Growth of Complex 2D Material-Based Structures with Naturally

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Growth of Complex 2D Material-Based Structures with Naturally Formed Contacts Shrouq H. Aleithan,† Thushan E. Wickramasinghe, Miles Lindquist, Sudiksha Khadka, and Eric Stinaff* Department of Physics and Astronomy, Nanoscale and Quantum Phenomena Institute, Ohio University, Athens, Ohio 45701, United States

Downloaded by 95.181.177.143 at 10:59:36:203 on May 31, 2019 from https://pubs.acs.org/doi/10.1021/acsomega.9b00955.

S Supporting Information *

ABSTRACT: The difficulty of processing two-dimensional (2D) transition metal dichalcogenide (TMD) materials into working devices with any scalability is one of the largest impediments to capitalizing on their industrial promise. Here, we describe a versatile, simple, and scalable technique to directly grow self-contacted thin-film materials over a range of TMDs (MoS2, MoSe2, WS2, and WSe2), where predeposited bulk metallic contacts serve as the nucleation site for the TMD material to grow, forming naturally contacted device structures in a single step. The conditions for growth as well as optical and physical properties are reported. Because the material grows controllably around the lithographically defined patterns, wafer scale circuits and complex device geometries can be envisioned, including lateral heterostructures of different TMD materials.

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INTRODUCTION In the monolayer limit, some of the most studied transition metal dichalcogenides (TMDs), including MoS2, MoSe2, WS2, and WSe2, display a direct band gap, large spin−orbit coupling, and broken inversion symmetry. These characteristics, in addition to enhancing its linear and nonlinear optical and electronic properties, may provide novel material-enabled functionality in emerging areas such as spintronics or valleytronics.1−7 Early devices such as ultrasensitive photodetectors and high-mobility transistors have shown promising results;8−12 however, nearly every seminal result in twodimensional (2D) material-based device fabrication has followed a standard procedure, which is to isolate the monolayer material, often through mechanical exfoliation and produce a one-off device using e-beam lithography and metal deposition, an extremely challenging process to scale. Chemical vapor deposition (CVD), which had been successfully used for large-scale graphene production,13 has emerged as one of the most promising and preferred processes for TMD synthesis.14−21 Typically, because of random nucleation, the location, number, and size of the crystals grown are not controllable. However, CVD growth of continuous thin films has been reported using different techniques, such as seeding the substrate with aromatic molecules, which has been found to produce high-quality, large-area, monolayer growth on a variety of substrates.16,18,20,22−27 Another promising technique involves the deposition of thin films of metal or metal oxide, which are then sulfurized directly or in the presence of an additional oxide precursor.15,21,28−33 The predeposited films may be patterned, providing a degree of control over the TMD film location; however, the predeposited metal or metal oxide films are completely consumed during the growth, leaving an isolated, and uncontacted, TMD layer where the film had been. © 2019 American Chemical Society

Therefore, even using CVD techniques, devices are fabricated postgrowth and almost always in a manner which is neither straightforward nor scalable, typically involving a combination of etching, transfer, lithography and metal deposition, and often requiring further processing to obtain good metal− semiconductor contacts. We have reported a versatile, simple, and scalable method for reproducibly creating as-grown 2D material-based devices.34 However, the process demonstrated a moderate yield of around 50% for complete thin-film coverage between patterns and had only been confirmed for sulfur-based TMDs. We report here a study of modifications that result in a higheryield, large-area process to grow mono-to-few layer MoS2, MoSe2, WS2, and WSe2, where the shape, size, and location of the TMD material are determined by the predefined transition metal patterns. We also report the observation that, under specific conditions, the growth is selective regarding the type of transition metal used for the contact, for example with MoS2 growing only on Mo and WS2 on W. The materials produced display strong luminescence and monolayer Raman signatures, indicating high optical quality 2D material deterministically and selectively over large regions. While the primary focus of this work is describing the process for producing thin films over a range of TMDs and presenting the optical and physical characterization of the films, basic current-versus-voltage (IV) measurements show conduction through the material, indicating a path to producing as-grown metallic contacts for simple device fabrication and large-scale production. In the following sections, we present details of the growth process Received: April 3, 2019 Accepted: May 20, 2019 Published: May 31, 2019 9557

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Table 1. Parameters for TMD Growth from Bulk Metallic Contacts precursors

growth

material

MoO3/WO3 (mg)

S/Se (mg)

time (min)

flow (sccm)

temperature (°C)

flow gas

MoS2 WS2 MoSe2 WSe2

0.5 1 0.5 1

60 60 30 30

5 8 5 8

3 4 3 4

780 850 780 850

Ar Ar (15% H2)/Ar (25% H2)/Ar

sulfur vapor react while being delivered to finally adsorb and nucleate on the substrate. While the exact ratio is difficult to predict, the relative prevalence of both processes is affected by the temperature and location of the substrate and reactants.37 Presumably, it is also affected by the rate at which the reactants are delivered to each other and the substrate. One advantage of using bulk metallic patterns is that despite the complex nature of thin-film TMD growth via CVD, consistent and reproducible materials growth is achievable. After refining the process, over 90% of the growth runs resulted in mono-to-few layer material deterministically localized between, and attached to, the metallic patterns. Though there have been reports of using patterned graphene electrodes in a similar manner, to nucleate 2D TMD material,38−40 here we observe that the specific metal deposited has an additional effect on the growth process. Though the details are still being investigated, the metallic patterns are playing an active role beyond simply serving as a nucleation site.

and materials characterization for the four most prominent TMDs, MoS2, MoSe2, WS2, and WSe2, produced using this technique. Growth Process. Metallic patterns of either Mo or W were produced using direct current sputtering (thickness of 160 ± 10 nm) and a basic lift-off lithography process. After removal of the photoresist, the samples were cleaned using de-ionized water, acetone, and isopropanol. The TMD material growth was performed using a one-inch tube furnace at atmospheric pressure,34,35 and a summary of the parameters used for the growths characterized in this work is displayed in Table 1. A mass of 0.5 mg or less of MoO3 (1 mg or less of WO3) powder was placed on a graphite holder along with the target substrate, which was placed with the metallic pattern facing down on the same holder 1−2 cm downstream from the powder. The graphite holder was placed in the center of the furnace, while a boron nitride boat containing the chalcogen was positioned just outside the furnace. The furnace was ramped up to the growth temperature in three stages: first to 550 °C where it remained for 5 min, then to 650 °C for 10 min, and finally to the growth temperature, 780 °C for Mobased TMD growth and 850 °C for W-based TMD growth. When the furnace reached the growth temperature, the tube was shifted such that the chalcogen boat was at a predetermined distance from the edge of the furnace where the temperature was measured to be 200 °C for sulfur or 400 °C for selenium. A continuous flow of argon was maintained at 50 sccm during the first 5 min temperature ramp, and the flow was then lowered to 3 sccm until the sample is removed. For the selenide-based growth, a mixture of H2/Ar was used during the low flow period of the process. The growth time, which was typically between 5 and 10 min, was measured from when the selenium or sulfur started to melt. At the conclusion of the growth, the furnace was turned off and allowed to cool naturally back to room temperature. This process is based on a typical CVD reaction involving a chalcogen and transition metal oxide-based precursor. For example, when utilizing an oxide precursor for growth of MoS2, the process relies on the reduction of MoO3 to the suboxide MoO3−x, followed by the replacement of oxygen by vaporized sulfur. This multistep reaction is shown in eqs 1 and 2 below.36 MoO3 +

x x S → MoO3 − x + SO2 2 2

(1)

MoO3 − x

7−x 3−x + S → MoS2 + SO2 2 2

(2)

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RESULTS AND DISCUSSION Optical images in Figure 1 provide a comparison of the four TMD material growths done following the parameters outline

Figure 1. Optical images for the four TMD growths. (a) MoS2 grown on Mo contacts display a region of monolayer material approximately 5 μm out from the contact. (b) WS2 grown on W contacts, (c) MoSe2 grown on Mo contacts, and (d) WSe2 grown on W contacts all show large areas of continuous monolayer material between the contacts where the material meets, unlike the MoS2 example shown. All images show multilayer material growing out from the contacts. The scale bars are all 10 μm.

in Table 1. For MoS2, shown in Figure 1a, optical contrast indicates a region of monolayer material roughly 5 μm out from the molybdenum contacts with few-to-multilayer material near the metallic contacts as well as at the edge of the material. For WS2 and WSe2, the optical images [Figure 1b,d] show two tungsten contacts separated by 16 and 20 μm, respectively, with films of WS2 and WSe2 extending out and joining. In these two tungsten-based growths, significant monolayer regions are seen with multilayer material becoming predominant near the contact and spreading out. Similarly, for MoSe2, shown in Figure 1c, a significant amount of monolayer material

Interactions between the initial reactants in eq 1 take place after both are heated from their solid form, after which an inert gas delivers the reactants to each other. It has been theorized that there are two possible and perhaps simultaneous deposition scenarios taking place: one in which the MoO3−x suboxide is adsorbed on the substrate prior to their sulfurization and another in which the suboxide vapor and 9558

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Figure 2. (a) PL spectra taken from representative regions. The wavelengths listed in each panel designate the A peak for each material. (b) Raman spectra from representative regions. The Si peak (520 cm−1) is labeled for reference. All spectra were acquired using a Witec confocal Raman instrument with 532 nm excitation held at ∼0.5 mW.

excitation laser (532 nm).18,45 Monolayer WS2 displays a strong 2LA Raman mode at 351 cm−1 and a weak A1g mode at 417.5 cm−1, where, with increasing number of layers, the 2LA mode is red-shifted while the A1g mode is blue-shifted, causing the separation between the two modes to increase from 66.5 cm−1 for monolayer to 69 cm−1 for multilayer.46 The PL spectrum of MoSe2 [Figure 2a] shows the characteristic A excitonic state for different numbers of layers. The low energy A exciton is typically the most intense peak, centered around 821 nm, and shows a more than 1 order of magnitude increase in intensity when comparing a few-layer region to a monolayer region. For MoSe2, Figure 2b displays the out-of-plane A1g Raman mode located around 240.5 cm−1 in the case of monolayer, which is blue-shifted to 241.3 cm−1 for few-layer material, and the in-plane E12g mode (287 cm−1 for monolayer) with weaker intensity that is red-shifted to 285.8 cm−1 for fewlayer material, in agreement with previous studies.47 Finally, for WSe2, the A exciton is located at 760 nm for the monolayer, while the B exciton around 600 nm is significantly weaker. The Raman signal displays the expected modes for monolayer WSe2, namely E12g at 250 cm−1 for monolayer that blue-shifts for 2 layers and A1g at 260 cm−1 for monolayer which shows a small red shift for 2 layers.48 Unlike traditional device fabrication where the material synthesis and contact formation are separate processes, here both are produced concurrently. This provides a potentially significant advantage for large scale production; however, it does make isolating the contact versus material properties a challenge. For example, it is likely that oxidation of the transition metal pattern during the process, along with the potential formation of other transition metal complexes, adds complexity to the contact structure. Additionally, with the current sample structures, the metallic patterns in the regions with thin-film growth extend out to large (50 μm × 50 μm) contact pads, all consisting of the same transition metal, and it is found that layers of semiconducting TMD form on the metallic contacts. This, along with the possible formation of a transition metal-based silicide under the patterns, make it difficult to produce gated devices and to isolate the metal− TMD contact. We are currently investigating the incorporation of a diffusion barrier and methods to maintain pristine surfaces using nonreactive metals in selective areas for external contacting along with systematically studying the formation of oxides on the transition metal patterns. Despite these complications, simple current versus voltage (IV) measurements clearly show current passing through the material,

is observed with multilayer material forming near the contacts and spreading outward. A pixel-by-pixel analysis of the optical contrast for the images in Figure 1 gives ratios of monolayerto-multilayer material of 21% for MoS2, 32% for WS2, 56% for MoSe2, and 30% for WSe2. Qualitatively, the ratio of the monolayer-to-multilayer material is consistent for multiple samples grown under the described conditions. The competing processes of lateral versus vertical growth of the TMD material is still under investigation; however, for different growth parameters (not shown here), such as shorter growth time or different contact separations, a predominantly monolayer region between the contacts is achievable as is observed in the WS2, MoSe2, and WSe2 growths. Polarization-resolved second harmonic generation on samples grown under similar conditions has shown that the films are polycrystalline with domain sizes on the order of a few microns squared.34 As seen in Figure 1, for all the growths, the material grows out from the contacts and merges in the middle. Though the atomistic details of this interface are currently under investigation, optically they often appear continuous. Simple IV measurements, discussed below, also indicate that they are electrically connected and support transport through the as-grown film. In bulk form, TMDs have an indirect gap with energy in the near-infrared. As the number of layers is reduced, the indirect gap shifts to higher energy with an indirect-to-direct gap crossover occurring in the monolayer limit with two distinct excitonic features, A and B, which arise from the large spin− orbit splitting in the valence band. This results in the excitonic photoluminescence (PL) intensity increasing, typically by at least an order of magnitude, when comparing monolayer material to multilayer material. Additionally, certain Raman modes of each TMD are highly affected by the number of layers; therefore, both PL and Raman are widely used for quantifying the layer number. PL and Raman measurements taken in representative areas are shown in Figure 2 and were performed using a Witec confocal Raman system using a 532 nm laser with the power held at ∼0.5 mW. The PL intensity is plotted on a log scale to reveal any relatively weak exciton peaks, such as the B exciton for MoS2. The PL for the monolayer regions of MoS2 shows strong PL with a very low B/A intensity ratio indicative of a high-quality growth with low defect density.41,42 The A1g and E12g lines in the Raman spectra show an absolute separation of 21 cm−1 for the monolayer regions consistent with other CVD grown monolayer material.43,44 For WS2, the intense PL for the A exciton is located at 630 nm while the B exciton is higher in energy than our 9559

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junction. However, the fact that only MoS2 (WS2) grows around the Mo (W) implies that there is selectivity with regard to the nucleation and growth process. In a previous study, under different growth conditions, it was demonstrated that using one type of transition metal contact, W for example, with a different transition metal oxide, MoO3 for example, would result in the formation of vertical heterostructures.34 These results indicate that complex structures may be selectively grown through systematic control of the metal, precursors, and growth conditions.

confirming that electrical contact is being made between the deposited patterns of transition metal and the subsequently grown thin-films. These measurements were performed on a set of materials grown on quartz substrates to eliminate the possibility of leakage current through the SiO2 to the doped silicon substrate. Though all the samples displayed a general Schottky contact, metal−semiconductor−metal, type of behavior as shown in Figure 3, given the unique nature of

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CONCLUSIONS In summary, we report a versatile, simple, and scalable technique to directly grow self-contacted thin-film devices for TMD materials including MoS2, MoSe2, WS2, and WSe2, where the shape, size, and location of the TMD material are determined by the predefined transition metal patterns. The materials produced display strong luminescence and monolayer Raman signatures, indicating high-quality materials with significant percentages of monolayer coverage. The material grows out from the metallic contacts with large crystal domains, merging and forming a continuous film as determined optically and electronically. It was also observed that, under certain conditions, the growth is selective regarding the type of transition metal used for the contact, with MoS2 growing only on Mo and WS2 on W, opening the possibility for complex device geometries including heterostructures of different TMD materials.

Figure 3. Current vs voltage (IV) measurements demonstrate a Schottky contact, metal−semiconductor−metal, behavior. Though the absolute variation between the various types of TMD is large, variations within a specific type are typically within a factor of 10.

this growth/contacting method, further consideration of contact formation and device structure, beyond the scope of the current work, are necessary to fully understand the electrical properties. An intriguing prospect for this technique is the ability to produce complex device architectures, including lateral heterostructures. An example of this is shown in Figure 4, where separate Mo and W contacts were produced using a two-step lithography process [Figure 4a inset]. This structure was subjected to a hybrid growth process, where MoO3 and WO3 precursors were simultaneously placed in the furnace along with the substrate and sulfur in a configuration like that described in the above Experiment section. With a growth temperature of 850 °C, used for W-based materials, it was found that lateral heterostructures would form [Figure 4a]. PL and Raman measurements indicate that the material around the Mo contact, such as at Position 1 in Figure 4b, is purely MoS2 and around the W contact, such as at Position 3 in Figure 4b, is WS2. The transition region, where both MoS2 and WS2 Raman peaks appear [Position 2 in Figure 4b], occurs over a length comparable to our experimental resolution (∼1 μm), so we are currently unable to determine the details of the

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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/acsomega.9b00955. Illustration of the TMD crystal structure, Raman spatial map of the heterostructure sample, and description and images of the synthesis setup (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: stinaff@ohio.edu. ORCID

Eric Stinaff: 0000-0001-7485-0348 Present Address †

College of Sciences, Physics department, King Faisal University, Al-Ahsa, Saudi Arabia.

Figure 4. (a) Lateral heterostructure formed from two different transition metal patterns. Here, Mo and W contacts were subjected to growth parameters used for a typical WS2 growth but with both MoO3 and WO3 precursors present. (b,c) Raman and PL spectra indicate an abrupt transition, where the spatial separation between the three positions indicated is 1 μm. 9560

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Author Contributions

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ohio University Innovation Strategy as well as the Ohio University CMSS and NQPI programs. This research was supported by the deanship of scientific research at King Faisal University through Nasher track under serial number 186128.

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DOI: 10.1021/acsomega.9b00955 ACS Omega 2019, 4, 9557−9562