Electron microscopy studies of defect structure and correlated impurity incorporation in silicon and germanium nanowires

Conventional and scanning transmission electron microscopy (TEM) were employed to investigate defect structures and catalyst impurity incorporation in silicon and germanium nanowires grown via the vapor-liquid-solid (VLS) mechanism. Novel synthesis and sample preparation procedures were developed that enabled advanced characterization of nanowire composition and structure. For the first time, conclusive evidence was shown for a finite concentration of Au impurities in Si nanowires grown by the VLS mechanism. Ordered arrays of stacking faults in Si nanowires were found to give rise to new polytypes that had not been previously identified.;A novel syntaxial growth mechanism, resulting in simultaneous one-dimensional growth of Mn11Ge8/Ge nanowire heterostructures from a shared growth interface, was investigated. Analytical TEM, electron diffraction and high resolution TEM (HRTEM) were used to identify the germanide and to establish the crystallographic relationships between the two crystals, including epitaxial and fiber textures.;Aberration-corrected scanning TEM (STEM) was used to show that individual gold catalyst atoms are incorporated into Si nanowires on sites associated with planar defects and in defect-free regions. Cross-sectional imaging of individual nanowires was used to establish that gold atoms were incorporated on internal grain boundaries in bicrystalline nanowires, and in some cases appear to be associated with dislocations at the defect plane. STEM tomography revealed that defective nanowires appear to crystallize from a faceted growth front and suggested a correlation between interface shape and the position of Au columns in the wire interior.;HRTEM and electron diffraction were used in correlation with Raman spectroscopy to study the ordering of {111} defect arrays in Si nanowires. Stacking faults were found to occur in periodic arrangements corresponding to the 9R rhombohedral and 2H hexagonal polytypes. Cross-sectional imaging was used to establish the origins and strengths of forbidden reflections arising from ordered stacking faults and twinned nanowires.