Research On The Optical And Dielectric Properties Of Two-Dimensional MoS₂ By Spectroscopic Ellipsometry

Two-dimensional（2D）materials are currently a hot research topic in the fields of materials,optoelectronics,and condensed matter physics.Two-dimensional transition metal dichalcogenides（TMDCs）family is another nanomaterial system that has been eagerly paid attention and extensively studied after graphene due to the controllable bandgap,large on（14）off ratio,moderate mobility,novel valley selection achromatic,etc.Recently,2D MoS₂ has been adopted in many devices such as field-effect transistors,photodetectors,and solar cells.Additionally,with the gradual improvement in the quality and dimensions of p-type doped 2D MoS₂,it is expected that 2D MoS₂ will participate in the development of next-generation large-scale integrated circuits,thereby greatly realizing its industrial and commercial value.The devices made by the intrinsic or doped2D MoS₂ can be miniaturized,integrated,and flexible,while being endowed with extraordinary functions.The performance of these emerging devices is strongly associated with the optical and dielectric properties of 2D MoS₂,especially the inherent optical constants and dielectric functions.Further,the optical/dielectric properties of 2D MoS₂ are sensitive to the number of layers（thickness）and doping concentration resulting from the intense quantum confinement effect.Therefore,studying the dependence of optical/dielectric properties of 2D MoS₂ on the layer number and doping concentration can advance the understanding of the electronic structure and many-body effects in intrinsic and extrinsic 2D TMDCs,guide the conceptual design of corresponding devices,and lay the foundation of future industrial research and development.Accordingly,this dissertation aims to research the layer-dependence（scale effects）and doping-concentration-dependence of the optical and dielectric properties of 2D MoS₂via the spectroscopic ellipsometry（SE）.The innovations achieved mainly include:An optical transition analysis method combining ellipsometry,critical point（CP）analysis theory,and first principles is designed.CPs in dielectric functions of materials are determined by CP analysis.The energy bands and particles involved in the optical transitions occurring at CPs are identified in the simulated band structure and density of states by ensuring the momentum and energy regions match each other in these figures.The intuitive relationships between energy bands,density of states,and CPs are derived.Experimental results suggest the method is suitable for tracing the optical transition mechanisms of varieties of materials including ultra-thin films such as 2D materials.An analytical calculation method for the optical constants of ultrathin films based on the second-order Taylor expansion of ellipsometric ratio is proposed.The ellipsometric ratio is calculated from Drude reflection coefficients and approximately expanded.A solvable equation with the complex refractive index of ultra-thin film as the unknown quantity is constructed.The complex refractive index is analytically determined by involving the physical constraints.This method can be used to analyze ultra-thin films with known thickness but unclear dispersion features,effectively solve the problem of local optimum or non-convergent in SE fitting caused by the parameter coupling.The optical constants（refractive index,extinction coefficient,and absorption coefficient）and dielectric parameters（dielectric fucntion and energy loss function）of 2D MoS₂ over the UV-Vis-NIR spectral region are determined,confirming the method we proposed can calculate the optical constants of ultrathin film with a higher precision and extend the applicative spectral region.The optical conductivity of 2D MoS₂ is determined by the classic plate model and the surface current model,and the applicability of these two models in calculating the optical conductivity of 2D materials is evaluated.The layer dependent optical/dielectric properties of 2D MoS₂ is studied via SE.The critical points（CPs）in dielectric functions are determined by CP analysis method,and the“W”like layer-dependent evolution of dielectric fucntions at CPs is explained as the alternative domination of exciton effects,joint density of states,and mass density.The feature peaks in optical conductivity spectra of 2D MoS₂ are distinguished by the differential spectral method,whose different layer-dependent shifts are interpreted as the competition between the decreasing E_b and the band shrinkage.A study on the impurity-modulated optical and dielectric properties of niobium-doped p-type 2D MoS₂ is attained by SE.The dielectric functions and complex refractive indices of 2D MoS₂ with different doping fractions are obtained.The CP parameters of p-type 2D MoS₂ are determined by CP analysis methods,finding that the two exciton peaks near the bandgap gradually merge into a broad peak with the increasing doping concentration.These phenomena are clarified from the perspective of impurity-induced band renormalization and exciton effects according to the simulated results of first principles.A p-type FET based on the 2D Nb-doped MoS₂ is fabricated and its working performance is analyzed.