Reactive magnetron sputtering of tungsten disulfide (WS2−x) films: Influence of deposition parameters on texture, microstructure, and stoichiometry

Abstract

Tungsten disulfide (WS2−x) films (0.07⩽x⩽0.7) were prepared by reactive magnetron sputtering from a tungsten target in rare gas/H2S atmospheres and at substrate temperatures up to 620°C. The nucleation and growth of the films were investigated by in situ energy dispersive x-ray diffraction (EDXRD) and by ex situ techniques such as electron microscopy, elastic recoil detection analysis, and x-ray reflectivity. From the EDXRD analysis it was found that the films always nucleate with the (001) planes, i.e., the van der Waals planes, parallel to the substrate surface. For high deposition rates and/or low substrate temperatures a texture crossover from the (001) to the (100) crystallite orientation occurs during the growth. High deposition rates, low substrate temperatures, or low sputtering pressures lead to a significant lattice expansion of the crystallites in the c direction (up to 3%). This is most probably caused by a disturbed or turbostratic film growth induced by the energetic bombardment during film deposition. Reflected and neutralized energetic ions (Ar0,S0) from the tungsten target and negative ions (S−) accelerated in the cathode dark space constitute the main sources of the energetic bombardment leading to crystallographic defects. The energy of these particles can be tailored by (i) thermalization between target and substrate in the sputtering gas or (ii) by a reduction of the discharge or target voltage, respectively, by high frequency excitation of the plasma. Films deposited under favorable conditions with respect to low particle energies and at substrate temperatures higher than 200°C exhibit a significant sulfur deficiency of up to about 5at.% compared to the stoichiometric composition of WS2. This is ascribed to an energetic particle bombardment-induced sulfur desorption from the growing films.