Molecular Dynamics Simulation Study Of Nucleation Mechanism Of Grown-in Dislocations In Directional Solidification Of Silicon

Multicrystalline silicon cells are widely used in photovoltaic power generation because of their high cost performance.Dislocations are the main factor affecting the photoelectric conversion efficiency of multicrystalline silicon solar cells.It has always been an important research topic in the field of photovoltaics to improve the conversion efficiency of multicrystalline silicon solar cells by reducing dislocation density.As the source of dislocation multiplication,the grown-in dislocations,formed in the process of directional solidification of silicon crystal,lead to a great increase of dislocation density in subsequent production or process,which leads to a decline in the efficiency of solar cells.However,the nucleation and evolution of grown-in dislocations formed at the front of solid-liquid interface during the solidification are difficult to observe and study directly through experiments.Molecular dynamics simulation,providing all the information of the evolution process about the structure at the atomic/molecular scale,is a powerful tool to study the crystal growth dynamics and the formation of related defects from the microscopic point of view.In this paper,the mechanism of grown-in dislocation nucleation during directional solidification of silicon is studied by method of molecular dynamics simulation.Tersoff potential is selected to describe the interaction between silicon atoms and between silicon and carbon impurity atoms.We investigated the elastic and plastic properties of silicon crystals at high temperature near melting point,the accompanying relationship between grown-in dislocations and twins,the solidification process under stress and the nucleation mechanism of grown-in dislocations,the formation mechanism of grown-in dislocations near the segregation position of carbon impurity atoms at Si grain boundary during solidification.The simulation results show that:(1)At the high reduced temperature(T/Tm)of 0.88,the anisotropic Young's modulus and yield strength of silicon crystals are observed.In the yield stage,Shockley partial dislocations nucleate and the stress is released.In the plastic deformation stage,as the strain increase,dislocations continue to form,and the dislocations cross each other to form dislocation network.The main types of dislocations formed during plastic deformation are Shockley partial dislocations with Burgers vector < 112>/6,ordinary dislocations with Burgers vector < 100>/3 and Lomer-Cottrell dislocations with Burgers vector < 110>/6.(2)Many grown-in dislocations and twins occur in the growing silicon crystals,and two typical configurations are determined.One configuration is sandwich structure,which consists of two twin boundaries and layers of {111} atoms stacked normally.At the end of the structure is Shockley partial dislocation with Burgers vector < 110 >/6.The other configuration consists of two interacting stacking faults and a Lomer-Cottrell dislocation with Burgers vector < 110>/6 at the intersection point.The formation process of dislocation and twin configurations can be described as follows: before dislocation nucleation,{111} facets of solid-liquid interface first appear a twin,and then Shockley partial dislocations nucleate with the formation of another twin;After two successive twin crystals form stacking faults,another one appears at the edge of the stacking fault,which result in nucleation of Lomer-Cottrell dislocations.(3)At the initial stage of solidification,the growth of silicon crystals is inhibited by compressive stress.The greater the stress,the smaller the growth rate.The effect of stress not only limits the transverse diffusion of liquid atoms during the growth of two-dimensional crystal nucleus,but also affects the nucleation rate of two-dimensional crystal nucleus.The FCC configuration usually dominates the whole(111)surface and form a normal stacking structure without stress.In applying compressive stress,the chance of nucleation and growth of HCP configuration increases,and consequently stacking fault area forms.A linear defect is formed on the boundary between the normal stacking area and the stacking fault area.The defect core is located on the glide(111)plane and lie linearly along the direction of <110>.The core is composed of 5-5-8 double periodic atom rings.The linear defect is a typical 30 °Shockley partial dislocation in silicon crystals.(4)The grain boundary grooves consisting of {111} facets form at the intersection of grain boundary and solid-liquid interface during the solidification of Si ?27 bicrystal model.Because the {111} surface has the lowest interfacial energy,the carbon impurity atoms segregate on the {111} facet of the grain boundary groove.Under the action of external stress,a stress concentration is formed near the carbon impurity atoms,and a large strain energy is generated,which provides a driving force for the nucleation of twin and stacking fault on {111} facet of grain boundary groove.Shockley partial dislocation is formed between stacking fault area and normal stacking area.If stacking faults are formed on two interacting {111} facets of grain boundary grooves,Lomer-Cottrell dislocations are produced by two Shockley partial dislocation reactions at the interaction.These grown-in dislocations originate at grain boundary,cross the crystal and extend to the solid-liquid interface.