Investigation of Polarization Effects in Nitride Semiconductors and Heterointerfaces

本课题致力于GaN薄膜极性的测定和控制。由于缺乏天然衬底，氮化物通常生长在蓝宝石等外来衬底上。这种衬底限制加上氮化物生长的极性性质导致了两种可能的表面终止，Ga或N。这两种终止的混合导致反转磁区的存在，从而引起极化效应。这些效应有两个来源：离子氮化键引起的异质界面自发极化和晶格失配或热应变引起的应变诱导压电极化。反转结构域可能会对异质结器件中的传输特性、带对齐和载流子浓度产生不利影响。在这项研究中，我们将研究与磁区结构和薄膜极化相关的基本材料科学现象，以便更好地理解和控制。方法是研究衬底表面和生长起始点对薄膜极性的影响，自发极化和层有序对能带不连续性的影响，以及压电极化对电学和光学性质的影响。我们的目标是实现基本的理解，从而能够生产单域薄膜，并生长异质结构，例如量子井，其性质取决于自发极化和压电极化效应的相对大小。将使用包括分子束外延和金属有机气相外延在内的生长技术。分析技术，如原位反射高能电子衍射，非原位原子力显微镜/扫描隧道显微镜，高分辨率X射线衍射，光电发射，和连续波/时间分辨发光将被用来研究薄膜的形貌，应变，能带不连续性和发射能量。%该项目致力于具有高潜在技术相关性的材料科学的主题领域的基础研究问题。一旦对异质结器件中的偏振场有了基本的了解，就有可能设计出更全面地实现GaN基电子/光子器件和电路潜力的结构。这个合作的跨学科项目的一个重要特点是通过在一个具有根本意义和技术意义的领域对学生进行培训，将研究和教育整合在一起。***

This project addresses the determination and control of GaN film polarity. Due to the lack of native substrates, nitrides are typically grown on foreign substrates such as sapphire. This substrate limitation coupled with the polar nature of nitride growth leads to two possible surface terminations, Ga or N. A mixture of these two terminations leads to the presence of inversion domains, which cause polarization effects. These effects arise from two sources: spontaneous polarization at heterointerfaces due to the ionic nitride bonds, and strain-induced piezoelectric polarization caused by lattice misfit or thermal strain. Inversion domains can adversely affect transport properties, band alignments, and carrier concentrations in heterojunction devices. In this research, fundamental materials science phenomena associated with domain structure and film polarization will be investigated for greater understanding and control. The approach is to study the influence of substrate surface and growth initiation on film polarity, the role of spontaneous polarization and layer ordering on band discontinuities, and the effect of piezoelectric polarization on electrical and optical properties. The goal is to achieve fundamental understanding and thereby be able to produce single-domain films, and to grow heterostructures, e.g., quantum wells whose properties depend on the relative magnitudes of spontaneous and piezoelectric polarization effects. Growth techniques including Molecular Beam Epitaxy with in-situ monitoring and Organometallic Vapor Phase Epitaxy will be used. Analysis techniques such as in-situ reflection high energy electron diffraction, ex-situ atomic force microscopy/scanning tunneling microscopy, high-resolution x-ray diffraction, photoemission, and CW/time-resolved luminescence will be employed to investigate film morphology, strain, band discontinuities, and emission energies.%%%The project addresses basic research issues in a topical area of materials science having high potential technological relevance. Once a fundamental understanding of the polarization fields in heterostructure devices is obtained, it will be possible to design structures that more completely realize the potential of GaN-based electronic/photonic devices and circuits. An important feature of this collaborative, interdisciplinary program is the integration of research and education through the training of students in a fundamentally and technologically significant area. ***