A Study On Electronic Structures And Magnetism Of Surface In Wide Band-gap Semiconductors

Wide band-gap semiconductors (E_g> 2.3eV), including diamond, GaN, titanium dioxide etc., are known as the third-generation semiconductor materials. As compared with the first and second generation semiconductors, the wide band-gap semiconductors pocess many superior properties. They exhibit high breakdown electric field strength, high saturated electron drift velocity, high thermal conductivity, small dielectric constant, strong anti-radiation ability and outstanding chemical stability, which would made them excellent candidates for the production of anti-radiation, high-frequency, high power and high-density integration of optoelectronic devices, and have a wide range of applications in the national defense industry, biological medicine, electronic circuits, energy storage and environmental protection.However, former theoretical studies mainly foucus on the bulk system, and little information is available about the electronic and magnetic characteristics on the surface. In fact, the surface of the wide band-gap semiconductors would possess some new and special features in the electronic and magnetic properties, which are crucial for further applications, and dramaticaly influence the chemical reactions. Therefore, it is of great significance to study the surface properties for these wide band-gap semiconductors. Based on the density functional theory, we have studied the details of diamond (100) surface, GaN (10(1|-)0) surface, and TiO₂ (110) surface. The dissertation is arranged as follows:In chapter 1, we introduce the background and significance of this dissertation. The recent experimental and theoretcial progress for diamond, GaN, and TiO₂ are presented, respectively.In chapter 2, we briefly introduce the basic concept of DFT and review its recent progress. Finding good approximation for exchange-correlation functional is one of the main targets in DFT research. With the development of functional, DFT leads to more and more accurate results from the initial LDA, GGA to hybridization functional. Besides the improvement of exchange-correlation, extension of DFT to the time dependent region and combination of DFT with dynamic mean field theory (DMFT) are also active topics recently. All these progress leads DFT application to a broad range of problems.In chapter 3, based on DFT calculations, we simulate the process for ethylene moclecule adsorbing on diamond (100) surface, and study the possible adsorption ways. In addition, the electronic structures are also investigated, which would provide a reference to further experimental studies.In chapter 4, we analyze the origin of room-temperature ferromagnetism in GaN nanoparticles without any doped magnetic ions. Our results indicate that the room-temperature ferromagnetism of undoped GaN nanoparticles originates from the nitrogen dangling bonds associated with the surface Ga-vacancies. The spins of the nitrogen dangling bonds couple ferromagnetically by through-bond spin polarization, and this ferromagnetic coupling is effective even when the vacancy separation is as long as～8 (A|°).In chapter 5, the geometric and electronic properties of various B doping defects in rutile TiO₂ (110) surface are investigated in detail. Our results indicate that the implanted B impurity prefers forming strongly covalent bonds with O atoms in the lattice, which will result in significant structure distortion and surface rumpling. Moreover, the cost energy of the formation of a bridging oxygen vacancy is enlarged in the presence of B impurities in the surface layers. The calculated results demonstrate that only when the B-Ti bonds are formed can the incorporation of boron induce some localized states in the mid-band gap, and thus result in the redshift of adsorption spectra.In chapter 6, we summarize the conclusions of this dissertation and preview the further studies.