Atomic Layer Deposition of High-k Insulators on Epitaxial Graphene: A Review

Round 1

Reviewer 1 Report

Review Report_ applsci-753241

The manuscript entitled “Atomic layer deposition of high-k insulators on epitaxial graphene: a review” written by Giannazzo et al. gave an overview of the current research on ALD of high-k insulators on epitaxial graphene. The review article is well structured and includes different approaches towards the ALD deposition, such as ALD on pristine EG and on EG either with a seeding layer or with pre-functionalization.

To give the readers a more general view on this field, the authors may consider to add a new section to describe two aspects: (1) comparison and discussion of high-k insulators (on EG) prepared by other methods other than ALD; (2) comparison and discussion of high-k insulators on other types of graphene other than epitaxial graphene. Their perspectives shall also be discussed in this new section. Of course the last section, i.e. conclusions and perspectives, will be rather short without additional references.

Line 93: “Van der Walls bonds” shall be corrected as “van der Waals forces”.

Author Response

The manuscript entitled “Atomic layer deposition of high-k insulators on epitaxial graphene: a review” written by Giannazzo et al. gave an overview of the current research on ALD of high-k insulators on epitaxial graphene. The review article is well structured and includes different approaches towards the ALD deposition, such as ALD on pristine EG and on EG either with a seeding layer or with pre-functionalization.

1. To give the readers a more general view on this field, the authors may consider to add a new section to describe two aspects: (1) comparison and discussion of high-k insulators (on EG) prepared by other methods other than ALD; (2) comparison and discussion of high-k insulators on other types of graphene other than epitaxial graphene. Their perspectives shall also be discussed in this new section. Of course the last section, i.e. conclusions and perspectives, will be rather short without additional references.

We thank the referee for this suggestion. We have added a new section entitled: 6. Open research issues and perspectives. This section includes a comparison with insulators on graphene prepared by other deposition methods, e.g. physcal depositions, and some references. Furthermore, a discussion about  ALD of high-k insulators on other types of graphene, more specifically CVD grown graphene on metals, is presented. The perspectives on the next developments of ALD for the deposition of alternative insulator/semiconductor films on graphene have been included at the end of this section. Finally, a more succinct Conclusion section has been created.

2. Line 93: “Van der Walls bonds” shall be corrected as “van der Waals forces”.

We made the indicated correction.

Reviewer 2 Report

This manuscript describes that some ways of ALD for high-k insulators on epitaxial graphene. These works contribute to device application of two-dimensional graphene. However, the reviewer has a few questions, as follow.

L. 241-244   It is difficult understand why does increase residence time of physisorption on water molecules by doping. The reviewer wants a description that is easy for readers to imagine. For example, is it possible to add from the viewpoint of surface energy and/or activation energy?

L. 245  Doesn't the incomplete oxidation occur in case of metal seed?

Although its dielectric constant is comparably low, h-BN is an insulator for layered materials. h-BN can utilize buffer layer for ALD on graphene because adsorption of species enhances due to spontaneous polarization on surface. (Nanotechnology, 2015, 26, 175708.) Of course, no damage to graphene layer exist. Are there any advantage for using a seeding layer compared to that?

Are there any results for the viewpoint of device performances such as transistor and capacitance? They are important knowledges for the device application,

Author Response

This manuscript describes some ways of ALD for high-k insulators on epitaxial graphene. These works contribute to device application of two-dimensional graphene. However, the reviewer has a few questions, as follow.

1. Lines 241-244:   It is difficult understand why does increase residence time of physisorption on water molecules by doping. The reviewer wants a description that is easy for readers to imagine. For example, is it possible to add from the viewpoint of surface energy and/or activation energy?

We thank the referee for this observation. We discussed more in details the effect of the epitaxial graphene n-type doping on the adsorption energy of water molecules and added a further reference supporting this model:

Recent experimental and theoretical investigations demonstrated that the interaction of polar water molecules with graphene depends on the Fermi level of graphene, i.e. its doping [53]. In particular, ab-initio calculations of the adsorption energy (E_a) for water molecules (the co-reactant of the ALD growth) on 1L graphene as a function of doping predicted an increase of E_a from ~127 eV for neutral graphene to ~210 meV for highly n-type doped (~10¹³ cm^-2) graphene [21]. The time of residence of a water molecule on graphene at a temperature T depends on E_a as τ∼exp(E_a/k_BT), where k_B is the Boltzmann constant. As a result, for the ALD process temperature (T = 250 °C), the residence time of physisorbed water molecules on the highly n-type doped EG is approximately six times higher than in the case of neutral graphene. The longer residence time provides, in turns, a larger number of reactive sites for Al₂O₃ formation during subsequent pulses of the Al precursor.

[53] Hong, G. ; Han, Y.; Schutzius, T. M.; Wang, Y.; Pan, Y.; Hu, M.; Jie, J.; Sharma, C. S.; Muller, U.; Poulikakos, D. On the Mechanism of Hydrophilicity of Graphene. Nano Lett. 2016, 16, 4447.

[21] Schilirò, E.; Lo Nigro, R.; Roccaforte, F.; Deretzis, J.; La Magna, A.; Armano, A.; Agnello, S.; Pecz, B.; Ivanov, I. G.; Giannazzo, F. Seed-Layer-Free Atomic Layer Deposition of Highly Uniform Al₂O₃ Thin Films onto Monolayer Epitaxial Graphene on Silicon Carbide. Adv. Mater. Interfaces 2019, 1900097, 1-11. 

2. Line 245  Doesn't the incomplete oxidation occur in case of metal seed?

We thank the reviewer for this remark. This aspect has been pointed out at the end of section 4.1 as follows:

In spite of the limited increase in the EG defectivity, a reduction of the electron mobility has been reported in most of the cases after ALD of high-k dielectrics seeded by an oxidized metal [34]. This was ascribed to the poor structural quality and sub-stoichiometric composition of the seed layer (typically due to an incomplete oxidation), leading to charge trapping phenomena and increased electron scattering by charged impurities.

3. Although its dielectric constant is comparably low, h-BN is an insulator for layered materials. h-BN can utilize buffer layer for ALD on graphene because adsorption of species enhances due to spontaneous polarization on surface. (Nanotechnology, 2015, 26, 175708.) Of course, no damage to graphene layer exist. Are there any advantage for using a seeding layer compared to that?

We thank the reviewer for this suggestion. We discussed the potential role of h-BN as an interfacial layer for ALD growth on graphene in the new Section 6. Open research issues and perspectives. The suggested reference has been also included:

Hexagonal boron nitride (h-BN) is an insulating layered material with lattice structure similar to graphite. Due to the atomically sharp interface formed between h-BN and graphene, it is considered as the ideal insulator to achieve the intrinsic graphene mobility [65]. However, due to its low dielectric constant (k≈4), the h-BN interfacial layer must be combined with a high-k dielectric overlayer in order to reduce the effective oxide thickness (EOT) for realistic integration in graphene FETs. Recently, the direct ALD growth of a high-k insulator (Y₂O₃) on the sp² h-BN surface has been demonstrated, and the deposition mechanism was explained by the enhanced adsorption of the Y precursor on h-BN due to the polarization [66]. The challenge in this research field is the large area growth of h-BN on graphene by scalable approaches.

[65] Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K. L.; Hone, J. Boron nitride substrates for high-quality graphene electronics. Nat. Nanotechnology 2010, 5, 722

[66] Takahashi, N.; Watanabe, K; Taniguchi, T.; Nagashio, K. Atomic layer deposition of Y2O3 on h-BN for a gate stack in graphene FETs. Nanotechnology 2015, 26, 175708.

4. Are there any results for the viewpoint of device performances such as transistor and capacitance? They are important knowledges for the device application

We thank the referee for this observation. Unfortunately, data on the relevant electrical parameters, such as transistors’ field effect mobility, dielectric permittivity and breakdown field were reported only by some of the reviewed papers, making difficult a systematic comparison. However, when these data were available, they have been included in our paper. As an example, Fig.8 (b) shows a quite comprehensive description of the correlation between the Hall mobility and carrier density in pristine EG and after the ALD of several insulators with different seeding layers. In Fig.6, current-voltage characterization by C-AFM allowed to extimate the breakdown field (>8 Mv/cm) of thin Al₂O₃ films deposited on monolayer EG by direct thermal ALD.