First Principle Study On Magnetism Of Semiconductor Nanostructure

Electrons have two intrinsic properties which are charge and spin. The chargehas a wide range of applications in the semiconductor material, which makes theelectronic technology, the computer technology and communication technologydeveloping very fast. The electron spin also can be used as a carrier of information, soas to realize information transmission, processing and storage. The rapid developmentof spintronics and its usefulness in the field of electronic devices makes the magneticsemiconductor become a research hotspot. Dilute magnetic semiconductors (DMS) aresemiconductors doped with magnetic impurities. Typical examples are transitionmetals or rare earth elements doped conventional semiconductors. Recently,unexpected high temperature “d0ferromagnetism” has been observed in a series ofsemiconductors, which do not contain ions with partially filled d or f bands. Nanomaterials have small dimension effect, quanta dimension effect, surface effect andmacroscopical quanta tunnel effect, so they manifest a series of specific physical andchemical characters. In this thesis, the first principles calculation was performed toinvestigate the electronic structure, magnetic origin, magnetic coupling and magneticcontrol in the semiconductor nanostructure. The thesis is organized as follows:In Chapter1, we introduce the definition, essential propertiy and the research indomestic and abroad of the DMS, point out the theories meaning and the practicevalue of studying on the origin of ferromagnetism and the magnetic coupling in theDMS nanostructure.In Chapter2, we introduce the basic principles, current situation anddevelopment trend of the band theory and the density functional theory (DFT).Finding good approximation for exchange-correlation functional is one of the maintargets in DFT research. With the development of functional, DFT leads to more andmore accurate results from the initial local density functional (LDA), generalizedgradient approximation (GGA) to hybridization functional. However, it sets higherdemands for the computing resources.In Chapter3, the unconventional magnetism related to VGain GaN isinvestigated by using DFT calculations. It is found that VGaformation-energy couldbe reduced by adding electrons, by nanostructure engineering, or by co-doping donor-like defects. The low VGaformation-energy results in the high concentrations ofVGain the GaN material. The VGainduced colossal magnetic moment in GaN:Gd canbe modulated by codoping the donor-like defects.In Chapter4, we used DFT to understand the origin of magnetism in Mg-dopedpassivated AlN NWs. The results show that surface passivation affects the impurityposition and leads to unequal characteristics of magnetic coupling. FM couplingexists in double Mg atom doped passivated AlN NWs with1.80Bper96-atom unitcell excluding the pseudohydrogen atoms. The magnetic moment mainly comes fromN-2p states that bond to Mg atoms. We also predict that the FM stability and Curietemperature of the passivated AlN NWs are much higher than those ofthree-dimensional bulk structures.In Chapter5, we investigated the electronic structures and magnetic properties ofhalf-bare zigzag SiC nanoribbons (ZSiCNRs) by using HSE06hybrid functional. Theresults show that there is a ferrimagnetic ground state with ferromagnetic couplingwithin the C edge and the antiferromagnetic coupling within the Si edge in ZSiCNRswith terminated C edge. While, there is a antimagnetic ground state within two edgeand ferromagnetic coupling within each edge in ZSiCNRs with terminated Si edge.Energy calculations show that the ferrimagnetism are stable in the ZSiCNRs withterminated C edge. Energy calculations also show that the magnetism of the C edgesare stable in the ZSiCNRs with terminated Si edge. By comparing analysis results, wefind that electronic structures and magnetic coupling of half-bare ZSiCNRs depend onthe type of bare edge atoms, regardless of the type of termination atoms.In Chapter6, we used DFT calculations to investigate the origin of magnetism incarriers and shallow donor impurities (Sn) codoped passivated In2O3:Co nanocrystal.The results show that the carriers and shallow donor impurities can enhance the totalmagnetic moment in passivated In2O3:Co nanocrystal, which is in good agreementwith a recent experimental. The electron mediated ferromagnetism is greatly enhancedby the shallow donor level, which is explained well by the levels’ coupling model.In Chapter7, we use density functional theory calculations to understand theelectronic structures and magnetic properties in the triple-decker Gd-phthalocyanine.The ferromagnetism of neutral triple-decker Gd-phthalocyanine is unstable. However,there is strong ferromagnetic coupling between two Gd atoms in the triple-deckerGd-phthalocyanine with two additional electrons per Pc ring. The additional electronsprefer to locate the innermost conjugate Pc ring, noticeably enhances the pz-f Zener exchange interaction. Controlled ferromagnetism of triple-decker Gdphthalocyanineby additional electrons make it have potential applications in organic spintronics.Finally, we make briefly summarizes of our work and gives an outlook of theproject prospects.