| Epitaxial SiGe (silicon-germanium) heterostructure films obtained by epitaxial growth are proposed to have great potential in the field of Si-based high performance devices. Recently, with development of the method with use of the low-temperature Si (LT-Si) buffer layers, it is possible to obtain the high quality SiGe film with low dislocation density. However, there is still a debate about the mechanism of the method with use of the LT-Si buffer layes which can reduce the dislocation densities in the epitaxial layers.The lots of vacancy defects in LT-Si have effect on the decrease of dislocation density in epitaxial layers. Recently, scholars inside and outside China have different views on that the vacancies would promote or hinder the dislocation nucleation. Many scholars believe the vacancies would promote the dislocation nucleation, however, it is unclear that the 90 degree or 60 degree dislocation would be nucleated. Moreover, there is still a debate about that the vacancies would promote the dislocation glide or climb.In this thesis, the SiGe heterosturcture model which consists of dislocation dipoles and vacancy defects was constructed. The molecular dynamics simulation method has benn used to investigate the dislocation nucleation in Si crystals, the dislocation mobilities in Si crystals and SiGe/Si heterostructures, and dislocation interaction with the vacancy defects. The SW and EDIP potential was used and the periodic boundary condition was used. The processes of the dislocation evolutions, the interaction between the dislocation and the vacancies have been obtained, which can not be obtained by experiments. The mechanism of the method with use of the LT-Si buffer layers reducing the dislocation densities in epitaxial layers was established.Firstly, the stress-strain relationship was calculated under uniaxial tensile. The ideal strength and elastic constants of Si crystals in [111] direction are obtained. Then, the influence of point defect on the strength and elastic constants was studied. It can be found that the variation of the strength versus the point defect fraction conincides a decaying exponential function, while the elastic constants vary with the defect fraction in a linear fashion. Finally, the elastic constants of macroscopically isotropic Si was obtained, which could be used in the later investigationsWith use of the atomic configurations of 90o and 60o dislocation dipoles, the formation energies of 90o and 60o dislocation were calculated with EDIP potential and the foremer is lower than the later. So, the 90 odislocation is easier to be nucleated. The divarication of dislocation nucleation is solved from the energy perspective.In order to study the dislocation mobilities in SiGe/Si heterostructure, Firstly, the dislocation mobilties in Si crystal has been investigated with use of SW potential. The Peierls stress which the dislocation must overcome before it glides was obtained. The Peierls stress decreases as the temperature increases, which confirms the result predicted by the Peierls-Nabarro theoretical model. When the influence of the temperature and the shear stress applied on the dislocation velocity was studied, the phonon damping effect was observed, that is, the dislocation velocity decreases as the temperature increases. This is the basement of the following studies.Then, the configuration of SiGe/Si heterostructure with the LT-Si buffer layers was constructed and the dislocation mobilities in the heterostructure was investigated by using the SW potential. It can be found that the dislocation velocity will increase remarkably when the temperature of the LT-Si buffer layers ranges from 300℃to 450℃. The LT-Si buffer layers can promote the dislocation glide from SiGe layers into LT-Si buffer layers, and at the same time, prevent the dislocation gldie from LT-Si buffer layers into SiGe layers. In this thesis, from the point of view of the dislocation annihilation reducing the dislocation densities, it can be concluded that the SiGe films with low dislocation densities can be growed when the LT-Si buffer layers is in the temperature range 300℃-450℃, which is consistent with the experimental results. In addition, the influence of the Ge content in the epitaxial SiGe films on the dislocation densities has been investigated. The foundation on which the parameters are choosed in the epitaxial growth is laid.The dislocation interaction with the vacancy defects in the LT-Si buffer layers has been studied by the MD simulations. Firstly, the hexagonal ring vacancy defects ( V6 ) was constructed with SW potential. The pinning effect can be observed when the dislocation is pinned by the V6 . The process that the V6 results in the 60o dislocation dissociation into 30o and 90o Shockley dislocations was investigated. It can be found that the V6 can hinder the dislocation glide and lower the dislocation velocity remarkably. Secondly, the pinning effect during the dislocation interaction with the V6 was carried out again using the EDIP potential and the result can be reproduced and validated. Lastly, the model of divacancy defects was constructed with SW potential. The result show that the divacancy can be absorbed by the dislocation and the energy of the system will decrease. It is found that the divacancy can promote the dislocation climb, but it has no effect on the dislocation velocity.Based on the above results, the mechanism of the method with use of the LT-Si buffer layers which can reduce the dislocation density in the epitaxial layers is established, that is, the LT-Si buffer layers promote the dislocation glide from the SiGe epitaxail layers into the LT-Si buffer layers. The large numbers of vacancy defects in the LT-Si buffer layers is divacancy and the divacancies promote the dislocation climb which makes the dislocation easier to annihilate. So, the misfit strain is relaxed and the dislocation densities in the epitaxial SiGe layers are redueced. These results have the certain instruction significance for the design and preparation of the semiconductor heterostructure materials.
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