The primary hurdle in commercializing these materials is the difficulty in producing their large wafer-scale forms, which may not necessarily be single crystalline. Researchers have the flexibility to choose a fabrication process for TMDs, whether single-crystalline or polycrystalline, based on the needs of the application and cost factors. Significant advancements have been achieved recently using chemical vapor deposition (CVD) and metal organic chemical vapor deposition (MOCVD) techniques. For instance, 12-inch polycrystalline monolayer MoS2 by CVD (Y. Xia et al., Nat. Mater. 22, 1324–1331; 2023) and low-temperature growth of 8-inch monolayer MoS2 by MOCVD for back-end of line (BEOL) integration (J. Zhu et al., Nat. Nanotechnol. 18, 456–463; 2023) have been realized in 2023. When asked about the most promising technique, Wang feels that both techniques hold industrial potential. The CVD method currently has higher materials quality, but MOCVD catches up quickly and has advantages in wafer-scale uniformity and repeatability. “We recently developed a halide vapor phase epitaxy method for TMD epitaxy, which is widely used in the III–V semiconductors industry and may bring a universal way to realize wafer-scale epitaxy of single-crystal TMDs,” says Wang (T. Li et al., Natl Sci. Open 2, 20220055; 2023).
Ensuring low-defect semiconductor 2D materials is one of the primary objectives pursued by the community, considering the defects introduced during the manufacturing process that can be detrimental to device performance. “The point defects density currently amounts to about 1012–1013 cm–2 in CVD-grown materials. This value should be reduced by 3 orders of magnitude,” adds Wang. Ideally, like silicon, single nucleation growth should be applied to achieve high-quality TMD materials, but seamless merging of multiple nuclei is a more practical way for wafer-scale growth, because of the 2D nature of TMDs. Great effort has been devoted to understanding the growth mechanism and controlling the growth conditions. For unidirectional epitaxy, for example, various mechanisms have been proposed, such as surface symmetry, surface chemistry, surface steps, and buffer layers. By far, most epitaxial growth mechanisms are explained from thermodynamic considerations. Wang believes that more studies on growth kinetics: the reaction pathway, surface migration, and reaction rate on specific substrate surfaces, should be carried out. To this end, in situ characterizations at the atomic level, combined with advanced simulations, will provide powerful toolkits for unveiling the real-time growth mechanism.
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