Wafer-Scale Epitaxial Growth of Unidirectional WS₂ Monolayers on Sapphire

Abstract

Realization of wafer-scale single-crystal films of transition metal dichalcogenides (TMDs) such as WS 2 requires epitaxial growth and coalescence of oriented domains to form a continuous monolayer. The domains must be oriented in the same crystallographic direction on the substrate to inhibit the formation of inversion domain boundaries (IDBs), which are a common feature of layered chalcogenides. Here we demonstrate fully coalesced unidirectional WS 2 monolayers on 2 in. diameter c-plane sapphire by metalorganic chemical vapor deposition using a multistep growth process to achieve epitaxial WS 2 monolayers with low in-plane rotational twist (0.09°). Transmission electron microscopy analysis reveals that the WS 2 monolayers are largely free of IDBs but instead have translational boundaries that arise when WS 2 domains with slightly offset lattices merge together. By regulating the monolayer growth rate, the density of translational boundaries and bilayer coverage were significantly reduced. The unidirectional orientation of domains is attributed to the presence of steps on the sapphire surface coupled with growth conditions that promote surface diffusion, lateral domain growth, and coalescence while preserving the aligned domain structure. The transferred WS 2 monolayers show neutral and charged exciton emission at 80 K with negligible defect-related luminescence. Back-gated WS 2 field effect transistors exhibited an I ON / OFF of ∼10 7 and mobility of 16 cm 2 /(V s). The results demonstrate the potential of achieving wafer-scale TMD monolayers free of inversion domains with properties approaching those of exfoliated flakes.

Water dissociation and association on mirror twin boundaries in two-dimensional MoSe2: insights from density functional theory calculations

“…Not only point, but also line defects have been reported to exist in TMDs. [17][18][19][20][21] Among them, mirror twin boundaries (MTBs) of various types 22 with relatively low formation energies have been found. Moreover, contrary to serpentine or low-angle grain boundaries, 17,18 which appear during the growth when domains with arbitrary crystal orientation coalesce, the formation of straight MTBs is energetically favorable in non-stoichiometric chalcogen-decient TMDs.…”

Controllable p‐Type Doping of 2D WSe2 via Vanadium Substitution

“…transistor at back-end-of-line (BEOL) [5][6][7] for advanced semiconductor technology nodes. To date, wafer-scale synthesis of near electronic-grade intrinsic TMDCs has been successfully realized at FEOL (>700 °C) [8][9][10] and BEOL(<500 °C) [11][12][13][14] compatible temperatures. However, while various doping techniques can tune the electrical conductivity of TMDCs (e.g., surface charge transfer doping, [15] electrostatic doping, [16] intercalation, [17] and substitutional doping, [18,19] ) progress is still limited in truly scalable doping.…”

Toward Wafer‐Scale Production of 2D Transition Metal Chalcogenides

“…Gaseous phase precursor also provides better controllability than the solid precursor such as metal oxide and sulfur powder, resulting in a better uniformity of the supply of precursors. [221][222][223] Wang et al used CS 2 as sulfur source instead of sulfur powder for higher controllability to grow WS 2 and obtained larger domain size WS 2 with more regular shape. [224] Additionally, MOCVD, with the reactants delivered in gaseous phase, offers another route to supply the precursor uniformly.…”