Study Of Electronic Properties Of Diamond And Zinc Nitride Semiconductor Materials

The dissertation is devoted to the properties study of diamond and zinc nitride semiconductor materials from first-principles, molecular orbital theory and semiconductor theory. With the progress in density functional theory (DFT) and its numerical methods, DFT method on the basis of first-principles has become one of the important tools, which was applied to the condensed matter physics and material science etc. In this dissertation, we studied the geometric structure, electronic structure and optical properties of diamond and Zn₃N₂ semiconductor materials by means of first-principles calculations.Diamond as a photoelectronic material possesses outstanding physical and chemical properties, which can be applied to the electronic and electrochemical devices enduring high frequency, high temperature and large power. Diamond film terminated with hydrogen and oxygen can exhibit different electronic behaviors, which affect the quality of devices directly. Vacancy defect is another important factor affecting the quality of diamond films and become more and more dominative with the size scale downward of devices and the increase of integration degree. Therefore, it is important to study the micro-structure and electronic properties of impurities and defects on diamond surface, which can be of great significance for its application in the field of electronics, and becomes a frontier issue of the carbon materials. For the bulk materials, with the discovery of high temperature superconducting phenomenon the electronic properties and superconducting transition mechanism in heavily B-doped diamond have been intensively studied, which also leads to the study interest in related properties of heavily B-doped silicon due to similar crystal structure between diamond and silicon. Thus, does it mean that effects of B in the two doped systems are similar? If not, what's the difference? Consequently, it is necessary to investigate the electronic properties of B-doped diamond (silicon) for understanding the superconducting transition mechanism. Zinc nitride film is one of the rare touched zinc binary compounds. It's a potential material to fabricate emitting diode and it can be also used to fabricate ZnO film by thermal oxidation method. So far, the experimental band gap of zinc nitride appeared differences. Furthermore, the as-grown samples were found to exist accidental n-type conductivity. First-principle can provide relevant properties in the absence of experimental data. It can not only explain experimental phenomena, but also provide new ideas.In chapter 1, we introduce the background and significance of the dissertation. Subsequently, we present the recent progress of diamond and zinc nitride materials in experiment and theory and point out the problems and its solving possible ways. We also elucidate the theoretical and actual significance of the dissertation.In chapter 2, we briefly introduced the basic concept of DFT and reviewed its recent progress. Finding good approximation for exchange-correlation functional is one of the main targets in DFT research. With the development of functionals research, 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 lead DFT application to a broad range of problems.In chapter 3, based on GGA method, we studied geometrical structures and electronic properties for a series of C (100) surface terminated with hydrogen and oxygen, and determines their stable configurations. The calculated results indicated that bridge oxygen is more stable than top oxygen, and the most stable configuration is hydroxyl terminated surface. The results are also consistent with the analysis from the viewpoint of HOMO-LUMO theory. The electronic structures show that surface polarization is stronger in the O-terminated surface than in H-terminated surface due to the more charge transfer in former case than latter case, which is one of the reasons that the H-terminated diamond surface can be easily oxidized under normal conditions. Analysis of density of states indicated that the p-type surface conductivity in the hydrogenation diamond surface may be ascribed to the electron localization weakening near the Fermi level and the electrons transformation from the interior to the surface to form a hole accumulation layer in the subsurface. The oxygenation diamond surface may weaken the surface conductivity due to more introduced localized states near the Fermi level, whereas the increasing of the surface states up the top of the valence band may lead to the hopping conductivity, which explains the recent experimental observation. In addition, we calculated the geometrical and electronic structures of neutral and charged monovacancy and divacancy on C(100)(2xl) surface from first-principles. Calculation results indicated very low formation energies for divacancies on the C(100) surface, implying a high concentration of vacancies is expected on the C(100) surface. The monovacancy on the first layer is hard to migrate into the second layer because of a large barrier about of 2.7 eV. The total energies demonstrated that neutral vacancies are the most stable defect. Moreover, the results have also offered strong evidence for the existence of multiple charge states whose individual stabilities depend on the position of the surface Fermi level. The diffusion of vacancy can be mediated by charged defects.In chapter 4, we studied of the electronic properties in boron heavily boron doped diamond (silicon). The influence of isolated substitutional B and B clusters on electronic structure of diamond was systemically investigated. We confirmed that the lattice parameter increases with the boron concentration in a proximity linear relation. By study of the electronic properties of heavily B-doped diamond (silicon), we discussed the effects of B on high-temperature superconducting and the relationship between B and superconducting transition temperature in heavily B-doped diamond and silicon. We obtained the stable configurations of B-pair and three-substitutional-B in two doped systems. The electronic structures showed that the impurity band induced by B cluster (except B dimer in diamond) mixes with valence band and the Fermi level locates in the valence band in heavily boron-doped diamond and silicon, which support that the superconducting mechanism is metal-superconductor transition. Comparing their electronic structures, we found that the influence of B on the states around Fermi level E_F in silicon is less than that in diamond at the same doping level. So, superconducting critical temperature T_C is related to the host material. Furthermore, T_C is only related to B content in silicon but plus B configurations in diamond.In chapter 5, we performed electronic structures calculations for oxygen and intrinsic defects in zinc nitride using first-principles to discuss the origin of the various band gap values appeared in different experimental samples. We investigated electronic properties for various possible defects based on the formation energy and found that substitutional oxygen for nitrogen and nitrogen vacancy are the reasons, which should be responsible for n-type conductivity. We also discussed possible photo-transitions to probe the origin responsible for the different optical band gap experimentally.In chapter 6, we summaried the conclusions of this dissertation and previewed the further studies. Within the framework of DFT, we explain the influences of oxygen on the properties of C( 100) surface. The research points out that top oxygen can lead to hopping conductivity. We firstly discussed that the superconducting critical temperature dependence of B content and B configurations on heavily B-doped diamond and silicon and drawn the conclusion that it depends on B content and B configurations in the former case, wheaers only relates to B content in the latter case. Finally, we have investigated geometrical, electronic and optical properties of oxygen and intrinsic defects in zinc nitride and given reasonable explainations to experimental phenomena.