Collaborative Research: Analysis of Defects and Their Causes in Bulk Aluminum Nitride Crystals

该项目旨在更好地理解和控制与升华法生产的块状氮化铝晶体生长机制相关的缺陷。缺陷如位错、堆垛层错、晶界、多型体和反转畴将与晶体中的杂质、生长条件以及热应力和机械应力相关。缺陷的类型，密度和空间分布将通过X射线形貌术，透射电子显微镜，缺陷选择性蚀刻和装饰性氧化来测量。 将确定由氧和碳引起的特定类型的缺陷。 这些杂质通常以相对高的浓度（~1019 cm-3）存在于AlN晶体中，这是由于它们在原始源AlN粉末、石墨加热元件和炉夹具中的高浓度。这些元素引起堆垛层错、沉淀和其他缺陷的倾向将通过比较具有宽范围杂质浓度的晶体来研究。 还将确定由于晶体取向和极性引起的差异。温度和氮气压力对各向异性生长速率和随后存在于晶体中的缺陷的影响将得到解决。将检查在不同温度和氮气压力下产生的一系列晶体，以确定存在哪些缺陷，以及它们对表面台阶的影响。 将检查样品安装和其他物理约束对应力和随后的空间缺陷分布的影响。晶体中的残余应力将被测量和建模。还将测量由抛光AlN晶体的机械作用引起的表面和亚表面损伤。从这方面的反馈信息的缺陷，在氮化铝预计将允许晶体生长过程中进行修改，在一个知识渊博的方式，使晶体质量可以得到改善。该项目解决了与电子/光子材料相关的基础研究问题，具有技术相关性，并整合了研究和教育活动。该项目为研究生提供了独特的机会，学习晶体生长和缺陷表征的基本方面。通过建立的合作，学生将能够更深入地研究研究问题，而不是单独调查。 学生将参观国家实验室，获得宝贵的经验，与国家的最先进的设备和经验丰富的科学家互动。研究结果将通过在高质量科学期刊上发表和组织材料研究学会专题讨论会的方式广泛传播。学生将通过在专业协会会议上展示他们的发现积极参与。将为妇女和代表性不足的高中学生安排讲习班，学习氮化铝晶体生长和表征。主要研究人员及其学生还将访问初中和高中，讨论这一项目，并特别鼓励妇女和少数民族学生加入科学和工程。***

This project aims for greater understanding and control of defects associated with growth mechanisms of bulk aluminum nitride crystals produced by sublimation. Defects such as dislocations, stacking faults, grain boundaries, polytypoids and inversion domains will be correlated to impurities in crystals, growth conditions, and thermal and mechanical stresses. The types, densities, and the spatial distribution of defects will be measured by x-ray topography, transmission electron microscopy, defect selective etching, and decorative oxidation. Specific types of defects caused by oxygen and carbon will be determined. These impurities are typically present at relatively high concentrations (~1019 cm-3) in AlN crystals as a result of their high concentrations in the original source AlN powder, graphite heating elements, and furnace fixtures. The tendency for these elements to cause stacking faults, precipitates, and other defects will be investigated by comparing crystals with a broad range of impurity concentrations. Differences due to the crystal orientation and polarity will also be determined. The impact of temperature and nitrogen pressure on anisotropic growth rates and defects subsequently present in the crystals will be resolved. A series of crystals produced at different temperatures and nitrogen pressures will be examined to determine which defects are present, and their resulting influence on surface steps. Effects of sample mounting and other physical constraints on stress and subsequent spatial defect distribution will be examined. Residual stress in crystals will be measured and modeled. Surface and subsurface damage caused by mechanical action of polishing the AlN crystals will also be measured. Feedback from this information on defects in AlN is expected to allow the crystal growth process to be modified in a knowledgeable way so that crystal quality can be improved. %%% The project addresses fundamental research issues associated with electronic/photonic materials having technological relevance, and integrates research and educational activities. The project provides graduate students with unique opportunities to learn fundamental aspects of crystal growth and defect characterization. Through the established collaborations, students will be able to study research issues in greater depth than would be possible under separate investigations. Students will visit national laboratories, gain valuable experience working with state-of-the-art equipment and interacting with experienced scientists. The results of the research will be broadly disseminated by publication in high quality scientific journals and by organizing a materials research society symposium. Students will be actively involved by presenting their findings at professional society meetings. Workshops will be arranged for women and under represented group high school students to learn about AlN crystal growth and characterization. Principal investigators and their students will also visit middle school and high school to discuss this project, and to provide special encouragement to women and minority students to join science and engineering. ***