Chemically Tailored and Phase-Selective Growth of 2D Semiconductors via Hybrid Metal-Organic Chemical Vapor Deposition

Two-dimensional (2D) semiconducting transition-metal dichalcogenides (TMDCs) are a leading platform for excitonic physics and next-generation electronics, creating a strong demand to understand their growth, doping, heterostructures and polytypes. Despite significant progress in solid-source (SS-) and metal-organic chemical vapor deposition (MOCVD), further optimization is necessary to grow highly crystalline 2D TMDCs with controlled doping and heterostructures. Here, we report a hybrid MOCVD (Hy-MOCVD) growth method that combines liquid-phase metal precursor deposition and vapor-phase organo-chalcogen delivery to leverage the advantages of both MOCVD and SS-CVD.¹ Using our hybrid approach, we demonstrate WS₂ growth with tunable morphologies – from separated single-crystal domains to continuous monolayer films – on a variety of substrates, including sapphire, SiO₂, and Au. These WS₂ films exhibit narrow neutral exciton photoluminescence linewidths down to 27 – 28 meV and mobility up to 34 – 36 cm²V^-1s^-1 at room-temperature. Through simple modifications to the liquid precursor composition, we demonstrate the growth of V-doped WS₂, Mo_xW_1-xS₂ alloys, and in-plane WS₂-MoS₂ heterostructures.<br/>Moreover, rhombohedral polytype (3R) TMDC multilayers exhibit non-centrosymmetric interlayer stacking, which yields intriguing properties such as ferroelectricity, a large second-order susceptibility coefficient χ⁽²⁾ for nonlinear optics, valley coherence, and a bulk photovoltaic effect. However, phase-selective growth strategies for 3R TMDCs are still underdeveloped and are especially lacking for multilayer films which strongly exhibit the desired optoelectronic characteristics. We facilitate multilayer growth of WS₂ by introducing a confined space into the Hy-MOCVD process.² The confined-space Hy-MOCVD method preferentially grows 3R multilayer WS₂ films with thickness up to 130 nm. We confirm the 3R stacking structure <i>via</i> polarization-resolved second harmonic generation characterization and <i>via</i> the threefold symmetry revealed by anisotropic H₂O₂ etching. The multilayer 3R WS₂shows a dendritic morphology which is indicative of diffusion-limited growth. Multilayer regions with large, stepped terraces enable layer-resolved evaluation of the optical properties of 3R-WS₂ <i>via</i> Raman, photoluminescence, and differential reflectance spectroscopy. These measurements confirm the interfacial quality and suggest ferroelectric modification of the exciton energies.<br/>References<br/>[1] Z. Zhang^#, L. Hoang^#, M. Hocking, Z. Peng, J. Hu, G. Zaborski Jr., P. Reddy, J. Dollard, D. Goldhaber-Gordon, T. F. Heinz, E. Pop and A. J. Mannix*<i>.</i> “Chemically Tailored Growth of 2D Semiconductors via Hybrid Metal-Organic Chemical Vapor Deposition” arXiv:2403.03482 (2024). #These authors contributed equally to this work (Z.Z., L.H.).<br/>[2] Z. Zhang, M. Hocking, Z. Peng, M. Pendharkar, E. D. S. Courtney, J. Hu, M. A. Kastner, D. Goldhaber-Gordon, T. F. Heinz, and A. J. Mannix* “Phase-Selective Synthesis of Rhombohedral WS₂ Multilayers by Confined-Space Hybrid Metal-Organic Chemical Vapor Deposition” <i>under review </i>(2024)