PREPARATION AND MICROSTRUCTURE OF WS2 THIN-FILMS

Thin tungsten films were ion beam sputter deposited onto quartz slides and then reacted at temperatures from 500 to 1000-degrees-C in an open system under a gas flow consisting of a mixture of H2S and forming gas. The reaction products were examined by X-ray diffraction, transmission electron microscopy, electron probe microanalysis, Auger electron spectroscopy, optical transmission spectra and sheet resistivity measurement. The onset of the reaction between tungsten and H2S to give WS2 thin films was found to be 650-degrees-C. Orientation of the WS2 crystallites could be controlled by choice of reaction temperature and sulphur concentration in the gas flow: low reaction temperatures (up to 900-degrees-C) and high sulphur concentrations lead to films where the van der Waals planes are perpendicular to the substrate. while high temperatures (more than 950-degrees-C) and low sulphur concentrations result in the van der Waals planes being parallel to the substrate. These results are explained on the basis of a competition between the reaction rate and the rate of crystallization. The importance of these results lies in the fact that the latter orientation is needed for solar cells and optimum lubrication uses, but the former has been found in the majority of reported cases.