Conventional epitaxy plays a crucial role in current state-of-the art semiconductor technology, as it provides a path for accurate control at the atomic scale of thin films and nanostructures, to be used as the building blocks in nanoelectronics, optoelectronics, sensors, etc. Four decades ago, the terms "van der Waals" (vdW) and "quasi-vdW (Q-vdW) epitaxy" were coined to explain the oriented growth of vdW layers on 2D and 3D substrates, respectively. The major difference with conventional epitaxy is the weaker interaction between the epi-layer and the epi-substrates. Indeed, research on Q-vdW epitaxial growth of transition metal dichalcogenides (TMDCs) has been intense, with oriented growth of atomically thin semiconductors on sapphire being one of the most studied systems. Nonetheless, there are some striking and not yet understood differences in the literature regarding the orientation registry between the epi-layers and epi-substrate and the interface chemistry. Here we study the growth of WS2via a sequential exposure of the metal and the chalcogen precursors in a metal-organic chemical vapor deposition (MOCVD) system, introducing a metal-seeding step prior to the growth. The ability to control the delivery of the precursor made it possible to study the formation of a continuous and apparently ordered WO3mono- or few-layer at the surface of a c-plane sapphire. Such an interfacial layer is shown to strongly influence the subsequent quasi-vdW epitaxial growth of the atomically thin semiconductor layers on sapphire. Hence, here we elucidate an epitaxial growth mechanism and demonstrate the robustness of the metal-seeding approach for the oriented formation of other TMDC layers. This work may enable the rational design of vdW and quasi-vdW epitaxial growth on different material systems.
Bibliographical noteFunding Information:
The authors gratefully acknowledge the very generous support from the Israel Science Foundation, Projects # 2171/17 (A.C., P.K.M., and O.D.), 2549/17 (A.P.), and 1784/15 (A.I.). S.H. and R.A. acknowledge funding from the European Unions’ H2020 research and innovation programme under Marie-Skłodowska Curie (Grant Agreement No. 889546), from the European Union H2020 program “ESTEEM3” (Grant Agreement No. 823717) as well as from the Spanish MICINN (Project Grant PID2019-104739GB-100/AEI/10.13039/501100011033) and from the Government of Aragon (Project DGA E13-20R). The TEM measurements were performed in the Laboratorio de Microscopias Avanzadas (LMA) at the Universidad de Zaragoza (Spain). A.I. and B.Z. acknowledge the support provided by the Crown Family Fund, Northwestern University. J.C. and J.M.R. were supported by the National Science Foundation (NSF) under Award Number DMR-2011208. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by NSF Grant Number ACI-1548562.
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- metal organic chemical vapor deposition
- quasi-van der Waals epitaxy
- surface modification
- transition metal dichalcogenides
- tungsten trioxide