Experimental And First-principles Investigation Of The Spin-polarization In Non-magnetic Doped Semiconductors

To promote the development of spintronic devices, searching for new spin-polarized materials to inject polarized current into semiconductors efficiently becomes a research focus. In recent years, the existence of ferromagnetism can also be observed in the system without doping with transition metal or rare-earth metal elements, which is mainly related with point defects, such as s or p electrons. In wide band-gap semiconductors, the s orbitals are related to the doner valence electron and p states are corresponding to the acceptor hole. Therefore, the investigation of the effects of intrinsic defects on magnetic mechanism, will not only deepen the understanding of spin coupling in sp-electron semiconductors, but also has a certain guidance for the new spin injection device.In the investigation of donor defects, polycrystalline SnNx films were fabricated by reactive sputtering with various substrate temperatures. The film has a hexagonal tin-rich structure and decomposition temperature is above 500 °C. The band-gap enlarged to 3.10 eV due to the complex defects and the conductivity of the films shows metallic-like behavior, which has the opposite variation trend with ferromagnetism as the substrate temperature changes. The origin of ferromagnetism may be from the intrinsic point defects, which result in a net moment by unpaired spin electrons from grain-boundary or interface. Effects of isovalent doping on electronic and magnetic properties in sputtered polycrystalline Ce_1-xSn_xO₂ films were investigated. The Sn doping helps to boost the magnetism in CeO₂, but has no significant impacts when the doping content exceeds 6%（x > 0.06）. Experimental analysis indicates that the magnetism may be directly associated with the structure distortion as well as the concentration of Ce3+ and oxygen vacancies. The superexchange mechanism through Ce-O-Ce interaction and F+ centers can ease the mediation of ferromagnetic coupling in the system.For the study of acceptor defects, introducing holes into intrinsic n-type materials can be achieved by doping with low-valence cation or high-valence anion. In epitaxial Sn_1-xMgxO₂ and Sn_1-xK_xO₂ films, the alignment of magnetic moments is established through p-p coupling interaction between the impurity p states and host p states. Due to the limit of solubility, the genetation of interstitials which act as donor may combine with the holes created by substituted incorporation, decreasing the magnetism. The defects in interstitial sites can irreversibly transform to lattice site after air annealing. Lattice distortion induced by large ionic radius also has great influence on magnetism. N-doped SnO₂ films were synthesized by oxidative annealing of sputtered SnNx with various temperatures. All the films show p-type conduction and the highest hole concentration is 2.08×1019 cm-3, indicating an effective way to alleviate the self-compensation effect. The substituted incorporation of N can also enhance the ferromagnetism. Meanwhile, the ferromagnetic coupling competes with antiferromagnetic coupling, causing the nonmonotonic variation of saturation magnetization.In the study of polarization in intrinsic p-type semiconductor, we have calculated the electronic structure and magnetic properties of the first-row element-doped CuCl semiconductors. The electronic correlations in both 2p and 3d orbitals are enhanced by adding the on-site Coulomb repulsion. After a comparative study, we find that the enhanced electronic correlation not only corrects the error of band-gap, but also influences the magnetic ground state and the distribution of local magnetic moments. The location of impurity bands with different dopants is related with the elements’ electronegativity. Strong hybridization between the dopant’s 2p states and the filled 3d orbitals of adjacent Cu yields the main contributions to magnetization.To make an ordered distribution of defects, we investigate the sp-electron half-metallic superlattices of wurtzite nitrides and rock-salt sulfides. Both of them can maintain their half-metallic property with an integer magnetic moment, which is mainly carried by the anions. The hybridization of p-d in nitrides and p-p in sulfides are shown to be essential for the formation of localized orbitals and spin polarization. Anion atoms in different layers show distinct polarized behaviors with the spin density distribution parallel or perpendicular to the z direction of superlattices. Moreover, the discussion of deformations further illustrates the feasibility of sp-electron half-metallic superlattices in practical applications.