Optical Characterization of H-Free a-Si Layers Grown by rf-Magnetron Sputtering by Inverse Synthesis Using Matlab: Tauc–Lorentz–Urbach Parameterization

Round 1

Reviewer 1 Report

In the submitted manuscript, the authors report on the impact of defects on the Raman spectroscopy acquisitions for amorphous silicon films. Subsequently, the authors have analyzed the optical responses of these films.  

In general, the scalable formation of amorphous coatings is crucial to the development of optical devices and such analyses will significantly push forward the current status quo. The topic studied, the novel synthesis procedure discussed, and the quality of the experiments are good, however, it lacks in-depth analysis for the represented area of silicon-based optics and optoelectronics. My main concerns regarding the manuscript are mentioned below:

1. The introduction is lacking in certain aspects regarding the nonequilibrium methods of amorphous Si fabrication. The general method of equilibrium-based processing leads to solid-phase epitaxial regrowth of Si. However, by employing non-equilibrium methods like ion implantation(Narayan, 1981), sputtering(Basnar et al., 2003; Rajsiri, Kempshall, Schwarz, & Giannuzzi, 2002) and laser annealing(Gupta & Narayan, 2019; Wood, Lowndes, & Narayan, 1984), it is possible to achieve amorphization of single-crystal Si.
2. The abstract is too verbose. The authors need to thin the abstract down, and mention only the key aspects and motivation for the study rather than delving deeper into the underlying aspects. The abstract does not have the reason or motivation for “why this experiment is being performed or needed”. Please address this.
3. Given the methods mentioned in point 1. why the authors chose ion beam sputtering as the film fabrication technique? Why was glass chosen as the substrate, and not quartz or sapphire?
4. Are the first-order peaks in the Raman spectrum for sample #1 arising from localized structural ordering, or impurities present inside the film? Please comment and provide XRD to eliminate the presence of impurities.
5. Please do not tabulate the conclusions and paraphrase them while reducing the length. Once the manuscript is reduced in size and authors present their results and analyses cohesively and briefly, then further publication can be considered.

These concerns need to be addressed before publication in an archival journal like MDPI Coatings.

Basnar, B., Lugstein, A., Wanzenboeck, H., Langfischer, H., Bertagnolli, E., & Gornik, E. (2003). Focused ion beam induced surface amorphization and sputter processes. Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing, Measurement, and Phenomena, 21(3), 927-930.

Gupta, S., & Narayan, J. (2019). Reduced Graphene Oxide/Amorphous Carbon P–N Junctions: Nanosecond Laser Patterning. ACS Applied Materials & Interfaces, 11(27), 24318-24330. doi:10.1021/acsami.9b05374

Narayan, J. (1981). Interface instability and cell formation in ion‐implanted and laser‐annealed silicon. Journal of Applied Physics, 52(3), 1289-1293.

Rajsiri, S., Kempshall, B., Schwarz, S., & Giannuzzi, L. (2002). FIB damage in silicon: Amorphization or redeposition? Microscopy and Microanalysis, 8(S02), 50-51.

Wood, R., Lowndes, D., & Narayan, J. (1984). Bulk nucleation and amorphous phase formation in highly undercooled molten silicon. Applied Physics Letters, 44(8), 770-772.

 

Comments for author File: Comments.docx

Author Response

We express first our gratitude for his comments.

1. The abstract has been completed rewritten. In the abstract is not used any formula. Moreover, the importance of the present work has been clearly stated.
2. Figures 4 (a) and (b) are enlarged in order to be clearly seen all the many enriching details of this very relevant figure. Its corresponding figure caption has been modified in order to explain in a much better way all the many contents of this figure #4. It has to be emphasized that the two insets shown in this figure present the cross-sectional SEM images for Si#1 and Si#2, thus corroborating the computed values of the average film thickness. The adopted GUI environment shown in Fig. 4(b) is certainly a key part of the devised Matlab-coded program “AdjustTransIS”.
3. In the legend of Fig. 5, it has been included that the “n” is the refractive index and “k” the extinction coefficient, in order to help the reader not familiar with the topic.
4. We have removed all the bullet points from the section, “Conclusions”. In this section, after very careful revision, the authors have highlighted the main perspective for the future, also indicating the pros and cons of the adopted inverse-synthesis method.
5. The authors have changed/reduced the title of their article, and they have removed the “registered symbol, R” in the whole paper.

Reviewer 2 Report

Authors prepared a set of several, nearly-1-µm-thick, pure, unhydrogenated (using Ar-plasma) amorphous-silicon (a-Si) thin layers, by rf-magnetron sputtering (RFMS), onto room-temperature non-absorbing glass substrates. They used normal-incidence optical transmittance spectrophotometry (wavelength range from 300 up to 2500 nm) to measure the samples. The optical responses of the H-free RFMS-a-Si thin films investigated, were parameterized using Tauc-Lorentz oscillator dispersion model, with the Urbach absorption tail, and also employed the Wemple-DiDomenico (WDD) single-effective-oscillator model.

It is not obvious why different from each other the four RFMS-a-Si samples (Si#1, Si#2, Si#3, Si#4) which were prepared in the same chamber with the same conditions: Ar-gas flow 70 sccm,  Ar-pressure  4.4 Pa, rf power of
525 W, power density 3.0 W/cm2. Why different the optical response and other things? This is only statistical difference?

In Table 2 and the text, Authors write 20 % void-fraction. How should I imagine this? This is too high for me! Is there any other independent measurement or proof for this values? As I know, the density difference between i-a-Si and c-Si is only 3 %. This 20 % needs some explanation or a mass-density measurement.

I suggest some revision to answer this questions.

Author Response

First, we express our gratitude for his very useful comments.

1. We have addressed in the revised version of the paper this very interesting remark regarding the variations in the optical responses of the samples.
2. We have given information about the values of porosity already reported in the literature.

Reviewer 3 Report

In this article, the authors study the optical characterization of H-free a-Si layers grown by

rf-magnetron sputtering.

The article has several issues.

• The abstract should be completely rewritten. In the abstract it should be not used formula. Moreover, the main highlights and the importance of their work should be stated.
• I think that Figure 4 should be put in the supplementary materials and not in the main article or not included in the article at all.
• In the legend of Fig. 5, please include that “n” is the refractive index and “k” the extinction coefficient to help a reader not familiar with the topic.
• Usually, it would be better not to use a bullet point list for the section “Conclusions”.
• In the conclusion section , after a careful revision, the authors should highlight the main perspective for the future, indicating the pros and cons of their method.
• The authors should change the title of their article, which is too long, trying not to use the  registered symbol ®.

Author Response

We first express our gratitude for the referee’s comments.

1. We have introduced several sentences in the INTRODUCTION, indicating some remarks concerning his point #1.
2. We have thinned the abstract down and addressed the question about “why the experiments are needed”.
3. We have chosen RF-MAGNETRON SPUTTERING (not ion-beam sputtering, as mentioned by the referee #3), for the reasons clearly pointed out, and the inexpensive/low-cost glass substrates.
4. We already provided the GIXRD measurements, and we now also mentioned our EDAX measurements, excluding the presence of impurities.
5. We have strongly reduced the conclusions.

Round 2

Reviewer 1 Report

The authors have responded properly to all the points raised in my review of the original version of the paper. The paper is much clearer than the original version. I recommend the publication of the paper.

Author Response

Thank you very much for helping me to improve the paper.

Reviewer 2 Report

I accept the answers and modifications.

Author Response

Thank you very much for your very-useful comments concerning the paper.

Reviewer 3 Report

I recommend a minor revision before the publication of this article. My only concern is for Fig. 4, where the PC interface is visible. Could the authors insert this image in the supplementary materials ?

Author Response

I have modified the paper taking into account your very-valuable remarks.