A Theoretical, Experimental, and Atomistic Investigation of Surface Energy Anisotropy Effects on Grain-boundary Grooving

固体薄膜是大多数微电子和光电子器件的基本结构。在多晶薄膜退火过程中，晶界沟槽会加深形成孔洞，而孔洞的生长会破坏薄膜。研究人员的初步理论工作表明，表面能各向异性可以显著降低沟槽速率。然而，如果暴露的表面取向不包含facet取向，则各向异性凹槽是光滑的，并且看起来与各向同性凹槽完全相同；只有开槽速率降低了。本研究旨在通过三种方法：连续统分析、实验和原子模拟来研究表面能各向异性与晶界沟槽之间的关系。研究人员的初步工作假设双晶的晶体取向在晶界上是对称的。然而，大多数双晶不太可能是对称的。因此，自相似连续统分析将扩展到不对称双晶。研究人员还将考虑晶界匀速迁移，并研究晶界槽与晶界剖面之间的耦合。由于开槽是自相似的，在开槽发展的初期开槽率非常大，原子方法非常适合于捕捉槽的形成。研究人员将对铝和硅双晶体进行分子动力学研究。通过动力学蒙特卡罗模拟研究热槽的长期演化过程。倾斜和扭转边界都将被考虑。系统地研究了表面和晶界能量各向异性及其对面形和生长速度的影响。在实验部分，将使用两种材料建立并测量表面能各向异性。具有对称晶体取向的双晶样品将由市售材料制备。然后，在晶界处演变的凹槽形状将在显微镜下作为热退火时间-温度循环的函数进行检查，以将其形状与理论预测进行比较。晶界开槽是测量表面扩散系数最常用的方法。然而，表面能各向异性的阻滞效应在测量技术中从未被考虑过。该研究将阐明表面能各向异性对开槽的影响，并提高该技术的准确性。更好地理解和控制沟槽将使更小的微电子和光电子器件得以制造。这意味着更快、更小的计算机芯片，这将维持信息技术的不断进步。该项目对材料教育也有广泛的影响。目前，在整个路易斯安那州没有提供材料学位课程。然而，当地工业需要接受材料培训的学生，路易斯安那州立大学（LSU）正在建立一个材料项目来满足这一需求。该提案是路易斯安那州立大学两个部门之间的合作成果，将加强校园材料研究活动。它将支持研究生和本科生，并使他们接触到两个系的材料研究。它还将允许路易斯安那州立大学提供更多的材料课程。

Thin solid films are the basic structure in most microelectronic and optoelectronic devices. During annealing of a polycrystalline film, grain-boundary grooves can deepen to form holes, which tend to grow to breakup the film. The investigators' preliminary theoretical work has shown that surface energy anisotropy can reduce the grooving rate significantly. However, if the exposed surface orientations do not contain a facet orientation, then the anisotropic groove is smooth and looks exactly the same as an isotropic groove; only the grooving rate is reduced. This proposal aims to investigate this intriguing relationship between surface energy anisotropy and grain-boundary grooving using three methods: continuum analysis, experiment, and atomistic simulations. The investigators' preliminary work assumes that the crystallographic orientations of the bicrystal are symmetric about the grain boundary. However, most bicrystals are unlikely to be symmetric. Thus, the self-similar continuum analysis will be extended to asymmetric bicrystals. The investigators will also consider grain boundaries migrating at constant speed and study the coupling between the grain-boundary groove and the grain-boundary profile. Because grooving is self similar, the grooving rate is very large at the beginning of groove development, and atomistic methods are ideally suited to capture the groove formation. The investigators will perform molecular dynamics on aluminum and silicon bicrystals. The long-time evolution of the thermal groove will be studied by kinetic Monte Carlo simulations. Both tilt and twist boundaries will be considered. Surface and grain boundary energy anisotropy and their effect on faceting and growth rate will be systematically investigated. In the experimental part, two materials will be used for which surface energy anisotropy is established and measured. Bicrystal samples with symmetric crystallographic orientations will be prepared from commercially available material. The evolving groove shape at the grain boundary will then be microscopically examined as a function of thermal annealing time-temperature cycle to compare its shape with theoretical predictions.Grain-boundary grooving is the most commonly used technique for measuring surface diffusion coefficients. However, the retardation effect of surface energy anisotropy has never been considered in the measurement technique. The proposed investigation will clarify the effects of surface energy anisotropy on grooving and improve the accuracy of the technique. Better understanding and control of grooving will allow smaller microelectronic and optoelectronic devices to be made. This translates into faster and smaller computer chips, which will sustain continuous advancement of information technology. This project has also broad impact on materials education. Presently, there is no Materials degree offering program in the whole State of Louisiana. However, local industries need students with materials training, and Louisiana State University (LSU) is building a materials program to meet that demand. This proposal is a collaborative effort between two departments at LSU and will enhance materials research activities on campus. It will support graduate and undergraduate students and expose them to materials research in both departments. It will also allow more materials courses to be offered at LSU.