Preparation And Characteristics Of Layered Transition Metal Sulfides

Layered Transition metal disulfides（TMDs）are a class of two-dimensional semiconductor materials.The semiconductor properties of TMDs have led to a research upsurge in two-dimensional materials,also provide the possibility to study narrower channels and thinner transistors.So far,the synthesis of a continuous film of molybdenum disulfide is a problem,especially the synthesis of a continuous film having large grain size.This thesis focuses on the controllable preparation and characteristics of the MoS₂continuous membranes.The MoS₂ film was prepared by CVD method.The prepared sample has good uniformity,the thickness of the single layer MoS₂is 0.7 nm,and the Raman peak difference of the single layer MoS₂is 18.2～19.5cm-1.The continuous film of MoS₂prepared by PVT method and characterized by optical microscopy,Raman spectroscopy,PL spectrum and SEM..The effects of growth temperature,growth time,carrier gas flow rate and Mo source temperature on the growth of MoS₂films were studied by CVD.And the parameters were optimized,the continuous MoS₂film with a large grain size was prepared by CVD.The continuous film is formed by combining triangular and hexagonal single crystals,and the grain boundaries are relatively small.The optimum parameters for the continuous film are a temperature of 800°C for the Mo O₃powder,a temperature of 1000°C for the sapphire substrate,and a carrier gas flow rate of 150 sccm.Using method of separating Mo O₃from the substrate is because the relatively low Mo O₃powder temperature（while maintaining the substrate temperature at 1000°C）can reduce the evaporation rate and provide sufficient time for atom diffusion on the substrate surface,a higher substrate temperature will reduce the nucleation density,which is beneficial to the increase of single crystal grains.The Mo S2 hexagonal single crystal has a large average size and a side length of 200μm or more.A continuous film of large single crystal domain size prepared by optimizing the optimum parameters of CVD,the size of the continuous film reaches the inch level.A bottom-gate transistor was fabricated using the prepared large grain size MoS₂continuous film.The MoS₂film grown on the sapphire substrate was transferred to a Si O₂/Si substrate.The bottom-gate MoS₂MOSFET was fabricated using photolithography and its electrical characteristics were investigated.The MoS₂film is an n-type semiconductor.The on/off current ratio is approximately 10⁴.The field effect mobility of the MoS₂channel is about 10 cm²/V·s,which is higher than MoS₂films on Si O₂substrates by chemical vapor deposition but lower than that of the mechanically exfoliated MoS₂.The present mobility is probably limited by substrate effects and the local electrostatic environment.The MoS₂-WS₂ heterostructures was prepared by growing WS₂on the pre-prepared MoS₂single crystal domain film using a two-step method.WS₂grows on the edge of the triangular MoS₂.The main reason is that there are some burrs on the edge and secondary nucleation of MoS₂ as the nucleation point of WS₂.