Effect Of Grain Boundaries On The Formation Of Twins In Silicon Crystal Of Growth From Melt By A Molecular Dynamics Study

There are a large number of grain boundaries(GBs)and twin boundaries in Multicrystalline silicon(mc-Si)which is the main stream product in the current photovoltaic market.The twin boundaries that terminate inside the crystal will induce the formation of dislocations,reducing the photoelectric conversion efficiency of mc-Si solar cells.However,during the directional solidification of silicon,it is not clear whether the formation of grain boundaries and twin boundaries will affect each other,and how they are affected.In order to further improve the quality of mc-Si,this article uses the molecular dynamics method to simulate single crystal,double crystal silicon with random grain boundaries(R-GBs)and coincidence site lattice(CSL)grain boundaries at different temperatures,and to investgate the influence of GBs on the formation of twins during the solidification and growth of silicon through describing the interaction force between silicon atoms by using the Tersoff potential function.It was found that:(1)During solidification and growth of silicon crystals,high-density parallel twins are formed,which penetrate through the grains or intersect with other defects,and often appear in the form of stacking faults.During the formation of stacking faults,two structural configurations will appear:one is a configuration in which dislocation loops surround stacking faults,and the other is a configuration in which two parallel dislocations sandwich stacking faults.The first configuration has a critical value on the stacking fault plane.When the stacking fault plane is smaller than this critical value,the stacking fault will gradually disappear during the growth process;we have estimated this critical value in a round manner,The critical radius of the intrinsic stacking fault circle is about 6.46 nm;the critical radius of the extrinsic stacking fault circle is about 3.20 nm.The stacking fault in the second configuration has a balanced width with a value of approximately 4.25 nm.(2)Comparing the content of twins in single crystal silicon and bicrystal silicon with R-GBs,it was found that R-GBs promote the formation of twins.Among the three bicrystal silicon with R-GBs,[112]-[100]bicrystal form the most twins,[112]-[100]bicrystal and[110]-[100]bicrystal form less twins.The R-GB will form a GB groove at the solid-liquid interface during the double-crystals' growth process,which is a zigzag structure composed of {111} facets and rough faces.Except that twins formed in single crystal silicon,twin crystals also induce twin formation due to the {111} facets on the GB groove.(3)The influence of CSL GBs on twin formation is similar to that of R-GBs,but different kinds of CSL GBs have different effects on the probability of twin formation.In the bicrystal with ?3{112} GB,E9{221} GB,and ?27{115} GB growing along the[110]direction,the order of induced twin formation probability from small to large is:?9{221} GB<?3{112}GB<E27{115} GB.The reason is related to the geometrical relationship between the crystal interface,the {111} facets and the growth plane.Taking the intersection of the crystal interface and the growth plane as the reference line,for grains in the same growth direction,the smaller the angle between the intersection and the {111} plane is,the easier it is to form twins.(4)Temperature affects the ability of GBs to induce twinning during the growth of silicon crystals.The higher the temperature is,the smaller the probability of GB induced twin formation.The reason is that at high temperatures,the GB grooves at the growth surface of the silicon crystal will evolve from a jagged facet structure to a rough structure,and the twins are difficult to nucleate due to the lack of {111} facets.