Study On Photoelectrical Properties Of Monolayer Transitional Metal Dichalcogenides Devices Modulated By Different Doping Methods

Two-dimensional transitional metal dichalcogenides（TMDCs）materials have the tunable bandgaps,electrical and optical properties,which have great application potentials in sensing,energy,biomedicine,integrated circuit and other fields.As direct band gap semiconductors,monolayer MS₂（M:Mo,W,Re）can be utilized as new optoelectronic device,valley electronics device,photocatalytic electrode.However,it remains the key challenge to regulate the physical properties of single atomic layer MS₂materials,which limits their further applications.In order to solve this problem,this thesis mainly studied the atomic substitution doping method during chemical vapor deposition（CVD）to regulate the doping of the monolayer MS₂（M:Mo,W,Re）.The electrical and optical properties of MS₂ devices were studied in detail after doping,which showed that the atomic substitution doping method obvious improved the carrier mobility,photoelectric response and hydrogen evolution reaction efficiency of the field effect transistors（FETs）based on monolayer MS₂ material.The work mainly includes the following aspects:1.Utilizing CVD methods to deposit monolayer MS_2.M:Mo,W,Re)materials.Especially,monolayer WS₂ materials were fabricated in a furnace with two heating zones by the CVD method.The PPh₃ and Au Cl₃solutions were used to produce the n-type and p-type doping effects,respectively,for the FET based on monolayer WS₂.Furtherly,a parallel homojunction was fabricated by half covering and half p-type doping a single WS₂flake,which showing the excellent photoelectrical response at 532nm illumination.However,this doping method is not stable.2.In order to achieve the stable doping effect,oxygen-assisted CVD method was used to deposit the oxidized Mo S₂.The sulfur atoms in the crystal can be partly replaced by oxygen atoms,and thus,induce weak p-type doping effect.Besides,defect lines appeared in the basal plane of the synthesized Mo S₂ samples,when the oxygen concentration exceeded certain threshold during CVD growth.Such defect lines not only quenched the PL of the monolayer Mo S₂,but also obstructed the free movement of the carrier to a large extent,which further degraded the electrical properties.3.Atomic substitution doping method was used to deposit Re doped Mo S₂during the CVD growth.Mo atoms in the crystal can be partly replaced by Re atoms.The concretions of Re in the crystal can be tuned by controlling the mass of Re O₃.By angle resolved polarization Raman spectrum and second harmonic generation technologies,the Re doped Mo S₂ were found to have the anisotropic property as the lattice strain generated by atomic substitution doping process.Furtherly,circularly polarized PL measurement demonstrated that the Re-doped Mo S₂ preserved valley dependent optical response,which also suggested that the 2H lattice structure optical helicity dependent valley selection rules were largely preserved during the doping.4.In order to realize the high concentration doping effect,CVD technology with a two heating zones furnace was used to deposit the Re doped Mo S₂ materials on mica substrates.The 2H Mo S₂ can be converted to 1T’Mo S₂ by controlling the relative mass ratio of Re O₃ and Mo O₃ during the deposition.PRS technology was used to analyze the anisotropy during the doping process,which found that the anisotropic ratio can be improved as the Re concentration increased in the crystal.Besides,it was also observed that the electrical properties showed increased angle dependence as the Re concentration increased,which finally resulted in phase change.