Study Of The Jahn-Teller And Strain Effects On The Silicon Crystal With Defects

With the increasing environment pollution and energy source shortage in current society. Developing and utilizing the renewable energies is imperative. Therefore, the research on renewable clean energy has become one dominating fields for the governments around the world. The solar photovoltaic technology is a representative in the field of renewable energy. Silicon is regarded as the body materials of the solar battery research and development, due to their high reserves, suitable band gap (high conversion efficiency), non-toxic, stable physical properties (long lifetime), and other obvious advantages. The quality of silicon wafer affects the performance of the solar cell, such as short circuit current and open circuit voltage, which determines the efficiency and lifetime of the solar cell to a large extent.Based on this background, in this thesis the influences of the defects on the electronic properties of silicon crystal are studied by using first-principles calculations. Firstly, the periodic supercell models of crystalline silicon with monovacancy (V1), divacancy (V2) and ring hexavacancy (V6) are constructed. Secondly, atomic and electronic structures of crystalline silicon with V1、V2 and V6 defects are studied. The impact of the Jahn-Teller (JT) distortion on electronic structures are analyzed simultaneously. Then the electron (hole) effective masses of perfect crystalline silicon and silicon with various vacancy defects near Fermi levels are investigated. Lastly, the influences of stress (strain) on the electronic structures of silicon with various vacancy defects are researched. According to our studies, the main results are as following:1. It is found that the effects of JT distortion on the structural stability of defects and the electronic properties are significant. After relaxation, the symmetries of V1、 V2 is lowered due to the JT effect, while the symmetry of V6 remains the same as unrelaxed structure as no JT distortion could be found. Our results show that the V6 defect is the most stable among V1、V2 and V6 defects, and the V2-RB structure is a little more stable than the V2-LP structure due to the lower vacancy formation energy. VI and V2 have deep levels in the gap, whereas V6 has tailing bands in the gap. The JT distortion also has the influences on the band decomposed charge density. The decomposed charge densities for the three gap bands of V1 and V2-LP have obvious directivity, while the gap bands and the tailing bands of V2-RB and V6 have not, and mainly being contributed by the nearest neighbors. Additionally, the similar influences of the JT distortion effects on the differences of charge density could be found.2. The effective mass of electron (hole) in silicon crystal with various kinds of vacancies are affected by the vacancy defects. Our results show that the band structures of the perfect crystalline silicon and the silicon crystal with hexavacancy defect have cubic symmetry. However, the systems with monovacancy and divacancy defect exhibit typically asymmetric characteristics due to the presence of defect bands that are caused by the JT distortion. The effective mass of electron that corresponds to the intrinsic band in the perfect crystalline silicon has high symmetry. The property of the electron effective mass in crystal silicon with vacancy defect is similar to that in perfect system except for the larger effective mass in crystal with hexavacancy defect. The effective mass of hole that corresponds to the intrinsic band in perfect crystalline silicon also has high symmetry, and the values for the high symmetry points are almost equivalent. Moreover, the values and symmetries of the hole effective masses basically are kept in the crystals with vacancy defects, implying that the non-localized band symmetry of VBM and CBM in monovacancy and divacancy defects is not changed by JT distortion. The effective masses of electron (hole) for the localized state bands at VBM and defect states in the crystal with monovacancy defect are not large and exhibit directivity. The effective masses of electron (hole) that correspond to the localized bands in the crystals with LP and RB divacancy defects also exhibit apparent directivity. On the other hand, the corresponding effective masses of electron (hole) in the crystal with hexavacancy defect exhibit clear symmetry. It shows that the localized state bands at VBM and defect states in the crystals with monovacancy and divacancy defects can be affected by the JT distortion. By analyzing the influence of vacancy defects on the band structures and the effective masses, and further investigating the effect of vacancy defects on conductivity and carrier mobility in crystal, we can deeply reveal the intrinsic relationship between vacancy defects and the electronic properties of crystalline silicon.3. The results show that the total energy increases as the strain (including compressive and tensile) increase and the structures becomes instable. At the same time, there are different effects of strain on the energy band of the crystalline silicon with various vacancy defects. Specifically, the symmetry of the energy band structures of the silicon crystal with V6 and V1 are not affected by strain. However, the situation is different for the energy bands of the silicon crystal with V2. The gap bands of the silicon crystal with V2-LP are sensitive to the compressive strain, and the gap bands of the silicon crystal with V2-RB are sensitive to the tensile strain. Moreover, the band gaps of the silicon crystal with V1, V2 and V6 defects can all be modulated by stress, and the relationship between the types of vacancy and the modulation is not close.