ZNS Theoretical Study On Electronic Structure And Optical Properties Of Doped System

Before 1990,silicon(Si) and germanium(Ge) based semiconductors have the dominant position in the so called first generation semiconductor materials.With the advent of information age,large demands for capacity storing,fast transmitting and treating have been put forward, being the representative of the so called second generation of semiconductor materials,gallium arsenide(GaAs) shows significant advantages.Now the third generation semiconductor materials marked with wide band gap,such as gallium nitride(GaN),silicon carbide(SiC), diamond(C),zinc sulfide(ZnS) and so on,show superior physical and chemical properties,and have attracted much attention.Especially,ZnS is cheap and nontoxicity,and has excellent optical performance,the researches on it have been a focus task.ZnS is aⅡ-Ⅵsemiconductor with energy gap of 3.68eⅤ,there are two different structures (blende and wurtzite).Doped with impurities,the conductive properties,luminous efficiency, luminous quality,and emission spectrum of the system can be improved for different practical needs.Combined with computer techniques,material computation and design play an important role in modem material science.In this thesis,we study the electronic structure and optical properties of the blende ZnS systems in different doping cases by the first-principles approach based on the density functional theory(DFT).The main contents are as follows:(1) The structure,basic physical properties,research situation,and applications of ZnS system are introduced.In addition,our calculation tool-CASTEP and its theoretical foundation are briefly discussed.(2) The electronic structure and optical properties of pure ZnS are investigated.The density of states,energy band,geometry parameters and absorption coefficient are calculated.The obtained results indicate that ZnS is a direct semiconductor with wide energy gap of 3.68eⅤ. There is little absorption in the energy range below the 4eⅤ;the absorption edge locates at about 3.6eⅤ(345nm) derived from the transition between valence band to conduction band.The main absorption peak locates at about 8.3eⅤ.(3) The electronic and optical properties of ZnS systems doped with Al and Ag are studied. The density of states,energy band,geometry parameters and absorption coefficient are calculated.The obtained results show that Al doping is n-type.Doped with Al,Mott transition takes place and the system transforms from semiconductor to metal;Ag doping is p-type.The band gaps of the doped systems become smaller compared with that of the pure ZnS,the absorption edges shift obviously to the infrared region,and strong absorption exists in the visible light distinct. (4) The electronic and optical properties of ZnS systems doped with different 3d transition metal elements are studied.The density of states,absorption coefficient,energy band,and geometry parameters of different doped systems are calculated.The obtained results indicate that the bottom of the valence band and the conduction band of all the doped systems shift to the low-energy area compared with that of pure ZnS,and the band gap reduces.The doping of Fe, Mn,Cr,and V are the n-type doping,and the doping of Cu,Ni,and Co are the p-type doping. Absorption edges of all the doped systems shift obviously to the infrared region;there is strong absorption in the green light distinct for all the systems.Furthermore,there is strong absorption in the far ultraviolet region for V and Cr doped systems.Our results are in agreement with experiments.