First-principles Calculation Of Electronic Structures Of The Wurtzite Cd_xZn_1-xO And Zincblende Cd_xZn_1-xSe

In this dissertation, firstly the development trend of Quantum Chemical theory and method was introduced; furthermore, calculation methods and status of Computational Materials Science based on the theory were narrated too. Then, the firt principles based on DFT were expatiated. Finally, we investigated the band structure of the wurtzite Cd-doped ZnO and the zincblend Cd-doped ZnSe by using CASTEP and DMOL3 base on the DFT.The electronic structures of pure and Cd-doped wurtzite ZnO have been investigated by using first-principles ultasolft pseudopotential approach of the plane wave. The calculation results indicate that the valence band maximum is determined by the O2p state and the conduction band minimum is determined by Zn4s. The band gap (Eg) of ZnO reduces and the lattice constants increase due to Cd doping . There are two mainly reasons about the decreasing band gap; On the one hand, with increasing Cd-doping concentrations, the interaction between the Zn and O becomes stronger so that the lower energy states of Zn4s orbital can take part in hybridization and the conduction band of minimum determined by the antibonding Zn4s states can shift to the lower energy. On the other hand, the O anion with nearest Cd has a stronger influence on the t2 of the Zn3d and the repulsion of p-d becomes stronger between O-Cd after Cd-dopping, thus the antibonding pd states which control the valence band of maximum shift to the higher energy position. In addion, the result indicates that the totall energy(Et) is increasing and the binding energy(Eb) is reducing with increasing Cd-doping concentrations; the Eg,Eb decrease linearly with the increase of Et.The electronic structures of zincblend ZnSe,CdSe and CdxZn1-xSe have been studied by the above method. The result indicates that CdxZn1-xSe belongs to the semiconductive materials of direct band gap; The top position of the valence band is occupied by Se4p and the bottom position of the conduction band is occupied by Se4s. With the quantity of Cd doping ZnSe, the lattice constants become longer and the Eg becomes narrower. By comparing the electronic structures of ZnSe undoped Cd with doped Cd, the stronger repulsion of p-d states causes that the energy of Se4p orbitals which determine the VBM become larger. As we all known that the sp3 hybridization obital energy of Cd is lower than Zn's, as the lower energy obitals of Cd is hybridizing with the sp3 hybridization obital of Se, whose antibonding and bonding orbitals shift to lower energy, so that the CBM shifts to the lower energy. The band gap narrows due to the increasing VBM and the decreasing CBM, which explains the red-shift phenomenon in experiment.