| Using special physical properties of semiconductors, people have invented and manufactured a variety of devices with specific photoelectric functions. Among the numerous semiconductor optoelectronic devices, solar cells and light detectors are two kinds of representative devices. Silicon solar cell is a large area device, covering a wide range of solar spectrum, has provided a lot of clean energy for human beings. At present the main research goal is how to effectively improve the absorption capacity of photons and reduce the recombination of photogenerated carriers, so as to improve the photoelectric conversion efficiency of the cell. The photodetectors, such as photodiodes, operate in a similar manner to solar cells and respond to the optical signals of a specific frequency. So the current research focuses on exploring a variety of materials and regulating their electronic structures to respond to a specific optical frequency. Two-dimensional materials with various electronic structures cover the electromagnetic spectrum range of microwave to ultraviolet light. Although two-dimensional materials are atomic layer thickness, the interaction between light and the materials is strong. Two-dimensional semiconductors can be fabricated as unique optoelectronic devices, which have drawn attention in recent years.The crystal structure of semiconductor is always imperfect and inevitably there are various defects. Defects can significantly change the electronic structure of materials, thus affecting the photoelectric properties of devices. There are two strategies for controlling the defects in semiconductor. On the one hand, we should try to eliminate the adverse effects of defects on the device as much as possible. On the other hand, we design the specific microstructure to regulate the photoelectric properties of materials. To obtain the excellent performance of optoelectronic devices, the properties of the defects in semiconductor should be thoroughly studied. Using density functional theory, we directly calculate the physical properties of the defects in semiconductor. In this paper, the point and surface defects in semiconductors are studied by theoretical calculations. Twin boundary is a kind of special interface defect, because the crystal structure on both sides of the twin interface is mirror-symmetric. There are many types of defects in two-dimensional materials, and we chose the most common ones as the research objects.In the first chapter, the twin interface in solids and the defects in two-dimensional materials are reviewed, including experimental findings and theoretical exploration. The second chapter mainly introduces the first-principles method and numerical algorithm used in our calculations. In addition, the first-principles molecular dynamics and Elastic Band Nudged method in transition state search are also introduced. In the third chapter, we calculate the electronic structure and optical properties induced by the twin boundary in silicon. In the fourth chapter, the work of identifying the type of topological defect in monolayer arsenide by optical method is introduced in detail. In the fifth chapter, we summarize and prospect the research.Our main research results include the following three parts:1. The energy band structure, density of states and optical properties of crystalline and twinned silicon are calculated by using the first-principles software package CASTER First, the superlattice structure of twinned silicon is constructed, and the stable crystal structure is obtained by energy optimization. The silicon bonds in the interface are stretched, which indicates that the mutually exclusive potentials on both sides of the interface are generated. By comparing the projected energy band of the crystalline silicon with the energy band of the twinned crystalline silicon, the positions of the defect states are precisely determined. The effect of twin boundary on the density of states is mainly in the valence band and the change of the conduction band edge is not obvious. In the visible light range, the optical absorption coefficient increases dramatically when the twin boundary exists, however, is not reduced when there are common types of vacancies and impurities in twinned silicon. It is inferred that twin boundary can improve the absorption of visible light and improve the photoelectric conversion efficiency of silicon-based solar cells.2. First, we construct the supercells of monolayer arsenene with various defects. By optimizing the geometric structures, we calculate the formation energy of every type of defect. MV-1, MV-2 and MV-3 represent the three types of monovacancy defect ?MV?, respectively. The MV-3 configuration with the minimal defect formation energy is the most stable, In addition, the reconstructed structures of the divacancy ?DV? and Stone-Wales ?SW? defects are determined. The first-principles molecular dynamics simulations show that MV-1 and MV-2 easily convert to MV-3 at low temperature, and DV and SW can keep thermal stability at room temperature. By using the transition state searching method, some energy barriers and reaction paths for the structural transition are found. The diffusion coefficient of MV-3 is equal to 1.33×10-11 cm2/s, which is slightly lower than that of the single vacancy in silicene, and higher than that of the single vacancy in graphene. Producing a SW defect requires overcoming a barrier of 1.136 eV while eliminating a SW defect is much easier. Therefore, it is possible to eliminate the SW defects in arsenene by rapid annealing.3. The calculation results of the imaginary part of the dielectric function and absorption coefficient show that the single vacancy defect can enhance the optical transition in the forbidden band of the arsenic crystal, and two new characteristic peaks appear in the spectrum of the single vacancy. The spectral characteristics are significantly different from those of the double vacancy and SW defects. These spectral differences can provide information related to topological defects in arsenene. |
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