<figure><img src="image_1.png" /></figure>

# Investigation of MoS₂ growth on GaN/sapphire substrate using molecular beam epitaxy

Mohamed Al Khalfioui*, Minh Tuan Dau, Zineb Bouyid, Ileana Florea, Philippe Vennéguès, Julien Brault, Stéphane Vézian, Adrien Michon, Yvon Cordier, Philippe Boucaud

Université Côte d'Azur, CNRS, CRHEA, rue B. Grégory, 06560 Valbonne, France

## ARTICLE INFO

Communicated by Maria Hilse

**Keywords:**

A1. Characterization

A1. Nucleation

A1. Interface

A1. Substrate

A3. Molecular beam epitaxy

B2. Semiconducting materials

## ABSTRACT

The growth of two-dimensional molybdenum disulphide (MoS₂) layers on Gallium Nitride/Sapphire (GaN/ Al₂O₃) substrates using molecular beam epitaxy (MBE) growth technique has been investigated. We show that optimized conditions at a substrate temperature around 700 °C lead to full surface coverage of MoS₂ on GaN. The growth proceeds through the initial formation of triangular-shaped MoS₂ flakes/domains on the GaN surface. Raman spectroscopy and high-resolution scanning transmission electron microscopy reveal the formation at the wafer-scale of a MoS₂ monolayer and few bilayers. A closer analysis of regions with two MoS₂ layers reveals the coexistence of both 1 T and 1H phases, showing strong bonds between the top layer of GaN and the first MoS₂ layer, and van der Waals interactions between the first and second MoS₂ layers.

## 1. Introduction

Since the first preparations of graphene in 2004 [1,2], the interest in two-dimensional (2D) materials has surged, drawing particular attention towards 2D semiconductors such as Transition Metal Dichalcogenides (TMDs). These materials are particularly noted for their exceptional electronic, mechanical and optical properties, which become increasingly remarkable as they are thinned to atomic scale [3,4]. Among TMDs, molybdenum disulphide (MoS₂) [5,6], composed of a single layer of molybdenum atoms sandwiched between two layers of sulphur atoms, forming a monolayer with a thickness of ~ 0.65 nm, presents a strong interest. As for all 2D materials, the monolayers are held together by van der Waals (vdW) forces, which allows the fabrication of vdW heterostructures. Furthermore, the bandgap of MoS₂ can be tuned by varying the number of layers. A single layer of MoS₂ exhibits a direct bandgap of approximately 1.8 eV [5]. The ability to adjust the bandgap by varying the number of layers adds a versatile parameter, thus allowing the material's properties to be finely tuned for specific applications. This characteristic renders MoS₂ a promising material for future advanced spintronic, electronic, and optoelectronic devices [7]. Besides, MoS₂ offers a significant advantage over silicon when it comes to electron mobility, especially when the thickness of the material decreases to a few monolayers [8]. Monolayer TMDs can exhibit two

common structural phases, depending on their atomic stacking configuration. These phases are characterized by either trigonal prismatic (1H) or octahedral (1T) coordination of the metal atoms. It is important to note that the octahedral phase (1T) is metallic, whereas the trigonal prismatic phase (1H) is semiconducting [9–11]. Controlling the phase during growth is crucial for exploring the properties of TMDs. For multilayers, other phases such as 2H and 3R can occur, depending on the arrangement of chalcogen and molybdenum (Mo) atoms, as well as the number of MoS₂ layers. [12].

The synthesis of 2D materials, despite their revolutionary potential, encounters significant challenges, particularly when scaling up for industrial applications. Traditional methods, such as mechanical and chemical exfoliation [13], though effective for producing high-quality samples in small size are difficult to integrate on large-surface. This limitation significantly impedes the pathway towards widespread industrialization of 2D materials. In contrast, bottom-up methods, notably Chemical Vapor Deposition (CVD), offer a more scalable and industrially feasible route. CVD is a widely used technique for synthesizing MoS₂ monolayers [14–19], although it faces challenges in controlling thickness uniformity and substrate temperature. Additionally, precursors used in the CVD method can sometimes limit the purity of the 2D layers grown, affecting their physical property and applicability in advanced technologies. Pulsed Laser Deposition (PLD) is another

<footnote>* Corresponding author.
E-mail address: mohamed.al.khalfioui@crhea.cnrs.fr (M. Al Khalfioui).</footnote>
promising method for synthesizing 2D materials. However, this tech-
nique does not allow to precisely control the layer thickness, purity or
stoichiometry [20–22].

Among the various growth techniques, molecular beam epitaxy (MBE) has emerged as an advanced technique for the growth of semiconductor materials, including TMDs [23–29]. The MBE technique distinguishes itself through the precise control over the deposition, enabling the synthesis of high-quality, atomically thin MoS₂ layers. The high-quality material is achieved by evaporating ultra-pure molybdenum and sulphur within an ultra-high vacuum chamber, where growth parameters such as, temperature and atomic or molecular beam fluxes are rigorously controlled to facilitate the formation of high-quality layers. Another advantage of MBE is the possibility for in situ monitoring of the growth of layers. Characterization technique such as Reflection High-Energy Electron Diffraction (RHEED) shows real-time insights into surface reconstructions, growth rates, and atomic arrangement of the layers during the growth. In this context, MBE growth which, enables the optimization of the MoS₂/GaN interface quality with a precise control over growth parameters, plays a crucial role in domain formation and phase differentiation. Mastering this interface via MBE would make possible to produce MoS₂ films with finely tuned characteristics and properties.

MoS₂ layers have been grown on various substrates, including sapphire [23,30], h-BN [31,32], mica [33] and SiO₂/Si [34]. While TMDs deposited on h-BN exhibit excellent optical properties [32–35], the combination of MoS₂ and gallium nitride (GaN) substrate to form 2D/3D heterostructures appears to be promising due to their low lattice parameter mismatch [36] (less than 1 %) and similar thermal expansion coefficients, which facilitate epitaxial coherence between the materials. Publications have reported the growth of MoS₂ on GaN using PLD [21] and CVD [37,38] techniques. However, it is noteworthy that the growth of MoS₂ on GaN substrates using MBE is an unprecedented achievement, as it has not been previously reported in the scientific literature, to the best of our knowledge.

This article reports on the epitaxial growth of MoS₂ on GaN surfaces, aiming to optimize the growth conditions necessary for achieving a high-quality MoS₂ layer on GaN. Particular attention during the study was given to understanding the MoS₂/GaN interface and how the MoS₂-GaN interactions operate. Publications have reported that Ga termination does not favor a vdW interface but rather strong bonds between Ga and S [37]. Additionally, the GaN surface and growth conditions can favour different polytypes, such as 1H, 1 T, or 2H. Some studies have also indicated the possibility of coexistence of the 2H and 1 T phases [39–42]. A key objective of this research was therefore to systematically examine how various growth parameters, including temperature and growth duration, influence the quality of the produced material. Indeed, understanding the growth mechanisms and the MoS₂/GaN interface will make these 2D/3D heterostructures a useable system for various electronic and optoelectronic applications [43–46]. It is worth mentioning

that the study of interface is crucial for understanding the growth
mechanisms of MoS₂ and its impact on the surface morphology.

## 2. Experimental details

The samples, each measuring 10 mm x 10 mm, underwent a thor-
ough preparation as follows. Firstly, the GaN layers on sapphire sub-
strates were grown using by Metal-Organic Vapor Phase Epitaxy (MOVPE) in an Aixtron reactor, using trimethyl gallium and ammonia as precursors, and hydrogen as the carrier gas. The total thickness of the GaN layer is about 3.5 µm.

Prior to the MoS₂ growth, the GaN surfaces were cleaned with acetone and isopropyl alcohol, in ultrasonic agitation for 10 min. This was followed by a drying step using nitrogen gas. To further prepare the GaN surfaces, a deoxidation step was carried out, involving treatment with Buffered Oxide Etch (BOE) solution, and then rinsing with deionized water to ensure a clean surface for the MoS₂ growth, and finally a drying step under nitrogen.

The growth of MoS₂ layers on GaN/sapphire templates was carried out using a Riber 32P MBE reactor equipped with a Telemark electron beam evaporator, specifically designed for the deposition of molybdenum, along with an H₂S line and a plasma cell designed for the generation of atomic sulfur. The growth of MoS₂ was carried out at a substrate temperature of around 700 °C, selected to accommodate the GaN substrate. Note that GaN surface begins to decompose at temperatures above 750 °C under UHV. At temperatures below 700 °C, the formation of nucleation centers increases and leads to the development of smaller domains, resulting in a higher defect density during coalescence. The H₂S flow rate was set at 3 sccm, and the plasma power was maintained at 400 W at a growth pressure of 8x10⁻⁶ Torr. The electron beam evaporator was operated at a voltage of 10 kV and an emission current of 90 mA to ensure a stable low deposition rate. The growth rate was kept low, less than one monolayer per hour under sulphur-rich conditions to promote lateral growth of MoS₂ and limit the generation of sulfur vacancies in the MoS₂ layers.

The thickness, surface morphology and surface coverage of MoS₂ films deposited on GaN surfaces were characterized using an Edge AFM system from Bruker, operating in tapping mode, with silicon tips having radii of curvature of less than 10 nm. The surface coverage of MoS₂ was determined from AFM images using the ImageJ software, which detects the domain contours, calculates the total surface area of the MoS₂ domains, and then determines the coverage rate. Raman spectroscopy was employed to assess the structural quality, the degree of strain, number of layers and the phase of MoS₂. The focus was on the resonance modes E₁g² and A₁g characteristic of in-plane vibration and out-of-plane vibration modes of the trigonal prismatic MoS₂ [47]. We analysed as well the resonance modes J₁, J₂, J₃ of 1 T-MoS₂ located around 150 cm⁻¹, 230 cm⁻¹ et 330 cm⁻¹ [11,41]. Other vibration mode were observed at 734 cm⁻¹, 568 cm⁻¹, and 144 cm⁻¹, corresponding to the GaN resonance

<figure><img src="image_2.png" /><figcaption>Fig. 1. 1 µm x 1 µm AFM image showing the GaN surface after cleaning with acetone and isopropanol, followed by deoxidation using a BOE solution. This image clearly shows the characteristic atomic steps of GaN synthesized by MOVPE on sapphire. Right: AFM profile of the GaN layer.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** 1 µm x 1 µm AFM images showing the surface morphology of MoS₂ deposited on GaN at different growth durations. The images (a), (b), (c), and (d) correspond to growth times of 30 min, 1, 2, and 3 h, respectively, highlighting the evolution of the MoS₂ surface morphology as a function of growth time. (a) Nucleation step, (b) and (c) triangular lateral growth of MoS₂, and (d) coalescence of MoS₂ domains. On the right side of each image, AFM profiles of the MoS₂ layers grown on GaN are presented: the thickness is below 2 layers on all samples. In Fig. 2b and 2c, the dotted lines show the triangular domains and the junction between them.</figcaption></figure>

modes $A_1(LO)$, $E_2$ (High Energy), $E_2^{\dagger}$(Low), respectively [37]. This analysis was conducted using a Renishaw inVia spectrometer, operating with a 532 nm laser excitation source. Structural analyses were performed using a ThermoFisher Titan SPECTRA 200 transmission electron microscope (TEM) operating at 200 kV equipped with a cold Field emission Gun (FEG) and a Cs aberration probe corrector together with dual Energy Dispersive X-ray (EDX) detector presenting a high sensitivity corresponding to a solid angle of 1.8 sr. All the analyses were performed using a probe convergent semi-angle of 20 mrad, with the current set to 50 pA (Fig. 4) and 67 pA (Fig. 5), and a camera length of 200 mm. This setup resulted in acceptance angles between 11 and 43 mrad, allowing simultaneous STEM – high-angle annular dark field (HAADF) imaging and STEM – integrated differential phase contrast (iDPC), a recently developed imaging mode. In contrast to, the HAADF imaging mode that uses an annular detector to collect the high angle

scattered electrons, the iDPC imaging mode uses a four segmented detector to collect almost all the coherently scattered electrons for imaging. For thin specimens, iDPC-STEM is a direct phase imaging technique since the phase of the transmission function is directly proportional to the projected electrostatic potential. The iDPC-STEM image is a single image that represents the electrostatic potential field of the sample and provides direct information about the structure and composition of a specimen [48–50]. This technique is particularly suited for imaging light elements, thus allowing for a complete imaging of the atomic structure. Prior to the HR-STEM observations cross-sections were prepared using standard polishing TEM preparation method. The cross-section was thinned to electron transparency using a Gatan PIPS I polishing system at different accelerating voltages from 4.5-0.2 kV under Argon ions. Before insertion in the electron microscope column the samples were plasma cleaned for 5 min under a mixture of Argon and Oxygen.
<figure><img src="image_4.png" /><figcaption>Fig. 3. Raman spectra of MoS₂ layers grown for different growth durations. We can identify the presence of several peaks, namely J₁, J₂, J₃, E¹₂g, A₁g. The J₂ resonance appearing at 228 cm⁻¹ is characteristic of the 1 T phase of MoS₂. The E¹₂g and A₁g peaks are characteristics of the trigonal prismatic phase. E¹₂ (Low) corresponds to a resonance mode of GaN.</figcaption></figure>

### 3. Results and discussion

AFM topography image (Fig. 1) shows the morphology of the GaN substrate prior to the deposition of MoS₂. A step-like morphology of Ga-terminated surface is observed, with terraces width of approximately 100 nm and steps height of a few angstrom, which represents a monolayer or bilayer of GaN. This morphology, with step height (maximum bilayer GaN ~ 5 Å) less than the thickness of a MoS₂ single layer (~7 Å), would facilitate the nucleation of MoS₂ and also promote lateral growth continuous films. This morphology of GaN (0001) is characteristic of layers grown by MOVPE. The GaN surface exhibits a very smooth morphology with a root mean square (RMS) roughness of about 2 Å.

Fig. 2a shows AFM image and height profile of the first stage of MoS₂ growth, which reveals a high density of nucleation centers or crystallites of MoS₂ that are found on both the terraces and along the terraces edges. Fig. 2b, 2c and 2d shows AFM images and the height profile of three MoS₂ films, corresponding to growth durations of 1, 2 and 3 h respectively. These different growth durations have led to a MoS₂ surface coverage of 45%, 84% and 100%, respectively, on the GaN surface. The root mean square (RMS) roughness measured by AFM is less than 0.3 nm, confirming the formation of a smooth MoS₂ layers. Fig. 2b reveals the triangular shape of MoS₂ crystallites on the GaN surface that was obtained after one hour of growth. Since MoS₂ monolayer has a hexagonal crystal structure, the threefold rotational symmetry [51,52] often leads to triangular shapes, especially when the growth conditions are energetically favourable along certain crystallographic directions, dictated by the underlying lattice symmetry. Steps heights of approximately 7 Å were measured, corresponding to a single layer of MoS₂. Fig. 2b and 2c show larger triangular-shaped MoS₂ domains that exhibit different orientations. The triangular MoS₂ domains align according to the rotational symmetry of the GaN space group P63mc [53]. Specifically, we observe a relative angle of 60° between triangular MoS₂ domains [36]. These shapes are typical features previously observed on sapphire and h-BN substrates [30,53,54]. The height profile in Fig. 2c shows areas containing a single layer (i.e. with a height of ~ 7 Å), whereas at the junction between the two domains, a thickness equivalent to two layers is measured (i.e. with a height of ~ 14 Å). The AFM topography image in Fig. 2d illustrates the coalescence of these domains, forming a continuous MoS₂ layer over the entire GaN surface, with some areas comprising two layers, that will be later confirmed by the scanning

transmission electron microscopy (STEM) observations. The roughness of the layer, measured by AFM, is the same as the one of initial GaN (RMS = 0.2 nm), indicating a smooth growth. Due to the 0° and 60° orientations of triangular MoS₂ domains, the formation of inversion domain boundaries (IDBs) at the frontier between the different domains could emerge.

The Raman spectra for different growth durations are presented in Fig. 3. The characteristic Raman modes E¹₂g at 382.5 cm⁻¹, which correspond to the in-plane vibration, and A₁g at 404.5 cm⁻¹, associated with the out-of-plane vibration of Mo and S atoms in trigonal prismatic MoS₂, are observed for all the samples, except for the one with a growth duration of 30 min, which corresponds to the formation of nucleation centers and the initial stage of MoS₂ domain formation. Importantly, the Raman shift difference (Δω) between E¹₂g and A₁g modes of MoS₂ depends on the number of layers and the strain in the material [55,56]. The intensity and width of the peaks can provide information about the quality of the MoS₂ layers, the presence of defects, and the uniformity of the material. For these samples, we observe that Δω is approximately 22 cm⁻¹. This value corresponds to the limit between single and bilayers, as reported in the literature [57,58]. It remains almost unchanged for a growth duration up to 3 h, indicating the lateral growth dominance over a multilayer growth. This result is in good agreement with the AFM images presented in Fig. 2. However, it appears that the Δω value for the synthesized layers is greater than that for exfoliated layers [59]. This difference could be attributed to the interaction between the Mo and S atoms of MoS₂, and the Ga and N atoms of the GaN substrate. It has been reported that under certain conditions, the Ga termination can bond with sulphur, forming strong bonds and shifting the Raman resonance modes of MoS₂ to a larger value of Δω [37].

Furthermore, the intensity of the E¹₂g and A₁g Raman peaks increases as a function of the growth duration, which is indicative of an increase of MoS₂ coverage on the GaN surface. Δω remains constant as a function of growth time, indicating that the number of layers is consistent with AFM observations and does not exceed two layers under these growth conditions. In addition to the resonance modes of GaN and sapphire, a resonance mode at J₂ = 228 cm⁻¹, corresponding to the 1 T phase, was observed. These results suggest the coexistence of both 1 T and 1H phases.

To get further information on the structural and chemical composition of the near interface region between the grown MoS₂ layer and the GaN substrate, high resolution STEM analyses were performed simultaneously in the STEM-HAADF and STEM-iDPC imaging modes. To this end, we prepared a sample equivalent to the one made with a 3 h duration growth but followed by a 3 nm-thick amorphous molybdenum capping to protect the sample from oxidation.

Fig. 4 present a STEM-HAADF (a) and a STEM-iDPC (b) image recorded simultaneously on a selected area within the TEM cross-section exhibiting the interface between the MoS₂ monolayer and the GaN substrate which was oriented in [11-20] zone axis. The growth direction is [0001]. The STEM-HAADF image is readily interpretable, as the atomic columns in the crystals are always imaged as bright dots whose intensity is directly proportional to atomic number Z of the probed element. Consequently, the bright contrasts in Fig. 4a are associated to the heavy atoms (i.e. Ga and Mo) whereas the light atoms (i.e N and S) are difficult to distinguish in the image. As shown, Ga atoms present in the GaN structure form a periodic pattern, with a straight and flat monolayer of MoS₂ on top. This leads us to conclude that the interface is sharp, with a small spatial gap estimated to be 2.21 Å, which is much smaller than the vdW gap [60]. An interesting observation from this image is that no intermediate layer generated from interface reaction is visible.

The interface between MoS₂ and GaN is better detailed in Fig. 4b and c, which corresponds to an iDPC images taken on the same area as the STEM-HAADF image. The intensity in the STEM-iDPC image varies linearly with the atomic number of the probed element and thus the contrast variation can reveal structural and composition details. In this
<figure><img src="image_5.png" /><figcaption>**Fig. 4.** HR-STEM–HAADF (a) and HR-STEM-iDPC (b) images taken on a cross-section sample consisting of a GaN / sapphire substrate and 1 monolayer of MoS₂; c) zoom image taken in an area indicated by the blue rectangle in (b). The distance measured from the bottom of the MoS₂ layer to the upper top Ga layer is ~ 2.21 Å, as is denoted on the Figure; d) Area selected in the STEM iDPC image on which an intensity profile was performed in the blue rectangle and illustrated in e) showing the position of S and Mo atoms within the MoS₂ layer.</figcaption></figure>

image, the atomic columns of Mo and S can be clearly distinguished
providing a clear view of the atomic planes (Fig. 4d and e). The atomic
structure of GaN is also clear with the successive arrangement of Ga
atomic planes. The sharp interface with a small distance between MoS2
and GaN layers is confirmed. The interface free of intermediate layer is
also evidenced by the chemical contrast of the STEM-iDCP image. Here,
the small gap, in the range of covalent bond length, at the MoS2/GaN
interface is again retrieved featuring the strong chemical interaction,
showing a non-vdW characteristic of the 2D materials growth.

A detailed investigation of whole observable area of the TEM cross-
section revealed that there might be some areas within the same sample
where two layers of MoS₂ can be observed. Fig. 5a and b show STEM-
HAADF and STEM-iDPC images taken on an area within the cross-

section containing 2 ML of MoS₂ on top of GaN substrate with the 2nd
layer which is not continued (Fig. 5a). We believe that these areas
remain very isolated at the substrate scale. As for the area with 1ML
MoS₂ on GaN substrate, similar features could be retrieved regarding the
interface between the first MoS₂ layer and the GaN substrate. Specif-
ically, the first MoS₂ is bonded to the GaN substrate layer by strong
bonds formed between Ga and S planes, with a separation distance
estimated to be d = 2.25 Å, which is close to the distance measured for
the previous area using Fig. 4b.

Moreover, more insights on the S-Mo-S distances within each layer as
well as the distance between the 1st and 2nd layer could be deduced
from the iDPC images. Firstly, the distance between the Mo planes
belonging to the 1st and 2nd layer was estimated to be d = 6.61 Å, value
<figure><img src="image_6.png" /><figcaption>**Fig. 5.** HR-STEM–HAADF (a) and HR-STEM-iDPC (b) images taken on a cross-section sample consisting of a GaN / sapphire substrate and 2 ML of MoS₂, c) A zoomed-in image of the area indicated by the blue rectangle in (b) with the schematic representation of the two coexisting phases. The first ML MoS₂ exhibits a 1 T phase, while the second ML of MoS₂ exhibits a 1H phase. Van der Waals bonds exist between the two layers, with a separation distance of 6.61 Å between Mo atoms. As observed, the second ML is discontinuous.</figcaption></figure>

characteristic to the vdW gap. Secondly, within each layer the measured distances found were: d<sub>S1-Mo1</sub> = 1.62 Å, d<sub>Mo1-S1</sub> = 1.48 Å, and d<sub>S2-Mo2</sub> = 1.75 Å and d<sub>Mo2-S2</sub> = 1.77 Å respectively. Finally, the difference between the S1-Mo1-S1 distances within the 1st MoS<sub>2</sub> layer make us assume that the corresponding phase for the first layer is a 1 T phase which is maintained on all areas within the sample. However, the close S2-Mo2-S2 distances within the 2nd MoS<sub>2</sub> layer make us assume that the corresponding phase of this latter layer is rather 1H. These cross-section HR-STEM analyses clearly illustrate that both the 1 T and 1H phases coexist. These findings are consistent with the results obtained by Raman spectroscopy analyses.

## 4. Conclusion

In summary, we successfully grew MoS₂ on GaN/sapphire surfaces using the MBE technique at a growth temperature around 700 °C. Through a combination of different characterization techniques such as Raman spectroscopy, AFM and HR-STEM, we were able to evidence the uniform growth of MoS₂ on the GaN/sapphire substrate with the presence of one or two MoS₂ layers. AFM characterization revealed that the MoS₂ layer consists of triangular crystallites approximately one hundred nanometers wide. Through Raman spectroscopy analyses a Raman shift difference (Δω) between the E<sub>2g</sub> and A<sub>1g</sub> modes of approximately 22 cm<sup>-1</sup> was obtained which indicates the growth of 1ML to 2MLs. Moreover, the presence of the J<sub>2</sub> resonance mode observed in Raman spectroscopy, suggests the coexistence of 1 T and 1H phases within the sample. The coexistence of these two phases was observed through a detailed HR-STEM analyses in both STEM-HAADF and STEM-iDPC

imaging modes. From these atomically resolved analyses we were able to evidence that the 1 T phase is more characteristic of the 1st MoS₂ layer which is linked by strong bonds with the GaN substrate, and the 1H phase is generally present between two consecutive MoS₂ layers. Significantly, the capability of imaging the heavy and light elements within both the GaN substrate and the MoS₂ layers demonstrates the ability and the prospects of the STEM-iDPC as a robust imaging mode for the study of other materials like oxides or nitrides for example.

These results demonstrate the feasibility of growing MoS₂ layers on GaN surfaces using an MBE approach under specific growth conditions. With the synthesis of layers with controlled thickness and high purity, the MBE paves the way to the development of 2D materials for advanced applications and the combination of 2D materials with 3D heterostructures. Further research needs to be done in order to favour the growth of the MoS₂ layers presenting only the 1H phase which is desired for electronic and optoelectronic application.

## CRediT authorship contribution statement

**Mohamed Al Khalfioui:** Writing – original draft, Visualization, Supervision, Investigation, Conceptualization. **Minh Tuan Dau:** Visualization, Investigation. **Zineb Bouyid:** Investigation. **Ileana Florea:** Visualization, Investigation, Conceptualization. **Philippe Vennéguès:** Investigation, Conceptualization. **Julien Brault:** Visualization, Investigation. **Stéphane Vézian:** Investigation. **Adrien Michon:** Investigation. **Yvon Cordier:** Visualization, Resources. **Philippe Boucaud:** Conceptualization, Investigation.
Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was partly supported by ANR in the framework of the FlagERA JTC 2019 project Epitaxial Transition Metal dichalcogenides Onto wide bandgap hexagonal Semiconductors for advanced electronics "ETMOS" (ANR-19-GRF2-0001), by the NANOFUTUR project handled by ANR in the framework of Programme d'Investissement d'Avenir (ANR-21-ESRE-0012), by the Region Sud "CROISSANT" project, by the ANR through PEPR Electronique, "ADICT" (ANR-22-PEEL-0011) and the "MoirePlusPlus" (ANR-23-CE09-033) project. MTD thanks the support from the French government through the France 2030 investment plan managed by the ANR, as part of the Initiative of Excellence Université Côte d'Azur, France under reference number ANR-15-IDEX-01. The authors thanks Benjamin Damilano, Eric Frayssinet, and Pierre-Marie Coulon for the growth of GaN layers on sapphire by MOVPE and Thierry Michel and David Maurin for help in Raman experiments in Laboratoire Charles Coulomb (L2C).

Data availability

Data will be made available on request.

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