| Wide band gap semiconductors, also known as the high temperature semiconductors, simply relate to a class of semiconducting materials with a band gap larger than2.2eV, partly involving group-IV semiconductors (diamond, SiC), Ⅲ-nitrides (BN, AlN, GaN), transition metal oxides (TiO2, ZnO) and some ternary oxides (SrTiO3, BiVO4). On one hand, the considerably large band gap makes these semiconductors be ideal candidates for the electronic devices to work under high temperature and also be featured with high frequency, high power, high temperature-resistance and radiation-resistance as well. On the other hand, a large band gap also facilitates the absorption and emission of high-energy photons, which is a preferred character for developing devices of short-wave blue and green light-emitting diodes, high-energy lasers and ultraviolet detectors, and thus can be widely used in the fields of microelectronics and optoelectronics. The recently discovered unexpected room-temperature d0magnetism in GaN, diamond, SiC, ZnO and TiO2, and the realization of H2-production by photocatalytic water-splitting and degeneration of organic pollutants by using TiO2, SrTiO3and GaN further extend the application of wide band gap semiconductors into the fields of spintronics and photocatalysis.The devices of conventional microelectronics only exploit the charge property of electrons. While in the case of spintronics, both the charge and spin of electrons are together utilized to fabricate nonvolatile devices with high processing speed, low energy consumption and versatile functionality. The corresponding progress will lead to revolutionary impacts on the development of information technology. Dilute magnetic semiconductor (DMS), which is a kind of semiconductor material that exhibit both dilute ferromagnetism and semiconductor properties, is one of the most important materials for spintronics applications. The Curie temperature of the DMS is very low in the early stage. Within the frame of Zener model, in2000, Diet1predicted that wide band gap semiconductors are much easier to realize their Curie temperature at or even beyond the room temperature than Ga1-xMnx,As system. Subsequently, it was found that when doped with transition metal ions, a range of wide band gap DMSs, such as SrTiO3, GaN, TiO2, and ZnO, show ferromagnetic behaviors at room temperature. Particularly, the d0ferromagnetism, which has been found in various wide band gap DMSs by doping non-transition metal ions or introducing vacancies, may also survive around room temperature, which strongly challenges the common understanding on the magnetism of semiconductors. Importantly, one obvious advantage of d0ferromagnetism (FM) stems from the fact that it is free of aggregation of impurities and generation of secondary phases by doping magnetic ions. The d0FM not only provides new ideas to develop novel DMS-based devices but also renew people's fundamental understanding on the magnetism theory. However, through introducing impurities or vacancies, it is quite a challenging task to control the locations of the defects to realize a large-scale and high quality spin sequence in materials. In contrast, for low-dimensional materials, there are plenty of dangling bonds on the surface or grain boundaries, the unpaired electrons of which may also introduce spontaneous local magnetic moments. These surface defects also suggest a possible way to realize a controllable magnetism resulted from certain spin structure. However, up to date, the physical picture of interactions among the dangling bonds and their effects on the surface magnetism remain unclear. Therefore, in view of the currently pending questions, further investigation is urgently needed to deepen the understanding of d0magnetism and to promote the development of spintronics.Wide band gap semiconductors, such as TiO2, SrTiO3, and solid solution ZnO/GaN are also good candidates for photocatalyst that have potential use for H2-production and decontamination of organic compounds under sunlight irradiation. Due to their considerable large gaps, these materials are only of high photocatalytic activity under ultraviolet radiation that accounts for no more than5%in energy of the whole sunlight. Thus, it is a solid challenge forcing us to enhance the photocatalytic efficiency of these materials to respond to the visible light, which possesses a substantial portion up to43%in energy of the sunlight. In general, the way of band structure engineering by impurity doping can be effective to extend the range of the absorbed sunlight. The defect states introduced by mono-doping usually locate in the mid-gap. For example, cation-doping introduces defect states near the conduction band minimum. However, the lowering of the band edge suffered from mono-doping may weaken the deoxidization ability of the system. For anion-doping, the defect states often emerge in the vicinity of valance band maximum, which might act as compensating levels and therefore result in poor photocatalytic efficiency due to the possible recombination of photogenerated electrons and holes. Prior to mono-doping, the synergistic effects of co-doping can not only enhance the solubility of the doped impurities but also prevent the occurrence of the above-mentioned disadvantages, and thus benefit the goal of achieving the enhanced response to the visible-light as well as the photocatalytic efficiency.The solid geometries and electronic structures of materials are the underlying basis of their electronic, magnetic, and photocatalytic properties. Therefore, it is of great significance to explore the correlation between these intrinsic factors and the related properties, which is helpful to make physical phenomena clear, grasp the underlying mechanism and rationalize the experimental design. In this dissertation, to exploit the potential use of wide band gap semiconductors in the fields of spintronics and photocatalysis, we systematically investigate the role of the factors including geometric structures, topology, impurity defects as well as the synergistic effects of co-doping, on the electronic structures, magnetic properties and photocatalytic activity of some wide band gap semiconductors (such as TiO2, GaN and so on) within the framework of first-principles-based density functional theory. The correlation of geometries-electronic structures-properties is particularly concerned and explored. This dissertation consists of seven chapters. In Chapter One, we give a brief introduction to the background and current scientific status of wide band gap semiconductors in the fields of spintronics and photocatalysis. In Chapter Two, we simply describe the basis of the density functional theory as well as the software employed in our simulations. In Chapter Three, we present our calculated results on the magnetic behaviors associated with the2p-orbital dangling bonds deriving from different diamond surfaces and owning different geometric and topological characters. In Chapter Four, we show our study of Si dangling bonds of SiC nanosheet systems with C atoms passivated by H atoms. The engineering effects induced by changing the thickness (the number of layers) and external stress have been investigated in detail. In Chapter Five, we show our study of mono-doping effects on the band structure modification and photocatalytic property of TiO2. In Chapter Six, we report the synergistic effects of impurity co-doping on the band structure and photocatalytic activity. In Chapter Seven, we make a simple summary of our studies, and outline some pending questions as well as possible future plans. The Index section shows some data processing scripts developed by our group. The main content and results are listed as follows:(1) We investigate the correlation between the magnetic properties and the dangling bonds of different geometric and topological structures of several low Miller index surfaces of diamond, and unravel the underlying mechanism as well as its applicable condition. The geometric and topological structures of the surfaces impact the competition between the exchange, bonding and transfer of the electrons in different dangling bonds. The local magnetic moments induced by the unpaired2p electrons of the dangling bonds on the (001) and (011) surfaces form an antiferromagnetic coupling, while those on the (111) surface form a FM coupling. The dangling bonds on the (001)-(2×1) surface reorganize and lead to a nonmagnetic insulating state, while those on the (111)-(2×1) surface tend to form extended π bonds, resulting in a nonmagnetic metallic state. For these low-dimensional materials in which the properties of the system can be determined by the surface dangling bonds, we can predicate their properties according to the geometric and topology structures of the dangling bonds.(2) We investigate the spin polarization and its long-range coupling induced by the unsaturated3p electrons of Si sites in the (SiC)n-H monolayer and multilayer systems with C atoms passivated by H atoms, as well as the effect of the layer thickness and anxious strain on the magnetic properties. In the studied systems, the magnetic moments are mainly contributed by the dangling bonds, and the AFM coupling is favorable only for the case of monolayer, while for the other thicker cases the FM coupling is stable. For a small strain (<±3%for the monolayer,<±2%for the double layer), the ground coupling state is sustained, and the ground state is much more stabilized by increasing the compress strain. Thus, it can be expected that the ground states of FM and AFM can be turned by the controlling the layer thickness of (SiC)n-H.(3) We investigate the effect of group-IV element doping on the band structures of TiO2as well as the underlying mechanism, and summarize the electronic structures and related properties of non-metal elements (N, B, S, Si, P, C, F, Cl, Br, I)-and Cr-doped TiO2. We find that the non-metal elements (F and Cl) with strong electronegativity tend to substitute O; while elements (Cr, Si, Ge, Sn, Pb) with weak electronegativity tend to substitute Ti; the incorporation of otherwise elements (B, C, N, S, P, Br, I) can be strongly influenced by the experimental conditions, namely, substituting Ti at O-rich condition and substituting O at Ti-rich condition. The impurity states emerge near the bottom of conduction band for the case of substituting Ti; the impurity states arise near the top of valance band for the case of substituting O. As increasing the concentration of the dopants, the impurity states gradually hybridize with the band edge state that contributes to the narrowing of the gap to increase the adsorption efficiency.(4) We reveal the synergistic engineering effect of co-doping of vanadium and oxygen on the electronic structure of GaN system. The introduced O species by co-doping can enhance the solubility of V in the GaN lattice. Non-compensating co-doping of V and O species narrows the band gap and thus renders the doped system show visible light response and makes the Fermi level pinned within the conduction band, which lowers the combination rate of photogenerated carriers and thus promote the photocatalytic activity. Co-doping of H, F, Sc and Y can improve the solubility of N impurity in SrTiO3. Co-doping of N with La can reduce the formation energy of N incorporation by47%compared to the mono-doping of N dopant. The disordered local structure around the dopant and nearest Ti is more significant in the co-doped SrTiO3than that in the mono-doped SrTiO3. Therefore, the inner field in the co-doped SrTiO3can enhance the separation of the carries. Except the N/Sc co-doping, the reduction of the gap can be observed and the localized impurity states can be passivated in other cases of co-doping. It is obvious that the synergistic effects of Co-doping can significantly enhance the adsorption intensity and reduce the recombination rate.In this dissertation, we have explored the correlation of geometry-electronic structure-property of the wide band gap semiconductors, unveil the effect of surface structures on the properties of dangling bonds and magnetic properties and the underlying mechanism, and clarify the band structure engineering of mono-doping and co-doping effect on the wide band gap semiconductors. The present results in this dissertation are of significant instruction to the application of wide band gap semiconductors in the fields of spintronics and photocatalysis.
|
PDF Full Text Request