Transition metal dichalcogenides (TMDs) form a compelling class of 2D materials with potential applications in optoelectronics, flexible electronics, chemical sensing and quantum technologies. At the monolayer limit, the semiconducting TMDs (e.g., MX₂ where M = Mo/W and X = S/Se) exhibit direct band gaps within the visible range, large exciton binding energies and valley polarization. The lack of out-of-plane bonding on the van der Waals surface of these materials enables heterostructure formation without the constraints of lattice matching. Current research on TMD monolayers is often carried out using flakes exfoliated from bulk crystals which is a limiting factor for device fabrication.
Our research is aimed at the development of an epitaxial growth technology for TMDs based on metalorganic chemical vapor deposition (MOCVD). The high growth temperatures (>700^oC) and chalcogen/metal precursor ratios (>10,000) attainable in an MOCVD environment are advantageous for promoting metal surface diffusion needed for epitaxy while maintaining growth stoichiometry.
Our studies employ metal hexacarbonyls and hydrides (H₂S, H₂Se) as precursors in an H₂carrier gas. Substrates include c-plane sapphire and hBN flakes. A multi-step precursor modulation growth method was developed to independently control nucleation density and the lateral growth rate of monolayer domains on the substrate for growth on 2” diameter sapphire substrates. Using this approach, uniform, coalesced monolayer and few-layer films of WS₂, MoS₂, WSe₂ and MoSe₂ were obtained on 2” sapphire at growth rates on the order of 0.02-0.1 monolayer/min. The films are epitaxially oriented with respect to the sapphire as determined by in-plane x-ray diffraction. Post-growth dark-field transmission electron microscopy carried out on WS₂ monolayers removed from the sapphire demonstrate that the films are nominally single orientation with translation boundaries that arise from coalescence of WS₂ domains with lattices that are offset by sub-unit cell lengths. The WS₂ monolayers exhibit intense room temperature photoluminescence (PL) at ~2.0 eV and variable-temperature measurements reveal PL associated with excitons and trions with negligible defect-related emission. The field effect mobility in the WS₂ monolayer, as measured at room temperature in back-gated devices that employ a 50 nm Al₂O₃ dielectric, is in the range of 20-30 cm²/Vs.

Joan Redwing

The Pennsylvania State University - University Park Campus

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