Electronic Structures And Optical Properties Of ZnS And Doped ZnS: Calculation And Experimental Veriifcation

As a wide band gap semiconductor （3.68 eV in bulk）, ZnS has two different structures:blende and wurtzite type. For its application limitations in luminescenct devices, doped ZnSshowed improved luminous quality, luminous efficiency and emission spectrum. Based oncomputer techniques, material design and computation played a very important role inmodern material science, which can predict the macro properties of materials. In this thesis,the electronic structures and optical properties of ZnS and doped-ZnS system have beeninvestigated using the first-principle plane-wave pseudopotential method. The experimentresults of polycrystalline ZnS prepared by chemical synthesis has verified the theoreticalpredictions. The main contents are as follows:（1） The electronic structures and optical properties of ZnS and ZnS with intrinsic defectsare investigated. Calculated results indicate that sulfur vacancy defect （Vs） produced a deepdonor level below the bottom of conduction band and Zn vacancy defect (V_Zn) is thought tobe above the top of valance band for direct wide band gap semiconductor of ZnS. Thecalculated optical properties show that, with Vs or VZnincluded in ZnS crystal, thepermittivity shows a trend of enhanced with consistent to the refractive index, absorptioncoefficient and loss function. Absorption and fluorescence spectra of ZnS prepared bychemical method with different kind of intrinsic defects are in agreement with that fromcalculations. The emission peaks at 430 nm are considered relate to the level of S vacancy.（2） The electronic structure and optical properties of Zn_1-xCd_xS ternary mixed crystals arecalculated, and the impact of variable x to the bandgap and mechanism of the formation ofdefect level of mixed crystals are discussed. Also the S vacancy level is calculated and thevariation of defect level with variable x is investigated. Moreover, the XRD, fluorescencespectra and experimental absorption spectra of Zn_1-xCd_xS prepared by chemical synthesis andannealing treatment are obtained. The results exhibit agreement with the calculations.（3） The electronic structures and optical properties of Cu-doped, Al-doped and Cu-Alcodoped ZnS are calculated. The results revealed that Cu-Al codoping is benefit to formn-type ZnS. The influence of cations doping on experimental fluorescence spectra and decaycurves of ZnS samples are discussed. It is considered that doped Cu produced an acceptotlevel (Cu_Zn) in band gap, and doped Al produced a donor level (Al_Zn). The blue shift in peakposition is related to the difference in defects level positions by different intrinsic defect.（4） The fluorescence characteristics of ZnS and doped ZnS nanocrystals capped bymercaptoacetic acid （TGA）, polyvinyl alcohol （PVA） and （3-mercaptopropyl）trimethoxysilane （MPS） are compared and discussed.