Preparation and characterization of highly oriented,photoconducting WS2 thin films

A novel combination of methods is shown to produce semiconducting WS₂ thin films with properties close to those of a single crystal. The first step requires the deposition of a very thin Ni layer on a quartz substrate. On top of it an amorphous, sulphur rich, (WS_{3 +x}) thin film is deposited by reactive rf sputtering. The final annealing step in an argon atmosphere yields 200 nm thick WS₂ films. X-ray diffraction shows that the films crystallize in the 2H-WS₂ phase and are perfectly oriented with the (002) basal planes parallel to the substrate. Residual W₁₈O₄₉ needles and-NiS grains are detected by transmission electron microscopy. The dc conductivity and its activation energy have values typical of bulk crystals. The optical absorption spectrum measured at Room Temperature (RT) shows excitonic peaks at the same energies as in a single crystal. RT photoconductivity measured as a function of wavelength is shown to result from interband transitions.