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.
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.
