Research On The Dielectric Properties Of GaN At THz Frequencies And The Fabrication And Properties Of Iron Nitrides

Gallium Nitride(GaN) is a kind of direct wide band semicouductor, which has multiple applications in blue and ultraviolet light-emitting diodes and laser diodes. In addition, it is one of the promising materials for high frequency transistors operated within gigahertz and terahertz frequency ranges. Recnetly, with the development of the terahertz time-domain spectroscopy, the optical and dielectric properties of GaN have been received many attentions.The ferromagnetic iron nitrides include ε-Fe3N, a"-Fe16N2and y’-Fe4N. They have high saturation magnetic moments and spin polarization. Therefore, these materials are potential as the spin injection electrodes in spintronic devices. Among them, the lattice of ε-Fe3N matches quite well with GaN and AlN, and thus has been studied intensively.Until now, there are some key issues that have not been desolved in the two areas of THz properties of GaN and properties of iron nitrides. For these issues, the researches of this paper include two parts:(1) The oscillatory behavior of the dielectric function of GaN in THz frequencies has been studied and discussed at different temperatures. Furthermore, the properties of the point defects in GaN have been obtained through the fitting parameters in the physical model of dielectric functions.(2) The magnetic structures of iron nitrides have been theoretically studied. In experiments, the s-Fe3N films have been fabricated through nitridation. The properties of the films are characterized and analyzed. Through the systemic studies, the main conclutions of this paper are as follows:1. The terahertz time-domain spectroscopies of the unintentionally doped, c-plane GaN thin film have been obtained by room-temperature and poikilothermal THz-TDS technology from10K to300K. Through the analysis of the signals and the extraction of the optical parameters, the dielectric functions of GaN film are carried out for the temperature and frequency ranges of10-300K and0.3-1.0THz. It is observed that the real and imaginary parts of the dielectric functions are evidently oscillating at various tempertures. According to the physical analysis, the oscillation behavior has been attributed to the resonance states arosed from the point defects. The contribution to the dielectric function by the resonace states is characterized by the classical damped oscillator model. Furthermore, the dielectric functions at various temperatures have been well fitted. From the fitting process, it is found that the contributions by the free carriers and resonace states have different temperature dependences, the reason of which is also analyzed. Finally, according to the fitting parameters, the concentration and electron lifetime of the point defects in GaN film have been determined at various temperatures. It indicates that the concentration decreases with the rise of the temperature, while the electron lifetime shows positive temperature dependence.2. By means of the Landau theory, the ferromagnetic point group of s-Fe3N is determined to be exactly62’2’. It indicates that the magnetic structure of s-Fe3N is uniaxially anisotropic, and the easy axis is along the c-axis of the hexagonal structure. The ferromagnetic point group of α"-Fe16N2can be classified into two categories, i.e.4/mm’m’and m’m’m. The magnetization for4/mm’m’ is along [001] direction of the tetragonal structure. As to the magnetization of m’m’m, it can take on two different orientations, one is along [100] direction of the tetragonal structure, and the other along [110]. In any case, there is no possibility for the magnetization along [111], which explains the puzzle why the easy axes of the samples of a"-Fei6N2on InGaN and Fe are different from each other. Finally, the magnetic structure of y’-Fe4N is clarified to be4/mm’m’, or m’m’m, or3m’, the corresponding magnetization is along [100], or [110], or [111] direction of the cubic structure, respectively.3. Under the opticmal conditions, the ε-Fe3N films on GaN (0002) templates have been fabricated by means of long time nitridation of amorphous Fe films. The structures of the ε-Fe3N films include two crystal orientations, i.e.(002) and (111), which indicates that the Fe film has almost been nitridated. The binding energies of Fe ions are investigated by XPS; the peaks at709.7eV and723.4eV are observed, they correspond to Fe-N2p3/2and2p1/2, respectively. That further verified the formation of the phase of iron nitrides. The surface of the ε-Fe3N film has island-like appearance, and is relatively rough when compared to the initial Fe film. In particular, the sample exhibits good magnetic and electrical properties. Firstly, it is of room-temperature ferromagnetism. Secondly, the coercivity of the ε-Fe3N film (200Oe) is much higher than that of the Fe film (67Oe). Finally, the ε-Fe3N film shows a metallic behaviour, according to the Ⅰ-Ⅴ curves. These properties indicate that the ε-Fe3N film could be considered as the potential material of spin injection.