The Formation And Evolution Of Twins And Dislocations In Growth Of Silicon From Melt By A Molecular Dynamics Study

Solar cells based on multicrystalline silicon have been widely used because of well-balanced high conversion efficiency and low production cost.However,the photoelectric conversion efficiency of the multicrystalline silicon solar cells is lower than that of monocrystalline silicon due to the existence of the crystal defects,such as dislocation and twin.Reducing the defect density is one of the goals in the Solar energy industry,but it is difficult to observe the formation of the defects during the Si crystal growth,so that we are lack of the knowledge of the defect formation mechanism at the atomic level and need to insight into it.In this paper,the most important defects,dislocation and twin in silicon have been carried out by molecular dynamics simulation,with Tersoff potential to describe the interatomic force.The results showed that:(1)The twin was formed on the close-paged(111)plane during the silicon crystal growth.It was a competitive growth process between HCP structure and FCC structure,and the HCP structure was gradually dominant to occupy the whole area.Through energy analysis,the stacking fault energy on the close-paged(111)plane was significantly low,the probability of the two kinds of structures formed was high at the same time.When the silicon crystal grew along a(111)plane,the(111)liquid-solid interface remained smooth relatively,on which the formed twin boundary was parallel to this interface and the probability of twin nucleation was high;When the silicon crystal grew along a(110)or(112)plane,a zigzag faceted interface bounded by {111} facets was expected to form gradually.On the zigzag {111} facets,the twins still were formed but the nucleation probability was lower relatively.When the silicon crystal grew along a(100)plane,the liquid-solid interface was stable,the {111} facets and twins were not formed during the silicon crystal growth process.(2)The nucleation of dislocation during the silicon crystal growth was accompanied with the process of the twin formed,it generated between these two regions,which has occupied by HCP and FCC structure respectively.The difficulty degree of dislocation nucleation from big to small at different growth planes was:(100)plane>(110)plane>(112)plane>(111)plane.The silicon crystal growth was at the same plane,the higher the temperature is,the more difficult the formation of dislocations.(3)During the process of silicon crystal growth,it was easy to form three types of dislocation.Their burgers vector were 1/6<112>,1/12<111>,1/6<110> respectively.The first two is partial dislocation;the last one is Stair-Rod dislocation.These different kinds of dislocation could be synthetic or dissociated with complicated changes.For example,the burgers vector of dislocation was 1/2<110>,it usually dislocated two partial dislocations with 1/6<112> burgers vector;the burgers vector of the partial dislocation was 1/6<112>,which also dislocated two metastable dislocation with 1/12<111> burgers vector;and two partial dislocation with 1/6<112> burgers vector could make a stair-rod dislocation,its burgers vector was 1/6<110>.(4)The generation of the dislocation usually had an influence on the silicon crystal growth.When the silicon crystal grew along a(100)plane,an edge dislocation which is perpendicular to liquid-solid interface had a little effect on the growth rate.However,the screw dislocation and 60°dislocation had an impact on the {111} facets formed at the liquid-solid interface,reducing the silicon crystal growth rate.