Interface Properties of MoS2 van der Waals Heterojunctions with GaN

The combination of the unique physical properties of molybdenum disulfide (MoS2) with those of gallium nitride (GaN) and related group-III nitride semiconductors have recently attracted increasing scientific interest for the realization of innovative electronic and optoelectronic devices. A deep understanding of MoS2/GaN interface properties represents the key to properly tailor the electronic and optical behavior of devices based on this heterostructure. In this study, monolayer (1L) MoS2 was grown on GaN-on-sapphire substrates by chemical vapor deposition (CVD) at 700 °C. The structural, chemical, vibrational, and light emission properties of the MoS2/GaN heterostructure were investigated in detail by the combination of microscopic/spectroscopic techniques and ab initio calculations. XPS analyses on as-grown samples showed the formation of stoichiometric MoS2. According to micro-Raman spectroscopy, monolayer MoS2 domains on GaN exhibit an average n-type doping of (0.11 ± 0.12) × 1013 cm−2 and a small tensile strain (ε ≈ 0.25%), whereas an intense light emission at 1.87 eV was revealed by PL analyses. Furthermore, a gap at the interface was shown by cross-sectional TEM analysis, confirming the van der Waals (vdW) bond between MoS2 and GaN. Finally, density functional theory (DFT) calculations of the heterostructure were carried out, considering three different configurations of the interface, i.e., (i) an ideal Ga-terminated GaN surface, (ii) the passivation of Ga surface by a monolayer of oxygen (O), and (iii) the presence of an ultrathin Ga2O3 layer. This latter model predicts the formation of a vdW interface and a strong n-type doping of MoS2, in closer agreement with the experimental observations.


Introduction
In the last decade, molybdenum disulfide (2H-MoS 2 ) emerged as the most investigated two-dimensional (2D) semiconductor material of the transition metal dichalcogenides (TMDs) family, due to its unique physical properties, combined to a good chemical stability and its abundance in nature [1].MoS 2 crystals (and in general all TMDs) are characterized by strong in-plane bonds between the chalcogen (X) and the transition metal (M) atoms and weak van der Waals (vdW) interactions between the stacked layers [2][3][4][5].In particular, the 2H-MoS 2 polytype exhibits a tunable bandgap as a function of the thickness, i.e., an indirect bandgap of 1.2 eV in the bulk form and a direct bandgap of 1.8-1.9eV for a monolayer (1L) MoS 2 [6,7].One layer and few-layers MoS 2 have been employed as channel materials in field-effect transistors (FET), showing very promising performances in terms of the on/off state current ratio (≥10 8 ) and decent mobility values (up to ~200 cm 2 /Vs under proper conditions) [1].These properties make MoS 2 one of the potential replacements of Si for the continuation of the Moore scaling law in digital electronics [8].Furthermore, 2H-MoS 2 is very appealing for a wide range of More-than-Moore applications [1,[9][10][11][12], including sensing [13,14], photocatalysis [15,16], photovoltaics [17,18], and photonics [19,20] until reaching the more exotic spin-valley physics [21][22][23].
In this context, the dangling bonds-free MoS 2 surface allows the creation of several vdW heterostructures by the combination of various 2D materials (2D/2D vdW heterostructures) [24][25][26][27], by integration of MoS 2 with semiconductor nanowires (1D core-shell heterostructures) and with bulk semiconductors (2D/3D vdW heterostructures) [1,[28][29][30][31][32].In particular, increasing research efforts have been directed in the last years to the integration of MoS 2 with wide-bandgap (WBG) semiconductors, including silicon carbide (SiC), gallium nitride (GaN), and related group-III nitrides (AlN and AlGaN alloys).The combination of the unique physical properties of MoS 2 with the robust properties of highly mature WBG semiconductors (such as high breakdown field and electron saturation velocity [33,34]) set the basis for the realization of new heterojunction diodes that exploit the vertical current at the MoS 2 /WBG interface [35][36][37] and of advanced photodetectors covering both the UV and the visible spectral range [38][39][40].
Nevertheless, understanding and controlling the interface properties of the MoS 2 /GaN heterostructures represent the key steps to optimize the performances of demonstrated devices and, eventually, to demonstrate new ones.In fact, the interface of such vdW systems plays a crucial role in terms of electronic transport and carrier transfer.As an example, Poudel et al. reported an increase in photoluminescence (PL) emission from MoS 2 and a consequent decrease from GaN, which was attributed to electron-phonon coupling and energy transfer at the MoS 2 /GaN interface [57].Furthermore, angle-resolved photoemission spectroscopy (ARPES) measurements performed on n-MoS 2 flakes transferred on p-doped GaN displayed a modification of the band structure caused by the formation of an interface dipole of 0.2 eV [58].Recently, Zhang et at.[59] performed a nitridation of the GaN surface by N 2 plasma treatment before transferring MoS 2 on top of GaN.A modification of the MoS 2 /GaN band structure with respect to a not-nitridated surface and a corresponding enhancement of photo-catalytic properties of the heterojunction were demonstrated, which can be exploited for hydrogen generation.
Besides experimental studies, several theoretical investigations based on the density functional theory (DFT) approach have been performed during the last few years to predict the interfacial properties and energy band-alignment in MoS 2 /GaN heterostructures.Most of these simulation studies considered an ideal lattice-matched interface between monolayer MoS 2 and the Ga-terminated GaN(0001) surface, which resulted in the prediction of a covalent-like bond at the interface [57,59].These theoretical results contradict the experimental evidence of a van der Waals (vdW) bond between MoS 2 and GaN, reported by different authors [36,37,42].Clearly, studies combining experimental investigations and more refined modeling of the interface are necessary to better understand the properties of this heterostructure.
In this paper, we combined experimental characterizations and DFT calculations to provide a detailed evaluation of the MoS 2 /GaN interface structure and the strain, doping, and the optical emission properties of MoS 2 domains grown by CVD on GaN.X-ray photoelectron spectroscopy (XPS) displayed the formation of stochiometric MoS 2 ([S]/[Mo] ≈ 2) without the presence of Mo-oxide (MoO x ) components.Raman mapping showed that the MoS 2 domains mainly consisted of monolayers, with a small bilayer fraction, consistently with the intense light emission peak revealed by micro-photoluminescence (µ-PL) mapping.Furthermore, an average n-type doping of (0.1-0.2) × 10 13 cm −2 and a very low tensile strain of ~0.25% was evaluated by the correlative plot of the E' and A' 1 Raman peaks.The obtained strain was in perfect agreement with the one derived by the exciton peak positions obtained by µ-PL spectra.Cross-sectional scanning transmission electron microscopy (STEM) measurements confirmed both the monolayer MoS 2 thickness, the presence of a van der Waals (vdW) gap at the interface with GaN, and a modification of the topmost GaN layers with respect to the bulk crystal.Finally, we employed DFT calculations to better understand the structural and electronic properties of the interface between 1L of MoS 2 and GaN.In particular, three configurations of the GaN surface were considered within this heterostructure: (i) an ideal Ga-terminated GaN(0001) surface, (ii) the passivation of Ga terminations with a monolayer coverage of oxygen (O) atoms, and (iii) the presence of an ultrathin Ga 2 O 3 film on the GaN surface.The first two configurations resulted in a strong covalent bond at the interface, very different from the experimentally observed vdW interaction.On the other hand, the formation of a vdW gap of 3.05 Å and a significant n-type doping of 1L of MoS 2 was predicted in the presence of an ultrathin Ga 2 O 3 film at the MoS 2 /GaN interface, which is in close agreement with the experimental results.

Materials and Methods
The starting material for these experiments was an unintentionally doped GaN(0001) template grown on a c-sapphire substrate.The pristine GaN surface showed a low root mean square (RMS) roughness of ~0.3 nm, evaluated from the AFM image in Figure S1 of the Supplementary Materials.
MoS 2 was grown on a GaN/c-sapphire substrate by single step CVD at a temperature of 700 • C for 10 min at atmospheric pressure.The process was carried out in a quartz tube furnace with two-heating zones, the first employed for the evaporation of the sulfur powders (7-10 mg) at 150 • C, and the second for the evaporation of the MoO 3 powders (2-3 mg) at 700 • C. The GaN substrate was placed in the second zone of the furnace above the MoO 3 crucible.The reaction between the S vapors (transported by an Ar flux of 100 sccm) and MoO 3 vapors occurred in the gas phase close to the GaN surface, resulting in the nucleation and growth of MoS 2 domains.
The MoS 2 domain coverage on GaN was evaluated by scanning electron microscopy (SEM) using a ThermoFisher Scios 2 dual-beam microscope.X-ray photoelectron spectroscopy (XPS) analysis was carried out by using Escalab Xi+ equipment by Thermo Fisher (Waltham, MA, USA), with a monochromatic Al K X-ray source (energy = 1486.6eV).The spectra were collected at a take-off angle of 90 • relative to the sample surface and pass energy of 20 eV.The instrument resolution was 0.45 eV (FWHM of the Ag 3d5/2 peak).The spectra were aligned using C1s (285 eV) as a reference.
High-resolution transmission electron microscopy (HR-TEM) and high-angle annular dark field scanning transmission microscopy (HAADF-STEM) were carried out with an aberration-corrected Titans Themis 200 microscope by Thermo Fisher.For the crosssectional analysis, a focused ion beam (FIB) was used to prepare lamellas from the sample.
Micro-Raman (µ-Raman) spectra were acquired employing both WiTec Alpha equipment by WiTec (Ulhm, Germany) and a Horiba Raman system with a confocal microscope (100×) and with a laser excitation wavelength of 532 nm.The second Raman setup was also employed to acquire the micro-photoluminescence (µ-PL) spectra changing the grating from 1800 L/mm to 600 L/mm.In all the configurations, the laser power was filtered with a neutral density (ND) filter at 1%.
Calculations of the MoS 2 /GaN(0001) interface were performed within the density functional theory (DFT).We used the plane-wave Quantum Espresso code [60] with Hamada's van der Waals exchange-correlation functional [61] and standard solid-state pseudopotentials [62,63].The latter was based on the Perdew-Burke-Ernzerhof functional [64].To have a broader comparison with the experiment, monolayer MoS 2 interfaces with ideal Ga-terminated GaN surfaces and with oxidized GaN surfaces were considered.A slab model comprising 16 bilayers of Ga-N, whose bottom termination was passivated with hydrogen, was used to model the GaN substrate.On the top termination instead, the Ga-terminated GaN(0001) surface interacted with the MoS 2 layer.The quasi-commensurate lattice constants of MoS 2 with respect to the surface vectors of GaN(0001) allowed for the construction of an interface model with unit-cell periodicity.This (1 × 1) interface model resulted in a small tensile strain for the MoS 2 layer (1.7%), whereas the GaN substrate was unstrained.The plane-wave cut-off kinetic energy was set to 50 Ry and the augmented charge density cut-off was set to 400 Ry, respectively.A (12 × 12 × 1) Monkhorst-Pack grid [65] was used for the sampling of the Brillouin zone.To avoid spurious interactions between the periodic replicas of the system perpendicular to the interface, a vacuum space of 20 Å was inserted in the simulation supercell.

Results
The nucleation and growth of MoS 2 on the GaN surface was preliminarily investigated by SEM images collected immediately after the deposition of the samples on different areas.Figure 1 shows a representative image on a 6 µm × 6 µm area, which demonstrates a very dense coverage with MoS 2 domains (dark contrast).The insert of Figure 1 allows us to better distinguish the domain's size and coverage, with typical sizes ranging from 50 to 150 nm and an estimated surface coverage ~35%.Furthermore, the typical triangular shape of these domains can be deduced clearly by a plan view TEM image, as reported in Figure S2 of the Supplementary Materials.also employed to acquire the micro-photoluminescence (µ-PL) spectra changing the grating from 1800 l/mm to 600 l/mm.In all the configurations, the laser power was filtered with a neutral density (ND) filter at 1%.
Calculations of the MoS2/GaN(0001) interface were performed within the density functional theory (DFT).We used the plane-wave Quantum Espresso code [60] with Hamada's van der Waals exchange-correlation functional [61] and standard solid-state pseudopotentials [62,63].The latter was based on the Perdew-Burke-Ernzerhof functional [64].To have a broader comparison with the experiment, monolayer MoS2 interfaces with ideal Ga-terminated GaN surfaces and with oxidized GaN surfaces were considered.A slab model comprising 16 bilayers of Ga-N, whose bottom termination was passivated with hydrogen, was used to model the GaN substrate.On the top termination instead, the Ga-terminated GaN(0001) surface interacted with the MoS2 layer.The quasi-commensurate lattice constants of MoS2 with respect to the surface vectors of GaN(0001) allowed for the construction of an interface model with unit-cell periodicity.This (1 × 1) interface model resulted in a small tensile strain for the MoS2 layer (1.7%), whereas the GaN substrate was unstrained.The plane-wave cut-off kinetic energy was set to 50 Ry and the augmented charge density cut-off was set to 400 Ry, respectively.A (12 × 12 × 1) Monkhorst-Pack grid [65] was used for the sampling of the Brillouin zone.To avoid spurious interactions between the periodic replicas of the system perpendicular to the interface, a vacuum space of 20 Å was inserted in the simulation supercell.

Results
The nucleation and growth of MoS2 on the GaN surface was preliminarily investigated by SEM images collected immediately after the deposition of the samples on different areas.Figure 1 shows a representative image on a 6 µm × 6 µm area, which demonstrates a very dense coverage with MoS2 domains (dark contrast).The insert of Figure 1 allows us to better distinguish the domain's size and coverage, with typical sizes ranging from 50 to 150 nm and an estimated surface coverage ∼35%.Furthermore, the typical triangular shape of these domains can be deduced clearly by a plan view TEM image, as reported in Figure S2 of the Supporting Information.XPS analyses provided surface-sensitive chemical information on the stoichiometry of MoS 2 domains.A near-stoichiometric [S]/[Mo] ≈ 2 ratio was deduced by a preliminary elemental analysis.To obtain more detailed information on the Mo oxidation state and Mo-S bonding, the Mo 3d5/2, Mo 3d3/2, and S 2s core-level spectra (located at 229.3, 232.5, and 226.5 eV, respectively) were collected, as reported in Figure 2. In particular, the Mo 3d spectrum confirms that Mo atoms exhibit only the Mo4+ oxidation state associated with the 2H-MoS 2 [66], without any contribution at higher oxidation states correlated with the presence of MoO 3 [67].Subsequently, Raman spectroscopy was exploited to evaluate the vibrational features of the MoS2/GaN heterostructure.In particular, the characteristic Raman peaks of GaN, i.e., E2 low-high energy and A1(LO), and of MoS2, i.e., E2g and A1g, can be observed in the wavenumber range between 100 and 1000 cm −1 , as shown by the blue and red lines in Figure 3a.The very narrow and intense A1(LO) peak is consistent with the low n-type doping (~10 16 cm −3 ) of the GaN substrate [36].Focusing on the wavenumber region between 365 and 425 cm −1 reported in Figure 3b, a baseline subtraction and a normalization of the A1g peak were applied with the purpose of extrapolating detailed information on the crystal quality of the CVD-grown MoS2 flakes.Despite a low-medium A1g/E2g intensity ratio (~0.5), the two main Raman modes could be fitted by narrow single Gaussian peaks.In addition, the deconvolution analysis revealed the presence of a small LO(M) component near the E2g mode, associated with defects or with the domain boundaries [68].Subsequently, Raman spectroscopy was exploited to evaluate the vibrational features of the MoS 2 /GaN heterostructure.In particular, the characteristic Raman peaks of GaN, i.e., E 2 low-high energy and A 1 (LO), and of MoS 2 , i.e., E 2g and A 1g , can be observed in the wavenumber range between 100 and 1000 cm −1 , as shown by the blue and red lines in Figure 3a.The very narrow and intense A 1 (LO) peak is consistent with the low n-type doping (~10 16 cm −3 ) of the GaN substrate [36].Focusing on the wavenumber region between 365 and 425 cm −1 reported in Figure 3b, a baseline subtraction and a normalization of the A 1g peak were applied with the purpose of extrapolating detailed information on the crystal quality of the CVD-grown MoS 2 flakes.Despite a low-medium A 1g /E 2g intensity ratio (~0.5), the two main Raman modes could be fitted by narrow single Gaussian peaks.In addition, the deconvolution analysis revealed the presence of a small LO(M) component near the E 2g mode, associated with defects or with the domain boundaries [68].
To obtain statistically relevant information, a wide number of Raman spectra were collected in a 10 µm × 10 µm sample.From this array of spectra, we evaluated the homogeneity of the MoS 2 number of layers, by extracting the wavenumber difference of the A 1g and E 2g Raman modes (∆ω = ω A1g − ω E2g ), which is known to be dependent on MoS 2 thickness [69].In particular, the statistical distribution of the MoS 2 thickness was obtained from the ∆ω histogram in Figure 3c, which shows a mean value of ∆ω = 20.9cm −1 with a standard deviation of 0.9 cm −1 .This distribution shows that the MoS 2 mainly consisted of monolayers, with a small percentage of bilayers.
In addition to the thickness assessment, the A 1g and E 2g Raman peak positions provide information on the strain and doping of the thin MoS 2 domains, according to the procedure discussed in several recent papers [9,35,70,71].These doping and strain effects can be due to the CVD growth conditions and to the interaction with the GaN substrate.Figure 4a shows a correlative E 2g vs.A 1g plot, where the graph is separated in four quadrants by the intersection of the ideal strain (red) and doping (black) lines.The intersection point represented by the light blue square corresponds to the ideal (E 2g , A 1g ) Raman modes of unstrained and undoped monolayer MoS 2 .To this aim, the literature values of the (E 2g ,A 1g ) peak positions (ω E2g = 385 cm −1 ; ω A1g = 405 cm −1 ) for suspended MoS 2 flakes [5,72] were taken as the best approximation to this ideal condition, since the effects of the interaction with substrate are excluded in that case.To obtain statistically relevant information, a wide number of Raman spectra were collected in a 10 µm × 10 µm sample.From this array of spectra, we evaluated the homogeneity of the MoS2 number of layers, by extracting the wavenumber difference of the A1g and E2g Raman modes (Δω = ωA1g − ωE2g), which is known to be dependent on MoS2 thickness [69].In particular, the statistical distribution of the MoS2 thickness was obtained from the Δω histogram in Figure 3c, which shows a mean value of Δω = 20.9cm −1 with a standard deviation of 0.9 cm −1 .This distribution shows that the MoS2 mainly consisted of monolayers, with a small percentage of bilayers.
In addition to the thickness assessment, the A1g and E2g Raman peak positions provide information on the strain and doping of the thin MoS2 domains, according to the procedure discussed in several recent papers [9,35,70,71].These doping and strain effects can be due to the CVD growth conditions and to the interaction with the GaN substrate.Figure 4a shows a correlative E2g vs. A1g plot, where the graph is separated in four quadrants by the intersection of the ideal strain (red) and doping (black) lines.The intersection point represented by the light blue square corresponds to the ideal (E2g, A1g) Raman modes of unstrained and undoped monolayer MoS2.To this aim, the literature values of the (E2g,A1g) peak positions ( 385 cm ;  405 cm ) for suspended MoS2 flakes The red and black arrows indicate the directions of tensile strain and n-type doping regions, respectively, while the opposite side of the red and black lines correspond the compressive strain and p-type doping.The n-type doping region was indicated by the yellow area to better distinguish it from the p-type region (white) in the upper-side of the graph.The experimental values of the peak positions from the same array of Raman spectra used in Figure 3 are reported by the empty circles in the graph of Figure 4a.The corresponding histograms of the E 2g and A 1g peak values are also reported on the upperside and right-side of the graph (grey bars).In Figure 4a, the blue and green points correspond to the peaks' positions obtained in the 1L and 2L (or multilayer) regions, respectively, as determined in the histogram of Figure 3c.For 1L of MoS 2 , an average tensile strain of around 0.2% and light n-type doping (<0.1 × 10 13 cm −2 ) is deduced from the correlative plot in Figure 4a.A more precise evaluation was obtained by evaluating the strain and doping for each data point of 1L-MoS 2 and by building the histograms of the strain and doping distribution, as reported in Figure 4b,c.A tensile strain of 0.25 ± 0.10% and a n-type doping of (0.11 ± 0.12) × 10 13 cm −2 were deduced from the mean value and the standard deviation of these two distributions.Notably, nearly unstrained monolayer MoS 2 on GaN has been recently reported also under different CVD growth conditions, resulting in the formation of micrometer size triangular domains [42] or continuous monolayer MoS 2 films [36,42].These observations confirm the key role played by the low mismatch of the in-plane lattice constants (<1%) and of thermal expansion coefficients between MoS 2 and GaN.Furthermore, the low n-type doping is consistent with the typically reported unintentional n-type doping of MoS 2 obtained by exfoliation from bulk crystals, probably induced by the presence of native defects (such as sulfur vacancies) [73,74].On the other hand, nor p-type doping behavior has been reported for MoS 2 grown by CVD approaches, depending on several factors, such as the content of MoO 3 residues in the films (typically responsible for p-type doping) or the peculiar interaction with the underlying substrate.In this regard, the average n-type doping of the CVD-grown MoS 2 on GaN in the present work is consistent with the absence of MoO 3 residues, as indicated by XPS analyses in Figure 2.
Nanomaterials 2024, 14, x FOR PEER REVIEW 7 of 15 [5,72] were taken as the best approximation to this ideal condition, since the effects of the interaction with substrate are excluded in that case.The red and black arrows indicate the directions of tensile strain and n-type doping regions, respectively, while the opposite side of the red and black lines correspond the compressive strain and p-type doping.The n-type doping region was indicated by the yellow area to better distinguish it from the p-type region (white) in the upper-side of the graph.The experimental values of the peak positions from the same array of Raman spectra used in Figure 3 are reported by the empty circles in the graph of Figure 4a.The corresponding histograms of the E2g and A1g peak values are also reported on the upper-side and right-side of the graph (grey bars).In Figure 4a, the blue and green points correspond to the peaks' positions obtained in the 1L and 2L (or multilayer) regions, respectively, as determined in the histogram of Figure 3c.For 1L of MoS2, an average tensile strain of around 0.2% and light n-type doping (<0.1 × 10 13 cm −2 ) is deduced from the correlative plot in Figure 4a.A more precise evaluation was obtained by evaluating the strain and doping for each data point of 1L-MoS2 and by building the histograms of the strain and doping distribution, as reported in Figure 4b,c.A tensile strain of 0.25 ± 0.10% and a n-type doping of (0.11 ± 0.12) × 10 13 cm −2 were deduced from the mean value and the standard deviation of these two distributions.Notably, nearly unstrained monolayer MoS2 on GaN has been recently reported also under different CVD growth conditions, resulting in the formation of micrometer size triangular domains [42] or continuous monolayer MoS2 films [36,42].These observations confirm the key role played by the low mismatch of the in-plane lattice constants (<1%) and of thermal expansion coefficients between MoS2 and GaN.Furthermore, the low n-type doping is consistent with the typically reported unintentional n-type doping of MoS2 obtained by exfoliation from bulk crystals, probably induced by the presence of native defects (such as sulfur vacancies) [73,74].On the other hand, n-or p-type doping behavior has been reported for MoS2 grown by CVD approaches, depending on several factors, such as the content of MoO3 residues in the films (typically responsible for Figure 5a shows a representative PL spectrum of the MoS 2 /GaN obtained with a laser excitation wavelength of 532 nm.The intense PL emission is a further confirmation of the good MoS 2 crystal quality achieved by CVD.In fact, a high density of defects in MoS 2 films would involve non-radiative recombination of excitons, causing PL quenching [75,76].In detail, a deconvolution analysis performed on the spectrum of Figure 5a revealed the coexistence of three components.The A 0 and B peaks located at 1.87 eV and 1.91 eV correspond to the excitonic emissions due to the spin-orbit coupling splitting of the MoS2 valence band [77].Differently, the red-area convoluted peak at lower energy (1.79 eV) is related to the trion (also known as charged exciton) contribution, consisting of the bound In detail, a deconvolution analysis performed on the spectrum of Figure 5a revealed the coexistence of three components.The A 0 and B peaks located at 1.87 eV and 1.91 eV correspond to the excitonic emissions due to the spin-orbit coupling splitting of the MoS 2 valence band [77].Differently, the red-area convoluted peak at lower energy (1.79 eV) is related to the trion (also known as charged exciton) contribution, consisting of the bound state between an electron (or hole) and an exciton [78,79].This deconvolution analysis confirms the absence of the defect-related peak X D , typically located at lower energy with respect to the trionic component.After a statistical analysis of different MoS 2 /GaN areas, we built a histogram of the excitonic peak energy A 0 , as reported in Figure 5b.This distribution showed a standard deviation of 10 meV around a mean value of 1.87 eV, indicating a spatially uniform PL emission from the sample surface.The energy of the main PL peak (A 0 ) has been shown in the literature to be dependent on the strain of MoS 2 , with a red shift of the peak at increasing strain with a rate of −99 meV/% [72].By applying this linear relation, the values of the tensile strain were calculated from the experimental values of the A 0 peak energy, as reported in Figure 5c.From this analysis, strain values in the range between 0.08 and 0.3% were deduced, with a mean value of ε = 0.19 ± 0.05%, in good agreement with the previous estimation by Raman measurements.
The interface properties of the 2D/3D vdW heterostructure were characterized by cross-sectional transmission electron microscopy analyses.Figure 6a is an overview HR-TEM image, showing a monolayer MoS 2 conformal to the crystalline GaN substrate, similarly to other reports for MoS 2 grown by CVD approaches on GaN or other crystalline hexagonal substrates [36,37,80,81].Furthermore, the presence of a vdW gap between the single layer of MoS 2 and GaN surface is clearly demonstrated by the high-resolution HAADF-STEM image in Figure 6b.This is a direct indication of a weak bond between MoS 2 and the underlying GaN crystal.Notably, this high-resolution STEM analysis reveals a different structure of the first crystalline planes of GaN with respect to the underlying bulk crystal.As reported in previous structural investigations of CVD MoS 2 /GaN heterostructures [37], such differences can be attributed to partial oxidation of the GaN surface during the MoS 2 growth process or some form of surface reconstruction.In the last section of this paper, DFT calculations have been performed to obtain a deeper physical understanding of the type of interaction and electronic properties of the MoS2/GaN interface.In this context, it is worth mentioning that DFT calculations of this kind of heterostructure have been recently reported in the literature [57,59].A S-Ga equilibrium distance of 0.232 nm in Ref. [57] and 0.237 nm in Ref. [59] was evaluated for the ideal case of a lattice-matched interface between MoS2 and Ga-terminated GaN, indicating the formation of a covalent-like bond at the interface.Clearly, those calculation results do not match with the results of atomic resolution TEM analyses of the MoS2/GaN heterostructure obtained in the present work and with those recently reported by different re- In the last section of this paper, DFT calculations have been performed to obtain a deeper physical understanding of the type of interaction and electronic properties of the MoS 2 /GaN interface.In this context, it is worth mentioning that DFT calculations of this kind of heterostructure have been recently reported in the literature [57,59].A S-Ga equilibrium distance of 0.232 nm in Ref. [57] and 0.237 nm in Ref. [59] was evaluated for the ideal case of a lattice-matched interface between MoS 2 and Ga-terminated GaN, indicating the formation of a covalent-like bond at the interface.Clearly, those calculation results do not match with the results of atomic resolution TEM analyses of the MoS 2 /GaN heterostructure obtained in the present work and with those recently reported by different research groups [37,47], which showed the presence of a larger vdW gap separating S from Ga atoms.
As a matter of fact, under real experimental conditions, the GaN(0001) surface can be subjected to reconstructions or to oxidations.Hence, to provide a more complete description of the MoS 2 /GaN system, we performed DFT calculations of the heterostructure considering three different model configurations of the GaN surface (see Figure S3 of the Supplementary Materials): (i) the ideal Ga-terminated GaN, analogous to the one reported in the literature; (ii) the passivation of the Ga termination with an oxygen coverage of one monolayer; and (iii) the formation of an ultra-thin crystalline Ga 2 O 3 oxide.The analysis of the DFT predictions for these three configurations has been compared with the experimental results for our system.
Figure 7a shows the most stable configuration obtained by DFT calculations of the ideal Ga-terminated GaN surface, where a Ga-S equilibrium distance of 0.241 nm was estimated, in close agreement with recent literature reports [57,59].Furthermore, the calculated band structure for this system (reported in Figure 7b) shows a high n-type doping of MoS 2 and strong perturbation of its energy bands.As a matter of fact, this ideal configuration of the MoS 2 /GaN(0001) interface does not provide a real representation of the system.For this reason, we performed DFT calculations considering the presence of O atoms bonded to the GaN(0001) surface.Figure 8a,b shows the results for oxygen-passivated Ga terminations with one monolayer O surface coverage.Also in this case, a covalent interface interaction was obtained, which again deviates from the experimental evidence of a weak van der Waals bonding.The theoretically calculated strong interface coupling had a structural impact only on the topmost Ga layer of the substrate and perturbed the MoS2 bands with respect to those of a freestanding MoS2 layer (see Figure S4 of the Supporting Information).Figure 8a,b shows the results for oxygen-passivated Ga terminations with one monolayer O surface coverage.Also in this case, a covalent interface interaction was obtained, which again deviates from the experimental evidence of a weak van der Waals bonding.The theoretically calculated strong interface coupling had a structural impact only on the topmost Ga layer of the substrate and perturbed the MoS 2 bands with respect to those of a freestanding MoS 2 layer (see Figure S4 of the Supplementary Materials).
We thereon considered the formation of an ultra-thin layer of surface native oxide Ga 2 O 3 , which significantly reduces the surface energy of GaN(0001) as compared to other oxidized reconstructions [82].This layer is characterized by an O−Ga−O trilayer which inverts the polarization of the GaN layer along the [0001] direction, followed by a Ga−O bilayer that terminates the oxidized surface (Figure 9a).The interaction of this Ga 2 O 3 -terminated GaN surface with MoS 2 gave rise to a van der Waals interface with an oxygensulfur interface distance of 3.05 Å.This distance is significantly larger than the one reported in the literature for the ideal MoS 2 /GaN(0001) system [57,59] and it is in better agreement with the experimental observations of a vdW gap at the interface.Figure 9b-e shows the total and partial electronic contributions (Mo d and Ga s orbitals) in the density of states of the heterosystem, plotted at the close Γ-M-K-Γ path of the Brillouin zone.The pristine bands of MoS 2 are clearly preserved in this case, showing a direct band gap of 1.7 eV at the K point of the Brillouin zone.We note that such an interface induces a significant n-type doping for the MoS 2 sheet, due to a shift of surface Ga s states deriving from the oxide layer towards lower energies (because of Ga-O bonding).Such a shift brings the Fermi level of the system close to the conduction band of the MoS 2 layer.Overall, the theoretical calculations indicate that a van der Waals interface at the MoS 2 /GaN(0001) heterosystem is expected when an ultra-thin Ga 2 O 3 native oxide forms at the substrate's surface, whereas it is rather improbable for low surface oxygen coverages.Figure 8a,b shows the results for oxygen-passivated Ga terminations with one monolayer O surface coverage.Also in this case, a covalent interface interaction was obtained, which again deviates from the experimental evidence of a weak van der Waals bonding.The theoretically calculated strong interface coupling had a structural impact only on the topmost Ga layer of the substrate and perturbed the MoS2 bands with respect to those of a freestanding MoS2 layer (see Figure S4 of the Supporting Information).We thereon considered the formation of an ultra-thin layer of surface native oxide Ga2O3, which significantly reduces the surface energy of GaN(0001) as compared to other oxidized reconstructions [82].This layer is characterized by an O−Ga−O trilayer which inverts the polarization of the GaN layer along the [0001] direction, followed by a Ga−O bilayer that terminates the oxidized surface (Figure 9a).The interaction of this Ga2O3-terminated GaN surface with MoS2 gave rise to a van der Waals interface with an oxygensulfur interface distance of 3.05 Å.This distance is significantly larger than the one reported in the literature for the ideal MoS2/GaN(0001) system [57,59] and it is in better

Conclusions
The structural, chemical, vibrational, and light emission properties of the CVDgrown MoS2 heterostructure with GaN have been investigated in detail by several microscopic and spectroscopic techniques and by DFT calculations.XPS analyses on as-grown samples showed the formation of stoichiometric MoS2.According to micro-Raman spectroscopy, monolayer MoS2 domains on GaN exhibit an average n-type doping of (0.11 ±

Figure 1 .
Figure 1.Overview SEM image and magnification (insert) of CVD-grown MoS2 domains on GaN.XPS analyses provided surface-sensitive chemical information on the stoichiometry of MoS2 domains.A near-stoichiometric [S]/[Mo] ≈ 2 ratio was deduced by a preliminary

Figure 1 .
Figure 1.Overview SEM image and magnification (insert) of CVD-grown MoS 2 domains on GaN.

Nanomaterials 2024 ,
14,  x FOR PEER REVIEW 5 of 15 elemental analysis.To obtain more detailed information on the Mo oxidation state and Mo-S bonding, the Mo 3d5/2, Mo 3d3/2, and S 2s core-level spectra (located at 229.3, 232.5, and 226.5 eV, respectively) were collected, as reported in Figure2.In particular, the Mo 3d spectrum confirms that Mo atoms exhibit only the Mo4+ oxidation state associated with the 2H-MoS2[66], without any contribution at higher oxidation states correlated with the presence of MoO3[67].

Figure 2 .
Figure 2. XPS spectra of Mo 3d and S 2s core levels were collected on the as-grown MoS2 on GaN.The binding energies of the S 2s peak (226.5 eV), Mo3d3/2 (232.5 eV), and Mo3d5/2 (229.3 eV) peaks associated with Mo atoms with Mo 4+ oxidation are indicated.

Figure 2 .
Figure 2. XPS spectra of Mo 3d and S 2s core levels were collected on the as-grown MoS 2 on GaN.The binding energies of the S 2s peak (226.5 eV), Mo3d 3/2 (232.5 eV), and Mo3d 5/2 (229.3 eV) peaks associated with Mo atoms with Mo 4+ oxidation are indicated.

Nanomaterials 2024 , 15 Figure 3 .
Figure 3. (a) Raman spectrum of the 2D/3D van der Waals heterostructures, showing the GaN and MoS2 vibrational modes by blue and red lines, respectively.(b) Detail on the MoS2 peaks after a deconvolution analysis, which pointed out the presence of a further LO(M) peak at lower wavenumbers.(c) Histogram of the difference between the two peaks (Δω = ωA1g − ωE2g).

Figure 3 .
Figure 3. (a) Raman spectrum of the 2D/3D van der Waals heterostructures, showing the GaN and MoS 2 vibrational modes by blue and red lines, respectively.(b) Detail on the MoS 2 peaks after a deconvolution analysis, which pointed out the presence of a further LO(M) peak at lower wavenumbers.(c) Histogram of the difference between the two peaks (∆ω = ω A1g − ω E2g ).

Figure 4 .
Figure 4. (a) Correlative E2g vs. A1g plot to evaluate the strain and doping effects induced on the MoS2 flakes by the growth conditions and the interaction with GaN substrate.Blue and green points in panel (a) correspond to the peak positions obtained in the 1L and multilayers regions, respectively.Evaluated strain (b,c) doping distributions for 1L of MoS2.

Figure 4 .
Figure 4. (a) Correlative E 2g vs.A 1g plot to evaluate the strain and doping effects induced on the MoS 2 flakes by the growth conditions and the interaction with GaN substrate.Blue and green points in panel (a) correspond to the peak positions obtained in the 1L and multilayers regions, respectively.Evaluated strain (b,c) doping distributions for 1L of MoS 2 .

Nanomaterials 2024 ,
14, x FOR PEER REVIEW 8 of 15good MoS2 crystal quality achieved by CVD.In fact, a high density of defects in MoS2 films would involve non-radiative recombination of excitons, causing PL quenching[75,76].

Figure 5 .
Figure 5. (a) Photoluminescence spectrum of MoS2 on GaN, where the two excitons (A 0 and B) and trion (A − ) components were extracted after deconvolution analysis.(b) Distribution of the A 0 exciton peak positions evaluated at different points of the sample.(c) A 0 peak positions as a function of the tensile strain.

Figure 5 .
Figure 5. (a) Photoluminescence spectrum of MoS 2 on GaN, where the two excitons (A 0 and B) and trion (A − ) components were extracted after deconvolution analysis.(b) Distribution of the A 0 exciton peak positions evaluated at different points of the sample.(c) A 0 peak positions as a function of the tensile strain.

Figure 6 .
Figure 6.(a) Overview the HR-TEM image showing 1L of MoS2 conformal to the (0001) basal plane of GaN.(b) Atomic resolution STEM image showing the presence of a van der Waals gap between 1L of MoS2 and the underlying GaN surface.

Figure 6 .
Figure 6.(a) Overview the HR-TEM image showing 1L of MoS 2 conformal to the (0001) basal plane of GaN.(b) Atomic resolution STEM image showing the presence of a van der Waals gap between 1L MoS 2 and the underlying GaN surface.

15 Figure 7 .
Figure 7. (a) Structure of the interface between Ga-terminated GaN and monolayer MoS2 seen from the (112 0) plane and (b) the energy band structure of this heterostructure.EF refers to the calculated Fermi level of the system.

Figure 7 .
Figure 7. (a) Structure of the interface between Ga-terminated GaN and monolayer MoS 2 seen from the (1120) plane and (b) the energy band structure of this heterostructure.E F refers to the calculated Fermi level of the system.

Figure 7 .
Figure 7. (a) Structure of the interface between Ga-terminated GaN and monolayer MoS2 seen from the (112 0) plane and (b) the energy band structure of this heterostructure.EF refers to the calculated Fermi level of the system.

Figure 8 .
Figure 8.(a) Structure of the MoS2/GaN interface with oxygen passivation of Ga terminations with monolayer O surface coverage, seen from the (11 00) plane.(b) Electronic contributions of Mo d orbitals in the wavevector-resolved projected density of states along the Γ-M-K-Γ Brillouin zone path.EF refers to the calculated Fermi level of the system.

Figure 8 .
Figure 8.(a) Structure of the MoS 2 /GaN interface with oxygen passivation of Ga terminations with monolayer O surface coverage, seen from the (1100) plane.(b) Electronic contributions of Mo d orbitals in the wavevector-resolved projected density of states along the Γ-M-K-Γ Brillouin zone path.E F refers to the calculated Fermi level of the system.

Figure 9 .
Figure 9. (a) Structure of the oxidized MoS2/GaN(0001) interface, showing the formation of an ultrathin layer of native oxide Ga2O3, seen from the (112 0) plane.Wavevector-resolved projected density of states along the Γ-M-K-Γ Brillouin zone path for (b) all electronic states, (c) contributions of Mo d orbitals, (d) contributions of surface Ga s orbitals, and (e) contributions of bulk Ga s orbitals.EF refers to the calculated Fermi level of the system.

Figure 9 .
Figure 9. (a) Structure of the oxidized MoS 2 /GaN(0001) interface, showing the formation of an ultra-thin layer of native oxide Ga 2 O 3 , seen from the (1120) plane.Wavevector-resolved projected density of states along the Γ-M-K-Γ Brillouin zone path for (b) all electronic states, (c) contributions of Mo d orbitals, (d) contributions of surface Ga s orbitals, and (e) contributions of bulk Ga s orbitals.E F refers to the calculated Fermi level of the system.