Strain and modulation doping in epitaxial Si/Ge core-shell nanowire heterostructures
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For over five decades, silicon based electronics relied on scaling of individual field-effect transistors (FETs) for improvements in integrated circuit performance. Recently, however, further enhancement of packing density and switching speed was limited by the increase in power consumption of short channel devices. New materials and device geometries were introduced to help expand CPU performance while also decreasing power dissipation. Semiconducting nanowires have also been recognized for potential applications as channel material in highly scaled FETs. These structures present opportunities for strain and energy band engineering through the use of radial, or core-shell, heterostructures. To fully exploit the benefits of radial heterostructures, however, requires knowledge of elastic strain distributions and energy band alignments, necessitating the development of new characterization methods. This is especially true in Si/Ge material systems, where a large lattice mismatch over 4% is possible. In this thesis, we grow Si/Ge core-shell nanowires and demonstrate multiple techniques to characterize the nanoscale heterostructure, including strain measurements and extraction of valence band offsets. We grow Ge-SixGe1-x core-shell nanowires and measure the elastic strain using Raman spectroscopy. The Ge core’s Raman spectrum is consistent with a compressive strain in this region due to lattice mismatch with the SixGe1-x shell. The strain distribution and expected Raman peak positions are calculated using continuum elasticity models and lattice dynamic theory, finding excellent agreement to experimental data. We also demonstrate radial modulation doping in Ge-SixGe1-x core-shell nanowire heterostructures by doping a portion of the SixGe1-x shell with boron during growth. The modulation doped nanowire FETs show an enhanced low temperature hole mobility and also a decoupling of transport between core and shell. Through comparison to finite-element calculations, we extract the valence band offset at the core-shell interface. Lastly, we grow coherently strained Si-SixGe1-x core-shell nanowires and characterize the structure using Raman spectroscopy. We first optimize the Si nanowire growth process to favor the diamond crystal structure and to minimize sidewall coverage by Au catalyst, followed by epitaxial growth of the SixGe1-x shell using the Si nanowire as substrate. Raman measurements on core-shell samples indicate a tensile strain in the Si core and a compressive strain in the SixGe1-x shell, both consistent with calculations of the strain and the strain-induced shift of the Raman peaks in this structure.