Thermally Induced Reactions of Monolayer WS2 with Au-Ti Substrates

Abstract The thermal stability of WS2 flakes is studied after integration into a device structure with a Si/SiO2 substrate, a Ti adhesion layer and a Au-contact layer. WS2 /Au/Ti is commonly used in catalytic and electronic devices, and its thermal stability is important for extended operation and mitigation of breakdown at local hot-spots during operation. The thermal stability is severely impacted by rapid diffusion of Ti into and through the polycrystalline Au. Thin film samples of Au/Ti, with and without an exfoliated WS2 monolayer, were annealed sequentially from 275- 625 °C in vacuum. The near-surface chemical information was extracted with X-ray photoelectron spectroscopy for each annealing step. Ti from the sticking layer, which is buried under 25 nm of Au, diffuses into the near surface region, and initiates a wide range of reactions forming: Ti oxide, carbide, sulfide and a Ti-Au alloy, mainly Au2Ti. WS2 experiences S loss for T>350 °C and forms Ti-sulfide for T>600 °C. Higher thermal loads spanning the same temperature range lead to coalescence within the nanocrystalline Au layer, limiting grain boundary diffusion of Ti. The results provide insight on the stability of the WS2/Au/Ti material system which impacts its performance in catalytic and electronic applications.