Thin Solid Films, 217 (1992) 91 97 91 Preparation and microstructure of WS 2 thin films M. Genut, L. Margulis, G. Hodes and R. Tenne Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100 (Israel) (Received October 29, 1991; accepted February 24, 1992) Abstract Thin tungsten films were ion beam sputter deposited onto quartz slides and then reacted at temperatures from 500 to 1000 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 trans- mission spectra and sheet resistivity measurement. The onset of the reaction between tungsten and H2S to give WS2 thin films was found to be 650 °C. Orientation of the WSz crystallites could be controlled by choice of reaction temperature and sulphur concentration in the gas flow: low reaction temperatures (up to 900 °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'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. I. Introduction Increasing interest has been recently shown in the preparation of polycrystalline thin films of semicon- ducting transition metal dichalcogenides, e.g. WS2, WSe2, MoS2 and MoSe2, for use as electrochemical and photovoltaic solar cells [1-3], lubricants [4], battery cathodes [5], catalysts [6] and recently for imaging of atomically smooth surfaces by scanning microscopy with atomic resolution [7, 8]. They all possess a layered- type structure with strong anisotropy of their mechani- cal and electrical properties. The chemical bonding is much stronger within the layers (covalent bonds) than between them (van der Waals (vdW) bonds) [9]. These polycrystalline thin films usually grow with a lamellar structure in which the basal (or vdW) planes are perpendicular to substrate [10, 11]. However, for photovoltaic as well as for lubrication purposes, growth with the vdW planes parallel to the substrate is desired [3, 12]. These thin films have been reported to be prepared mainly by sputtering the transition metal dichalco- genides [10, 13], by electrochemical deposition [14] and by soft selenization as was recently reported by J~ger- Waldau et al. for tungsten [15] and molybdenum [16] diselenides. In the soft selenization method, thin sput- tered tungsten or molybdenum films were reacted with selenium in a closed tube system for tens to hundreds of hours. As a result, two types of orientations were obtained, with the basal plane of the crystallites being oriented either predominantly perpendicular or pre- dominantly parallel to the substrate surface. Following refs. 17 and 18 we shall designate these textures as type I and type II respectively. For large-scale photovoltaic applications, where the cost:efficiency ratio is very important, a continuous open system preparation is preferable over a batch method. For this reason we have selected an atmo- spheric growth technique. In this paper we report on the microstructure of WS2 thin films grown by sulphiding of thin sputtered tung- sten films, using H2S, with emphasis on control of the orientation of the WS2 crystallites. 2. Experimental details Thin films of tungsten (about 50 nm thick) were ion beam sputtered onto quartz slides, at an argon pressure (in the chamber) of 2 x 10 4 Tort. The sputtering rate was about 10 nm min -~. The samples were reacted in an open quartz furnace with a flow of H2S (5-50 ml min ~) and forming gas (5%H2,95%N2,150ml min-l). The reaction temperatures were controlled in the range from 500 to 1000 °C. After the furnace reached the desired temperature and a uniform gas flow was maintained, the samples were introduced into the hot zone of the reaction chamber (by a magnet from outside the quartz tube) for times between 5 min and 5 h. After reaction the samples were moved back to the cold zone of the reaction chamber, where a little above room temperature (about 60 °C) was maintained. 0040-6090/92/$5.00 ~' 1992 Elsevier Sequoia. All rights reserved