Chalcogen Precursor Effect on Cold-Wall Gas-Source Chemical Vapor

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Chalcogen precursor effect on cold-wall gassource chemical vapor deposition growth of WS

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Tanushree H Choudhury, Hamed Simchi, Raphaël Boichot, Mikhail Chubarov, Suzanne E. Mohney, and Joan M. Redwing Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00306 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

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Crystal Growth & Design

Chalcogen precursor effect on cold-wall gas-source chemical vapor deposition growth of WS2 Tanushree H. Choudhury,1,2 Hamed Simchi,2 Raphaël Boichot,3 Mikhail Chubarov,1 Suzanne E. Mohney2 and Joan M. Redwing1,2* 1

2D Crystal Consortium–Materials Innovation Platform, Materials Research Institute 2 Department of Materials Science and Engineering The Pennsylvania State University, University Park, PA, USA 3 Univ. Grenoble Alpes, CNRS, Grenoble INP*, SIMAP, 38000 Grenoble, France Institute of Engineering Univ. Grenoble Alpes *Email: [email protected]

Abstract Tungsten disulfide (WS2) films were grown on c-plane sapphire in a cold-wall gas source chemical vapor deposition (CVD) system to ascertain the effect of the chalcogen precursor on the film growth and properties. Tungsten hexacarbonyl (W(CO)6) was used as the tungsten source and hydrogen sulfide (H2S) or diethyl sulfide (DES-(C2H5)2S) were the chalcogen sources. The film deposition was studied at different temperatures and chalcogen-to-metal ratios to understand the effect of each chalcogen precursor on the film growth rate, thickness, coverage, photoluminescence and stoichiometry. Larger lateral growth was observed in films grown with H2S than DES. The reduced lateral growth with DES can be attributed to carbon contamination, which also quenches the photoluminescence. Thermodynamic calculations agreed well with the experimental observations suggesting formation of WS2 with both sulfur precursors and additional formation of carbon when deposition is done using DES.

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Introduction WS2 is a semiconducting transition metal dichalcogenide (TMD) with a layer-dependent tunable band gap, where monolayer WS2 has a band gap of 2 eV.1,2 It is predicted to have the highest mobility in the class of 2D TMD materials due to low electron mass3 and a strong spinorbital coupling leading to a valence band splitting of ~426 meV.3,4 In recent years, WS2 monolayers have been investigated as photodetectors,5 field-effect transistors6 and for valleyelectronics.6 One major challenge in harnessing the potential of WS2, like any other TMD, is the uniform growth of high quality monolayer and few-layer films over large substrate areas. WS2 has been grown with considerable success by powder-source chemical vapor deposition (CVD) techniques,7 but this process does not readily lend itself to large area uniform growth. Large domain sizes have been realized,8 but simultaneous uniform coverage over large area is still elusive. As interest prevails in layered TMD heterostructures, ingenious methods have also been developed to synthesize heterostructures without ambient exposure using powder source precursors.9,10,11 However, as the precursors are located upstream in the tube along with the substrates, it is difficult to control the precursor fluxes independently. This is a particular impediment for the growth of heterostructures, which requires precursor modulation during growth. Conventional gas source chemical vapor deposition (CVD), with precursors placed outside the reactor, allows for flexibility in precursor selection and precise gas phase concentration control. In CVD, the precursors are crucial in determining the film properties. A variety of tungsten and sulfur precursors have previously been employed to deposit WS2 films by gas source CVD. For example, WS2 deposition has been demonstrated using W(CO)6, WCl6 and WOCl4 and organic sulfur sources such as 1,2 ethanedithiol [HS(CH2)2SH]and 2-


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methylpropanethiol [HSC(CH3)3] on glass substrates in a cold wall CVD reactor at atmospheric pressure.12 The deposition was carried out over the temperature range of 300-600°C, which resulted in nanocrystalline films. Prior studies also investigated the use of W(CO)6 and H2S in a cold wall reactor for growth on Si(100) substrates, producing thick films oriented out-of-plane (basal plane nonparallel to the substrate).13 More recently, the focus has shifted to the growth of in-plane oriented (basal plane parallel to the substrate) monolayer and few-layer WS2 films with large domain size and control over the layer number. Kang et al. demonstrated the use of diethyl sulfide (DES-(C2H5)2S) along with W(CO)6 to grow monolayer WS2 films on 4” diameter substrates with domain size on the order of 1 µm.14 Recently, Park et al. deposited WS2 films with domain sizes on the order of 50-100 nm using WCl6 and H2S and demonstrated control over film thickness.15 In both of these cases, the growth was carried out in a hot wall CVD reactor which results in pre-heating of the gas phase and potential gas phase decomposition/reaction of precursors upstream of the substrate which complicates the direct comparison to prior work carried out in cold wall reactor configurations. As a result of the different reactor geometries and precursors that have previously been employed for WS2 growth, it is difficult to select an appropriate combination of metal and chalcogen precursors by merely an extensive literature survey. In this study, the sulfide precursors, H2S and DES, were investigated in combination with W(CO)6 to ascertain their effects on the growth and properties of WS2 films on sapphire as a function of temperature and chalcogen-to-metal precursor ratio in a cold wall CVD reactor. The results demonstrate that monolayer WS2 films can be grown on sapphire with both DES and H2S; however, the films grown with DES exhibited carbon contamination and smaller domain size than those grown with H2S. Thermodynamic calculations were performed to analyze the reaction



chemistry with both sulfur precursors for comparison to the experimental results. The presence of carbon contamination in the films grown by DES also quenches the photoluminescence and results in sub-stoichiometric WS2, which readily oxidizes.

Experimental Procedure Tungsten disulfide (WS2) was grown in a metalorganic chemical vapor deposition (MOCVD) system that was designed for the growth of 2D chalcogenides. A simplified schematic of the horizontal cold wall reactor is shown in Figure 1. The growth was carried out on nominally 1x1 cm2 pieces of c-plane sapphire. The substrates were cleaned by sonication in acetone, isopropanol and deionized water for 10 min. The substrates were dried in a N2 stream and then immersed in Nanostrip (Fischer Scientific), maintained at 90°C, for 20 min. The substrates were then rinsed and sonicated again in deionized water and finally dried in a N2 stream. The sapphire substrate was placed on a graphite susceptor inside the quartz tube that couples with an induction heater, enabling the substrate to be heated while keeping the tube wall at a lower temperature. The susceptor temperature, measured by a thermocouple, was varied from 600-850°C while the reactor pressure was held at 50 Torr. Tungsten hexacarbonyl (W(CO)6) was used as the metal precursor and hydrogen sulfide (H2S-99.5% Praxair) or diethyl sulfide (DES (C2H5)2S-99.98% Sigma Aldrich) were used as the chalcogen precursors. W(CO)6 (99.99 %, Sigma Aldrich) powder within a stainless steel bubbler was maintained at 760 Torr and 25°C, controlled by a heat tape. Ultra-high purity hydrogen was used as a carrier gas and was passed through the bubbler at flow rates of 20 sccm and 5 sccm to control the W(CO)6 precursor flux at 6.8×10-4 sccm and 1.7×10-4 sccm, respectively, where 1 sccm = 7.44.10-7 mol/s. To study the effect of chalcogen-to-metal ratio on the film growth, the metal precursor flow rate was maintained at 1.7 x 10-4 sccm while the chalcogen precursor was varied. DES liquid was 4 ACS Paragon Plus Environment

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kept in a bubbler at 760 Torr and 21°C, controlled by a constant temperature bath. The H2 carrier gas flow through the bubbler was varied from 6 to 22 sccm to obtain DES flow rates ranging from 0.034 sccm to 2.2 sccm. To obtain a DES flow rate of 0.034 sccm into the reactor, a dilution manifold was used in which 10 sccm H2/DES gas mixture coming out of the bubbler was further diluted with an additional 150 sccm of H2. Finally, 10 sccm of the diluted mixture carrying 0.034 sccm was introduced into the inlet gas stream. The vapor phase precursor concentrations over solid (W(CO)6) and liquid (DES) inside the bubblers were calculated from equilibrium vapor pressure data16. The H2S (99.5% Praxair) flow rate was varied from 2-10 sccm. Additional hydrogen was added to the inlet gas stream to maintain a constant total gas flow rate of 200 sccm through the reactor tube. Whenever the system was opened for sample loading/unloading, 300 sccm N2 was flowed through the reactor to minimize the tube exposure to air. To carry out WS2 growth, the substrate was placed on the graphite susceptor and loaded into the growth chamber, which was then pumped down to a base pressure of ~4×10-3 Torr. The pressure in the growth chamber was then increased to 50 Torr under H2 flow and then the heating was started. After reaching the growth temperature, the substrate was annealed for 10 min in H2. During this period, the metal and chalcogen precursors were purged to the vent line using a run/vent manifold. After the annealing period, the precursors were switched to the run line to initiate growth for a period of 30 min. A gas phase equilibrium model of the growth chemistry was developed to gain insights into the reaction products anticipated with the different sulfur precursors as a function of growth conditions. The model assumes infinite residence time, constant pressure (50 Torr) and equilibrium molar fractions of the inlet gas mixture similar to those used experimentally.



FactSage 7.0 software package, with the FACT database17 was used for the Gibbs free energy minimization calculations. All gas species, liquid compounds and solid phases from W-C-O-S-H system included into the FACT database have been considered in the thermodynamic calculations. It must be noted that the calculations at equilibrium do not take into account kinetics or mass transport processes that are relevant to CVD growth. Consequently, the calculations provide trends that are particularly relevant if the temperature is high or if the kinetics does not limit the extent of reaction but will not detect metastable phases. The morphology of the deposited films was investigated by scanning electron microscopy (SEM) (Zeiss Merlin) at accelerating voltages of 3 kV. Atomic force microscopy (AFM) (Bruker Icon) was used to determine the domain size and thickness. The AFM tip was a Scanasyst air probe with a nominal tip radius of ∼2 nm and spring constant of 0.4 N/m. Images of a 1×1 µm2 area were collected using peak-force tapping mode with a peak-force set point of 0.5 nN. The Raman spectra and photoluminescence (PL) response of the films was also probed at room temperature using a 488 nm laser (Horiba LabRAM). The composition of the films was investigated by X-ray photoelectron spectroscopy (XPS) (Phi Versa Probe II) with monochromatic Al Kα x-ray excitation at 20 kV, equipped with dual beam charge neutralization. Charging offsets were corrected by calibrating the XPS spectrum based on the C 1s peak position (284.8 eV). Peak fitting was done to confirm the stoichiometry and extent of oxidation of the films.

Results and Discussion The focus of the study was on the growth of ultrathin (monolayer or few-layer) WS2 films rather than thicker films, consequently, the growth conditions were initially set to provide a very low metal precursor flow rate in order to obtain a low nucleation density and high lateral


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growth rate. As a result of the many orders of magnitude difference in the vapor pressures of transition metals (W and Mo) and chalcogens (S, Se), TMD films are typically grown using large chalcogen-to-metal ratios with the chalcogen present in excess.18 Consequently, the nucleation and growth rate are primarily controlled by the concentration of the transition metal precursor. As shown in Figure 2(a-b), W(CO)6 = 6.8×10-4 sccm gives a coalesced multi-layer film in the case of DES (1.1 sccm) and H2S (10 sccm) at 800°C after 30 min. Vertical platelets (basal plane not parallel to substrate) are observed in both cases, about 10 – 25 nm thick. These platelets are characteristic of a large growth rate, which causes rapid coalescence of adjacent domains, and finally vertical out-of-plane growth of domains. On reducing the metal precursor flow to 1.7×104

sccm, the growth rate was reduced significantly, as evident by separated individual triangular

domains shown in Figure 2(c-d). The sapphire substrate underneath is evident in Figure 2 (c-d) even after the growth duration of 30 min. The effect of DES and H2S on WS2 growth was then studied and compared for the low metal precursor flow rate as described in the following sections. A. Diethyl Sulfide (DES) The effect of temperature on the surface morphology of WS2 is summarized in Figure 3(a-d) for a reactor pressure of 50 Torr and DES flow rate of 1.11 sccm. The AFM images show the surface coverage, domain size and film thickness. WS2 films grown at 650°C, as shown in Figure 3(a), are continuous with some nanocrystalline out-of-plane growth, as highlighted in Figure 3(a) by white circles, and nanocrystalline in-plane domains. On increasing the temperature to 750°C, Figure 3(b), the domain size increases. The out-of-plane growth is still present. On further increasing the temperature to 800°C and 850°C, the coverage on the substrate surface decreases with isolated triangular domains present ~ 50 nm in size and 0.8 – 1.5 nm in 7 ACS Paragon Plus Environment


height, as shown in Figure 3(c-d). This behavior is expected as the increase in temperature would also increase the desorption rate of species, resulting in lower coverage. The film thickness corresponds to monolayer to bilayer WS2.7 In addition to the morphology, the films were also investigated by Raman spectroscopy as shown in Figure 3(e). There is an overlap of the ‫ܣ‬ଵ௚ peak position of WS2 with that of sapphire, but the intensity of the sapphire is negligible and ଵ hence its contribution to the WS2 peaks have been ignored. The separation between the ‫ܧ‬ଶ௚ and

‫ܣ‬ଵ௚ is an indicator of the layer thickness of WS2.19 Based on comparison with literature, it can be ascertained that the layers grown are mono- to bi-layer in thickness. The full width at half maximum (FWHM) of the ‫ܣ‬ଵ௚ peak decreases from 7.4 cm-1 at 650°C to 6.9 cm-1 at 800°C. At 850°C, FWHM is however 7.2 cm-1. This slight variation in the FWHM is an indication that with DES, the film quality does not necessarily improve with an increase in temperature. One important factor is the presence of sp2-bonded carbon in the WS2 films grown with DES. As shown by the Raman peaks present at 1360 and 1600 cm-1, graphitic carbon is present at all temperatures, however, the carbon peak intensity relative to the WS2 peak is lowest at 800°C. The presence of carbon could also affect the crystalline quality as observed by the FWHM of the WS2 films. Further investigation into the effect of the S:W ratio was carried out at 800°C. The DES flow rate was varied from 0.03 sccm to 2.24 sccm to vary the S:W ratio from 200 to 13000. Figure 4(a-d) shows the variation in growth morphology for different S:W ratios. The corresponding Raman spectra are shown in Figure 4(e). At a very low DES flow rate, 0.03 sccm and S:W ratio of 200, WS2 crystals are not formed as shown by the absence of characteristic peaks in the Raman spectra. The darker areas in the film can be attributed to graphitic carbon, as indicated by the large Raman peaks at 1360 and 1600 cm-1. When the DES flux ratio was 8 ACS Paragon Plus Environment

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increased to 0.3 sccm (S:W = 1700), WS2 domains begin to grow. The significantly higher amount of chalcogen required to initiate the growth of WS2 is attributed to the high vapor pressure of sulfur at 800oC (~2250 Torr).20 Therefore, a larger sulfur over-pressure is needed to aid incorporation in the solid WS2 film. On further increasing the DES flux to 1.11 sccm (S:W = 6500), the density of WS2 domains increases. There is, however, no significant change in the domain size. A further increase in the DES flux to 2.24 sccm (S:W = 13000), however, shows a decrease in the nucleation density. An important observation made from the Raman spectra is that the area under the Raman peaks corresponding to the graphitic carbon phase is similar in all the cases. This indicates that at 800°C, even though the amount of carbon deposited is similar and independent of the DES flow rate, the rate at which this carbon layer forms can vary. At a higher DES flow rate, the carbon layer can form more rapidly due to the increased concentration of ethyl groups in the growth environment. The carbon deposition competes with WS2 growth and does not allow lateral growth needed for large domains. Similar graphitic carbon deposition and a resulting reduction in film growth rate was previously reported for WSe2 films grown using W(CO)6 and dimethyl selenide (DMSe) in a vertical cold-wall CVD reactor.21 It was reported that the domain size of WSe2 decreased as the DMSe flow rate was increased beyond a particular value. The presence of graphitic carbon in the WS2 films grown using DES in the cold wall reactor configuration is in contrast to that reported for MoS2 and WS2 grown in a hot wall system by Kang et al.14 The absence of carbon in WS2 deposited in a hot wall system may be attributed to gas phase decomposition of the DES precursor, which would begin as soon as it experiences an elevated temperature. In the hot wall system, this decomposition would initiate near the reactor inlet and by the time the species reach the growth front, the carbon could be deposited on



the upstream walls of the reactor or may have formed more stable C2H4 or C2H6 molecules in the gas phase.22 Kang et al. also used NaCl in the chamber, which is proposed to remove moisture. NaCl could also be playing a role in carbon removal by the formation of activated Cl species which could etch carbon from the surface to form CClx. In the cold wall configuration, however, the DES decomposition begins on or near the substrate surface, which does not allow for efficient carbon elimination. B. Hydrogen Sulfide (H2S) The effect of temperature on the surface morphology of WS2 is highlighted in Figure 5(ad) for the case of H2S at a pressure of 50 Torr and H2S flow of 10 sccm. As shown in Figure 5(a) at 650°C, a continuous film is deposited with increased surface roughness and domains of the order of 30 nm. At 750°C (Figure 5(b)), there is a continuous underlying film with multi-layer domains ~ 100 nm in size on top. On increasing the temperature to 800°C (Figure 5(c)), wellseparated domains, ~200 nm in size, are obtained. Some multi-layer growth is also observed. At 850°C (Figure 5(d)), a continuous film is observed. Additional features in the AFM image (Figure 5(d)), spaced ~50 nm apart, are attributed to wrinkles in the WS2 arising from thermal expansion mismatch between the film and substrate23,24,25 rather than grain boundaries, as the grain size at 850°C would be larger than that at 800 °C. The continuity in the films can be confirmed using the AFM adhesion profile, as shown in supplemental Figure S1. The Raman spectra (Figure 5(e)) show that these films are also mono- to bi-layer thick. The FWHM of the ‫ܣ‬ଵ௚ peak decreases from 7 cm-1 at 650°C to 6 cm-1 at 850°C, indicating an increase in the crystalline quality with an increase in temperature. The major difference is the absence of carbon-related peaks at all temperatures. The absence of graphitic carbon and a comparatively larger domain size lends weight to the hypothesis that carbon deposition competes with the 10 ACS Paragon Plus Environment

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lateral growth of WS2. As the films grown in H2S show negligible carbon content, higher WS2 lateral growth rates are possible. The chalcogen-to-metal ratio study for H2S was carried out at 800°C for a direct comparison to the results obtained with DES. The results are shown in Figure 6. When 2 sccm of H2S was used, there was no nucleation of WS2 (Figure 6(a)). In comparison to DES, where WS2 growth occurred at 0.3 sccm, a larger amount of H2S is required for the growth of WS2. The onset of WS2 nucleation and growth is, therefore, dependent on the chalcogen precursor chemistry. In the case of DES, graphitic carbon is deposited on the substrate, which could provide a higher dangling bond density, which, in turn, acts as nucleation sites and initiate the growth of WS2. In case of H2S, no such nucleation sites are possible and growth can occur only when sufficient sulfur is available. WS2 growth is observed when 5 sccm of H2S is used (Figure 6(b)). The domains are larger on increasing H2S to 10 sccm, Figure 6(c). The Raman spectra (Figure 6(d)), shows that at 5 sccm, there is a small amount of carbon present. The intensity in the range from 1100-1750 cm-1 has been multiplied by a factor of 100 to highlight the carbon peaks. The Raman peak area shows that the carbon amount in this case is considerably lower than that observed with DES. The probable source for this carbon is the W(CO)6 source itself. The carbon peak is suppressed on increasing H2S to 10 sccm. C. Thermodynamic calculations Thermodynamic calculation of the gas phase chemistry was employed to gain insight into the reaction products of two different chemistries involving H2S and DES as sulfur sources for the deposition of WS2. For the Gibbs free energy minimization calculations, the amounts of the input gases were as follows: 200 mol H2, 5.5×10-4 mol W(CO)6 and either 25.5 mol H2S or 5 mol DES. The relative ratios of the components are comparable to what has been used experimentally.



Calculations performed in FactSage software package indicated that the main gaseous species (out of 77 gaseous species taken into consideration) except hydrogen were H2S and methane (CH4) in the case of deposition performed from both H2S and DES precursors (Figure 7a-b). In the case of DES, the other significant carbon-containing gaseous species are CS2~1×10-4 mol above 640 °C and CO ~ 3×10-3 mol (but has slight increase with the temperature) over the entire temperature range investigated. The HS concentration increases above 1×10-4 mol when temperature exceeds 720 °C and gaseous S2 exceeds 1×10-4 mol above 740 °C. When H2S is used as the sulfur precursor, additional significant gaseous species in the investigated temperature range include H2O (3×10-3 -1.5×10-3 mol), H2S2 (4×10-4 - 7×10-3 mol), S2 (1×10-4 0.27 mol), HS (1×10-4- 4×10-2 mol above 640 °C), CO (1×10-4 -2×10-3 mol above 680 °C) and CS2 (>1×10-4 mol above 760 °C). However, the concentration of CH4 is considerably lower in case of H2S (Figure 7(a)) since carbon is only introduced into the system via W(CO)6. The actual amounts of the various dominant species at different temperatures are included in Table S1 in supplementary information. In case of DES, solid carbon is present at 600°C and the amount increases with an increase in temperature (Figure 7(b)). There is a simultaneous decrease in the gaseous CH4 concentration with temperature. Even though the actual mechanism cannot be established by the thermodynamic calculations, the correlation between the change of the amounts (mol) of carbon and methane in case of deposition from DES are suggestive of the role of CH4. This is assumed since calculations predict other individual gaseous hydrocarbon species amounts in the equilibrium to be below 1×10-5 mol. While there is no solid carbon formed in the case of the H2S precursor, the presence of CH4 in the gas phase may be the source for the very small amount of carbon observed experimentally in one of the samples (Figure 6(d)). In addition to WS2 deposition, formation of WC is expected at temperatures exceeding 850 °C when DES is


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used. The reduced WS2 formation above ~850 oC is also accompanied by an increase in other sulfur containing species like HS, H2S2, S2, CS2 (Table S1). While this is the probable path, the presence of kinetic barriers for the reaction could push the transition temperature higher. In the present work, 850 °C was the upper limit of the growth temperature attainable in the reactor. WC formation may occur in practice at higher deposition temperatures. In addition, it is interesting to note that according to the calculations, when DES is used as the sulfur precursor, the main sulfur-containing equilibrium gas phase species is actually H2S over the entire temperature window (600 – 1000 °C). This suggests that similar experimental results could be obtained for WS2 growth when growth is carried out using H2S and CH4 is added to the inlet gas. In addition, the experimental results show that nucleation onset when deposition is conducted from H2S requires higher S/W ratios compared to the case of DES precursor. This suggest that the deposition occurs through some intermediate sulfur or carbon containing gaseous species which forms from the pyrolysis of the DES molecule in the vicinity of the substrate surface which exhibits higher reactivity compared to H2S. This is, however, not possible to predict by the thermodynamic calculations, as only equilibrium phases are observed. D. Effect on Photoluminescence and Stoichiometry The choice of the chalcogen precursor affects not only the growth and phase purity of the films, but also the optical response and stoichiometry. The photoluminescence (PL) spectra of WS2 grown at 800°C, using DES (S:W = 6500) and H2S (S:W = 60000) are shown in Figure 8(a). The PL peak position is at 1.99 eV which is close to the “A - exciton” transition at 2 eV reported for monolayer WS2.26 This provides additional evidence for the monolayer thickness of WS2. The narrow peak at 1.79 eV corresponds to the sapphire substrate. The peak position for the films grown by DES are also at ~1.99 eV but the spectral intensity from the films is



significantly lower than that grown with H2S. It is widely known that the PL response of WS2 depends on the layer thickness.26 Typically, with an increase in layer thickness, the PL peak reduces in intensity and red shifts. However, the PL peak position in conjunction with the AFM images (Figure 3(c) and 5(c)) and Raman spectra (Figure 3(e) and 5(e)) of these films show that the WS2 grown in both the cases are of similar thickness. Therefore, the difference in PL intensity is not the result of differences in thickness. The domain size of the films deposited using DES is however considerably smaller. It has been reported that in nanocrystalline WS2 films, the PL intensity reduces and additional defect peaks are visible in the energy range of 1.81.95 eV.26 Even though the additional defects peaks are not observed, the limited domain size in the case of DES could reduce the PL intensity. An additional difference is the carbon content in the films. The films grown with H2S have a negligible amount of carbon as shown by the Raman spectra. It has been shown previously that the PL response of TMD films grown on graphene is greatly diminished.22 This is attributed to electron transfer from the TMD to graphene, which reduces the PL efficiency. A similar mechanism could be active in the films obtained using DES, resulting in a lower PL yield. In addition to the PL spectra, the stoichiometry of the films is also influenced by the choice of the precursor, as shown in Figure 8(b). Here the W 4f and S 2p spectra of films grown at 800°C for DES and H2S flow rates of 1.11 sccm (S:W=6500) and 10 sccm (S:W=60000), respectively, are compared. Peaks were fit by Gaussian-Lorentzian curves satisfying the following constraints: a) the doublets intensities have the ratio of 4:3 and 2:1 for the electrons coming from f and p orbitals, respectively; b) each doublet has equal full width at half maximum (FWHM); and c) the spin orbit splitting of the doublets are matched with the database.27 Peak positions for the core levels of each compound were compared with the National Institute of


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Standards and Technology (NIST) database for identification.27 The peak positions of W 4f7/2 at 33.1 eV and W 4f5/2 at 35.3 eV correspond to WS2. This is the most prominent peak in both cases. However, in the case of films grown using DES, there are additional W 4f7/2 and W 4f5/2 peaks present, at 36.6 eV and 38.6 eV, respectively, corresponding to the presence of oxide. In the S 2p spectra, S 2p1/2 and S 2p3/2 peaks present at 162 eV and 163 eV correspond to WS2. In the case of DES, it is evident that the sulfur has a different chemical environment. Peaks present at 168 eV and 169 eV correspond to SO2.28 The intermediate peaks at 164 eV and 165 eV could be attributed to a sub-stoichiometric oxysulfide or organic sulfur compound.28 The actual stoichiometry of the DES films could not be confirmed, but it is evident that the films grown with H2S are closer to stoichiometric WS2 (with less oxide/oxysulfide contribution). However, the lower sulfur concentration in the films grown by DES, could lead to a high concentration of S vacancies, which could make the sub-stoichiometric WS2 films more prone to oxidation as opposed to a stoichiometric WS2 film obtained using H2S. It must be noted that the ratio of S:W in the case of H2S is about 60000, while in the case of DES it is only 6500. Higher amounts of S would aid in maintaining the stoichiometry, but in case of DES, higher S precursor concentration leads to lower WS2 nucleation. Hence, H2S provides better control of the composition of the WS2 films.

Conclusions The effect of temperature and S:W ratio on the growth of WS2 on sapphire in a cold wall CVD reactor was investigated and compared as a function of sulfur precursor (DES versus H2S). The choice of the chalocogen precursor in the growth of WS2 is crucial to obtaining larger crystalline domains. DES results in the formation of graphitic carbon on the substrate, which inhibits lateral growth of WS2. H2S significantly reduces carbon impurities and results in larger



triangular domains. Gibbs free energy minimization calculations predict the formation of WS2 for both H2S and DES and, in the case of DES also predicts the formation of solid carbon in molar amounts similar to that of WS2 that is consistent with the experimental observations. The presence of graphitic carbon in the WS2 films grown with DES is likely responsible for the smaller domain size and reduced PL intensity compared to films grown with H2S. WS2 films grown with DES also oxidize more quickly in ambient air compared to those grown with H2S because of sub-stoichiometric films obtained with DES. H2S is therefore a preferred chalcogen source for WS2 growth on sapphire when a cold wall CVD reactor geometry is utilized.

Acknowledgements The work was financially supported by the National Science Foundation (NSF) through the Penn State 2D Crystal Consortium – Materials Innovation Platform (2DCC-MIP) under NSF cooperative agreement DMR-1539916 and the NSF EFRI-2DARE program (Grant EFRI1433378).

Supplementary Information. AFM Adhesion maps to show film coverage.

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Figures:

Figure 1: Simplified schematic of the gas-source cold-wall chemical vapor deposition system.


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Figure 2: SEM images of WS2 films grown at 800°C for 30 min with (a) W(CO)6 = 6.8×10-4 sccm and DES =1.1 sccm, (b) W(CO)6 = 6.8×10-4 sccm and H2S =10 sccm, (c) W(CO)6 = 1.7×10-4 sccm and DES =1.1 sccm and (d) W(CO)6 = 1.7×10-4 sccm and H2S =10 sccm. In (c) and (d) the regions of dark contrast are WS2 (as highlighted by the yellow outlined region) and the brighter regions in between are sapphire.



Figure 3: AFM images of films grown with W(CO)6 = 1.7×10-4 sccm and DES =1.1 sccm for 30 min at (a) 650 °C, (b) 750 °C, (c) 800 °C and (d) 850 °C. The platelets growing out-of-plane have been highlighted with white circles. The corresponding Raman spectra of the films at different temperatures along with bare sapphire are shown in (e).


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Figure 4: SEM images for WS2 growth at 800°C for 30 min with W(CO)6 = 1.7×10-4 sccm and different S:W ratios of (a) 200 for DES = 0.03 sccm, (b) 1700 for DES = 0.304 sccm, (c) 6500 for DES = 1.1 sccm and (d) 13000 for DES = 2.2 sccm. The corresponding Raman spectra are given in (e).



Figure 5: AFM images of films grown with W(CO)6 = 1.7 ×10-4 sccm and H2S =10 sccm for 30 min at (a) 650 °C, (b) 750 °C, (c) 800 °C and (d) 850 °C. The white circle and arrow shows the bare sapphire substrate. The corresponding Raman spectra of the films at different temperatures along with bare sapphire are shown in (e).


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Figure 6: SEM images for WS2 growth with H2S at 800°C for 30 min with W(CO)6 = 1.7 ×10-4 sccm and different S:W ratios of (a) 11000 for H2S = 2 sccm, (b) 30000 for H2S = 5 sccm, (c) 60000 for H2S = 10 sccm. (d) The corresponding Raman spectra are shown. The intensity in the range from 1100-1750 cm-1 has been multiplied by a factor of 100 to highlight the carbon peaks.



Figure 7: Temperature dependence of amount (moles) of main gas species and all solid phases present at equilibrium for (a) 200 mol of H2, 5.5×10-4 mol of W(CO)6 and 25.5 mol of H2S, and (b) 200 mol of H2, 5.5×10-4 mol of W(CO)6 and 5 mol of DES.


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Figure 8: (a) Photoluminescence spectra from the films grown with H2S and DES at 800 oC with S:W=60000 and 6500, respectively. (b) The corresponding XPS spectra showing the chemical composition of the films.



For Table of Contents Use Only

Chalcogen precursor effect on cold-wall gas-source chemical vapor deposition growth of WS2 Tanushree H. Choudhury,1,2 Hamed Simchi,2 Raphaël Boichot,3 Mikhail Chubarov,1 Suzanne E. Mohney2 and Joan M. Redwing1,2* 1

2D Crystal Consortium–Materials Innovation Platform, Materials Research Institute 2 Department of Materials Science and Engineering The Pennsylvania State University, University Park, PA, USA 3 Univ. Grenoble Alpes, CNRS, Grenoble INP*, SIMAP, 38000 Grenoble, France Institute of Engineering Univ. Grenoble Alpes *Email: [email protected]

In a cold wall system, choice of chalcogen precursor determines the properties of the deposited WS2 films. H2S leads to larger lateral growth and lesser carbon contamination. This in turn translates to higher photoluminescence yield from the films grown in H2S ambient.


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Chalcogen Precursor Effect on Cold-Wall Gas-Source Chemical Vapor

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