Considerations for Utilizing Sodium Chloride in Epitaxial Molybdenum

Nov 2, 2018 - ACS Applied Materials & Interfaces .... Materials Research Institute, ⊥Materials Characterization Laboratory, and ¶Department of Phys...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 40831−40837

Considerations for Utilizing Sodium Chloride in Epitaxial Molybdenum Disulfide Kehao Zhang,†,‡ Brian M. Bersch,† Fu Zhang,†,‡ Natalie C. Briggs,† Shruti Subramanian,† Ke Xu,§ Mikhail Chubarov,∥ Ke Wang,⊥ Jordan O. Lerach,⊥ Joan M. Redwing,†,∥ Susan K. Fullerton-Shirey,§,# Mauricio Terrones,†,‡,¶ and Joshua A. Robinson*,†,‡,∥

ACS Appl. Mater. Interfaces 2018.10:40831-40837. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/28/19. For personal use only.

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Department of Materials Science and Engineering and Center for Two Dimensional and Layered Materials, ‡Center for Atomically Thin Multifunctional Coatings, ∥2-Dimensional Crystal Consortium (2DCC), Materials Research Institute, ⊥Materials Characterization Laboratory, and ¶Department of Physics, Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States § Department of Chemical and Petroleum Engineering and #Department of Electrical and Computer Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15213, United States S Supporting Information *

ABSTRACT: The utilization of alkali salts, such as NaCl and KI, has enabled the successful growth of large single domain and fully coalesced polycrystalline twodimensional (2D) transition-metal dichalcogenide layers. However, the impact of alkali salts on photonic and electronic properties is not fully established. In this work, we report alkali-free epitaxy of MoS2 on sapphire and benchmark the properties against alkali-assisted growth of MoS2. This study demonstrates that although NaCl can dramatically increase the domain size of monolayer MoS2 by 20 times, it can also induce strong optical and electronic heterogeneities in asgrown, large-scale films. This work elucidates that utilization of NaCl can lead to variation in growth rates, loss of epitaxy, and high density of nanoscale MoS2 particles (4 ± 0.7/μm2). Such phenomena suggest that alkali atoms play an important role in Mo and S adatom mobility and strongly influence the 2D/ sapphire interface during growth. Compared to alkali-free synthesis under the same growth conditions, MoS2 growth assisted by NaCl results in >1% tensile strain in as-grown domains, which reduces photoluminescence by ∼20× and degrades transistor performance. KEYWORDS: 2D Materials, MOCVD, alkali-assisted synthesis, molybdenum disulfide, Properties

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synthesis of polycrystalline monolayer MoS2.11 Most recently, using NaCl helped exploring the novel 2D materials.18 The addition of NaCl is said to increase the single-crystalline domain size of monolayer MoS2 in metal−organic CVD (MOCVD) because of dehydration12 and nucleation suppression.13 For traditional solid-source (e.g., MoO3 and Mo foil) CVD vaporization, NaCl is demonstrated to increase the reactivity of metal and chalcogen elements, which is the key to enable the batch production of 6 in. uniform monolayer MoS211 and the exploration of an entire TMD library,18 where in some cases, large-area 2D films cannot be achieved without NaCl. Although >100 μm single-crystalline TMDs are readily synthesized via alkali-assisted methods, it is highly unlikely that nucleation can be suppressed to a single nuclei on a substrate that subsequently grows to multiple centimeters. Therefore, large-scale epitaxy has recently been the focus to realize largescale TMD films with minimum grain boundaries.14,19,20

INTRODUCTION Two-dimensional (2D) transition-metal dichalcogenides (TMDs)1,2 are primed for a wide variety of promising technologies in nanoelectronics [field-effect transistors (FETs)3,4 and phototransistors],5 photonics (photodetectors6 and light-emitting diodes7), and sensors (gas sensors8 and biosensors9). However, most devices are based on exfoliated flakes, which are not compatible with industrial needs. Therefore, many efforts continue to focus on the synthesis of large-scale TMDs.2,10−14 Ion-exchange methods enabled waferscale production of molybdenum disulfide (MoS2)15 and tungsten diselenide (WSe2),16 but these films suffer from poor electrical performance because of a nanocrystalline structure (30 μm.17 Recent utilization of alkali salts clearly demonstrates that its use enables 5−100× increases in domain size12,13 and is reported to be the key to realizing the first 100 mm wafer © 2018 American Chemical Society

Received: September 19, 2018 Accepted: November 2, 2018 Published: November 2, 2018 40831

DOI: 10.1021/acsami.8b16374 ACS Appl. Mater. Interfaces 2018, 10, 40831−40837

Research Article

ACS Applied Materials & Interfaces Building from this, it may be hypothesized that using alkaliassisted epitaxy may be a route to achieving epitaxial TMD films with large domain size. However, alkali metals are also proven impurities that must be avoided at all cost in traditional silicon-based technologies because of high rates of ion diffusion through the gate oxides, which leads to unreliable performance.21−27 Although the utilization of alkali-assisted growth continues to expand among the community,11−13,18,28 a direct comparison of the impact of alkali metals on the electronic and photonic properties of MoS2 films (with and without layer transfer) has not been achieved; therefore, a comprehensive study on the impact is merited. In this work, we explore the trade-offs of utilizing NaCl in the MOCVD synthesis of coalesced, epitaxial MoS2 monolayer films. We directly benchmark the optical, structural, and electronic properties of alkali-free and alkali-assisted MoS2 grown under the same conditions. Monolayer films grown without NaCl are found to be epitaxial and uniform across a 2 × 2 cm2 sapphire substrate, whereas NaCl-assisted growth is shown to enhance the growth rate and domain size by 20× for some domains. Meanwhile, residual strain in NaCl-assisted films leads to a 20× quench in photoluminescence (PL) of the large domains. Additionally, when using the same growth parameters as alkali-free MOCVD, we find a high density of multilayer MoS2 particles and an atomic layer of Na at the MoS2/sapphire interface that destroys the epitaxial relationship between MoS2 and sapphireprecluding the ability to achieve large-scale, epitaxial 2D layers. Moreover, we compare transport properties of films that transferred to SiO2/Si substrates to understand if the variations found in the asgrown films translate to films removed from the substrate. Utilizing NaCl introduces a 1.5× decrease in mobility, 2× increase in subthreshold slope, and 100× reduction in the on/ off ratio when directly compared to alkali-free growths. We also find a 3× higher variation (>15 V) in threshold voltage in transistors that utilize alkali-assisted MoS2, suggesting that any residual Na remaining from the transfer process dominates the transistor performance.

Figure 1. Large-area epitaxy of MoS2. (a) Schematic of the MOCVD reactor, the substrate is loaded in a hot wall tube reactor with Mo(CO)6 and DES as a precursor; (b) photograph of monolayer MoS2 grown on a 2 cm2 sapphire substrate; (c,d) SEM (c) and AFM (d) images of 1L MoS2, confirming the uniformity and low roughness; (e) XPS spectra of synthetic MoS2 in Mo 3d range, showing that the Mo−O bonding is below the detection limit (20× increase in domain size compared to the growth without NaCl (Figures 2a, S4, and S5); however, this trend does not extend beyond very short growth times. SEM images of MoS2 after a 2 min growth demonstrate that domains grown with NaCl are >1 μm in lateral size (Figure 2a) compared to ∼50 nm for alkali-free (Figure 2d). However, when the growths are extended beyond 2 min, we find that the large MoS2 domains (L-MoS2 hereafter) do not continue growing. Instead, small MoS2 domains (S-MoS2 hereafter) nucleate and grow to fill the gaps between L-MoS2 to form a coalesced monolayer film after 60 min (Figure 2b,c). In

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RESULTS AND DISCUSSION Optimization of alkali-free MOCVD (Figure S1) on c-plane (0001) sapphire enables the synthesis of highly uniform, stoichiometric, epitaxial MoS2. To achieve full coalescence, films are synthesized using 1.2 × 10−3 sccm of molybdenum hexacarbonyl [Mo(CO)6] and 2.71 sccm diethyl sulfide (DES) with 565 sccm Ar as a carrier gas at 900 °C, 10 Torr for 2 h. Figure 1a presents a schematic of the MOCVD reactor and photo of a typical sample (Figure 1b), demonstrating a uniform coverage across a 2 × 2 cm2. Low-magnification scanning electron microscopy (SEM) (Figure 1c) confirms the uniformity of the film, with atomic force microscopy (AFM) characterization, confirming that the film is particle-free with 80% (based on the intensity change) of the MoS2 film. Interestingly, after the first cycle, the Na signal (Figure 3f, red curve) increases nonuniformly across the surface, with a greater increase in S-MoS2 regions compared to L-MoS2 (Figure S8). Figure 3f (inset) is a Na signal intensity map after the first sputtering cycle, showing wide-spread, nonuniform coverage of Na ions on the surface, following the removal of the MoS2 layer from the surface. The higher Na concentration beneath S-MoS2 regions (Figure 3f inset, bright yellow area) indicates that a layer of Na is present at the MoS2/sapphire interface in these regions. However, the lack of a strong Na signal below L-MoS2 domains (Figure 3f inset, brown triangles) indicates a much lower density of Na in these regions. These findings correlate well with AFM data (Figure 3g) that shows the L-MoS2 has a lower z-height value than its surrounding S-MoS2 counterparts, indicating that the surrounding S-MoS2 is higher than L-MoS2likely from a Na interlayer. Attempts are made to probe if Na is filled in the MoS2 lattice by high-resolution scanning TEM (HRSTEM), but we do not see the clear evidence that suggests the substitutional doping of Na into the MoS2 monolayer lattice. Combining AFM, TEM, XPS, and TOF-SIMS (Figures 3, S7, and S8) leads to the hypothesis that Na plays multiple roles in the nucleation, growth, and properties of MoS2. At the very early stages of the growth, Na only partially passivates the sapphire surface with low-density Na−O bonds (based on XPS Figure S7) that initially disrupt the epitaxial relationship (based on AFM/SEM, Figures 2 and 3), but subsequently enhance the growth rate by lowering the energy barrier for domain-edge Mo−S bonding and catalyze stronger interfacial interactions (based on Raman, Figure 2).11 However, as the growth time goes beyond the initial stages, the substrate surface becomes fully saturated with Na−O bonds that passivate the in-plane L-MoS2 growth front (also contributed 40834

DOI: 10.1021/acsami.8b16374 ACS Appl. Mater. Interfaces 2018, 10, 40831−40837

Research Article

ACS Applied Materials & Interfaces

even after transfer. Similar to as-grown films, the heterogeneity of the film likely plays a role in the wide ranging variability in FET characteristics; however, one would expect the interface heterogeneity to be eliminated upon film transfer if Na is dissolved in water during transfer.13 We find that this is not the case; in fact, the device-to-device variation is magnified compared to as-grown films, suggesting that even if Na is removed, it has altered the intrinsic properties of MoS2, potentially creating charge trap states within the MoS2 layers themselves.43−46 To verify the discussed trends, we randomly measure 5 EG-FETs (due to the long charging/reset time) and 9 BG-FETs for each condition and create quartile plots of on/ off ratio and mobility (Figure 4d,e), validating the previous discussion on as-grown and transferred MoS2(NaCl). We note that the differences discussed here cannot be attributed simply to variations between gated measurements because the results are repeatable in both the EG, nontransferred devices and in BG, transferred devices, as shown in Figures S10 and S11. In the transferred device, the interaction between film and substrate is weaker; hence, the gate voltage has a stronger effect over the transport in the channel, and we were able to more easily observe the change in mobility and on/off ratio related to the addition of dopants. Unlike devices on exfoliated and transferred TMDs, synthetic TMDs on 3D crystalline substrates always exhibit stronger film/substrate coupling such as chalcogen passivating interlayer,14 charge transfer,29 and Coulomb screening.47 This unique coupling is a convoluting factor when trying to understand how dopants impact the threshold voltage, mobility, on/off ratio, and low-frequency noise in as-grown cases.29,47,48 Finally, we note that most highperformance synthetic MoS2 FETs are from powder-grown, single-crystalline grains with careful device fabrication process.29,48 Therefore, future works to remove the antiphase grain boundaries and optimize the device fabrication focusing on large-scale films may lead further improvements on the performance of synthetic 2D films.

by the enhanced tensile strain) and dramatically reduces the surface energy of the sapphire similar to that of polymer functional layers.35 With a reduction in surface energy, one would expect a dramatically reduced growth rate, preference for molecular cohesion over substrate adhesion, and the loss of an epitaxial relationship between the film and substrate. This is the case for S-MoS2. On the basis of TOF-SIMS (Figure 3f), SMoS2 nucleates and grows on top of the Na−O layer evident by the 0.2 nm height difference between the L-MoS2 and SMoS2 (Figure 3g). Because of this, the S-MoS2 exhibits a low growth rate (Figure 2), high density of multilayer particles (Figure 3b), and loss of epitaxy (Figures 2 and 3). Figure 3h presents a hypothesized schematic of NaCl-assisted CVDinduced growth heterogeneities. Non-uniformities in alkali-assisted MOCVD MoS2 films lead to degraded and variable electrical performance. This is true regardless of the device type based on the evaluation of asgrown films on sapphire with electrolyte-gated (EG) (gate materials: poly (ethylene oxide) and CsClO4) FETs (details about the electrolyte gate is described elsewhere36)14,29,36,37 and transferred films on SiO2/Si using back-gated (BG) FETs (see Figure S9 for transfer details). Figure 4a provides representative transport characteristics of as-grown, EG FETs. Clear differences in threshold voltage and subthreshold swing (SS) between the films grown with (noted with “NaCl” subscript) and without NaCl (no notation) are observed. Detailed FET analysis reveals that the field effect mobility is similar in both as-grown cases (μNaCl = 3.8 ± 0.8 cm2/V s; μ = 3.4 ± 1.3 cm2/V s). However, the NaCl-assisted films exhibit a degraded SS by 2× (SSNaCl = 504 ± 30 mV/dec; SS = 238 ± 6 mV/dec), with a threshold voltage (Vth) shift of +2 V. The increase of SSNaCl can be expected when considering: (a) additional capacitance due to the interfacial trap charges caused by the Na residual layer between MoS2 film and the substrate38 and (b) the high surface particle density (Figure 3b) that leads the nonconformal gate/film contact.39 The Vth shift is likely due to the presence of a Na−O interface layer. Typically, synthetic MoS2/c-sapphire exhibits significant charge transfer (electron transfer from sapphire to MoS2) that heavily n-type dopes the MoS2 because of strong film/ substrate coupling.29 In our case, when using NaCl during the growth, the Na−O interface likely suppresses charge transfer from the substrate and thereby decreases the electronic signature of n-type doping. Prior work claims that transferring MoS2 from the growth substrate to pristine SiO2−Si substrates eliminates adverse impacts that Na may yield.13 To directly test this theory, we fabricate conventional BG FETs from transferred MoS2 on a 300 nm SiO2−Si substrate. Although the performance of devices can vary widely based on the quality of the transfer technique, relative changes between NaCl-free and NaClassisted MoS2 can be readily extracted. In the transferred film case, we find that in addition to a degraded SS for MoS2(NaCl) (SSNaCl = 11.2 ± 1.5 V/dec; SS = 6.2 ± 1.1 V/dec), the field effect mobility of MoS2(NaCl) is degraded by >30% compared to MoS2 (μ = 6.0 ± 1.3 cm2/V s; μNaCl = 3.8 ± 0.7 cm2/V s) (Figure 4b,e). These mobility values fall in the range of reported room-temperature measurements (0.02−30 cm2/V s) on single- or polycrystalline MoS2.11−13,18,29,35,40−42 The most significant impact may be a >100× reduction of on/off ratio (on/offNaCl = 104 to 105; on/off = 106 to 107) (Figure 4b,d), a ∼20 V shift in Vth, and dramatic increase in Vth variability (Figure 4c), suggesting that Na may still impact the interface

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CONCLUSIONS In conclusion, our work highlights a variety of trade-offs that one must consider when utilizing NaCl in monolayer MoS2 epitaxy. Alkali-assisted growth clearly enables a significant increase in the growth rate and domain size by reducing energy barriers for precursor vaporization, enhancement of adatom mobility. However, this work also presents evidence that use of NaCl destroys the epitaxial relationship between MoS2 and sapphire and deposits a Na-ion interfacial layer that varies in density based on growth time and NaCl temperature, under slower growing domains, which impacts device performance in as-grown and transferred films. Although some heterogeneities in alkali-assisted MOCVD, such as domain growth rates and the density of surface particles, can be overcome by continued optimization of the MOCVD process, evidence suggests that regardless of growth parameters, the loss of epitaxy and Na interfacial layer cannot be avoided. Furthermore, before utilizing alkali metals in the synthesis of 2D semiconductors, one must consider if the end application can tolerate the presence of alkali metals in the required device architectures.

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EXPERIMENTAL SECTION

Brief descriptions of experimental details are discussed here. Further details can be found in the Supporting Information. MOCVD Synthesis. The MOCVD growth of MoS2 is conducted in a home-built MOCVD system. Two separate bubblers are filled 40835

DOI: 10.1021/acsami.8b16374 ACS Appl. Mater. Interfaces 2018, 10, 40831−40837

Research Article

ACS Applied Materials & Interfaces with 25 g molybdenum hexacarbonyl (Mo(CO)6) and 100 ml DES in the glovebox. Before the growth, the main chamber is pumped to vacuum (18 mTorr) for 10 min to remove contaminants, moisture, and so forth, followed by pressurizing the chamber to 10 Torr with Ar. The temperatures for Mo(CO)6 and DES bubblers are set at 24 and 22 °C, respectively. The pressure for both bubblers is 735 Torr. During the growth, the chamber is ramped up to the designated growth temperature 900 °C at 50 °C/min. When the temperature reaches the growth temperature, a 2 min nucleation step with 2 sccm H2 and 45 sccm H2 through Mo(CO)6 bubbler and DES bubbler, respectively, and a subsequent 2 min ripening step are carried out, as shown in Figure S1. After the ripening stage, the flow rate of H2 for Mo(CO)6 was increased to 5 sccm for the rest of the growth. Raman and PL Characterization. Raman and PL characteristics of MoS2 films are measured by a Horiba Raman system with a 532 nm laser at 1 mW power at room temperature. The integration time is 5 s, and the spectra shown in this work are integrated twice. XPS Characterization. The XPS characterization is carried out with a Physical Electronics VersaProbe II. The X-ray source is Al Kα (hν = 1486.7 eV). High-resolution spectra are taken over an analysis area of 200 μm at a pass energy of 23.5 eV and resolution of 0.1 eV. The data are analyzed in CASA software. Transfer and HRSTEM Characterization. The as-grown monolayer MoS2 film is transferred onto a TEM Quantifoil grid for TEM analysis and onto patterned Si for device fabrication via an etchant-free PMMA-assisted transfer method. Aberration-corrected (S)TEM imaging and microscopy are performed by a FEI Titan 60300 microscope at Penn State, operating at 80 kV with a monochromated beam and spherical aberration correction, providing sub-angstrom imaging resolution. A high-angle annular dark field detector with a collection angle of 51−300 mrad is used for the ADFSTEM imaging. A camera length of 115 mm, beam current of 45 pA, beam convergence of 30 mrad are used for STEM image acquisition. XRD Characterization. In-plane XRD measurements are performed using a PANalytical MRD diffractometer equipped with a Cu-anode X-ray tube operated at 45 kV accelerating voltage and 40 mA filament current.

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(LEAST), one of the six SRC STARnet Centers sponsored by MARCO and DARPA. K.X. and S.F-S. acknowledge partial support from the Center for Low Energy System Technology (LEAST), one of the six SRC STARnet Centers sponsored by MARCO and DARPA, and partial support from DMR-EPM under grant no. 1607935. F.Z. and M.T. acknowledge the support from the Center for Atomically Thin Multifunctional Coatings Project (S3-S16) and the Center for 2-Dimensional and Layered Materials at Penn State. K.W. acknowledges the support by the Pennsylvania State University Materials Characterization Laboratory Staff Innovation Funding (SIF).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b16374. Detailed experimental procedures, additional Raman/ PL, TEM, AFM, SEM, XPS, and device measurements (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kehao Zhang: 0000-0003-4405-2438 Ke Xu: 0000-0003-2692-1935 Mikhail Chubarov: 0000-0002-4722-0321 Joan M. Redwing: 0000-0002-7906-452X Susan K. Fullerton-Shirey: 0000-0003-2720-0400 Joshua A. Robinson: 0000-0001-5427-5788 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work at Penn State was conducted as part of the Center for Atomically Thin Multifunctional Coatings (ATOMIC), sponsored by the National Science Foundation (NSF) division of Industrial, Innovation & Partnership (IIP) under award # 1540018 and the Center for Low Energy System Technology 40836

DOI: 10.1021/acsami.8b16374 ACS Appl. Mater. Interfaces 2018, 10, 40831−40837

Research Article

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DOI: 10.1021/acsami.8b16374 ACS Appl. Mater. Interfaces 2018, 10, 40831−40837