6. Figure 8, revise the x axis label to “power”

I have revised the axis label to “power”.

7. In electrical properties section, include carrier concentration and mobility for a more detailed analysis

I would have liked to add carrier concentration and mobility to the paper, but my group lacks access to the necessary equipment to obtain those measurements. 

 

Reviewer 2 Report

The paper is devoted to comparison between two doped ZnO deposition techniques. The authors have presented the original research which could make a contribution to the development of the transparent conductive thin film technology. However the bandgap investigation results lack proper discussion and therefore appear ambiguous.

The authors suppose that the revealed ZnO films bandgap variation is due to the stress. However in the variation is too large to be attributed to the stress only. In the ref.18 the amplitude of stress related bandgap modulation is by the order of magnitude lower. In the ref.19 the gap is varied by about 0.65 eV which is even greater then 0.25 eV   in the paper under review. However the authors of ref.19 of consider not ZnO but CuGaO2 system and attribute the bandgap modulation with a number of factors including stress, composition change etc. Thus the bandgap change revealed in the present paper should be discussed in more details.

Also there are some minor remarks:

1. In the introduction section the authors should define the "Figure of merit" term.

2. The authors use word "depositions" very often referring to different deposition experiments. However plural form is not very often used in the articles, "deposition" word is more common.

Author Response

Dear Reviewer,

Thank you for your thoughtful comments and suggestions. I have made several revisions based on your feedback.

1. The authors suppose that the revealed ZnO films bandgap variation is due to the stress. However in the variation is too large to be attributed to the stress only. In the ref.18 the amplitude of stress related bandgap modulation is by the order of magnitude lower. In the ref.19 the gap is varied by about 0.65 eV which is even greater then 0.25 eV   in the paper under review. However the authors of ref.19 of consider not ZnO but CuGaO2 system and attribute the bandgap modulation with a number of factors including stress, composition change etc. Thus the bandgap change revealed in the present paper should be discussed in more details.
   
   

I have corrected the bandgap discussion. The variation in bandgap has been explored and discussed in other GZO papers and is attributed to the Burstein-Moss effect in which the concentration of dopant can significantly widen the bandgap. I have added two citations [22, 23] to support this, which detail the mechanism and report bandgap values in the same range. Reference 23 reports that the charge carrier concentration and hence band gap increase with the power of deposition, which mirrors my results.

 

2. In the introduction section the authors should define the "Figure of merit" term.
   
   

I have added a citation for the Figure of Merit definition and included the equation within my introduction. Figure of merit is defined as  , where T is the transmission of the thin layer at 550nm and Rsh is the sheet resistance.

 

3. The authors use word "depositions" very often referring to different deposition experiments. However plural form is not very often used in the articles, "deposition" word is more common.

I have removed many uses of the word “depositions” and have only kept it when referring to two categories of deposition.

References

22. Zhu, Yangfei, et al. "Ga-concentration-dependent optical and electrical properties of Ga-doped ZnO thin films prepared by low-temperature atomic layer deposition." Journal of Materials Science: Materials in Electronics 33.8 (2022): 5696-5706. Akin, N., et al. "Influence of
23. RF power on the opto-electrical and structural properties of gallium-doped zinc oxide thin films." Journal of Materials Science: Materials in Electronics 28.10 (2017): 7376-7384.

Reviewer 3 Report

The authors in the paper discussed the possibility of deposite GZO via HiPIMS technique. The topic is very important and innovative. The paper is well written and well argued with figures and comparisons between the two techniques mentioned in the paper, the one commonly used RF and the one proposed by the authors. 

Author Response

Dear Reviewer,
Thank you for your feedback on my paper. I have improved the in the next revision with improved figures, explanations, and grammatical editing. 

Reviewer 4 Report

In this paper, the authors compared the properties of gallium doped zinc oxide grown with a high impulse magnetron sputtering (HiPIMS) to those of the material fabricated with radio frequency (RF) sputtering. The resistivity, crystallinity, absorption coefficient, bandgap, and refractive index of the sputtered films were measured and compared. It was concluded that very similar results could be obtained with HiPIMS and RF sputtering processes under the same average power conditions. Still, it was found that the RF sputerring yields the material with a slightly higher bandgap and lower resistivity and optical absorption coefficient.

In the reviewers' opinion, the paper is well-written and conclusions are supported by the data. Still, two major issues arise:

1. The paper is submitted to Special Issue devoted to 'Laser-Assisted Processing and Treatment of Materials'. This issue considers 'obtaining/characterizing/applying coatings that include thermal treatment or laser-assisted process'. Neither of these treatments (laser-assisted or thermal) were used in the paper. Still, 'RF sputtering' is claimed to be welcome to the Issue, according to its website, so the decision is on the editors.

2. The authors should explain why they decided to apply HiPIMS process in the first place. Did they expect any advantage over conventional RF sputtering? Is HiPMS sputerring process cleaner, faster, simpler or cheaper? Without this explanation the motivation of the study is unclear.

 

Author Response

Dear Reviewer,
Thank you for your thoughtful feedback.

1. The editors of the journal recommended this special issue for my paper’s submission because of the RF sputtering category.
2. In this revision, I added more regarding the motivation for using HiPIMS in my introduction section. Primarily, similar papers for similar materials have achieved beneficial results such as lower resistivity. Since the properties of sputtered GZO are process-dependent, exploring HiPIMS sputtered GZO may yield a combination of properties that assist in the design of electro-optical devices.

 

Reviewer 5 Report

The manuscript entitled “Comparison of RF and High Impulse Magnetron Sputtered Gallium Doped Zinc Oxide Thin Films” by Phelps et. al. demonstrated the comparison of the gallium-doped zinc oxide (GZO) films deposited by HiPIMS and conventional RF sputtering technique in terms of their structural, optical and, electrical properties. In this study they found the GZO film deposited using RF sputtering technique outperformed the HiPIMS deposited film in various important aspects.

The work is definitely interesting as a negative result and the manuscript is well written. After addressing the following comments/questions, the revised manuscript can be considered for publication.

· The SEM images are not high quality, please improve the image quality.

· How is the grain size calculation done? Please elaborate on the process.

· The authors should perform AFM imaging of the GZO films deposited using different parameters in order to study the surface topology of the films and how it is changing with varying parameters.

· It is also suggested to perform the conducting AFM measurements to understand the homogeneity of the conductive region in the GZO films deposited at different processing conditions.

 

· Authors should also discuss the potential reason for the lower performance of HiPIMS deposited GZO films compared to the RF sputtered films.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors have made significant improvement to the paper clarifying most of the questions. 

However the new version is still lacking appropriate discussion of the increase band gap revealed from the figure 5. In the earlier version the authors have attributed the band gap change with the strain. This sounds reasonable but the strain is probably not the only mechanism since the gap increase value is too high. In the new version the authors do not even mention the strain discussing some other mechanisms. 

I would suggest the authors that they discuss the data from Fig.5 more carefully taking all possible mechanisms into account.

Author Response

Dear Reviewer,

Thank you for your thoughtful feedback. I have updated the band gap discussion pertaining to figure 5 to be much more in-depth than the previous revision. In addition to discussing the Burstein-Moss effect as a cause for the widening bandgap, I have added an improved version of the discussion on the impact stress can have on GZO thin films. In the stress discussion, I have added a citation that found a relationship between decreasing compressive stress causing an increase in the band gap for sputtered GZO films. This correlates well with my stress estimation from my XRD data. Based on the above, I have discussed why the HiPIMS deposition may have a narrower band gap as compared to the RF deposition.

Reviewer 5 Report

The revision done by the authors is satisfactory. The manuscript can be accepted in its present form.

Author Response

Dear Reviewer,

Thank you for your feedback, as it has greatly helped to improve the quality of the paper.