First Principles Study For ZnS And Doped ZnS

Wide band gapⅡ-Ⅵgroup semiconductor ZnS which is an important luminescent material has the particular properties of infrared transparence, fluorescence, phosphorescence and so on. So it is widely used in photoluminescence device, electroluminescence device, solar battery, infrared detector and laser device. Pure ZnS material has some flaws which hinder it from further applied research. So doping ZnS with right impurity and improving its optical properties and structural performance have great application prospect. We used the first-principles to simulate ZnS and the bulk of ZnS after importing impurity and calculate their optical properties and electronic structure with computers. With the corresponding calculations we can forecast various physical properties of ZnS material. And these results of computer simulation can provide a reasonable theoretical basis for the growth and synthesis materials in experiment.In this paper, We performed first-principle calculations based on pseudopotential plane-wave within the density functional theory(DFT) using the Cambridge Serial Total Energy Package(CASTEP) in ZnS super cell (2×2×2) and alternative-doping in ZnS 2×1×1,2×2×1 and 2×2×2 supercells. Ultra-soft pseudopotentials were used to desribe the electronion interaction. The exchange and correlation energy by the generalized gradient approximation (GGA) of Perdaw-Burke-Emzer was adopted for all elements in our models. The research can be divided into three areas:In order to ensure reliability of the results we must have convergence test of calculation quality and K-point sampling density before calculating properties. In the case that the precision is not too high, in order to save computing time and memory, the cutoff energy was set at 350eV and the 2x2x2 k-points were generated by using the Monkhorst-Pack scheme in the geometry optimization of ZnS super cell (2x2x2). The band structure and electronic density have been computed by means of plane wave ultra-soft pseudopotential method and generalized gradient approximation which is based on the density functional theory. The results showed that ZnS was direct band gap semiconductor. The thermal band gap of ZnS was 2.08eV and this theoretic calculational value was smaller than experimental value (Eg of ZnS is 3.68eV). Analysis of density of states found that Zn3d state electronic peak was significantly higher than any other peak. So it was highly localized. S3p states overlapped with all Zn states in upper and lower valence bands. A powerful hybrid phenomenon existed between them.This was partial reason of the stable existence of ZnS.The band structure, electronic structure, electron density differentce and optical properties (mainly the dielectric function and absorption coefficient) of ZnS importing Ag have been computed by means of plane wave ultra-soft pseudopotential method and generalized gradient approximation which is based on the density functional theory. Comparing before and after doping we found that by importing impurities Ag, the density of states of Zn and S had a shift about 0.26eV to the low-energy direction. The thermal band gap has narrowed. Meanwhile, according to the electronic structure, the acceptor impurity level was introduced by importing impurity Ag. It was pointed out that the acceptor level was the overlapping of the Ag 4d and the S 3p electron orbit that was the result of hybridization. Meanwhile the electronic density was analyzed. Zn-S bond had strong covalence in the ZnS original cells and the covalence of bond Ag-S was weakest after Ag-doped, since the population was smallest, bond length was longest. And Ag-S bond has the trend of broken. After Ag-doped the imaginary part of the dielectric function had a new rush between the energy of 0～1.88eV in the (1,0,0) direction, the peak between the energy of 1.88～11.67eV had a small shift, and the peak had also decreased. The absorption edge had a tiny blue shift. By analysis of different concentrations of Ag-doped ZnS we found that with the increase in the concentration of doped Ag, the optical band gap became bigger, optical absorption edge had a blue shift, the diffuse of valence band and the conduction band was stronger and electronic delocalization correspondingly increased. Ag-doping played an important role in conductivity of ZnS.The band structure, electronic structure, electron density differentce and optical properties (mainly the dielectric function) of ZnS importing Al have also been computed by means of plane wave ultra-soft pseudopotential method and generalized gradient approximation which is based on the density functional theory. The density of states of Zn and S had a shift about 2.74eV to the low-energy direction after Al-doped. The valence band introduced a donor energy level. The covalence of bond Al-S was strongest, the population was biggest and the bond length was shortest. When Al took replace of Zn the interaction of Al and the neighboring S strengthened, the electronic density of Al-S was obviously more than that of Zn-S and electron move to the low energy direction. The higher was concentration of Al-doped, the wider was optical band gap. Optical absorption edge had a blue shift.The density of states (DOS) of Zn atoms and S atoms move to the lower energy. Valence band and the conduction band were diffuser and electronic delocalization correspondingly increased. Because of Ag-doped the imaginary part of the dielectric function had a new rush between the energy of 0～1.91eV in the (1,0,0) direction, the highest peak had a small shift to the low energy. All these changes reflected the impact in the band structure and the electronic structure which was introduced by Al impurities.