The usage of molten salts, for example, Na2MoO4 and Na2WO4, has shown great success in the growth of two-dimensional (2D) transition metal dichalcogenides (TMDCs) by chemical vapor deposition (CVD). In comparison with the halide salt (i.e., NaCl, NaBr, and KI)-assisted growth (Salt 1.0), the molten salt-assisted vapor–liquid–solid growth technique (Salt 2.0) has improved the reproducibility, efficiency, and scalability of synthesizing 2D TMDCs. However, the growth of large-area MoSe2 and WTe2 is still quite challenging with the use of the Salt 2.0 technique. In this study, a renewed Salt 2.0 technique using mixed salts (e.g., Na2MoO4–Na2SeO3 and Na2WO4–Na2TeO3) is developed for the enhanced CVD growth of 2D MoSe2 and WTe2 crystals with a large grain size and yield. A continuous monolayer MoSe2 film with a grain size of 100–250 μm or isolated flakes up to ∼450 μm is grown on a halved 2 inch SiO2/Si wafer. Our study further confirms the synergistic effect of Na+ and SeO3 2– in the enhanced CVD growth of the wafer-scale monolayer MoSe2 film. Thus, the addition of Na2SeO3 and Na2TeO3 into the transition metal salts could be a general strategy for the enhanced CVD growth of many other 2D metal selenides and tellurides.
The structure and morphology of monolayer 2H-MoTe2 on GaAs(111)B grown by molecular-beam epitaxy have been studied using scanning tunneling microscopy, electron diffraction, and X-ray photoelectron spectroscopy. The MoTe2 film grown and annealed under the Te-rich condition is mainly composed of a pure 2H phase, while, under the Te-deficient conditions, the 2H-MoTe2 phase begins to evolve into nanowire-like structures owing to the desorption of Te. The 2H-MoTe2(0001) film on GaAs(111)B exhibits two types of energetically favorable epitaxial orientations; one is a perfect alignment of [11$$\overline{2}$$ 2 ¯ 0]MoTe2 // [1$$\overline{1}$$ 1 ¯ 0]GaAs, and the other shows a slight in-plane rotation of ± 0.77∘, which reduces the effective lattice mismatch between MoTe2 and GaAs.
source, ultrasensitive sensor, have been demonstrated using the mechanically exfoliated MoS 2 flakes from its bulk crystal. [1][2][3][4] Because of poor management in thickness, scalability, and uniformity, however, mechanical exfoliation is difficult for future practical applications. Therefore, new routes to synthesize TMDCs over a large area are actively explored mainly based on powder-source chemical vapor deposition (powder-source CVD) and metalorganic chemical vapor deposition (MOCVD). [5,6] Due to low-cost and straightforward experimental setup, powder-source CVD has been widely used for TMDCs growth, where both solid precursors of transition metal oxides (MoO 3 or WO 3 ) and elemental chalcogen (sulfur/ selenium) powders are evaporated, transported, and mixed in the heated furnace for TMDCs deposition. Powder-source CVD has promoted enormous advances in fundamental studies but lacks the ability in controlling the supply of these precursors precisely and independently, making it difficult to realize scalable growth on a large wafer. Moreover, the poisoning effect of transition metal oxides caused by chalcogen vapor is typically inevitable in powder-source CVD, [7] which further degrades the controllability and reproducibility of growth. Vapor-phase-assisted growth of MoS 2 is also reported recently, where MoO 3−x precursor film is pre-deposited on a substrate, followed by a postsulfidation process to form relatively uniform MoS 2 layers. However, flexible and precise control of the MoO 3 supply is still disabled for this method. [8] In contrast, MOCVD has been proven to be a scalable and industrial technique for the growth of conventional III-V semiconductors. [9,10] In recent years, MOCVD has also been used for 2D materials growth, including TMDCs. [6,11] In addition to the long deposition time commonly required for monolayer growth, MOCVD growth of TMDCs typically suffers from unintentional carbon contamination, leading to inferior crystallinity. [12,13] Hence, scalable growth of TMDCs with high crystalline quality and less carbon contamination is strongly desirable but still challenging. Besides the requests for growth techniques, the selection of substrate could be another crucial factor to boost and optimize 2D materials growth. A wide variety of substrates, such as sapphire, SiO 2 /Si, Ga 2 O 3 , etc., have been utilized for TMDCs growth. [14][15][16][17][18] Among them, glass substrate shows several advantages, other than its lower cost, both in growth processes and device applications of A newly developed oxide scale sublimation chemical vapor deposition (OSSCVD) technique for 2D MoS 2 growth is reported. Gaseous MoO 3 , which is supplied separately from H 2 S, can be generated in situ by flowing O 2 over Mo metal with oxidation and sublimation processes. In this method, particularly, controllably and abruptly modulating the supply of MoO 3 is achievable by precisely tuning O 2 flow. Having appropriate conditions, where the generation rate of MoO 3 on the Mo metal surface is not larger than its sub...
Highly efficient growth of a centimeter‐scale MoS2 monolayer film by oxide scale sublimation chemical vapor deposition (OSSCVD) in a time as short as 60 s is reported. Benefiting from the superior catalytic ability of Dragontrail glass (DT‐glass) substrate and the controlled large vapor supersaturation of the molybdenum source, the ultrafast deposition of MoS2 is realized with maintaining large‐sized single‐crystalline domains over 20 µm at maximum in the film. It is comparable to those reported for MoS2 grown in tens of minutes and even hours. Similar to the face‐to‐face precursor feed route, the gas‐controlled OSSCVD with a showerhead configuration facilitates a homogeneous and controllable source supply. It enables high‐quality monolayer MoS2 film deposition on 2 × 2 cm2 DT‐glass with centimeter‐scale uniformity confirmed by microscopic, spectroscopic, and electrical characterizations. Back‐gate MoS2 field‐effect transistors fabricated on polycrystalline continuous film exhibit the maximum field‐effect mobility of 5.1 cm2 V−1s−1 and a peak Ion/Ioff ratio of 5 × 108. They reach 40 cm2 V−1 s−1 and 1.2 × 109, respectively, on single‐crystalline domains. These results are even greater than those for MoS2 grown using 1–2 orders of magnitude longer deposition time and higher temperatures. This study highlights the opportunities for low‐cost high‐throughput production of large‐area high‐quality monolayer MoS2.
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