CVD Controlled Preparation And Characterization Of Tungsten Diselenide Thin Films

The discovery,large-scale preparation and application of graphene have opened up the research ideas of two-dimensional materials.Among them,transition metal sulfide compounds?TMDCs?are the most concerned by researchers.Compared with graphene,single-layer transition metal sulfide compounds,such as MoS₂,MoSe₂,WS₂ and WSe₂,are direct band-gap semiconductors,which fill the defect of zero band-gap of graphene.Mono-layer TMDCs film is a direct band-gap semiconductor material.The band-gap width of mono-layer to multi-layer TMDCs material varies from 1.1 to 1.9eV,and the band-gap width is adjustable with the number of layers.The band gap width of silicon material is 1.1eV.Due to its excellent electrical and optical properties,as well as the semiconductor properties of such materials,the transition metal sulfide is expected to be a new semiconductor material after graphene.As a representative of transition metal sulfide,MoS₂ has been extensively studied in its synthesis methods and related applications,while relatively few reports and researches on WSe₂ have been reported so far.Single-layer WSe₂ is a direct band-gap semiconductor with a band gap width of1.64eV,and has excellent electrical and optical properties,which can meet most requirements for electrical and optical devices.Compared with other transition metal sulfide compounds,WSe₂ is a transition metal sulfide material with both p-type and n-type conductive properties.The preparation of single-layer WSe₂ with high quality and large area is the premise of its application,but compared with graphene,there are still many problems to be overcome in the preparation.Chemical vapor deposition?CVD?is the main method to prepare WSe₂ films,and it is the most promising method to be used in industry.So far,it is quite difficult to produce large-area and high-quality single-layer WSe₂ by CVD method,and there is no accurate study on its growth mechanism.Therefore,it is of great significance to study the influence parameters of preparation of WSe₂ by CVD method and its growth mechanism.This paper mainly studied effects of such parameters as the relative ratio of Se and WO_2.9,pressure,temperature,growth time,growth temperature and hydrogen introduction time point on the growth of WSe₂ thin film.Through the regulation of these growth factors,triangular single-crystal monolayer WSe₂ film with side length of 150?m was grown on SiO₂/Si substrate with the thickness of SiO₂ is 300nm,and a monolayer polycrystalline WSe₂ thin film of irregular shape with multilayer points with an area of about 400?m×400?m.The prepared WSe₂ was characterized by optical microscope and Raman spectroscopy,the results show that the single layer WSe₂ film has smooth surface and good properties.At the same time,optical microscope and Raman spectrum were used to distinguish single-layer WSe₂ and multi-layer WSe₂.On this basis,the photoluminescence spectra?PL?of single-layer,double-layer and three-layer WSe₂ were compared,and the change rule of WSe₂ band structure with the change of layer number was distinguished.Here,the influence of hydrogen introduction time point on the growth results is mainly studied,It was found that the earlier hydrogen was introduced,the greater the influence on WO_2.9,by analyzing the X-ray diffraction?XRD?and growth results of the residual precursor introducing hydrogen at different time points,it was found that the earlier hydrogen was introduced,the less WSe₂ was deposited on the silicon wafer,and WSe₂ particles tended to form.This is mainly due to the advanced reaction of H₂ with the mixture of WO_2.9.9 and NaCl to generate Na_xWO₃,WO_2.9.9 is significantly reduced when the temperature reaches the growth temperature.