Quantum Confined Electron Systems in Coherently Strained Si-Ge Nanowire Heterostructures

非技术描述：半导体异质结构在许多科学发现和技术进步中起着重要作用。虽然大多数研究都集中在平面异质结构上，但非平面异质结构，特别是纳米线，最近获得了极大的兴趣。本研究项目解决了有关非平面半导体异质结构，特别是硅锗核壳纳米线的加工和表征的基本问题。这些研究活动旨在提高材料合成和表征方面的知识，从而帮助设计未来的高速、低功耗电子设备。研究活动与学生的教育和前沿研究培训很好地结合在一起。更具体地说，这项工作对研究生和本科生进行半导体生长、制造和表征技术的教育，研究成果被整合到课程材料和面向高中生的拓展讲座中，以鼓励学生从事科学、技术和工程方面的职业。技术描述：半导体纳米线为实现外延、非平面异质结构提供了一个通用的平台，与基础科学和技术应用相关。本研究项目的主要目标是：(1)结合能带工程和调制掺杂来促进IV族纳米线异质结构的生长；(2)表征其结构和电子特性；(3)实现和探索相干应变Si-SiGe核壳纳米线中的量子受限电子系统。更具体地说，这些纳米线异质结构是通过气相-液-固相生长的硅纳米线核心和超高真空化学气相沉积的外延SiGe外壳的组合来生长的。利用微拉曼光谱结合晶格动力学理论对硅锗核壳纳米线中的应变分布进行了实验研究，并与计算结果进行了比较。为了确定电子迁移率并将其与纳米线异质结构设计相关联，在低温条件下制备并表征了具有低电阻触点的纳米线异质结构场效应晶体管。将波段工程和径向调制掺杂相结合，提高了Si-SiGe核壳纳米线中一维电子的迁移率。利用自洽密度模拟和实验纳米线电导随密度的关系，提取了Si-SiGe核-壳纳米线中芯与壳之间的能带偏移量。实验研究了Ge-SiGe核壳纳米线中高迁移率一维空穴系统的磁输运性质，重点研究了自旋分裂和自旋轨道相互作用，以评估是否可以在实验中观察到自旋和动量对齐的螺旋态。

Non-technical description: Semiconductor heterostructures play an important role in many scientific discoveries and technological advances. While the majority of research has focused on planar heterostructures, non-planar heterostructures, particularly nanowires, have recently gained significant interest. This research project addresses fundamental questions regarding the processing and characterization of non-planar semiconductor heterostructures, particularly silicon-germanium core-shell nanowires. The research activities are designed to advance knowledge in materials synthesis and characterization, and thus assist in the design of future high-speed, low-power electronic devices. The research activities are well integrated with students education and training in cutting-edge research. More specifically, this effort educates graduate and undergraduate students in semiconductor growth, fabrication and characterization techniques, and the research results are integrated into course material and outreach lectures to high-school students to encourage careers in science, technology, and engineering. Technical Description: Semiconductor nanowires provide a versatile platform to realize epitaxial, non-planar heterostructures, with relevance for fundamental science and technological applications. The main objectives of this research project are (1) to advance the growth of group IV nanowire heterostructures by combining band engineering and modulation doping, (2) to characterize their structural and electronic properties and (3) to realize and explore quantum confined electron systems in coherently strained Si-SiGe core-shell nanowires. More specifically, these nanowire heterostructures are grown pseudomorphic using a combination of the vapor-liquid-solid growth for the Si nanowire core, and the ultra-high-vacuum chemical vapor deposition for epitaxial SiGe shell. The strain distribution in Si-Ge core-shell nanowires is experimentally probed using micro Raman spectroscopy coupled with lattice dynamic theory, and compared with calculations. Nanowire heterostructures-based field-effect transistors with low resistance contacts are fabricated and characterized under low temperatures, in order to determine the electron mobility and correlate it with the nanowire heterostructure design. Band engineering and radial modulation doping are combined to enhance the mobility of one-dimensional electrons in Si-SiGe core-shell nanowires. Using self-consistent density simulations and the experimental nanowire conductance dependence on density, the band offset between the core and the shell in Si-SiGe core-shell nanowires is extracted. The magnetotransport properties of high mobility one-dimensional hole system in Ge-SiGe core-shell nanowires are investigated experimentally, with an emphasis on spin splitting and spin-orbit interaction in order to assess if helical states with the aligned spin and momentum can be experimentally observed.