ZnO-based dilute magnetic semiconductor material microstructure

　　Title: ZnO-based dilute magnetic semiconductor material microstructure 　　
        Author: Wang Dong
　　Degree-granting units: Wuhan University
　　Keywords: ZnO: 7194, nanocrystals: 4031, emission: 3644, positron annihilation: 2272, nanocomposites: 1844, DMSs: 1522, annealing temperature: 1454, positron submerged: 1123, Doppler broadening : 801, ion implantation: 770, grain size: 763, doped: 746, ferromagnetic at room temperature: 647, nano-particles: 636, nano-powder: 609, Neodymium Magnet positron techniques: 460, high-resolution transmission electron microscopy: 433, single crystal materials: 424, hysteresis: 408, nano-materials: 350
　　Abstract:
　　Spintronics is a recent emergence of a new discipline, since the date of birth has become the focus of many researchers. This is due to the spin-based electronic multi-function devices with conventional devices based on charge effects do not have many advantages: such as high-speed data processing, non-volatile, higher integration and lower power consumption, etc. and so on. As a spin-electronic devices to achieve these the most promising candidate materials, ZnO-based diluted magnetic semiconductors are increasingly attracted more and more attention. Although it has been prepared by many of the ZnO-based diluted magnetic semiconductors at room temperature, but the source of ferromagnetism is still unclear. To understand the source of ferromagnetism, we need the microstructure of the material more in-depth analysis.
　　This use of positron annihilation spectroscopy for transition metal ions into the ZnO single crystal, pure and Fe-doped ZnO nanocrystals of ZnO nano-crystalline micro-structural defects for a more detailed study. The main research contents are as follows:
　　We use the distribution of ion implantation method to transition metal (Fe, Co) ion implantation into ZnO single crystal, the injection of ions from 50keV to energy 380keV, the total injected flux reached 1.25 × 1017cm-2. After the sample into the crystal quality by X-ray diffraction rocking curve and ω angles were measured. Fe ion-implanted samples after annealing at 700 ℃ observed a weak Fe phase peaks, indicating the formation of Fe nanoparticles. SQUID measurements show that the Fe ion implantation of ZnO when the ambient temperature lower than 250K showing ferromagnetic, while the Co ions into the sample at room temperature has shown a stronger ferromagnetism. Rocking curves, photoluminescence and Raman spectra of the test results are displayed in the ion implantation generated a lot of structural defects. Part of the defect as a non-radiative recombination centers and suppress the ZnO UV emission of visible light. These flaws also undermine the Raman selection rules, so some of the original activation pattern does not exist there. Doppler broadening spectrum of experimental results show that Fe ions into the sample, most of the irradiation induced defects after annealing at 700 ℃ restored in the post-annealing temperature above 1000 ℃ can be completely eliminated. As for the Co implanted sample, most of the defects at 900 ℃ was restored, and to eliminate these shortcomings, the need for temperatures above 1100 ℃ annealing. Fe ions into the sample CKS electronic defects observed recovery is not consistent http://www.everbeenmagnet.com/ with the magnetic changes that Fe ion implantation of ZnO single crystal in the magnetic part is likely due to Fe ions replace Zn ions in the lattice position.
　　We will high-purity ZnO nanometer powders pressed into discs and in the air from 100 ℃ to 1200 ℃ annealing. X-ray diffraction of the crystal quality of ZnO nanocrystals. Annealing makes the nanocrystals grow to some extent, in 400-1200 ℃ annealing stage, the grain size has been increased from 25nm to 165nm. High-resolution scanning electron microscopy and TEM images confirmed that annealing occurred during the grain growth. Positron spectrum measurements show submerged in ZnO nanocrystalline grain boundary there is a lot of defects, including Zn vacancies, vacancy clusters and microvoids. Which micro-holes by 100-700 ℃ annealing can be easily restored. The Zn vacancies and vacancy clusters above 500 ℃ was changed. When the annealing temperature above 1000 ℃, the samples are no longer in the positron capture by interface defects. Raman spectroscopy results also confirmed the annealed lattice has been restored. We are at 100 and 400 ℃ annealed samples observed hysteresis phenomenon, but when the annealing temperature exceeds 700 ℃ after the magnetic disappeared. Taking all measurements we believe that ZnO nanocrystals at room temperature ferromagnetism may be due to nano-crystalline surface defects (Zn vacancies, VZn).
　　We purity ZnO/Fe2O3 nanocomposites in air at different annealing temperature, annealing temperature from 100 ℃ up to 1200 ℃. Nanocomposite structure and grain size by X-ray diffraction patterns measured. Annealing makes the grain size increased from 25nm to 195nm, and produces zinc-iron spinel phase ZnFe2O4. Positron spectrum measurement shows the submerged ZnO/Fe203 nanocomposite interface areas there is a large number of vacancies type defects, which can be as the annealing temperature gradually restored. When the annealing temperature above 1000 ℃, the vacancy concentration decreased positive electronic detection limit of the following. After measuring the physical properties of multi-functional system testing, Fe-doped ZnO nanocrystals showed room temperature ferromagnetism. Annealing temperature rose above 1000 ℃, the magnetic remains, indicating the ferromagnetic samples has nothing to do with the interface defects.
　　Degree Year: 2010