MOCVD Growth of Tungsten Ditelluride Thin Films

Abstract

Abstract Body: Tungsten ditelluride (WTe2) is a layered, type-II Weyl semimetal transition metal dichalcogenide (TMD) typically observed in a distorted 1T (1T’) phase with an orthorhombic crystal structure comprised of planes of distorted triangular lattices of tungsten atoms sandwiched by tellurium atoms. The distortion pushes tungsten atoms closer together in the x-axis than the y or z-axis, generating quasi-one-dimensional chains of these atoms, leading to strongly anisotropic electronic properties throughout the material. It has been shown to have extraordinary physical properties, such as a high magnetoresistance, anisotropic ultra-low thermal conductivity and metal-insulator transition and it also exhibits interesting quantum phenomena such as the quantum spin-Hall effect and pressure-driven superconductivity. These unique properties make WTe2 an exciting candidate for emerging applications, including phase change memory electrodes, magnetic field sensors, biosensors, microelectromechanical systems, hard disk drives and quantum computing. While bulk crystals of the material have been available for many years, to date, thin films of WTe2 have only recently been synthesized using techniques such as molecular beam epitaxy and powder source chemical vapor deposition (CVD) that utilize Te powder and W-feedstock. These techniques, however, have a number of challenges including tellurium dimer formation, that can severely hinder growth with high dissociation energies, and the use of salt assisted growth promotors during CVD growth which could contaminate future device fabrication. Metalorganic CVD (MOCVD) growth is of interest as it enables precise delivery of precursors to the substrate at moderate growth pressures (100-700 Torr), while proper precursor selection mitigates the Te dimer formation, but has not yet been studied in detail. In this study, we investigate the use of MOCVD for the growth of WTe2 on c-plane sapphire substrates. The studies were carried out in a vertical cold wall MOCVD reactor using tungsten hexacarbonyl (W(CO)6) and diethyltelluride (DETe) as precursors for W and Te, respectively, in a H2 carrier gas. Initial studies, carried out at 100 Torr reactor pressure using a W(CO)6 flow rate of 1.3 x10-4 sccm and Te/W ratio of ~8000 : 1, demonstrate the growth of centimeter scale WTe2 as confirmed by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Additional studies to elucidate the growth mechanism and properties of MOCVD grown WTe2 were then undertook. The growth rate was found to decrease with increasing temperature over the range of 350 - 600oC. Similarly, higher growth rates were observed at lower growth pressures when examined over a range of 50 - 300 Torr. The effect of precursor flow rates was also investigated and demonstrated that the DETe flow rate had little effect on growth rate while changes in W(CO)6 flow rate can be used to directly control growth rate of films. Peaks at ~1340 cm-1 and ~1600 cm-1 were present in the Raman spectra of some films indicating the presence of carbon in the layers resulting from the DETe source. Higher carbon content was observed at higher growth temperatures and growth pressures. This suggests that our growths are becoming limited by parasitic prereactions of our precursors in the gas phase. When precursors react in the gas phase before reaching the substrate surface, they form carbon containing species which deposit, thus limiting the available reactants to interact on the surface and form WTe2. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis further indicate that films are hexagonal and exhibit the layered 1T’ structure. XPS studies over a period of a few weeks reveals that films do not readily oxidize and are quite stable.