Optimum conditions for deposition of amorphous WS2 thin films and changes in structure and optical properties during solid state crystallization

Abstract

Transition metal dichalcogenides exhibit a unique properties, which make them interesting for fundamental studies and for applications in many fields of daily life. Here, we report on optimum conditions for deposition of amorphous stoichiometric WS2 thin films by (non-reactive) magnetron sputtering using a stoichiometric WS2 target and subsequent application of solid state crystallization of amorphous WS2 thin films for fabrication of crystalline WS2 over a large area. By ex-situ annealing of samples up to 1000 degrees C, we found that the optical contrast change occurs in two steps, which are separated by a plateau that exhibits a gradual increase in optical absorbance. We suggest that the dissociation of W-W homopolar bonds, associated with the transformation of 1T' -like symmetry into 2H-like symmetry units, plays a key role in the first abrupt change in the optical properties of WS2 thin films between 300 and 400 degrees C while the formation of excitons occurred between 700 and 1000 degrees C is associated with crystallization, as confirmed by X-ray photoelectron spectroscopy, X-ray diffraction and Scanning electron microscopy studies.