Characterisation of r.f. sputtered tungsten disulfide and oxysulfide thin films

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Tungsten disulfide (WS 2) and tungsten oxysulfide (WO y S z ) thin films were prepared by reactive radio frequency magnetron sputtering using a WS 2 target and argon or a mixture of argon and oxygen as a discharge gas. For a total pressure of 1 Pa, a large range of composition, determined by Rutherford backscattering spectroscopy, can be obtained from WS 2.05 when no oxygen gas is introduced in the sputtering chamber to WO 3.04S 0.09 when the oxygen partial pressure is 10 −2 Pa. Scanning electron microscopy studies have shown that the film morphology depends on the sputtering conditions (oxygen partial pressure, total pressure and sputtering time). A structural analysis (X-ray diffraction and transmission electron microscopy) has highlighted that the tungsten oxysulfide thin films such as WO 1.05S 2.01, WO 1.35S 2.20 and WO 3.04S 0.09 are amorphous. Only the WS 2 and the WO 0.4S 1.96 films are made of small crystallites (length ≤80 nm and width ≤10 nm) which grow with their c-axis parallel to the substrate. An X-ray photoelectron spectroscopy study on both the core levels (W4f and S2p peaks) and the valence bands has shown that three different environments of the tungsten atoms exist inside the tungsten oxysulfide thin films: an oxygen and a sulfur environment, respectively as in WO 3 and WS 2, and a mixed oxygen–sulfur environment constituted of O 2−, S 2− and S 2 2−.