Electronic and Transport Properties of Epitaxial Graphene on SiC and 3C-SiC/Si: A Review

Round 1

Reviewer 1 Report

Nowadays, there are many topical-reviews concerning the transport properties of 2D-materials. This review focuses on the epitaxial graphene layers formed on SiC, SiC / Si and Si (001), which distinguishes it from many others reviews. A significant number (137) of references are given in the work, but only ~ 22 of them are dated later than year 2014. 

In my opinion, presented review can be accepted after a minor revision, concerning the following notes:

1) In my opinion, the review lacks a direct comparison of the electrophysical properties of epitaxial graphene films with the properties of free-stnding graphene sheets, for example [10.1021 / nl301922d]. Without that dicsussions on the properties of quasifreestanding graphene films is not clear to the reader.

2) Since the review is devoted to epitaxial structures, it requires a brief discussion about the morphology (SEM, AFM), crystal structure (LEED), and heterointerface properties of graphene structures obtained by different techniques.

3) Optical techniques for studying transport properties (Raman spectroscopy) deserve a slightly more detailed description with new references, see for example [10.1103 / PhysRevB.92.125437], [10.1088 / 2053-1583 / aa90b3]. One could also sign out the possibility of homogeneity studies by Raman micro-spectroscopy - see [10.1016 / j.physb.2019.411751], [10.1016 / j.mssp.2019.02.039] [10.1016 / j.carbon.2020.03.027 ].

4) An interesting topic is the discussion on the FET structures properties, however the review does not discuss the role of the gate dielectric layer material and its propertie - hBN/SiOx etc.. - see, for example, [10.1038 / s41928-019-0256-8], as well the review does not discuss the intriguing possibility of direct epitaxial growth of the gate dielectric on graphene, see for e.g. [10.1038 / s41427-019-0162-6].

5) The review could be improved with addition of links to recent reviews devoted to the transport properties studies of graphene — see, for example, [10.1103 / PhysRevMaterials.3.110301], [10.1103 / RevModPhys.83.407] and 2D materials [10.1146 / annurev-physchem-050317-021353].

Author Response

1) In my opinion, the review lacks a direct comparison of the electrophysical properties of epitaxial graphene films with the properties of free-standing graphene sheets, for example [10.1021 / nl301922d]. Without that discussions on the properties of quasi-freestanding graphene films is not clear to the reader.

Response: We thank the Reviewer for the comment. Section 2.1, lines 71-89 details the band structure of a free-standing monolayer graphene. Section 2.3, lines 146-147 compares the turbostratic graphene with an isolated monolayer graphene. Section 4.5 –discussion on the scattering mechanism also details the comparison of epitaxial graphene to the free-standing graphene. Additionally, we have now added a comparison of the electrical properties of epitaxial graphene on SiC with the free-standing graphene on page 13 with the reference advised by the Reviewer.

2) Since the review is devoted to epitaxial structures, it requires a brief discussion about the morphology (SEM, AFM), crystal structure (LEED), and heterointerface properties of graphene structures obtained by different techniques.

Response: We appreciate the Reviewer’s remark. This manuscript elaborates on low-energy electron diffraction observations for epitaxial graphene on SiC(000) and 3C-SiC(100) on pages 4, 11, 13, and 21. Given the focus on transport of this work, and the fact that a large number of reviews has been already devoted to the surface morphology of epitaxial graphene, we considered to be more practical referring to those available reviews rather than increasing the length of this manuscript.

3) Optical techniques for studying transport properties (Raman spectroscopy) deserve a slightly more detailed description with new references, see for example [10.1103 / PhysRevB.92.125437], [10.1088 / 2053-1583 / aa90b3]. One could also sign out the possibility of homogeneity studies by Raman micro-spectroscopy - see [10.1016 / j.physb.2019.411751], [10.1016 / j.mssp.2019.02.039] [10.1016 / j.carbon.2020.03.027 ].

Response: We appreciate the Reviewer’s suggestion. We have now extended the Raman spectroscopy section (3.2) on page 7 with the references mentioned by the reviewer –lines 218-225.

4) An interesting topic is the discussion on the FET structures properties, however the review does not discuss the role of the gate dielectric layer material and its properties - hBN/SiOx etc.. - see, for example, [10.1038 / s41928-019-0256-8], as well the review does not discuss the intriguing possibility of direct epitaxial growth of the gate dielectric on graphene, see for e.g. [10.1038 / s41427-019-0162-6].

Response: We have now added a paragraph on the influence of different gate dielectrics and its properties on the FETs on page 24.

5) The review could be improved with addition of links to recent reviews devoted to the transport properties studies of graphene — see, for example, [10.1103 / PhysRevMaterials.3.110301], [10.1103 / RevModPhys.83.407] and 2D materials [10.1146 / annurev-physchem-050317-021353].

Response: We thank the Reviewer for the suggestions. We have now referred to the recent reviews mentioned by the Reviewer on pages 18 [128], 22 [139] and 24 [151].

Reviewer 2 Report

The manuscript reviews comprehensibly the research work that has been done with respect to electronic and transport properties of epitaxial graphene grown on various SiC substrates.

The review is well organized and it will be instructive for those researchers  who needs precise and well documented  facts about the progress in this active and interesting field of scientific research with large technological opportunities.

Besides a few typos errors at the lines 234, 662, and 693 regarding the naming and the indexing of subsections, I do not have any other objections.

Author Response

The manuscript reviews comprehensibly the research work that has been done with respect to electronic and transport properties of epitaxial graphene grown on various SiC substrates. The review is well organized and it will be instructive for those researchers who needs precise and well documented facts about the progress in this active and interesting field of scientific research with large technological opportunities.

Besides a few typos errors at the lines 234, 662, and 693 regarding the naming and the indexing of subsections, I do not have any other objections.

Response: We thank the Reviewer for the feedback and for pointing out the typos. We have now corrected the errors mentioned by the Reviewer;

Line 240: ‘Dubstrate’ is corrected to Substrate

Line 606: The section number has been changed from 5.1 to 5.2

Line 669: The section number has been changed from 5.1 to 5.3

Reviewer 3 Report

The Review Manuscript applsci-834220-peer-review-v1 titled "Electronic and Transport Properties of Epitaxial Graphene on SiC and 3C-SiC/Si: A Review" submitted to Applied Sciences by F. Iacopi and co-workers describes the electronic and transport properties of epitaxial graphene grown from SiC-related substrates. Authors focalised in particular on the interplay between graphene and its surroundings, highlighting how interface interactions enable a fine-tuning of graphene properties.

Authors did a great job summarising from a considerable mass of literature research and reviews, the latest advanced in tuning epitaxial grow (pre- and post- sublimation) to govern the properties of the final material. 

The paper is well written and organised and may represent a useful reference for the specific research field. Despite the fact many of the highlighted points are available as well in other recently published works (e.g., the monumental review of C. Backes et al., 2020, 2D Materials), the main merit of this review is to make the specific knowledge readily available and concise.

I have particularly appreciated the linear and precise organisation of the manuscript, and the broad spectrum of factors investigated.

The Reviewer considers this work a valuable research and has just some points to rising:

1. The authors should have emphasised more the properties of unperturbed, suspended graphene as a reference to which compare the features of the different EG materials described. This comparison is present in some paragraph/chapter of the review (e.g., in some parts of chapter 5) but was not made systematically. Such reframing of the review could improve its appealing to a broader spectrum of readers;  

2. Some sentences in the manuscript are difficult to read and need to be rephrased (e.g. paragraph 4.2, rows 462-464 and paragraph 4.3 rows 486-488);

3. Chapter 5 paragraphs have to be corrected in their numeration (the feeling is that authors have not carefully revised the final version of the manuscript before submission);

4. In chapter 5, when describing the factors influencing graphene electrical properties, a paragraph about the effect on this of ion adsorption has to be added (e.g., surface adsorption of alkali metals ions).

Author Response

The Review Manuscript applsci-834220-peer-review-v1 titled "Electronic and Transport Properties of Epitaxial Graphene on SiC and 3C-SiC/Si: A Review" submitted to Applied Sciences by F. Iacopi and co-workers describes the electronic and transport properties of epitaxial graphene grown from SiC-related substrates. Authors focalized in particular on the interplay between graphene and its surroundings, highlighting how interface interactions enable a fine-tuning of graphene properties.

Authors did a great job summarizing from a considerable mass of literature research and reviews, the latest advanced in tuning epitaxial grow (pre- and post- sublimation) to govern the properties of the final material. 

The paper is well written and organized and may represent a useful reference for the specific research field. Despite the fact many of the highlighted points are available as well in other recently published works (e.g., the monumental review of C. Backes et al., 2020, 2D Materials), the main merit of this review is to make the specific knowledge readily available and concise.

I have particularly appreciated the linear and precise organization of the manuscript, and the broad spectrum of factors investigated.

The Reviewer considers this work a valuable research and has just some points to rising:

1. The authors should have emphasized more the properties of unperturbed, suspended graphene as a reference to which compare the features of the different EG materials described. This comparison is present in some paragraph/chapter of the review (e.g., in some parts of chapter 5) but was not made systematically. Such reframing of the review could improve its appealing to a broader spectrum of readers;  

Response: We thank the Reviewer for the comment, which is in line with the point 1 from Reviewer 1.  A comparison of epitaxial graphene with the suspended graphene has made in this review. Section 2.1, lines 71-89 describes the band structure of a free-standing monolayer graphene. Section 2.3, lines 146-147 compares the band structure of turbostratic graphene with an isolated monolayer graphene. A comparison of the electrical properties of epitaxial graphene on SiC with the free-standing graphene on page 13, (please also refer to answer 1 to Reviewer 1).

2. Some sentences in the manuscript are difficult to read and need to be rephrased (e.g. paragraph 4.2, rows 462-464 and paragraph 4.3 rows 486-488);

Response: Apologies for the lack of clarity on those instances. Those sentences have been rephrased as follows:

• Lines 470-472: ‘The most dominant mechanism of ambient doping is described as; ambient exposed graphene forms a thin film of water, which contain dissolved CO₂ that reacts with water forming H⁺’ Changed to ‘The mechanism of ambient doping is described as; ambient exposed graphene forms a thin film of water containing dissolved CO₂ that reacts with water forming H⁺ ions’.

 Lines 494-500: In the case of chemisorption, the band structure is disturbed, and the doping is estimated from the difference of work function between the graphene covered with metal and metal-free graphene. In the case of Ni, Co, Ti and Pd, the graphene is n-type doped [119]. Physisorption, e.g., interaction with metals such as Ag, Al, Cu, Au and Pt, involves only weak binding to graphene, so the electronic structure is preserved. In the case of physisorption, the electrons transfer from the graphene to the metal, causing the EF to move closer to the ED –and the hole doping is estimated from the change.

3. Chapter 5 paragraphs have to be corrected in their numeration (the feeling is that authors have not carefully revised the final version of the manuscript before submission);

Response: We thank the Reviewer for pointing this out. This issue has been rectified in the current version.

4. In chapter 5, when describing the factors influencing graphene electrical properties, a paragraph about the effect on this of ion adsorption has to be added (e.g., surface adsorption of alkali metals ions).

Response: We appreciate the Reviewer’s suggestion. We have now added a paragraph on the adsorption of alkali metal ions on page 22.

Reviewer 4 Report

Manuscript: "Electronic and Transport Properties of Epitaxial Graphene on SiC and 3C-SiC/Si: A Review"

Authors: Aiswarya Pradeepkumar, David Kurt Gaskill, Francesca Iacopi

My opinion, the manuscript under review is good enough. This is a good ballanced and comprehensive review. The authors' personal opinion is unbiased, it is expressed quietly and unobtrusively. The paper may be recommended for publication after minor revision. The minor revision is necessary, because there are some uncertanties and misprints. The authors are highly qualified specialists. In my opinion, no additional peer-review steps will be needed after revision.

Recommendation: ACCEPT.

COMMENTS:

line 4: *Francesca -> Francesca

line 40: SiC -> The polytype should be specified. Probably, 6H-SiC.

line 40: (the so-called Si-face) better (Silicon-terminated surface, Si-face) 

line 44: (aka the C-face) better (Carbon-terminated surface, C-face) 

line 44: (000-1) and more x35 times all throughout the manuscript. Comment: It is better to use overlined 1 instead of 'minus 1' for negative Miller indices https://en.wikipedia.org/wiki/Miller_index. 

lines 54-56: Reformulate the sentence.

lines 60-66: Not sure. As to me, the simple present tense sounds better in this paragraph.

line 73: [23], using the -> [23], who used the

lines 134-135: Reformulate the sentence.

line 234: Dubstrate -> Substrate

line 259: polytope -> polytype

line 658: And,

lines 724-7255: Si(1 1 0), Si(1 1 1) x2 times -> Si(110), Si(111)

Author Response

My opinion, the manuscript under review is good enough. This is a good balanced and comprehensive review. The authors' personal opinion is unbiased, it is expressed quietly and unobtrusively. The paper may be recommended for publication after minor revision. The minor revision is necessary, because there are some uncertainties and misprints. The authors are highly qualified specialists. In my opinion, no additional peer-review steps will be needed after revision.

Recommendation: ACCEPT.

COMMENTS:

line 4: *Francesca -> Francesca

Response: We thank the reviewer for pointing this out. The error has been rectified on line 4.

 line 40: SiC -> The polytype should be specified. Probably, 6H-SiC.

Response: We have now specified the polytype 6H-SiC(0001) on line 40.

line 40: (the so-called Si-face) better (Silicon-terminated surface, Si-face) 

Response: We have now amended the ‘Si-face’ to ‘silicon-terminated surface, Si-face’ in line 40 of the manuscript.

line 44: (aka the C-face) better (Carbon-terminated surface, C-face) 

We have now amended the ‘C-face’ to ‘carbon-terminated surface, C-face’ in line 45 of the manuscript.

line 44: (000-1) and more x35 times all throughout the manuscript. Comment: It is better to use overlined 1 instead of 'minus 1' for negative Miller indices https://en.wikipedia.org/wiki/Miller_index. 

Response: We thank the Reviewer for the suggestion - we have consequently changed (000-1) to (000) throughout the manuscript.

lines 54-56: Reformulate the sentence.

Response: To clarify the sentence, we have added the reference to our recent work of alloy-mediated graphitization of 3C-SiC/Si is added.

lines 60-66: Not sure. As to me, the simple present tense sounds better in this paragraph.

Response: We have now amended the paragraph on page 2 (lines 60-65) in the simple present tense.

line 73: [23], using the -> [23], who used the

Response: Based on Reviewer’s suggestion, we have now modified the sentence on line 73.

lines 134-135: Reformulate the sentence.

Response: We thank the Reviewer for their comment. We have now rephrased the sentence on page 4, lines 133-134.

‘This means the band gap can be varied by means of an external transverse electrical field to control the density of electrons; the gap can be varied from 0 to 0.3 eV by changing the doping level between the two layers, or by an external gate control [34, 45]. Thus the BLG transitions from a semi-metal to an insulator [34].’

line 234: Dubstrate -> Substrate

Response: We thank the Reviewer for pointing this out. This error is now corrected in the manuscript -line 240. 

line 259: polytope -> polytype

Response: We thank the Reviewer for pointing this out. This typo has now been corrected (line 265).

line 658: And,

Response: The new sentence on line 658 is ‘Other examples, Wong et al. [135] obtained a charge-neutral QFSG on SiC(0001) using fluorine from a fluorinated fullerene source, which was stable under ambient conditions, whereas resistant to temperatures up to 1200 °C, and Gierz et al. [136] demonstrated buffer-layer decoupling using gold intercalation of EG on SiC(0001).

lines 724-7255: Si(1 1 0), Si(1 1 1) x2 times -> Si(110), Si(111)

Response: We thank the Reviewer for pointing this out. We have now changed Si(1 1 0), Si(1 1 1) x2 times to Si(110), Si(111) from lines 741-743.