硅晶体凝固生长及位错形核的分子动力学模拟研究

发布时间：2018-02-04 21:37

本文关键词： 晶体生长位错形核应变硅分子动力学　出处：《南昌大学》2015年硕士论文　论文类型：学位论文

【摘要】：晶体硅是当前和未来相当一个时期主要的光伏材料。它一般通过直拉法和定向凝固法获得。前者得到单晶硅棒,后者得到多晶硅锭。后者因相对成本低而更具竞争力,但也因其结晶缺陷形成更难于控制而在技术上更具有挑战性。本文主要针对硅晶体的定向凝固生长进行研究,包括其生长动力学各向异性和对晶体硅电学性能影响最大的位错缺陷的形成条件与形成机制。采用的方法为分子动力学计算模拟,这是迄今为止唯一能够在原子尺度模拟跟踪位错形成动态过程的方法。本研究选择Tersoff势函数描述硅原子间的相互作用,构建了晶体硅定向凝固的NPT系综生长模型和外加应力施加方法,以Nose-Hoover算法控制和恒定温度,以Andersen算法控制和恒定压强,应用LAMMPS软件进行模拟实验。模拟研究结果显示,晶体硅凝固生长速率及其受应力影响情况有明显的各向异性,不同方向晶体生长速率大小关系为[100][110][112][111]???????;对于同一生长方向,在压应变条件下,随着应变的增大晶体生长速率降低,而在拉应变条件下,晶体生长速率先随着应变的增大而增大,再随着应变的增大而减小,不过110生长中,在压应变条件下晶体生长速率出现反常。模拟结果揭示,不同方向晶体生长过程中位错形核情况也存在各向异性。在100、110、111、112方向自然生长中,112和111生长过程中能观察到位错的形核,另外两个方向上,在多次模拟生长试验中都未出现位错;112生长中,固液界面呈{111}小面化特征,生长过程中会出现沿{111}面的堆垛层错,而位错即在{111}面上于层错边沿处形成;111生长中位错直接形核于固液界面。在外加应力条件下,100、110、111和112生长过程中均能观察到位错形核;在较大的应变条件下,100和110生长中的位错形核概率会显著提升;而一定的应变范围内,112和111生长中的位错形核概率却会降低。在应变下的晶体生长过程中会出现 V‖型固液界面,而位错形核于 V‖型凹槽附近的晶体无序化—重结晶的过程中,而且形成的位错是位于生长面内的垂直于生长方向的一对位错偶极子;但110生长中还有沿生长方向的位错形核。另外,对112晶体生长中的过冷度和温度梯度条件模拟结果表明,当过冷度和温度梯度达到一定值时,112方向自然生长中才会观察到位错形核。
[Abstract]:Crystal silicon is the main photovoltaic material at present and in the future. It is usually obtained by Czochralski method and directional solidification method. The latter is more competitive because of its low cost. However, the formation of crystalline defects is more difficult to control, so it is more challenging in technology. This paper mainly focuses on the directional solidification growth of silicon crystals. The formation conditions and mechanism of dislocation defects, which have the greatest influence on the electrical properties of silicon crystal, are discussed in this paper. The molecular dynamics method is used to simulate the formation of dislocation defects. This is the only way to simulate the dynamic process of dislocation formation at atomic scale. In this study, the Tersoff potential function is chosen to describe the interaction between silicon atoms. The NPT ensemble growth model of directional solidification of crystal silicon and the method of applying applied stress are constructed. The Nose-Hoover algorithm is used to control the temperature. Andersen algorithm is used to control and constant pressure, and LAMMPS software is used to simulate the experiment. The simulation results show that. The solidification growth rate of crystal silicon and its effect on stress have obvious anisotropy. The relationship between crystal growth rate and crystal growth rate in different directions is as follows. [100]. [110]. [112]. [111]? ? ? ? ? ? ? ; For the same growth direction, with the increase of strain, the growth rate of crystal decreases with the increase of strain, while under the condition of tensile strain, the growth rate of crystal increases with the increase of strain. With the increase of strain, however, the growth rate of the crystal is abnormal in the growth of 110. The simulation results show that the growth rate of the crystal is abnormal. Anisotropy also exists in the case of dislocation nucleation during crystal growth in different directions. Dislocation nucleation can be observed during the growth of 112 and 111. In the other two directions, there is no dislocation in many simulated growth tests. During the growth of 112, the solid-liquid interface has the characteristics of {111} facture, and the stacking faults along the {111} surface will occur during the growth process, while the dislocation will be formed on the {111} plane at the edge of the stacking fault. Dislocation nucleation was observed at the solid-liquid interface during the growth of 111. The probability of dislocation nucleation in the growth of 100 and 110 increases significantly under the condition of large strain. However, the probability of dislocation nucleation will decrease in a certain strain range. The dislocation nucleation is in the process of disorderation-recrystallization near the V-shaped grooves, and the resulting dislocation is a pair of dislocation dipoles perpendicular to the growth direction in the growth plane. However, there are dislocation nucleation along the growth direction in 110 crystal growth. In addition, the simulation results of undercooling and temperature gradient conditions in 112 crystal growth show that when the undercooling and temperature gradient reach a certain value. Dislocation nucleation is observed only in natural growth in the 112 direction.
【学位授予单位】：南昌大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TQ127.2

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