Molecular Simulation On The Evolution Of Shuffle60°Dislocation In Silicon

Si and Si-based materials are the major raw materials used in semiconductor micro-electronic industry. However various defects will be inevitably introduced into the semi-conductor materials in the process of fabrication and processing. Among them, the mostimportant one is the dislocation. On one hand, the presence of dislocations will introducea continuous series of energy levels in energy band gap, resulting hindering of carriersmovement, more power loss and decline of electrical properties of the device. On theother hand, as the microscopic carrier of material plasticity, dislocations reduce the yieldstrength and reliability of the device. The most important dislocations in silicon are the60dislocation, the screw dislocation and the corresponding extended dislocations (30and90partials). Compared to the extensive research work on partial dislocations, theresearch of60dislocation is still lacking. We put our eforts on the structures and the dy-namic evolution of shufe60dislocation. For dislocation structures, nucleation, motionand interaction with other defects, atomic-scale structure variation is involved, renderingthe classic theory of elasticity incapable in this field. In this thesis, first principles (DFT)and classic molecular dynamics (MD) simulations are utilized to study the dislocationcore structures and the corresponding mechanical behavior. The results are expected toprovide a theoretical guidance for artificial controlling of defects and the fabrication ofnew materials.First, the dislocation nucleations in bulk silicon with vacancy disk and in siliconnanowire with surface step are simulated with SW-based MD calculations. When the ap-plied strain reaches a critical value, both models show slip on (111) plane and dislocationsare nucleated at the defects (vacancy disk and step). The rising of temperature speeds upthe dislocation formation, resulting a decrease in crystal strength. The type of the dislo-cation formed is directly determined by stress state of the system. On (111) plane, whenthe shear stress lies in the11ˉ0direction is dominant, shufe full dislocations can beeasily generated. Partial dislocations will only appear when the shear stress lies in the112ˉdirection is dominant. In cases where stresses along the above two directions areapproximately of the same order, temperature plays the critical role because at low andhigh temperatures shufe full and partial dislocations are preferred, respectively. In addi-tion, at low temperatures under high stresses, nucleations of glide partial dislocations are observed, which is consistent with recent experimental results in literatures.Second, The DFT calculations are utilized to determine the possible stable corestructures of shufe60dislocation. With MD simulations, several core structures ofshufe60dislocation are calculated based on several empirical potentials, followed byfurther relaxations with DFT calculations. The final stable configurations have three d-iferent kinds, with two which can not glide in silicons already reported in literatures.The newly discovered structure has an energy between those of the two former config-urations, and it can be charactered as a complex of a meta-stable shufe60dislocationand a bond defect. With DFT and ab-initio MD calculations, the mechanical behavior ofthis structure is studied. It is shown that under loading the structure can release a glissilemeta-stable shufe60dislocation. Also, increase in temperatures can assist the motionof the dislocation. In addition, Ge atoms which are introduced as impurities can combinewith the meta-stable shufe60dislocation to form a stable, glissile structure. The resultsshow that the meta-stable shufe60dislocation may form stable structures with point orbond defects and maintain mobility.Third, the Peierls stress which is required for gliding of the meta-stable shufe60dislocation in silicon is calculated under0K directly with MD simulations using the con-jugate gradient method. In its calculation, to reduce the disturbance by the interaction ofdislocations under periodic boundary condition, the reasonable height to width ratio forthe cross section of the simulation box along direction perpendicular to the dislocationsis determined by dislocation interaction energy, which is calculated by anisotropic elas-ticity. Simulation results show that Peierls stress along [112ˉ] and [1ˉ1ˉ2] are diferent. MDsimulations at0.1K show that the required critical driving shear stress for dislocationslip is obviously lower than the Peierls stress, which means the Peierls stress shouldn’tbe calculated with almost zero thermal activation. It is also shown that when the appliedshear strain rate exceeds a critical value, the flow stress reaches almost a constant and thedislocation propagates with constant velocity.At last, the model containing the vacancy clusters and the shufe60dislocation isconstructed, the interaction between the dislocation and vacancies are calculated with theSW potential. From the calculation of the interaction energy between vacancy clustersand shufe60dislocation, it is found that the dislocation is attracted by vacancies. Theinteraction energy increases as the distance between hexavacancy and the dislocation de-creases. The energy per unit length decreases as the concentration of vacancy clusters decreases. The dislocation in overcomes the pinning of vacancies with a critical resolvedshear stress, which has a linear relationship with hexavacancy concentration. Simulationsof the interaction between the shufe60dislocation and the vacancy disk of various radiishow that the pinning efect of the vacancy increases with the radius of the vacancy disk.Also, the rising of temperature helps to overcome the pinning of dislocation. Finally, thedissociation of shufe60dislocation is discovered. Under a strong pinning and a highstress level, the60dislocation dissociates into the shufe30dislocation and glide90dislocation.