Manipulation Of Electronic Structures And Magnetism In Novel Dilute Magnetic Semiconductor Materials

Dilute magnetic semiconductors（DMSs） representing one of the more promising,challenging,and exciting materials have stimulated extensive interests and the possibility of fabricating spintronic devices.This dissertation presents a comprehensive investigation on the structural and magnetic nature of theⅡ-Ⅵ,Ⅲ-Ⅴ,Ⅳ,and oxide DMSs by using x-ray absorption fine structure（XAFS）,density functional theory,X-ray diffraction（XRD）,and superconducting quantum interference device（SQUID） methods.The local structure and occupation of the dopants in the DMSs were analyzed.On the basis of the electronic structures performed from first-principles,the magnetic origin of these DMS systems was explored.The findings provide theoretical and experimental support to search diluted magnetic semiconductors for practical spintronic applications.The main content in this dissertation is as follows:1.Structural and magnetic investigation onⅡ-ⅥDMSsX-ray absorption near-edge structure（XANES） and the first-principles calculations were employed to systematically study the structure and ferromagnetism origin of Zn_0.97Mn_0.03O and Zn_0.96Mn_0.04O:NⅡ-ⅥDMS thin films grown by MOCVD and ICP-CVD methods,respectively.The magnetization measurement indicates that both samples are ferromagnetic at room temperature. For the Zn_0.97Mn_0.03O film,the Mn ions are located at the substitutional Zn sites as revealed by the Mn K-edge XANES spectroscopy and the O K-edge XANES analysis indicates the substantial existence of Zn vacancy.Meanwhile,XANES analysis reveals that the codoped Mn and N impurities were substitutionally incorporated into the ZnO host in the Zn_0.96Mn_0.04O:N thin film.Based upon the first-principles calculations and XANES results,we propose that the Zn vacancy and the N co-substitution of O can induce the room-temperature ferromagnetism in Mn-doped ZnO systems.2.Structures and magnetism of Mn-dopedⅢ-ⅤDMSsXANES spectroscopy and the first-principles calculations were used to study the features of occupation sites and magnetic interaction of Mn dopants in theⅢ-ⅤGa_1-xMn_xN DMSs with zinc-blende structure.Theoretical XANES spectra are calculated for representative structure models of Mn atoms in the GaN lattice.It was shown that the substitutional Mn(Mn_Ga) in GaN is characterized by a pre-edge peak at 2.0 eV and a post-edge multiple-scattering peak at 29.1 eV.The peaks shift in position and drop in intensity dramatically for the interstitial Mn（Mn_I） and Mn_Ga-Mn_I dimer,and then disappear completely for Mn clusters.The experimental spectrum of Ga_0.990Mn_0.010N is almost reproduced by the calculated XANES spectrum of Ga_1-xMn_xN with Mn_Ga.First-principles calculation results suggested that the formation of Mn_Ga is preferable to that of Mn_I under low doping levels.With increasing Mn content,the 3d states of Mn_Ga are favored to mix with those of Mn_I, facilitating the formation of Mn_Ga-Mn_I dimer.The theoretical predictions are consistent with the experimental observations.3.Energetic stability,electronic structure,and magnetism inⅣ-group DMSs First-principles study on the energetic stability,electronic structure,and magnetic coupling in Mn-dopedⅣ-group silicon with various configurations has been performed systematically.The results show that the poly-Mn atoms prefer to aggregate in the Mn-doped silicon dilute magnetic semiconductor.It is found that the tetrahedral interstitial Mn（Mn_T） atoms assemble together via an intervening substitutional Mn(Mn_Si) ion,subsequently forming the generally favored -Mn_T-Mn_Si-Mn_T- type complexes.The dominant d-d exchange interactions between the Mn_Si and Mn_T make all Mn ions in the -Mn_T-Mn_Si-Mn_T- structures interact ferromagnetically with each other.4.Roles of charge-passivated codoping on oxide DMS surface The magnetism and electronic character of rutile Fe:TiO₂（110） oxide DMS thin films mediated by oxygen vacancies（OVs） and codopant were studied by using first-principles calculations.The results show the magnetic interaction between Fe ions is a ferromagnetic superexchange（FM-SC） nature,while the presence of OV makes it turn to an antiferromagnetic behavior.It is found when the system is codoped with oppositely charged P,the interactions are rehabilitated to FM coupling. The electronic structure analysis reveals that this is principally attributed to the sp-d hybridizations between P and Fe,which finally activate a long-ranged FM interaction between the Fe ions,and substantially facilitate the electron transport.