Epitaxial Growth and Characterization of MoS2 Monolayers on Sapphire Substrates

The epitaxial growth of monolayer (ML) MoS2 crystals was conducted on c-plane sapphire substrates with varying vicinal angles of 0°, 2°, 10°, and 15°. This process utilized a custom-built metal–organic chemical vapor deposition (MOCVD) system, where the substrates were heated to 850 °C for 20 minutes under a controlled pressure of 1.3 torr. The growth utilized precursors, specifically Mo(CO)6 and (C2H5)2S, with optimized flow rates of 0.3 s.c.c.m. and 0.6 s.c.c.m., respectively. The growth duration varied, with the longest time of 90 minutes observed on the 15° vicinal substrate due to atomic steps acting as diffusion barriers, while the 0° substrate required only 40 minutes.

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Following the growth, various fabrication techniques were employed to create devices from the MoS2 films. For instance, SiO2 windows were fabricated on sapphire wafers using conventional photolithography, followed by a lift-off process. The MoS2 films were then transferred onto copper transmission electron microscopy (TEM) grids using a polymethyl methacrylate (PMMA)-assisted wet-transfer method.

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To create Hall-bar devices, a thermoplastic methacrylate copolymer stamp was utilized to assemble hBN/MoS2/hBN stacks. The assembly involved heating the stamp and subsequently dissolving it in hot acetone. The MoS2 channels were patterned into Hall-bar structures using electron-beam lithography, followed by reactive ion etching to ensure clean edges.

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Characterization of the MoS2 films and devices was performed using several advanced techniques. Scanning tunneling microscopy (STM) was employed to investigate the single-crystal MoS2 ML under ultrahigh-vacuum conditions. The samples were annealed to remove contaminants before gold deposition. Electrical measurements were conducted using a four-probe vacuum station for room temperature and an Oxford Teslatron PT system for low-temperature measurements.

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Magnetoconductance data were analyzed using the Hikami-Larkin-Nagaoka (HLN) model to extract the phase coherence length, a critical parameter for understanding quantum interference effects in the material.

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The research team emphasized the significance of their findings in advancing the understanding of MoS2 materials and their potential applications in electronic devices. They noted that the optimized growth conditions and fabrication techniques could lead to improved performance in future applications.

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While the study presents promising results, some experts in the field have raised concerns regarding the scalability of the MOCVD process for commercial applications. Critics argue that the complexity of the growth conditions and the need for precise control over the substrate angles may limit widespread adoption.

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• “The growth of MoS 2 ML films on the various vicinal sapphire substrates was conducted for 40–90 min.” — Research Team

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This comprehensive study on the epitaxial growth and characterization of MoS2 monolayers provides valuable insights into the material's potential for future electronic applications, while also addressing challenges that may arise in scaling the process.

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