Large-Size Growth And Plasmon Effect Induced Photoelectric Studies Of MoS₂ Thin Films

Molybdenum disulfide（MoS₂）has nanoscale thickness and excellent electrical and optoelectronic properties,and thus provide a good choice for thin-channel transistors.At present,one of the biggest challenges for the large-scale application of MoS₂ is how to obtain high-quality,large-scale and uniform single-crystal MoS₂ films.Furthermore,the band gap of single-crystal MoS₂ monolayer films is 1.8 e V,which limits its light absorption in the near-infrared region.Therefore,a"face-to-face"confined space CVD method is proposed to realize the growth of centimeter-scale monolayer,bilayer,and multilayer MoS₂ thin films;On the other hand,this thesis proposes to integrate the WO₃ particle into the MoS₂ film,and then perform plasma treatment on it to improve the photoresponse time of WO_x/MoS₂ composite structure optoelectronic devices.The main contents of this paper are as follows.1.A"face-to-face"confined space CVD method is proposed to achieve centimeter-scale MoS₂ thin film growth on Si O₂/Si substrates.The layer number can be well controlled.1-layer,2-layer,3-layer and multi-layer MoS₂ films can be synthesized by varying the growth parameters.The confined space reduces the concentration and flow rate of the precursor,providing a relatively stable environment for the precursor.Furthermore,the"face-to-face"placement between the WO₃ powder source and the substrate allows greater contact between the source powder and the substrate area.Therefore,both effects promote the growth of large size MoS₂ films.In order to better control the layer number of MoS₂ thin films,the effects of growth temperature,time and carrier gas flow on the layer number of MoS₂ thin films were investigated systematically.The composition,morphology and optical properties of the as-grown MoS₂ films were characterized by x-ray photoelectron spectroscopy,atomic force microscopy and optical microscopy and Raman spectroscopy2.We use hydrogen（H₂）or nitrogen（N₂）plasma treatment to transform the WO₃ particle film into suboxide WO_x（x<3）,and improve and expand its light absorption to the near infrared region.After that,WO_x/MoS₂ composite structure based photodetector devices were fabricated by integrating the suboxide WO_x into MoS₂ thin film.By adjusting the RF power of the plasma,the treatment time and the evaporation time of the WO₃ particle film,it was demonstrated that the light absorption of the WO_x particle film in the visible-near-infrared region is significantly enhanced via UV-Vis spectroscopy measurement.Among them,the effect of H₂ plasma treatment is the most obvious.The light absorption of WO_x particle film formed after H₂ plasma treatment at 1100 nm is～70 times stronger than that of the original sample.The corresponding WO_x/MoS₂ based photodetector devices benefit from local surface plasmon resonance（LSPR）,and thus display a much faster photoresponse speed at 447 nm,520 nm and 637 nm.The rise time and fall time of the WO_x/MoS₂ photodetector device constructed after H₂ plasma treatment are 15 ms（original sample is 52 ms）and 11 ms（original sample is 154 ms）,respectively.3.Through the self-assembly method of high temperature thermal annealing,the WO₃particle film was first converted into WO₃ nanorods,and then was transformed into WO_x（x<3）nanorods by plasma treatment.Similarly,WO_x nanorods can induce localized surface plasmon resonance,consequently enhancing the light absorption at the wavelength region between 400nm and 1100 nm.The light absorption of H₂ treated WO_x nanorods at 1100 nm is～95 times stronger than that of the original sample,which further improves its light absorption in the near infrared region.The WO_x nanorods were integrated into MoS₂ films to fabricate photodetector devices.The rise time of the WO_x/MoS₂ nanorods based photodetector treated by H₂ is 16 ms and the fall time is 8 ms under 637 nm laser irradiation,which greatly improves the photoresponse time of the device.