Transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂) have gained significant attention due to their unique electronic, optical, and mechanical properties. These materials exhibit a layered structure with strong in-plane covalent bonds and weak interlayer van der Waals interactions, making them suitable for applications in transistors, photodetectors, and flexible electronics. Among the various synthesis techniques, metal-organic chemical vapor deposition (MOCVD) has emerged as a promising method for producing high-quality, large-area TMDC films with precise control over thickness and crystallinity.

MOCVD growth of TMDCs involves the decomposition of metal-organic precursors in a controlled environment to deposit thin films on a substrate. The process relies on volatile organometallic compounds as sources for transition metals (e.g., molybdenum or tungsten) and chalcogens (e.g., sulfur or selenium). Common precursors for molybdenum include molybdenum hexacarbonyl (Mo(CO)₆) and molybdenum pentachloride (MoCl₅), while tungsten hexacarbonyl (W(CO)₆) is often used for tungsten. For sulfur, hydrogen sulfide (H₂S) or organic sulfur compounds like diethyl sulfide ((C₂H₅)₂S) are employed. The choice of precursors influences the growth kinetics, purity, and stoichiometry of the resulting TMDC films.

The substrate plays a critical role in MOCVD growth, affecting nucleation density, orientation, and film uniformity. Common substrates include sapphire (Al₂O₃), silicon dioxide (SiO₂), and hexagonal boron nitride (hBN). Sapphire is widely used due to its lattice matching with TMDCs, promoting epitaxial growth. The substrate temperature is a key parameter, typically ranging between 500°C and 900°C for MoS₂ and WS₂. Higher temperatures enhance precursor decomposition and surface mobility of adatoms, leading to improved crystallinity. However, excessive temperatures can cause undesirable reactions or precursor depletion before reaching the substrate.

Gas flow rates and carrier gases are carefully controlled to ensure uniform precursor delivery and reaction efficiency. Hydrogen (H₂) or argon (Ar) is commonly used as a carrier gas, with flow rates adjusted to balance precursor transport and residence time in the reaction chamber. The ratio of metal to chalcogen precursors is critical for achieving stoichiometric films. For example, an excess of sulfur precursor is often required to compensate for its higher volatility and ensure complete reaction with the metal precursor. Deviations from optimal ratios can lead to sulfur vacancies or metal-rich phases, degrading electronic properties.

Layer thickness in MOCVD-grown TMDCs is controlled by growth time, precursor concentration, and deposition rate. Monolayer films are achievable with precise tuning, while multilayer films form through sequential nucleation and lateral growth. The transition from monolayer to few-layer growth depends on the balance between surface energy and interlayer interactions. In-situ monitoring techniques such as laser reflectometry or optical emission spectroscopy can provide real-time feedback on growth progression.

Crystallinity and domain size are influenced by growth conditions and post-deposition treatments. Large single-crystalline domains are desirable for high-performance devices but require optimized nucleation density and adatom mobility. Lower precursor fluxes and higher temperatures generally favor larger domains by reducing nucleation sites and enhancing surface diffusion. Post-growth annealing in a chalcogen-rich environment can further improve crystallinity and heal defects.

Despite its advantages, MOCVD faces challenges in TMDC synthesis. Precursor purity is critical, as impurities can introduce defects or dopants. Controlling stoichiometry is difficult due to the differing reactivities of metal and chalcogen precursors. Sulfur vacancies are a common issue, acting as charge traps and reducing carrier mobility. Additionally, achieving uniform film coverage over large areas requires precise control of gas flow dynamics and temperature gradients across the substrate.

Comparing MOCVD with other growth techniques highlights its strengths and limitations. Physical vapor deposition (PVD), such as sputtering or evaporation, offers simplicity but often produces films with poorer crystallinity and higher defect densities. Chemical vapor deposition (CVD) using solid precursors (e.g., MoO₃ and S powder) is widely used but lacks the precise control over precursor delivery offered by MOCVD. Molecular beam epitaxy (MBE) provides exceptional control at the atomic level but is expensive and less scalable for large-area growth. MOCVD strikes a balance between scalability, control, and film quality, making it suitable for industrial applications.

Recent advancements in MOCVD for TMDCs include the development of low-temperature processes compatible with flexible substrates and the integration of in-situ doping for tailored electronic properties. For example, nitrogen or selenium can be incorporated during growth to modify the bandgap or carrier concentration. The use of alternative precursors, such as metal-organic complexes with lower decomposition temperatures, is also being explored to reduce energy consumption and enable growth on temperature-sensitive substrates.

In summary, MOCVD is a versatile and scalable technique for growing high-quality TMDC films with controlled thickness, stoichiometry, and crystallinity. Precursor chemistry, substrate interactions, and growth parameters such as temperature and gas flow rates play pivotal roles in determining film properties. While challenges like stoichiometric control and defect management persist, ongoing research is addressing these limitations through advanced precursor design and process optimization. Compared to other growth methods, MOCVD offers a compelling combination of precision and scalability, positioning it as a key technology for the future of TMDC-based devices.