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First-Principles Investigation on Characteristics of Doped-ZnO

Author: XiongZhiHua
Tutor: JiangFengYi
School: Nanchang University
Course: Materials Physics and Chemistry
Keywords: ZnO Doping Surface Electronic Structure Optical Properties Magnetic Properties First-Principles Molecular Dynamics
CLC: TN304
Type: PhD thesis
Year: 2008
Downloads: 1390
Quote: 9
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Abstract


Computational materials and materials design combined with computer techniques are important contents in materials science.Here five main aspects of the dissertation have been achieved using first-principle theory based Density Functional Theory(DFT)and molecular dynamics methods:electronic structures,optical properties of perfect ZnO,ZnO with native defects and under strain ZnO,respectively; doping and alloy mechanisms in ZnO;the magnetism origin and mechanisms of ZnO-based DMSs doped with non magnetic metal;the metallic transition of ZnO(10-10)surface induced by hydrogen adsorption.1.The electronic structures of perfect ZnO and ZnO with native point defects were investigated with the methods of first principles.(1)Using GGA,the calculated results indicated that ZnO is a direct wide band gap semiconductor material with energy gap of 1.02eV.(2)The optical parameters including dielectric functions, absorption coefficient were calculated,which were furthermore identified with electronic structure information of ZnO and theory of electron inter-band transitions. (3)The electronic structure and optical properties of ZnO under strain were calculated and the bule-shift effects were also been studied in details.2.We studied the electronic structure of n-type,p-type and p-type codoping ZnO, such as:Ag,N,Al,Ga single doping and N-H codoping,respectively.Moreover,the action of H atom was studied using the methods of molecular dynamics.It was shown: (1)Ga was more suitable for fabricating n-type low resistance ZnO due to its better conductivity compared with Al.(2)Substitute Ag produced a deep acceptor level,and doping N also produced a deep acceptor level,which were hardly localized near the top of the valence band.Therefore,single doping of Ag and N made against to fabricate high quality p-type ZnO.(3)It was confirmed that H is the main factor to induce the native n-type coductivity in ZnO for its shallow acceptor level and large concentration in ZnO.(4)The formation energy of N-H complex was lower than that of N single doping,which indicated that some of H atoms-doped made for increasing the concentration of N.As well,it was found that N was passivated by H atom,which is obviously not benefit for p-type doping.Furthermore,using molecular dynamics simulation,we found that H was unstable thermodynamicly.It means we could obtain the high quality p-type ZnO by annealing for N-H codoped ZnO.3.The electronic structure of MxZn1-xO(M=Cd,Ca)were calculated to analyse the mechanism of ZnO band gap engineering.It was shown:(1)The band gap of ZnO narrows with increasing Ca-doping concentration.Our work shows that the top of valence band is determined by the O 2p electron,while the bottom of conduction band is occuupied by the hybrid of O 2p,Cd 5S and Zn 4s electrons which can lower the bottom of conduction band.(2)On the contrarly,the band gap of ZnO broadens with increasing Ca-doping concentration.Our work shows that the top of valence band is still determined by the O 2p electron,while the bottom of conduction band is only determined by the Zn 4s electron states which can shift to a higher energy due to Ca doping.4 We presented first-principles spin polarized calculations of the eleotronic structure and magnetic properties of Cu(Ti)doped in ZnO.We showed that:(1)It is likely for Cu(Ti)to order ferromagnetically in ZnO,forming a dilute magnetic semiconductor.The coupling between Cu(Ti)atoms is found to be ferromagnetic despite Cu(Ti)being nonmagnetic in its natural phase.Cu is acceptor and Ti is donor in ZnO,respectively.(2)The ferromagnetic ground state in Cu(Ti)doped ZnO can be explained in terms of Zener’s double exchange mechanism.(3)Cartier concentration can affect the magnetism of ZnO-based DMSs,for example,oxygen vacancies tend to destroy the ferromagnetism and therefore should be avoided during sample fabrication.These results are consistent well with the recent experimental discovery of ferromagnetism in Cu-doped ZnO.5.The clean ZnO(10-10)surface and hydrogen adsorbed ZnO(10-10)surface are studied by first principles.It was shown:(1)Zn atom move inside obviously after relaxation,which induced surface Zn-O dimer distorting.(2)The alteration of the charge density redistribution of the ZnO(10-10)surface shows the electron transfer from surface layer to the adsorbed H atoms,which leads to a metallization of the ZnO(10-10)surface.The mechanism of alteration for charge density redistribution is discussed,which provides a theoretical background of electrical properties of ZnO(10-10)surface.This work was supported by the 863 project(contract No.2003AA302160)and China foundation for development of electronic information technology.

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