The Molecular Simulation For Evolution Of The Partial Dislocation In Sige Heterostructures

Si and Ge are the basic matrerials in semiconductor industry. Since the SiGe/Si heterostructure can tailor the band structures of semiconductor devices, and has excellent electronic and optical properties, it is widely used in the novel devices.The SiGe/Si heterostructure is formed by the growth of SiGe epilayers on Si substrates. In the production of heterostructure, the misfit dislocations are produced because the lattice mismatch between Si and Ge. The primary mobile misfit dislocations are 60°dislocation in Si. With the dissociation of 60°dislocation and screw dislocation, the partial dislocations (30°partial dislocation and 90°partial dislocation) are the predominant dislocation. In the semiconductor devices, dislocations are not only related to the plastic deformation behavior, but also related to the electronic and optical properties of micro-electro devices. Therefore, the molecular simulation is used in this thesis to investigate the evolution of the misfit partial dislocation in SiGe/Si. This may provide theory support to obtain fully relaxed and high quality heterostructure with low dislocation density.Firstly, the dynamic properties of the 30°partial dislocation in Si have been investigated by the molecular dynamics (MD) method with environment-dependent interatomic potential (EDIP) and nudged elastic band (NEB) method with semiempirical potential (TB). The migration processes in one period of left kink (LK), right kink (RK), LK-reconstruction defect complex (LC) and RK-reconstruction defect complex (RC) are obtained through the MD simulations under different shear stress and temperature conditions. It is found that the motions of kinks are carried out by the transformation between kinks and their intermediate states under a majority of conditions. But for the LK, one or more kink pairs are produced under relatively higher temperature and shear stress conditions. Moreover, RK dissociates into the RC+ reconstruction defect structure under the similar conditions. All of them take part in the motion of LK and RK. The velocity curves of LK and RK indicate that the above phenomena can promote the motion of the 30°partial dislocation. Based on the MD results, the migration barriers of the four kinds of kinks in one period are obtained by the NEB method. All the migration barriers are in good agreement with these MD results. In addition, it is found that the reconstruction defect can lower the migration energies of LK and RK, and make the 30°partial dislocation move faster. This conclusion also explains why the presences of RC can greatly enhance the mobility of 30°partial dislocation.And then, the MD and NEB methods are used to investigate the dynamics properties of LK and RK in the 90°partial dislocation double period (DP) structure. The migration processes of DP-LK and DP-RK in one migration period are described in detail. Moreover, the formation energies of the two kinks are computed by the MD method with TB. On the base of the kink motion processes, the migration barriers of DP-LK and DP-RK in one period are obtained by the use of NEB together with TB. Finally, the activation energies for short dislocation segment and long dislocation segment, which determine the velocity of DP, are computed according to the activation theory.The structure properties and formation energies of monovacancy (V1), divacancy (V2) and hexavacancy (V6) have been comparatively studied with density functional theory (DFT), Stillinger-Weber (SW), EDIP and Tersoff (T3) methods in silicon. It is found that the DFT method may provide accurate descriptions of atomic structures and energies of vacancies. For the empirical potentials, they are unable to investigate quantum mechanical effects such as Jahn-Teller distortion by reason of the intrinsic disadvantage of classical potentials. It results in that EDIP and T3 are unsuitable to the structure property calculations. In the formation energy calculations of the three vacancies, only SW provides good results which are closed to the DFT results. Based on the structure property and formation energy results, it can be concluded that SW should be the best empirical potential to describe V1, V2 and V6.On the base of the partial dislocation dynamics properties and vacancies researches, the interactions of 30°partial dislocation with V1 and V2 in silicon are investigated by the molecular dynamics simulation method based on the SW. The results under different temperature and shear stress conditions show that the 30°partial dislocation is pinned when dislocation encounters vacancy. When the shear stress approaches to a critical value, the dislocation can overcome the pin and begin to move. As the temperature increases, critical shear stress decreases approximately as a linear function. Moreover, it is found that critical shear stress is mainly determined by the migration barrier of corresponding kink in the V2 calculations. In comparison of the results in two models with and without vacancy, it is found that the appearance of V1 and V2 may bring in the recovery stress and more kinks structures, which make the 30°partial dislocation move faster once dislocation moves through vacancy. These results also resolves the disputed point of view that vacancies make the dislocation move faster or slower.