A next-generation electronics and optoelectronics materials: WSe2, a semiconducting transition metal dichalcogenides
Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) exhibit a broad spectrum of properties attractive to the electronics industry. Additionally, in their ideal monolayer form, with inherently dangling bond-free surfaces, they enable atomic-level scaling and excellent electrostatic gate control. The properties that exist only in mono- or few-layer TMDs have driven extensive synthesis efforts of large-area, atomically thin TMDs, such as tungsten diselenide (WSe2), by a variety of thin film deposition techniques. Including powder vaporization (PV; often termed chemical vapor deposition), selenization of W/ WOx thin films predeposited on arbitrary substrates, molecular beam epitaxy (MBE), and metal-organic chemical vapor deposition (MOCVD). Among these options, owing to the ease of preparation and setup, PV using WO3 and Se powders has been widely adapted to produce large WSe2 domains with edge length between 1 and 100 μm on both crystalline and noncrystalline substrates.
Recently in the Journal of ACS Nano, R. M. Feenstra, S. K. Fullerton and colleagues have demonstrated the device-ready synthetic tungsten diselenide (WSe2) via metal−organic chemical vapor deposition. This work is a breakthrough in the knowledge for vapor phase epitaxy of epitaxial 2D films on sapphire or other insulating, crystalline substrates. The work has presented here demonstrates >50% reduction in elemental impurities compared to previous reports by utilizing hydrogen selenide (H2Se) and tungsten hexacarbonyl (W(CO)6) precursors to synthesize large-scale, coalesced, epitaxial WSe2 on sapphire (Al2O3) substrates.
Furthermore, they have demonstrated that substrate step edges are a major source of carrier doping and scattering. Even with 2D/3D coupling, transistors utilizing transfer-free epitaxial WSe2/sapphire exhibit ambipolar behavior with excellent on/off ratios (∼107), high current density (1−10 μA·μm−1), and good field-effect transistor mobility (∼30 cm2 ·V−1 ·s −1) at room temperature. This work establishes that realization of electronic-grade epitaxial TMDs must consider the impact of the TMD precursors, substrate, and the 2D/3D interface as leading factors in electronic performance.