Wafer-scale MoS2 with water-vapor assisted showerhead MOCVD

Among numerous thin film synthesis methods, metalorganic chemical vapor deposition performed in a showerhead reactor is the most promising one for broad use in scalable and commercially adaptable two-dimensional material synthesis processes. Adapting the most efficient monolayer growth methodologies from tube-furnace systems to vertical-showerhead geometries allows us to overcome the intrinsic process limitations and improve the overall monolayer yield quality. Here, we demonstrate large-area, monolayer molybdenum disulphide growth by combining gas-phase precursor supply with unique tube-furnace approaches of utilizing sodium molybdate pre-seeding solution spincoated on a substrate along with water vapor added during the growth step. The engineered process yields a high-quality, 4-inch scale monolayer film on sapphire wafers. The monolayer growth coverage, average crystal size and defect density were evaluated using Raman and photoluminescence spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and scanning transmission electron microscopy imaging. Our findings provide a direct step forward toward developing a reproducible and large-scale MoS2 synthesis with commercial showerhead reactors.


Introduction
From the broad family of atomically thin, two-dimensional (2D) materials, molybdenum disulphide (MoS 2 ) has been found particularly interesting in recent years. Being a member of transition metal dichalcogenides (TMDs), 1 the unique electronic 2,3 and optical properties 4 combined with high mechanical exibility 5,6 of MoS 2 make it an attractive candidate for a potential new generation of wearable devices, 7 exible sensors [8][9][10] or nanoelectronics 2,11 as well as heterostructure research and development. [12][13][14][15][16][17][18][19][20] Therefore, aer years of extensive study, the shi of academic focus from small, lab-scale synthesis methods towards prototype applications and scalable processes is inevitable. [21][22][23] One of the most vigorously investigated aspects of scaling up the research and application of MoS 2 is large-scale, uniform and high quality material synthesis. [24][25][26][27][28][29][30][31][32] Over the years, numerous efforts to understand, optimize and grow large-area 2D lms have been reported. 22,33 The most researched and versatile approaches are chemical vapor deposition (CVD) 25,30,31,34 and metalorganic chemical vapor deposition (MOCVD) processes, 24,32,35,36 both allowing atomic thickness-controlled growth. While CVD is considered the most common and straightforward method due to the use of commonly available powder precursors of sulfur and molybdenum, its metalorganic MOCVD variant is considered to be crucial to achieve the best results and highest reported material quality. 24 The most important feature of MOCVD is its high level of repeatability due to the use of gaseous precursors and wide array of tunable process parameters. Recent research showed the potential of MOCVD to obtain high quality monolayer TMD lms using hexacarbonyls (e.g. molybdenum hexacarbonyl -Mo(CO) 6 ) as a metal source 24,32,35,37 and either diethyl, 35,36 dimethyl 28 or chalcogenide hydrides as the chalcogen precursor (e.g. hydrogen disulphide H 2 S 24,37 ). This, combined with a thorough study of the precursor reactions and thermodynamics, gaseous parameters 38,39 and added growth promoters using liquid salt solutions 28,37,40 proved to be the key to obtaining superior material properties, large crystal size and uniform substrate coverage on large scale substrates.
However, the most promising results in the TMD synthesis eld are obtained using the hot-wall, tube furnace geometry. Although a robust and relatively cheap method of 2D material synthesis, tube furnaces do have a signicant size limitation prohibiting further fabrication upscaling and industrial adaptation of monolayer MoS 2 . Promising studies using much more versatile showerhead systems, typically used in the modern semiconductor's industry 41 for repeatable, large-scale manufacturing, have demonstrated the capability of MoS 2 and other TMD synthesis on a 4-inch wafer scale. Atomic layer deposition (ALD), 42,43 molecular beam epitaxy (MBE) 44,45 and MOCVD approaches 32,46-48 to synthesize TMD monolayers were reported. The material made in such processes however lacks the qualities of tube-furnace monolayers. Due to different furnace geometry (involving a laminar gas ow, shorter distance between the gas inlet and substrate, and heating directly at the substrate susceptor) the most successful tube-furnace methods have not been directly applied with showerhead systems. Instead, thin lms reported to date with showerhead reactors show much smaller domain size (from tens of nanometers 32,42,43,46,47 to tens of micrometers 48 ) as compared to their tube furnace counterparts, which signicantly impact the material properties. The high-temperature decomposition products of organic gaseous precursors tend to contaminate the material with carbonaceous impurities 49 and might lead to growth limiting or suppressing phenomena, 38,39 and thus need to be precisely controlled. On top of that, a typical process suffers from long deposition times, oen reaching tens of hours which makes the grown material expensive to obtain.
In this study we revise and compare the state-of-the-art tubefurnace MOCVD methods of large-scale monolayer MoS 2 synthesis: the all-gas-phase process, the use of salt solutions as growth and seeding promoters and the addition of water-vapor are adapted to the showerhead reactor geometry. We optimize these methods and demonstrate for the rst time their viability and practical use in an industrial-grade showerhead system. We further engineer the most efficient, combined showerhead MOCVD synthesis approach to achieve uniform, continuous wafer-scale monolayer MoS 2 with the largest reported grain sizes to date (>100 mm). Finally, we show and discuss how the material quality, uniformity and coverage can be dramatically improved by adding water vapor to the process gas mixture to eradicate growth-limiting factors such as carbonaceous and organic gas decomposition products. The results are obtained in an industrial-grade MOCVD system and are supported by systematic Raman and photoluminescence (PL) analysis as well as X-ray photoelectron spectroscopy (XPS) and defect density calculations based on a scanning transmission electron microscope (STEM) study. This work provides a direct methodology to widely adapt large-area, monolayer MoS 2 synthesis to industrial showerhead systems.

Experimental results and discussion
All experiments were performed on a commercial Aixtron BM Novo MOCVD system. The coldwall, showerhead reactor is designed to separate the chalcogen and metal precursor sources preventing the premature occurrence of the MOVCD reactions, before reaching the substrate surface (Fig. 1A) minimizing potential unwanted processes and reactor contamination. Such an adaptation is vital for a controlled and clean process reaction since the decomposition temperature of the gaseous molybdenum precursor used here (Mo(CO) 6 ) is relatively low (250 C (ref. 50)) and may lead to gas lines and showerhead contamination. All MOCVD growth variations are schematically shown in Fig. 1B-E and are discussed in detail in the following sections. All growth experiments are performed under constant parameters of 960 C, a pressure of 830 mbar, a growth step time of 150 min and by using a 4-inch c-cut sapphire wafer as a growth substrate. Argon is used as an ambient gas and precursor carrier, whereas H 2 is used as an active gas and carbonyl reduction agent. Flow parameters are detailed in Fig. S1 † and in the Methods section.

MoS 2 growth with gas-phase precursors
The rst investigated MOCVD process pathway for MoS 2 growth is gas-phase synthesis (Fig. 1B). It is considered as a benchmark process as it does exploit all tube-furnace method adaptations discussed further. During gas-phase MOVCD growth, all of the precursors are fed into the reactor chamber through the showerhead along with the carrier gas (argon) and additional process gases (hydrogen) necessary for efficient synthesis of the monolayer lm. Although a fundamentally scalable and clean synthesis approach, achieving large lateral MoS 2 domains in the showerhead system is challenging due to the large nucleation density and low reaction rate. 51 The typical crystal size reported in the literature reaches up to 100 nm. 32,43,46 In addition, only material thickness (and not single grain size) scales with the process time. In our optimized method, we have surpassed that limit reaching a size of 500 nm of single crystal monolayer material ( Fig. 2A and B) with gas-phase precursors only. It is evident that the crystal size is limited by the reaction byproducts 24,38,39 causing high temperature etching effects as well as high nucleation rate prohibiting the lateral crystal growth. The average substrate surface is thus composed of large crystals mixed with much smaller MoS 2 nuclei, constantly formed and etched during the process (Fig. 2B) by gaseous products of the Mo(CO) 6 /H 2 S/H 2 chemical reaction. 46 On top of that, since the process requires a steady ratio of Mo : S along the entire substrate area, even the smallest aws or imbalances in the gas feed or showerhead design may cause nonuniform growth conditions leading to uneven crystallization and local growth anomalies. Therefore, achieving stable coverage and substratewide uniformity, using this approach, is still challenging and far from the results reported in hotwall MOCVD systems. 24

MoS 2 growth with coated lm sulfurization
Another way of controlling the precursor coverage is to use a liquid source of molybdenum. In this approach, we are spincoating the substrate with sodium molybdate (Na 2 MoO 4 ) and sodium chloride (NaCl) water solution (Fig. 1C). Such a combination allows pre-seeding 28,35 of the surface with a controlled amount of molybdenum and uniformly coating it with NaCl, widely used in tube furnace MOCVD MoS 2 methods for increasing the reaction rate and decreasing the nucleation density leading to large single crystal formation. 28,35 The coated wafer is then sulfurized under a H 2 S gas ow without an external, gaseous source of molybdenum. The material tends to form large single crystals and continuous lms, the thickness and coverage of which depend on the spincoated mixture concentration, which controls the local Mo saturation during growth. The Na 2 MoO 4 /NaCl sulfurization approach was demonstrated to be efficient for continuous monolayer growth on 1-3 inch substrates; 35,36 however the use of the alkali salt solution heavily impacts the nal material quality. Intermediary  NaCl + MoS 2 reaction products 52 lead to a fast crystal growth rate but also have been found to remain underneath MoS 2 lms. This phenomenon, driven by excessive use of NaCl, negatively impacts the epitaxial connection between the crystal and sapphire substrate and may lead to grain boundary defects and uneven grain stitching inherently deteriorating large-area continuous lm cohesion and its electronic parameters. 52 To avoid this unwanted effect, we have limited the Na 2 MoO 4 /NaCl molar concentration to 0.03 M and 0.1 M respectively. This allowed the preservation of the MoS 2 /sapphire epitaxy and maintenance of high material coverage. In our coldwall, showerhead regime, we note lateral crystal size varying from 0.7 up to 3-4 mm (Fig. 2C and D) with sporadic events of >100 mm single crystals forming continuous lms locally. This growth method has an intrinsic self-limiting character due to a xed amount of Mo coated on the substrate. Tuning the precursor solution molar concentration allows the control over the coverage and nucleation density; however uniformity is problematic on large area substrates. The uneven lm thickness inherent to the spincoating process (especially emphasized in large-scale growth areas) leads to different local Mo : S concentrations during the process. Such a coverage issue is manifested in bilayer, multilayer or blank areas with the presence of unreacted, bulk molybdenum oxide particles on different substrate areas. The sulfurization of the spincoated Na 2 MoO 4 /NaCl lm is therefore not perfectly adjustable in the case of 4-inch substrates and requires careful molar concentration to balance between preserving epitaxy and promoting continuous lm growth to be useable.

MoS 2 growth with a hybrid process
Increasing the control over the material's coverage, crystal density and uniformity even further requires netuning the precursor ux i.e. by combining two precursor sources: adding a gas phase Mo(CO) 6 ow to the process with a spincoated mixture of Na 2 MoO 4 /NaCl (Fig. 1D) on the substrate. The combined, hybrid method in principle allows the compensation of individual limitations of small grain size present in the gas phase method and self-limiting Mo supply of the spincoated lm sulfurization approach. In this case, we consider Na 2 MoO 4 / NaCl as a Mo seeding/catalytic agent, leading to the formation of molybdenum oxychlorides, oxysuldes and suboxides 28 kickstarting and promoting the lm growth under the constant supply of Mo from the Mo(CO) 6 gas phase. Indeed, in our experiments the hybrid method yields single crystals of 3 mm average lateral size ( Fig. 2E and F) with the largest exceeding 100 mm locally. Adding Mo(CO) 6 however reintroduces the carbonyl reaction byproducts from the Mo(CO) 6 + H 2 S + H 2 reaction. Further optimization of the process (to improve the single crystal size, uniformity and monolayer coverage) does not intuitively scale with the Mo(CO) 6 nor Na 2 MoO 4 /NaCl concentration during the growth. The local imbalances of Na 2 MoO 4 and Mo(CO) 6 lead to multilayer island formation. We have found that reducing the molar concentration of Na 2 MoO 4 to as low as 0.005 M is enough to effectively pre-seed and nucleate Mo 2 and crucial to minimize the multilayer growth under a constant Mo(CO) 6 mass ux. As discussed later, in our experiments volatile carbonaceous reaction products (CO 2 and CH 4 , C) do not seem to dope MoS 2 nor contaminate the substrate; however they do contaminate the showerhead and MOCVD reactor heavily, signicantly suppressing the growth process by releasing carbonaceous products into the gas phase growth environment limiting the reproducibility. Thus, further increase of MoCO 6 concentration is unwelcome and counterproductive for obtaining high-quality, large-area lms. The scalability of the hybrid approach offers more process parameters and better coverage control; however it still requires signicant optimization to be viable on an industrial scale.

Water-assisted large grain growth
The presence of water vapor during the MOCVD reaction is widely considered as unwanted. Uncontrollable moisture can lead to a signicant decrease of material quality due to waterinduced oxidation and reduction during the growth process. 53 However, as was previously reported in tube-furnace, small scale processes, controlled addition of a small amount of water vapor was found to be benecial due to low oxidizing effects, which assist in the removal of volatile carbonaceous reaction products 48,54 and assists in the reduction of molybdenum oxides to suboxides and hydrogen promoting the lateral growth (formulas (1)- (3)) The use of oxidizing materials such as water vapor in an industrial-grade showerhead reactor is in general signicant safety concern. However, by maintaining proper safety precautions and source separation we have achieved a water-assisted method combining metalorganic gas supply and promoter lm coating (Fig. 1E). We found that a ow of 5 sccm of argon through a room temperature H 2 O lled bubbler is sufficient to signicantly remove impurities and promote monolayer crystals to reach an average lateral size of 25-50 mm (Fig. 2G and H) with the largest locally exceeding 100 mm (Fig. 3) and with a signicant increase in continuous lm coverage with further increase in the water vapor ow. The monolayer coverage peaked at a H 2 O vapor ow of 20 sccm (Fig. 3D and E) and its continuity was maintained across a 4-inch substrate area (Fig. 3D) with the exception of the substrate edge (see Fig. S2 †). We attribute this dramatic improvement of the crystal size and monolayer coverage to the self-regulating effect of H 2 O, similar to the growth-etch approach demonstrated in tungsten disulphide (WS 2 ) pulsed MOCVD process. 48 Due to gas-ow imbalances coming from subtle showerhead and gas-line imperfections as well as coated lm non-uniformity (amplied by the large-area substrate), different Mo(CO) 6 + H 2 S + H 2 reaction pathways may occur locally leading to the formation of volatile CH 4 , C, CO, CO 2 , COS, and CS 2 species in different concentrations. 38,46 Water vapor may react with and suppress the formation of excess carbon oxides and suldes negating the effects of local differences in reaction paths and their negative impact on MoS 2 grain lateral growth leading to overall better coverage as shown in Fig. 3D and E.

Spectroscopic characterization
Detailed material characterization was performed to compare the uniformity and crystallinity of MoS 2 synthesized by each growth approach. The analysis of the shape of characteristic E 2g and A 1g Raman peaks from monolayer areas from each method (Fig. S3A †) shows D E 2g -A 1g to be 20.9, 20.8 and 19.5 for sulfurization, hybrid and H 2 O respectively. The gas-phase growth method proved to be difficult to analyze due to the small grain size and low Raman signal. The full width at half maximum (FWHM) of E 2g and A 1g of each method is shown in Fig. S3B † and it was found to be narrowest for the H 2 O assisted approach with an E 2g and A 1g of 2.6 and 3.4, indicating higher crystallinity and good uniformity. In the case of all substrates, spectra taken from the 1000-1700 cm À1 range showed no distinguishable carbon peaks (Fig. S4 †), which suggests that all carbonaceous reaction products remain in the gas phase and do not deposit on the substrate. Raman spectra taken radially from 50 spots on each wafer conrm the self-regulating effect of H 2 O (Fig. 4A). The separation between E 1 2 g and A 1g peaks smaller than 20 cm À1 is regarded as a proof of a high-quality monolayer 55 lm. The positions of Raman peaks in the case of water-assisted growth remain constant which indicates uniformity in thickness and strain compared with other growth methods. Water vapor can promote further oxidation of thick particles from residual Mo oxides and etching of emerging multilayers during the growth process due to local H 2 generation (as presented in formulas (1)- (3)). The result is the formation of a continuous monolayer lm without thicker, seed particles on a wide area of the 4-inch substrate. Panel 4B shows the PL spectra of MoS 2 synthesized with sulfurization (red), hybrid (green) and H 2 O assisted (blue) methods taken from the monolayer MoS 2 sites from each substrate. Signicantly higher PL emission from water assisted growth indicates that water improves crystal quality by removing carbon contamination (which is known to quench PL 54 ) and enhances the efficiency of precursor ligand-reduction. The increase in PL intensity signies a lower impact of grain boundary defects and uneven grain stitching. 56 In this case, the obtained Raman and PL spectra are evidence of a high quality, large-area lm cohesion obtained with the water-assisted growth method through large crystals and potentially epitaxial grain stitching.

XPS analysis
An XPS measurement was performed to investigate the chemical states and stoichiometric ratio of the grown MoS 2 . Trace sodium was only detected on liquid-phase sulfurized MoS 2 substrates ( Fig. 4C and Table S1 †) as this process relies on coating the substrate with relatively concentrated Na 2 MoO 4 /NaCl solution compared to the hybrid approach (0.03/0.1 M as compared to 0.005/0.1 M). The detailed atomic ratios, molybdenum oxides and sodium contents of each growth method are shown in ESI Table S1. † The analysis of the atomic ratios of each growth method reveals the sulfurization method to yield an z2.22 Mo : S ratio, as compared to z6.1 for the hybrid and z3.4 for the H 2 O approach. However, compared to other growth methods, the H 2 O assisted method produces the least oxide states (Fig. S5A and C †), attributed to the Mo 6+ ratio. It is explained by waterassisted enhanced reduction of the metal precursor, which results in higher crystallinity. The lack of detected sodium within the measurement limit in the water-assisted method combined with the obtained Raman spectroscopy results discussed in the previous paragraph suggests the high quality of the monolayer material synthesized in this process approach.

STEM imaging
Aberration-corrected STEM imaging was used to image and characterize the structural properties of MoS 2 at atomic resolution. Small and large magnied high angle annular dark eld (HAADF) images ( Fig. 5A and B) show a clean and perfect hexagonal crystal lattice of the monolayer MoS 2 lm in the 2H phase. Due to the Z-contrast dependency of HAADF, the atomic intensities of Mo and 2S can be distinguished directly from the atomically resolved images (Fig. 5C). In the case of earlier reported exfoliated and MOCVD grown monolayer MoS 2 , different types of defects have been reported. However, here only sulfur mono-and divacancies can be observed which are very common due to their lower formation energies. Moreover, an electron beam can also induce sulfur defects even at lower acceleration voltages. Thus to extract the intrinsic S defect concentrations, the number of mono-and divacancies can be calculated with respect to the accumulated electron beam dose in an image series. 57 Further defect concentration analysis shows a pristine defect concentration of 1.4 Â 10 13 cm À2 for single sulfur vacancies (V S ) and 0.3 Â 10 13 cm À2 for double sulfur vacancies (V 2S ) as extrapolated from an E-beam dose rate test (Fig. S6 †). A distribution of V S , V 2S and sulfur-next-to-sulfur (V S V S ) defects taken from 11 STEM images (measured area of 925 nm 2 ) is presented in Fig. 5E. MoS 2 synthesized in the water-assisted process does not show any signs of unwanted etching or oxidative damage. Interestingly, the grain boundaries found during imaging are stitched seamlessly (Fig. 5E), demonstrating locally the epitaxial growth mode. Due to the reactive nature of grain boundaries, they tend to attract contamination along it. However, the ltered STEM image (highlighted in orange color) shows no disorientation angle between the stitched crystal areas. The fast Fourier transform (FFT) of the corresponding image shows only one set of hexagonal patterns, further con-rming the high-quality growth of MoS 2 .

Conclusion
All experimental evidence points to a strong advantage of combining the hybrid gas and liquid phase growth approaches in a coldwall showerhead system. We have successfully achieved a uniform and continuous MoS 2 lm over a large scale 4-inch substrate with spincoated Na 2 MoO 4 /NaCl solution and gasphase Mo(CO) 6 and H 2 S by limiting growth reaction imbalances with water vapor introduced during the growth phase. We found that the water regulates the growth reaction by removing carbonaceous products and reduces excess molybdenum oxides. Material characterization conrms the good material quality and no negative impact of using moderate and carefully controlled amounts of H 2 O.
The synthesized MoS 2 showed evidence of epitaxial connection with the sapphire substrate. Our ndings provide a direct step forward towards developing scalable, reproducible and large-scale MoS 2 synthesis with showerhead reactors and demonstrate the potential of combining key advancements in the eld of 2D material growth to overcome the process limitations.

MOCVD growth
All MoS 2 synthesis experiments were performed on an Aixtron BM Novo MOCVD system with a coldwall, showerhead, horizontal growth chamber. Prior to growth, 4-inch c-plane sapphire wafers (MTI) were cleaned in IPA, acetone and DI and annealed at 1000 C for 6 h to obtain an atomically smooth surface. For sulfurization, hybrid and H 2 O assisted growth methods, a sapphire wafer was additionally treated in 50 W O 2 plasma for 5 min to ensure a hydrophilic surface needed for efficient spincoating of a growth promoter (a solution of Na 2 MoO 4 (Sigma Aldrich, 98.0%) and NaCl (Sigma Aldrich, 99.5%) in DI with All growth processes started with heating the reactor with a sapphire wafer to 950 C in 60 min under a constant ow of 300 sccm of high purity argon (Carbagas 99.999%). The growth step started immediately aer reaching the set temperature. All substrates were exposed to 20 sccm of hydrogen gas (Vici DBS NM Plus 100 Hydrogen Generator, purity 99.999%) and 60 sccm of H 2 S (Carbagas, 99.9%). For gas-phase, hybrid and H 2 O assisted approaches, Mo(CO) 6 (Sigma Aldrich 99.9%) was supplied by 150 sccm of Ar gas owing through a bubbler kept at 10 C. In the H 2 O assisted variant, an additional 5-20 sccm of Ar owing through a DI-lled bubbler at room temperature was introduced. All MoS 2 growths were done at 830 mbar with growth steps set to 150 min (although the sulfurization method was completed in only 30 min). Aer that, the reactor was cooled under ambient H 2 S and Ar ows. Aer a growth run, the reactor chamber was thoroughly cleaned and annealed and all the ceramic elements were additionally air-baked to prevent unwanted carbon contamination.

Raman spectroscopy and PL
Raman and PL spectroscopy were done with a Renishaw inVia Confocal Raman Microscope with a 532 nm laser beam at a low power (<0.3 mW) to avoid defect nucleation and substrate damage. A diffraction grating of 3000 mm À1 was used for good spatial resolution.

Atomic force microscopy
Atomic force microscopy was performed on an Asylum Research Cypher AFM system in tapping mode.

SEM imaging
Scanning electron microscopy was done on a Zeiss Merlin SEM with a GEMINI II column. All imaging was done at 1 kV acceleration voltage to minimize substrate charging STEM imaging. The as-grown MoS 2 was transferred from the growth substrate using the wet-transfer method 36 on 5 Â 5 mm Si/SiN TEM grids (Norcada). Scanning transmission electron microscopy imaging was conducted using an aberration-corrected (with a double spherical corrector) FEI Titan Themis TEM 60-300 kV, equipped with a Schottky X-FEG electron source and a monochromator to reduce the effect of chromatic aberrations. To avoid the electron-beam induced knock-on damage, a low acceleration voltage (80 kV) was used for the imaging. 58 The electron probe current, C 2 aperture size, and beam convergence angle were 25 pA, 50 mm, and 21.2 mrad, respectively. Images were acquired with Velox soware (Thermo Fisher Scientic) using a 185 mm camera length, which corresponds to an angular range (49.5-198 mrad) in a HAADF detector. To avoid the sample dri, serial imaging was performed using 512 Â 512 pixels with an 8 ms dwell time. The images were aligned and processed using the "double-Gaussian ltering" method in ImageJ.

Conflicts of interest
The authors declare no competing nancial interest.