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

Tungsten disulfide (WS2) and tungsten oxysulfide (WOySz) thin films were prepared by reactive radio frequency magnetron sputtering using a WS2 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 WS2.05 when no oxygen gas is introduced in the sputtering chamber to WO3.04S0.09 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 WO1.05S2.01, WO1.35S2.20 and WO3.04S0.09 are amorphous. Only the WS2 and the WO0.4S1.96 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 WO3 and WS2, and a mixed oxygen-sulfur environment constituted of O2−, S2− and S22−.