Two dimensional atomically thin Transition Metal Dichalcogenides (TMDs) are of pronounced focus owing to their semiconducting nature. This article discusses the synthesis of TMDs via chemical vapor deposition, the challenges and future outlooks.
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2D Transition Metal Dichalcogenides
TMDs, or Transition Metal Dichalcogenides, are a novel class of two-dimensional materials with characteristics ranging from metals (e.g., NbTe2, TaS2) to superconductors (e.g., NbSe2, NbS2, PdTe2) to semiconductors (e.g., WSe2, MoS2, WS2).
Mechanical exfoliation was used in the early stages of development to prepare 2D TMDs. However, this method cannot produce the scalable films required for industrial applications. As a result, to fully benefit TMD-based devices from the laboratory to the industry, a scalable synthesis method must be developed.
Chemical Vapor Deposition Growth of TMDs
Chemical Vapor Deposition (CVD) is widely regarded as the most assured fabrication technique for producing large-area high-quality 2D TMDs at a competitive cost. TMDCs are grown via CVD by heating reaction precursors such as transition metal oxide and pure chalcogen in a furnace. The reaction precursors are exposed to the substrate (particularly SiO2/Si) at high temperatures (ranging from 650 to 1000 °C) in the atmosphere of N2 or Ar. Unlike graphene, TMDs can be grown directly on insulating substrates.
Plasma-enhanced CVD, atmospheric-pressure CVD, and low-pressure CVD are examples of CVD methods for TMDs film synthesis. These methods yield ultrathin 2D TMDCs nanosheets with outstanding electronic properties.
Metal-Organic Chemical Vapor Deposition of TMDs
Apart from conventional CVD, TMDs have been synthesized using metal-organic chemical vapor deposition (MOCVD). Unlike the traditional CVD mentioned above, MOCVD utilizes gaseous precursors rather than solid precursors, and MOCVD is typically performed at lower temperatures ranging from 300 to 900 °C. MOCVD has demonstrated more uniform and consistent wafer-scale growth than traditional CVD
Researchers, for example, utilized the MOCVD system to produce a 4-inch wafer-scale uniform MoS2 and WS2 monolayer on insulating SiO2 substrates. They chose tungsten hexacarbonyl (THC), molybdenum hexacarbonyl (MHC) and dimethyl sulphide (DES) as W, Mo, and S precursors, respectively. According to new findings, batch-producing 6-inch monolayer MoS2 on a soda-lime glass substrate is also possible.
Single Crystal Growth of TMDs
Grain boundaries may form during the CVD growth of 2D-films, weakening the electronic properties. One strategy to target this is to grow large single crystals from a single nucleus. Scientists reported that the size of monolayer MoSe2 single crystals on molten glass could reach ~2.5 mm. The molten glass formed a homogeneous and quasi-atomic smooth surface, reducing the nucleation sites for the growth of large MoSe2 crystals.
The use of Au substrate is another method for growing millimeter-scale single crystals. Because of tungsten's low solubility in Au, the precipitation mechanisms for growing multilayer crystals are significantly hampered.
Recent Advancement in CVD Growth of TMDs
Self-limited growth (SLG) has been proposed by scientists as a promising direction for achieving the high-quality large-area uniform TMDs film desired for optoelectronic applications.
Layer-dependent properties exist in 2D TMDs. Recently, some impactful strategies to control the second layer growth of TMDs have been developed. Scientists have reported controllable growth of bilayer MoS2 through reverse gas flow. The reverse carrier gas flow from the substrate to the precursor source hindered the first layer of MoS2 from nucleating.
Elemental doping is an efficient method for tuning the properties of TMDs. Doping heteroatoms during the CVD process is an efficient method for controlling the doping condition. Previous work reported in situ doping of monolayer MoS2 with manganese (Mn) using CVD techniques.
Manufacturers of TMDs CVD Equipment
FirstNano®, CVD Equipment's R&D product line, is one of the largest CVD manufacturers for wafer-scale growth of uniform 2D TMDs film.
PlanarTech, based in the United Kingdom, is a global leader in CVD for 2D material synthesis. They intend to facilitate the development of graphene and 2D material applications by providing high-quality materials synthesis and characterization equipment to researchers worldwide.
AIXTRON Ltd (UK) is one of the world's leading suppliers of CVD deposition equipment to the semiconductor industry and a key partner in the Graphene Flagship's 2D Experimental Pilot Line initiative.
Challenges for Scaling CVD grown TMDCs
One of the most difficult challenges in CVD growth TMDC is precisely controlling the vapor concentrations of reactants throughout the CVD process. Furthermore, continuous film growth is time-consuming, and precise control of film thickness across the entire wafer-scale substrate is difficult.
Contamination and defect risks, low reproducibility and limited grain size are all issues that must be addressed before the industrialization of 2D TMDs materials.
Future Outlooks
Despite the difficulties mentioned above, the novel design of a CVD arrangement with an appropriate selection of precursors, substrates, additives, and temperature may open up a plethora of opportunities for future TMDs applications.
There is no doubt that the CVD method for 2D TMDC growth has advanced significantly in the last decade, with applications in the semiconductor industry including flexible integrated circuits, optoelectronics, and flexible electronic devices.
Continue reading: How is Chemical Vapor Deposition Used to Create Thin Films for Semiconductors?
References and Further Reading
Wang, J., Li, T., Wang, Q., Wang, W., Shi, R., Wang, N., Amini, A. and Cheng, C., (2020) Controlled growth of atomically thin transition metal dichalcogenides via chemical vapor deposition method. Materials Today Advances, 8, p.100098. https://doi.org/10.1016/j.mtadv.2020.100098
Hoang, A., Qu, K., Chen, X. and Ahn, J., (2021) Large-area synthesis of transition metal dichalcogenides via CVD and solution-based approaches and their device applications. Nanoscale, 13(2), pp.615-633. https://doi.org/10.1039/D0NR08071C
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