Studies On The Hybridized Electronic States In The Band Gap Of ZnO-based Diluted Magnetic Semiconductor And The Correlation With Magnetic Properties By Synchrotron Radiation Photoemission Spectroscopy

The mass, charge and spin of electrons in solids have built up the foundation of information technology in the modern world. Semiconductor devices take advantage of charges, and magnetic materials take advantage of spins to record information. Nowadays, diluted magnetic semiconductor (DMS) incorporates transition-metal (TM) magnetic ions into semiconductor material. By combining charges and spins together, DMS is poised to create transformative changes in information technology and other fields. In recent years, the research about DMS has made great progresses. However, the origin of ferromagnetism (FM) is still controversial. What the mechanism that determines the ferromagnetism is, and how to prepare DMS that is ferromagnetic at room temperature (RT) are the most important problem that researchers concern.In this thesis, we have grown Fe-doped ZnO films by laser assisted molecular beam epitaxy (L-MBE) method and plasma assisted molecular beam epitaxy (P-MBE) method, respectively. Synchrotron radiation resonant photoemission spectroscopy (RPES) and x-ray adsorption (XAS), magnetic measurements and other related techniques are used to study the structure, electronic and magnetic properties of the films. Based on the results, we have analyzed the relationship between electronic structures and magnetic properties, and illustrated the origin of ferromagnetism systematicly.We first explored the growth condition of pure ZnO films by P-MBE method, and built up foundation to grow doped ZnO films. Three pure ZnO samples were prepared on Si (111) substrates by changing the Zn evaporation amount at a fixed O2flowing rate. The films are grown with c-axis preferred orientation. As the Zn/O ratio decrease, the quality of the films becomes lower. The images of scanning electronic microscope (SEM) show a nano-scale reticulation morphology under the Zn-rich condition. As the Zn/O ratio decrease, the surface turns flat with large amounts of grains. Under O-rich condition, there are a lot of defects in the film. The peak intensity of photoluminescence (PL) spectrum also becomes lower as the Zn/O ratio decreases. Under the O-rich condition, the structure of the peak is complicated with different features. Then we prepared ZnO film on ZnO (0001) substrate under O-rich condition. The film is well epitaxial grown on the same material. The high quality of the film indicates that the quality of the film is closely related to the substrates. In order to understand the electronic structure of Fe-doped ZnO at a first glance, we have built up a simple ideal model and calculated the density of states (DOS) using local spin density approximation (LSDA) method in density functional theory (DFT). The results show there are localized Fe3d electronic states near the Fermi level. Based on this finding, a Fe-doped ZnO film on Si (111) substrate is prepared by L-MBE method with Fe doping concentration of8.8%. The film is grown well with c-axis preferred orientation and extremely high qulity. The absorption peak of Fe L-edge X-ray absorption spectroscopy (XAS) is located at a photon energy of710eV, indicating the Fe valence state is Fe3+. Resonant photoemission spectroscopy (RPES) shows an obvious resonant enhancement effect at710eV. There are no electronic states near Fermi level. Magnetic measurements show a superparamagnetic property and no obvious hysteresis loops at room temperature. Ferromagnetism does not exist in a nearly perfect Fe-doped ZnO film with little defects and no electronic states near the Fermi level.In order to obtain a deep insight about the origin of ferromagnetism, Fe-doped ZnO films on Si (111) substrates with Fe doping concentrations of3%and10%are prepared under O-rich condition. The two samples are ferromagnetic at room temperature. The magnetic moment is higher in the film with lower Fe doping concentration and the paramagnetic contribution becomes larger in the film with higher Fe doping concentration. Fe2p PES and Fe L-edge XAS show that the dominant Fe valence state is Fe3+accompanied with some Fe2+. Fe3+ions and Fe2+ions coexist in the system. Furthermore, the position of Fe3+absorption peak varies as Fe concentration increases, indicating that the distribution of Fe3+ions in the crystal lattice differs from each other. It is observed from Fe2p-3d and Fe3p-3d RPES that the electronic states related to Fe2+ions exist near the Fermi level. As Fe doping concentration increases, the locations of Fe3+ions change from tetrahedral sites to octahedral sites. Under the O-rich condition, the large amount of defects like Zn vacancies and the electronic states related to Fe2+ions near the Fermi level are the reasons that facilitate the formation of ferromagnetism in Fe-doped ZnO films grown by P-MBE method. When Fe doping concentration is high, the Fe3+ions located at octahedral sites destabilize ferromagnetism.