Novel Approach to Synthesize Nanostructured Gallium Oxide for Devices Operating in Harsh Environmental Conditions

Round 1

Reviewer 1 Report

In this manuscript, Alhalaili et.al. has described a solution process to improve the wettability of liquid gallium on a rough silicon surface, which is a novel and significant research work. Based on the reviewer’s suggestion, this manuscript needs some revision before acceptance.

1. In this abstract, the author mentioned “because the presence of Ag catalyst has completely changed the morphology of Ga2O3, this method is recom mended for the sustainable and low-cost synthesis of nanostructured gallium oxide for applications in gas sensor”. It will lead this work more interesting if the author can provide some data regarding the gas sensors. I suggest the author provide at least one application regarding to this work.
2. The figures definition need to be adjusted, such as Fig. 1 and 5.
3. Some subscripts and superscript need to be corrected, such as “Ga2CO3” (line269. Page9).
4. The language in this manuscript should be polished.
5. The format of the references needs to be examined more carefully, such as the Ref. 16 in page 9.

Author Response

We thank reviewer for his/her thoughtful comments on the original version of the manuscript. We have revised the manuscript accordingly, making the required changes and additions that are highlighted in yellow in the resubmitted manuscript. In the pages below, we will respond in point-by-point fashion to the reviewer’s comments. 

In this manuscript, Alhalaili et.al. has described a solution process to improve the wettability of liquid gallium on a rough silicon surface, which is a novel and significant research work. Based on the reviewer’s suggestion, this manuscript needs some revision before acceptance.

1. In this abstract, the author mentioned “because the presence of Ag catalyst has completely changed the morphology of Ga2O3, this method is recommended for the sustainable and low-cost synthesis of nanostructured gallium oxide for applications in gas sensor”. It will lead this work more interesting if the author can provide some data regarding the gas sensors. I suggest the author provide at least one application regarding to this work.

R1: In this short Communication, we presented only a solution to how to overcome the poor wettability of Ga2O3. Because this subject is of outmost importance for the researchers and manufacturing community, we will follow up soon with another paper to discuss in more details applications of this method for gas sensors such as the detection of oxygen molecules at high temperatures.

Conventional silicon (Si)-based nanodevices are intolerant to all these extreme conditions. Even though silicon has a lot of scientific and commercial benefits such as it is inexpensive, abundant, dominance in semiconductor market, easy to process and dope by suitable dopants, and used in a wide range of electronics and photonic applications, it falls short in extreme conditions such as high temperature. For instance, silicon nanosensors can operate at a maximum temperature up to 250°C. Creating nanosensors that are capable of sensing at extreme temperatures and detecting gases and chemicals remains a challenge.

2. The figures definition need to be adjusted, such as Fig. 1 and 5.

R2: The resolution of all figures has been improved. In particular, Figures 1 and 5 have been modified to increase clarity.

3. Some subscripts and superscript need to be corrected, such as “Ga2CO3” (line269. Page9).

R3: The sub/superscripts in the References list cannot be adjusted manually since the references are automatically introduced by the EndNote software. Once the manuscript is approved, the appearance of these references will be fixed during the editorial process.

4. The language in this manuscript should be polished.

R4: The text has been polished according to the reviewer’s suggestion.

5. The format of the references needs to be examined more carefully, such as the Ref. 16 in page 9.

R5: The reference format is automatically done by the Endnote software when a citation is introduced in the text. The reference list will be re-formatted during the editorial process upon acceptance.

Reviewer 2 Report

In this work the authors made an attempt to study the growth of gallium (III) oxide on various substrates, indicating that the presence of silver has a positive effect on the wettability and uniformity of the coatings studied. The approach seems interesting; however, the work has several drawbacks (minor and major) which prevent it from being published without a major revision. My comments are the following:

• The authors write (lines 79-81): “Ga2O3 is a promising dielectric material because the dielectric constant of Ga2O3 (9.93 and 10.2) [26] is much higher than the dielectric constant of silicon dioxide (3.9)”.

 

Dielectric constant does not directly determine (and describe) the nature of materials as being insulators or semiconductors. For example, ε(GaAs) = 12.9, ε(Si) = 11.7 but they are classical semiconductors. The sentence should be rewritten accordingly. (minor)

 

• The authors write (lines 149-150): “The Ga nanospheres observed in Figure 2 b and c do not have wetting properties since the wetting angle (α) is closed to 180 °C.”

The word “since” is it not suitable within that context. Wetting angle does not determine wetting properties; it is simply a way to describe the surface affinity of one material for another. Wetting properties are determined by the intermolecular forces between the contacting materials. (very minor)

• All the presented pictures are of very low quality. It is even impossible to distinguish what is written. (minor)
• I would like to see some pictures captured at a low magnification to see that the coatings are uniform throughout the substrate, not just over a small area. This can be added in the supplementary file. (medium)

 

• The main disadvantage is the following. The authors assert to obtain Ga₂O_­3 without proving what they obtained in the end. The samples were annealed in 1000 °C in a NITROGEN atmosphere. So, where did the oxygen come from? Moreover, it is well known (and I observed it personally) that the formation of GaN whiskers is observed upon annealing Ga-containing films. Given that the annealing was conducted in a nitrogen atmosphere I would expect to obtain GaN, not Ga­­₂O_­3. The authors provide no information on the composition of the final material. So, I suggest that XPS data of the samples must be provided. (major).

 

Comments for author File: Comments.pdf

Author Response

We thank reviewer for his/her thoughtful comments on the original version of the manuscript. We have revised the manuscript accordingly, making the required changes and additions that are highlighted in yellow in the resubmitted manuscript. In the pages below, we will respond in point-by-point fashion to the reviewer’s comments. 

In this work the authors made an attempt to study the growth of gallium (III) oxide on various substrates, indicating that the presence of silver has a positive effect on the wettability and uniformity of the coatings studied. The approach seems interesting; however, the work has several drawbacks (minor and major) which prevent it from being published without a major revision. My comments are the following:

1. The authors write (lines 79-81): “Ga2O3 is a promising dielectric material because the dielectric constant of Ga2O3 (9.93 and 10.2) [26] is much higher than the dielectric constant of silicon dioxide (3.9)”. Dielectric constant does not directly determine (and describe) the nature of materials as being insulators or semiconductors. For example, ε(GaAs) = 12.9, ε(Si) = 11.7 but they are classical semiconductors. The sentence should be rewritten accordingly. (minor)

R1. We agree with the reviewer’s comment and modified the sentence to read: ”Ga₂O₃ is a promising gate dielectric material with a dielectric constant of 9.93 and 10.2 [26], which is much higher than the dielectric constant of silicon dioxide (3.9) [27].

2. The authors write (lines 149-150): “The Ga nanospheres observed in Figure 2 b and c do not have wetting properties since the wetting angle (α) is closed to 180 °C.”The word “since” is it not suitable within that context. Wetting angle does not determine wetting properties; it is simply a way to describe the surface affinity of one material for another. Wetting properties are determined by the intermolecular forces between the contacting materials. (very minor)

R2. We agree with the reviewer’s comment and modified the sentence to read: “The SEM observation of Ga nanospheres presented in Figure 2 b and c suggests a large contact angle (α) close to 180^o, which correspond to low wettability”.

3. All the presented pictures are of very low quality. It is even impossible to distinguish what is written. (minor)

R3: The resolution of all figures has been improved.

4. I would like to see some pictures captured at a low magnification to see that the coatings are uniform throughout the substrate, not just over a small area. This can be added in the supplementary file. (medium)

R4: In this short communication, we have shown that adding Ag catalyst allows for a conformal coating of Ga₂O₃ on Si. We have previously published (and cited in the manuscript) our work on the contribution of Ag catalyst in the growth mechanism and process of Ga₂O₃, when a homogeneous coating and denser nanowires were achieved due to the low contact angle [29]. The contact angle of Ga on silver film is 30^o [38] and on silicon substrate is 73.9^o [37], leading to better wetting of Ga on Ag surface and uniform growth of Ga₂O₃ nanowires [29].

Further, we will apply this technique to gas sensors to detect the oxygen at high temperatures. The success of this technique to provide a conformal coating on a large surface will be then demonstrated and correlated with the sensing parameters.

5. The main disadvantage is the following. The authors assert to obtain Ga₂O_­3without proving what they obtained in the end. The samples were annealed in 1000 °C in a NITROGEN atmosphere. So, where did the oxygen come from?

R5: The oxygen required for the oxidation reaction was the residual oxygen in the chamber, which interacted with Ga molecules to form Ga₂O₃. In our previous work, we showed using various investigation techniques that Ga₂O₃ was grown on the flat silicon surface [29]. The oxidation at 1000 °C in N₂ atmosphere led to the formation of dense Ga₂O₃ nanowires. In this paper, we aimed to demonstrate the effectiveness of the use of Ag catalyst in growing the Ga₂O₃ film on the PEC etched surface compared to a flat surface.

6. Moreover, it is well known (and I observed it personally) that the formation of GaN whiskers is observed upon annealing Ga-containing films. Given that the annealing was conducted in a nitrogen atmosphere I would expect to obtain GaN, not Ga­­₂O_­3. The authors provide no information on the composition of the final material. So, I suggest that XPS data of the samples must be provided. (major).

R6: We agree with the reviewer’s observation. We actually performed XPS on Ga₂O₃ grown by oxidation and the results have already been published. Ref [48] was added.

[48]   B. Alhalaili, H. Mao, D.M. Dryden, H. Cansizoglu, R.J. Bunk, R. Vidu, J. Woodall, M.S. Islam, “Influence of Silver as a Catalyst on the Growth of β-Ga2O3 Nanowires on GaAs”, Materials 2020, 13(23), 5377

Round 2

Reviewer 1 Report

The quality of the figures in Figure 1 and 5 must be improved before publication. 

Author Response

Figures 1 and 5 have been re-arranged to improve the resolution. Thank you.

Reviewer 2 Report

Dear authors,

thank you for reviewing and improving your manuscript.

Unfortunately, I am not convinced by the explanations you have provided for my last question (number 5) . My point is as following.

You claim to have obtained Ga₂O₃ films by annealing Ga films in a nitrogen atmosphere at 1000 °C without any information about the coatings composition. As a source of oxygen you indicate the residual oxygen in the chamber in where the annealing was carried out. In this case even that there is some oxygen in the chamber, the amount of nitrogen is substantially higher (by several order of magnitude) and given that I do not understand why you rule out the possibility of GaN formation. If the experiments were actually performed under the specified conditions I am certain that you must see GaN. 

You also give a reference to your previous work where a similar system was investigated. But in this case the annealing was carried out in argon atmosphere which does not react with gallium.

If you have difficulties in obtaining XPS data, you should, at least, provide an EDX spectrum (not a map) recorded at an accelerating voltage of not higher than 5 kV. 

 

Author Response

Please see attached file.

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

In replying to my questions, the authors constantly refer to their previous publications, which is generally normal practice. However, publication in a peer-reviewed journal (even in the form of a communication) implies that all controversial issues must be addressed before an article is published.

In this case the authors ignore my request of adding the evidence of having Ga2O3 in the films. To me the results seems to be doubtful, given that the samples were annealed in the absence of oxygen and given that the nitrogen reacts with Ga at 1000 C. If the authors believe that I am wrong, I would like to see some convincing proofs, not only references to their own works.

I do not want to be harsh with the authors, therefore I accept the article after minor revision giving the authors one last chance to be scientifically correct. 

Author Response

Reviewer 2 (round 3)

In replying to my questions, the authors constantly refer to their previous publications, which is generally normal practice. However, publication in a peer-reviewed journal (even in the form of a communication) implies that all controversial issues must be addressed before an article is published.

In this case the authors ignore my request of adding the evidence of having Ga2O3 in the films. To me the results seems to be doubtful, given that the samples were annealed in the absence of oxygen and given that the nitrogen reacts with Ga at 1000 C. If the authors believe that I am wrong, I would like to see some convincing proofs, not only references to their own works.

I do not want to be harsh with the authors, therefore I accept the article after minor revision giving the authors one last chance to be scientifically correct.

 

1. In our work, we observed only the formation of β-Ga₂O₃ in the process of oxidation of Ga at high temperatures. We believe that the high nitrogen pressure and high temperature condition (1-2GPa, 1400-1500 °C) to drive the reaction between Ga and N₂ were not achieved in our experiments. Moreover, the presence of the Ag catalyst, which has a high solubility for oxygen, could drive the reaction towards Ga oxidation rather than Ga nitridation.

The literature shows that the synthesis of GaN nanostructures from Ga always involves complicated processes, while the nitridation of β-Ga₂O₃ is the most common strategy to GaN. However, we did not treat/anneal the β-Ga₂O₃ in nitrogen gas. We just treated/oxidized Ga with and without Ag catalyst. On the other hand, it can be believed that once β-Ga₂O₃ is formed at high temperatures, it will react with nitrogen to form GaN. The conversion of β-Ga₂O₃ to GaN is not possible in the given conditions. In our experiments [1-4], if any of the nitrogen atoms were trapped in the β-Ga₂O₃ phase, nitrogen was not detected (see materials characterization data in [1-5]). Because neither GaN was never observed by XRD, nor elemental N, or Ga-N interactions were never observed by XPS, we did not specifically look further for nitrogen.

2. We believe that some confusion came from the fact that that the oxidation of Ga took place in N₂ atmosphere. The furnace was not hermetically sealed. The oxidation took place in N₂ flow at 20 sccm. Therefore, we changed the text to clarify the experimental procedure in L131 and add comments in L211-215.

We hope that this clarification will remove the reviewer doubt about our experiments: we did not treat/anneal the β-Ga₂O₃ in the absence of oxygen. However, residual oxygen was present in the chamber. In our previous work, we have shown that the oxidation occurred at a background oxygen concentration of 88 - 280 ppm [1]. Our group performed a detailed investigation of Ga oxidation at high temperatures, different substrates, in Ag and N₂ flow, with and without Ag catalyst.

We preformed experiments in both Ar and N₂ atmosphere and we observed that the influence of the oxidation atmosphere has significant impacts on the growth of nanowires due to the presence of Ag thin film. It has been observed that the motion of Ag NPs was influenced by the gas flow. The Ag films heated in N₂ gas dewet faster than the film in the Ar gas due to the difference in molecular mass of N₂ compared to Ar, leading to faster diffusion of Ag atoms. The higher molecular mass of Ar limits diffusion through it, changing the growth dynamics. Furthermore, heating Ag NPs in O₂ gas as opposed to vacuum increases the surface self-diffusion of Ag atoms. Hence, the diffusion mechanism of Ag NPs enhances the growth of nanocrystalline thin film.[2]

All our work on β-Ga₂O₃ has already been published [1-5]. The published papers present a comprehensive characterization of β-Ga₂O₃ obtained by oxidation using XRD, SEM, SEM-EDS, XPS, HRTEM, HRTEM images with the corresponding EDS mapping, TEM-SAED, electrical characterizations (I-V curves, transient time), optical characterization etc. We don’t see any reason to repeat the same information in this communication. The film obtained by oxidation on Si substrate was characterized in ref [4], although other substrates gave similar results in film composition and crystalline structure [1-5]. The repetition of the same results is redundant. In this communication, we present only an application of the technique we used to grow conformal β-Ga₂O₃ film obtained by oxidation using Ag catalyst. In our experiments [1-4], if any of the nitrogen atoms were trapped in the β-Ga₂O₃ phase, nitrogen was not detected.

In conclusion, we believe that the reviewer raised an interesting observation that should challenge us to design further experiments to elucidate whether GaN is formed along with β-Ga₂O₃ during heating Ga in N₂ flow, with and without Ag catalyst. Although the mechanisms of formation of GaN and Ga₂O₃ are completely different during heating Ga at high temperatures, i.e. solid diffusion for GaN and evaporation/suboxide formation for Ga₂O₃, characterization techniques such as SEM-EDS can be performed in the analysis of nitrogen to investigate the presence of GaN.

We hope that we answered reviewer’s concerns. We really appreciate reviewer’s comments that opened up for discussions the possibility of GaN formation during Ga oxidation in N₂ flow. 

References

 

[1]          B. Alhalaili, R. Bunk, R. Vidu, and M. S. Islam, "Dynamics Contributions to the Growth Mechanism of Ga2O3 Thin Film and NWs Enabled by Ag Catalyst," Nanomaterials, vol. 9, Sep 2019.

[2]          B. Alhalaili, R. Vidu, and M. S. Islam, "The Growth of Ga2O3 Nanowires on Silicon for Ultraviolet Photodetector," Sensors, vol. 19, Dec 2019.

[3]          B. Alhalaili, R. J. Bunk, H. Mao, H. Cansizoglu, R. Vidu, J. Woodall, et al., "Gallium oxide nanowires for UV detection with enhanced growth and material properties," Scientific Reports, vol. 10, Dec 2020.

[4]          B. Alhalaili, R. Vidu, H. W. Mao, and M. S. Islam, "Comparative Study of Growth Morphologies of Ga2O3 Nanowires on Different Substrates," Nanomaterials, vol. 10, Oct 2020.

[5]          B. Alhalaili, H. Mao, D. M. Dryden, H. Cansizoglu, R. J. Bunk, R. Vidu, et al., "Influence of Silver as a Catalyst on the Growth of beta-Ga2O3 Nanowires on GaAs," Materials, vol. 13, Dec 2020.