| Diluted m agnetic sem iconductors (DMSs) have attracted significant interests because of their unique spintron ic properties with potential applications in the field of spintronics devices. Theoretical calculations have predicted that ZnO based DMSs may be ferromagnetic at room temperature. Additionally, ZnO is a typical direct wide band gap semiconductor material with high exciton binding ener gy, which not only can em it high ultraviolet (UV) light at room temperature, but also be abundant in raw m aterials, cheap and environment friendly. Therefore, ZnO-based DMSs have becom e one of the hot topics in the field of condensed matter physics and materials science recently.In this the sis, the ef fects of intrinsic def ects and a variety of impurities on the electronic structure and m agnetic properties of ZnO-based DMSs were investigated systematically using first principles based on density functional theory . The origin of ferromagnetism and adjusting m ethods of Curi e temperature have been explored. The interaction m echanisms of dif ferent si ngle-doping and co-doping ZnO-based DMSs were also discussed in detail.Firstly, the systematic research about the effect of intrinsic defects on the electronic structure and magnetic properties of ZnO s hows that neutral oxygen vacancy in ZnO is nonmagnetic, but Zn vacancies can lead to magnetism. The magnetic moments mainly originate from the spin polarized ox ygen atoms around the Zn vacancies and it is the spin polarized Zn 3d state that modulates the ferromagnetic exchange interaction among oxygen atoms. In addition, it was also found that the substitution of one Zn atom by one Li atom in the system can generate holes an d reduce th e for mation energy of Zn vacancies, and thus s tabilize the Z n vacancy-including system and result in a larger magnetic moment.Secondly, the electronic st ructure and magnetic properties of N-monodoping, (Li, N)-codoping, and (Zn vacancy, N) codoping Z nO were studied systematically. It was found that monodoping of N in ZnO favors a spin-polarized state with a m agnetic moment of 0.95μB per superce ll and the m agnetic moment mainly stems from the unpaired 2p electrons of N and O atom s. Additionally, the ferromagnetic (FM) stability can be improved significantly by codoping with (Li, N) and (Zn vacancy, N). Thirdly, the electronic structure and m agnetic properties of ZnO:Cu and (C or N) codoped ZnO:Cu were investigat ed in detail. The results indicate that a single Cu substitution of Zn prefers a ferromagnetic ground state with a total magnetic moment of 1μB per supercell and leads to half-metallic ferromagnetism. The local moment of Cu is about 0.58μB and the rest m agnetic moment mainly arises from the spin polar ized O atoms. Besides, the FM stability is weaken ed as the Cu separation d istance increases. By doping with NO, the FM stability can be slightly enhanced, while it is weakened by doping with CO.Finally, the investigation on the electroni c structure and m agnetic properties of Cr-monodoped and (Cr , Al)-codoped ZnO indi cates that monodoping of Cr in ZnO favors a spin-polarized state w ith a magnetic moment of 7.50μB per supercell and the magnetic moment mainly comes from the unpaired Cr 3d electrons. And it was found that the FM exchange interaction between Cr atoms is short-ranged in Cr m onodoped ZnO and the FM stability can be enhanced significantly by codoping with (Cr, Al). |
PDF Full Text Request