A Molecular Dynamics Simulation Study Of Crystal Growth Of Silicon And Nucleation Of Dislocations

Crystalline silicon is the major photovoltaic material in the current and future periods. It’s generally obtained by the Czochralski and directional solidification methods. The former obtains the monocrystalline ingot while the latter obtains the multicrystalline ingot. The latter one is more competitive due to its low cost, but it’s more challenging too because of the difficulty in control of the formation of defects. This paper reports our studies of the directional solidification growth of silicon crystal, including the anisotropy of growth kinetics and the formation conditions and mechanism of dislocations which have the most serious effects on the electrical properties of the crystal silicon between the all defects. We use the molecular dynamics simulation method which is so far the only way to track the dynamic process of dislocation nucleation in the atomic scale.The Tersoff potential is employed for computing atomic interaction. The model of directional solidification of silicon under a NPT ensemble is built, and the method of adding external strain is acquired. Nose-Hoover algorithm is used for controlling the constant temperature while Anderson algorithm is used for controlling the constant pressure. LAMMPS software is used for the simulation experiments.The simulation results show that there is distinct anisotropy on the crystal growth velocities and the stress effects, the magnitude relation between the crystal growth velocities of the different crystal growth orientation is [100] [110] [112][111]? ?? ?? ??. To the same crystal growth orientation, the crystal growth velocity decreased with compressive strain increasing. However the crystal growth velocity didn’t decrease with tensile strain increasing, until it reached a certain value. But when the(110) as the crystal growth plane, the crystal growth velocity is abnormal under the compressive strain.The results also show that the nucleation of dislocation during the crystal growth from melt between the different crystal growth orientations is also anisotropic. Among the orientations of <100>, <110>, <111> and <112> naturally crystal growth, dislocations are observed during the process of crystal growth in the orientation <112> and <111>. And there are no dislocations occurring in the other two crystal growth orientations among the repeatedly simulation experiments. The crystal-melt interface with <112> growth orientation is being {111} facets, stacking faults will occur along the {111} facets, and dislocations nucleate beside the stacking faults in the {111} facets. And dislocations nucleate directly on the crystal-melt interface with <111> crystal growth. The dislocations could nucleate with the four growth orientations under the conditions of external strain; the probabilities of nucleation of dislocations observably increase with <100> and <110> crystal growth under larger strain while they decrease with <112> and <111> crystal growth under certain value of strain. The crystal-melt interfaces under strain are becoming notched V-shaped and the dislocation nucleating during the process of the crystal beside the notch disordering—recrystallization; the dislocation dipoles which are located in the crystal growth planes and perpendicular to the crystal orientations are formed, but the dislocations along growth orientation with <110> crystal growth are also formed under the conditions of tensile strain. In addition, we study the effects of undercooling and temperature gradient on the dislocation nucleation during the naturally crystal growth with <112> growth orientation. The results show that dislocations don’t nucleate until the undercooling or temperature gradient reaches a certain value.