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Electronic Structure and Magnetism of ZnO-based Magnetic Semiconductors

Author: LinXueLing
Tutor: MeiLiangMo；YanShiShen
School: Shandong University
Course: Condensed Matter Physics
Keywords: Spintronics Magnetic semiconductors ZnO First-principles calculations Electronic structure
CLC: O472.5
Type: PhD thesis
Year: 2011
Downloads: 267
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Abstract

As we all know, the electronic charge and spin with two kinds of degrees of freedom. Traditional microelectronics research and control the main characteristics and the charge transport properties, which greatly promoted the social progress and human development. However, the conventional microelectronics just use the electronic nature of the charge, while ignoring the electron spin properties. With the development of society, the electron spins gradually be taken seriously. The late 1980s, the discovery of giant magnetoresistance, which greatly raises interest in magnetic material scientists, and gradually formed a new discipline: spintronics. It is today a kind of condensed matter physics a hot area of ??research will be two electron charge and spin degrees of freedom as a carrier of information, through the charge and spin control, and thus achieve information transmission, processing and storage. Ferromagnetic semiconductor spintronics field is the key material, has attracted wide attention. The traditional method for preparing a transition metal ion doped into the semiconductor lattice, we expect to provide the magnetic moment transition metal ion, or a semiconductor carriers can cause some defects in ferromagnetic coupling between the metal ions role of the semiconductor material has iron Magnetic and retain key bandgap Seek to have the intrinsic, high Curie temperature (ie, ferromagnetic state with paramagnetic transition temperature) is a ferromagnetic semiconductor spintronics research priorities. The most common semiconductor material doped Si and Ge is difficult to achieve high Curie temperature ferromagnetism, mainly due to transition metal ions in the semiconductor material of these very low solubility defect states localized radius sufficient to make the transition more distant family ions ferromagnetic exchange interaction; addition, the nearest neighbor dopant ions tend to form between the antiferromagnetic coupling. Therefore, the experimental magnetic semiconductors Si and Ge-based study no significant progress. In the 1990s, first in Ⅲ - Ⅴ semiconductor material (such as GaAs) research progress, but still below its Curie temperature is room temperature, and can not reach the actual application requirements. After an oxide represented Ⅱ - Ⅵ group material (such as ZnO) have gradually taken seriously. ZnO is a wide band gap (3.4eV), high exciton binding energy (60meV) of Ⅱ - Ⅵ semiconductor material, but also a piezoelectric material, the optical also has many uses. In the transition elements doped ZnO materials, the experiments reported in the Curie temperature span is large, ranging from a few K to room temperature, and the origin of the magnetic explanations have different, and some reports obtained paramagnetic. How to get the intrinsic Curie temperature above room temperature magnetic semiconductors, as well as the physical mechanism of the magnetic source is our research focus. In the beginning of this century, for HfO2 materials research found that in non-doped HfO2 observed ferromagnetic signal that is a d0 ferromagnetism. In a later study, SnO2, TiO2, ZnO and other oxides were also found in similar non-doped ferromagnetic. Most researchers believe that the magnetic system is due to a defect, such as: anion vacancies, cation vacancies, vacancy clusters or interstitial ions and so forth. However, there are various defects valences, theoretical calculations show that, with a low defect formation energy is not necessarily neutral, and the magnetic properties on the valence state of the defect. Having ZnO as a promising semiconductor material for optical applications, the optical properties of the material by the stress. Most researchers believe that the tensile stress causes the optical band gap narrowing, while the compressive stress is to widen the band gap, band shift occurs. Stress as a technical means of transition metal doped ZnO magnetic materials will be affected, but the specific relevance is unclear. Current research on the theory of magnetic semiconductors mainly in two ways: (1) modeling studies; (2) based on the density functional theory Fan First-principles calculations. In this thesis, first-principles calculation software ZnO-based electronic structure of magnetic semiconductors, energy band from the perspective of transition elements in magnetic semiconductors the coupling effect of magnetic origin. Since the density functional theory and Van rapid development of computer technology, a variety of first-principles calculations have been born originals, such as Vasp, Castep, Siesta, Quantum-Espress and so on. In this thesis, using Quantum-Espresso and Vasp software. Mn doped ZnO-based semiconductor material Zn1-xMnxO magnetic medium, Mn atoms adjacent to each other easily and form the anti-ferromagnetic coupling of neighboring clusters, and in the case of Mn farther, disappears magnetic interaction, which is obtained the intrinsic, high Curie temperature of the magnetic semiconductor is very unfavorable. The doping of the C atoms in ZnO, the spontaneous polarization can be introduced, and the magnetic pole of the more widely distributed. Introduction of C atoms in Zn1-xMnxO, Mn between nearest neighbor magnetic coupling is ferromagnetic, and this ferromagnetic coupling effects can be far apart in the case of Mn is maintained, in order to achieve the intrinsic, Curie temperature of ZnO-based magnetic semiconductors. For the calculation of the electronic structure of ZnO materials, results show that non-doped ZnO, we found that neutral Zn vacancy and negative monovalent Zn vacancy, O interstitial moments can be introduced, in which the first two are spontaneous spin polarization, and the latter a non-spontaneous spin polarization. Thus, it is possible to introduce a non-doped intrinsic defects only magnetic neutral or negative monovalent Zn vacancy two possibilities. Further studies have shown that the magnetic coupling, magnetic coupling of the neutral Zn vacancy is paramagnetic, ferromagnetic and antiferromagnetic coupling of energy without significant advantage. While a negative one Zn vacancy house price is ferromagnetic coupling, and the ferromagnetic coupling can be maintained at above room temperature. And, in the n-type ZnO, the negative monovalent Zn vacancy formation energy is lower than neutral. Therefore, a non-doped ZnO negative origin of the magnetic monovalent ferromagnetic coupling between the Zn vacancy effect. Most experimental and theoretical studies have shown, Zn1-xMnxO in, p-type defects can be made for the ferromagnetic coupling between Mn. In the p-type intrinsic defects, a common neutral and negative monovalent Zn vacancy two kinds. In experimental studies, researchers believe that the neutral Zn vacancy can cause Zn1-xMnxO ferromagnetic; while in ferromagnetic Zn1-xMnxO materials, the experimental workers also observed negative monovalent Zn vacancy. In theory, most researchers have focused on the neutral Zn vacancy defects in the study. Our research found that the two defects are introduced Zn1-xMnxO ferromagnetism in Zn1-xMnxO the neutral Zn vacancy formation energy has a smaller, therefore, a ferromagnetic Zn1-xMnxO play a major role.

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