Study On D~0 Ferromagnetism In ZnO

ZnO is an important wide-band-gap semiconductor, the band gap of which is 3.37eV at room temperature (RT). It has a higher exciton binding energy of about 60meV. Therefore, ZnO is of great interest for its low-voltage and short wavelength (ultraviolet(UV), blue or green) light emitting devices such light-emitting diodes and diode lasers. On the other hand, ZnO is also an outstanding semiconductor. Computational studies have predicted ferromangnetism above room trmperature for several ZnO-based DMSs through doping Cu, Mn and Ni in ZnO film. Diluted magnetic semiconductors (DMSs) are considered to be very important materials in future semiconductor spintronics applications due to the simultaneous control of electrons'charge and spin. So ZnO has been studied extensively both experimentally and theoretically. In this dissertation, the density functional method based on the first principle calculation are used to study the crystal structures, electronic and magnetic properties ZnO. Calculations were performed on a 72 atoms wurtzite ZnO supercell using gradient corrected density functional theory as implemented in VASP.In this thesis, magnetic properties and electronic structure of ZnO with vacancies have been studied firstly. And secondly, we have been studied the effects of doping on magnetic properies of non-TM-doped ZnO, here C-doped ZnO, N-doped and P-doped ZnO systems have been studied. The thesis is arranged as follows:Firstly, some basic knowledge of Spintronics and diluted magnetic semiconductor(DMS) were introductored. Some questions about diluted magnetic semiconductor were also presented here. As the theoretic foundation of this paper, basic description of first principles calcution based on density functional theory was also included in the first part of this thesis. at last of this part, the software package VASP we used in this thesis was also introduced briefly.Secondly, the current interest in ZnO is largely driven by potential applications in optical and optoelectronic devices Since many properties of ZnO are highly sensitive to point and line defects tresented in the material, the defect physics of ZnO has been extensively studied in the past. Recently, some experiments show that Nonmagnetic oxides such as TiO2,ZnO,In2O3,SnO2,CeO2,Al2O3 and HfO2 thin films as well as nanoparticles, CaB6, SrB6 and BaB6 with the structure of CsCl and radiated graphite are ferromagnetic and all of which exhibit Tc above room temperature. They all don't contain transition metal or rare earth element in which electrons partially filled d or f shell, Generally, In DMS materials the delocalized conduction band electrons and valence band holes interact with the localized magnetic moments associated with the d or f electrons in magnetic atoms.But in these systems above, since they all don't contain transition metal or rare earth element in which electrons partially filled d or f shell, the origins of ferromagnetism which are different from conventional ferromagnetic materials have aroused public interest. So in the part of the thesis we studied the influence of vacancies on the magnetic properties of ZnO. The result showed that the ZnO with one O vacancy was nonmagnetic. While the ZnO with one Zn vacancy was magnetic. The magnetic moment was form the neighbouring 2p orbital of O atoms. From the calculation, we can draw a conclusion that Zn vacancy is the origins of magnetism in ZnO films. We also calculated and compared the total energy of different distance of Zn vacancies with the magnetism moment coupled ferromagnetically and antiferromagnetically. We found that the ferromagnetic configuration has smaller energy than anti-ferromagnetic configuration, so the ferromagnetic coupling is stable. In order to study the effect of distance of Zn vacancy on the electronic structures and magnetism in ZnO by comparing the total energy of three kinds of distance, we found that as the distance of two Zn vacancies becomes longer, the total energy becomes samller, which means the system with longer distance of two Zn vacancies can keep stable. All the result show that the origin of ferromagnetism of ZnO film is from the ferromagnetic coupling between two Zn vacancies.Thirdly, Dilute magnetic semiconductors (DMSs) has attracted interest and there has been a major effect to produce DMSs with Curie tempratures at or above room temperature. DMSs is usually produced by doping semiconductors with transition metals (TMs). ZnO and GaN were theoretically predicted to be ideal candidates for room-temperature DMS. Even though ferromagnetism has been observed in a number of systems, experimental studies on TM-doped ZnO have producted inconsistent results and the mechanism of ferromagnetism in TM-doped ZnO remains unclear. It was speculated that TM dopants in ZnO form clusters or secondary phases, which are detrimental to applocations of DMS. However, the non-TM dopants would not suffer from problems related to precipitates of dopans since they do not contribute to ferromagnetism. For example,copper doping in ZnO and GaN have been investigated both experimentally and theoretically. And it has been confirmed experimentally that both Cu-doped ZnO and GaN are room-temperature DMSs. Ferromagnetism was also reported in a number of carbon systems. Some of these studies have speculated that intrinsic carbon defects could be responsible for the observed magnetic propersities. So, C-doped ZnO is introduced in this part. The result of the calculations show that when C is doped into ZnO, it becomes spin-polarized and couple with each other ferromagnetically though p-p exchange mechanism. So C-doped ZnO system can be a promising DMSs.Fourthly,N-doped ZnO,another novel DMSs with half-metallicity and strong ferromagnetism, is also introduced , N dopants become spin polarized when substitutionally incorporated into ZnO and induce finite magnetic moments which render a ferromagnetic ground state in the host ZnO semiconductor,So N-doped ZnO system can be a promising DMSs.Finally,we also calculated P-doped ZnO system,The result of the calculations show that when P is doped into ZnO, it becomes spin-polarized and and induces finite magnetic moments which render a ferromagnetic ground state in the host ZnO semiconductor,So P-doped ZnO system can be a promising DMSs.In conclusion, if the p oribit has not been occupied by electrons in ZnO system. Then the p oribit can introduce magnetic moment, and p oribit magnetic moment may induce long distance ferromagnetic coupling.