ABSTRACT
Hole accumulation in Ge/Si core/shell nanowires (NWs) has been observed and quantified using off-axis electron holography and other electron microscopy techniques. The epitaxial [110]-oriented Ge/Si core/shell NWs were grown on Si (111) substrates by chemical vapor deposition through the vapor-liquid-solid growth mechanism. High-angle annular-dark-field scanning transmission electron microscopy images and off-axis electron holograms were obtained from specific NWs. The excess phase shifts measured by electron holography across the NWs indicated the presence of holes inside the Ge cores. Calculations based on a simplified coaxial cylindrical model gave hole densities of (0.4 ± 0.2) /nm(3) in the core regions.
Subject(s)
Germanium/chemistry , Holography/methods , Microscopy, Electron/methods , Nanostructures/chemistry , Nanostructures/ultrastructure , Silicon/chemistry , Crystallization/methods , Macromolecular Substances/chemistry , Materials Testing , Molecular Conformation , Nanotechnology/methods , Particle Size , Porosity , Surface PropertiesABSTRACT
Vapor-liquid-solid growth of high-quality Si nanowires relies on the stability of the liquid metal seed. In situ transmission electron microscopy shows that liquid AuSi seed spreads along the sidewalls of Si nanowires for some growth conditions. This liquid thin film phase separates to form solid Au clusters as the nanowire is quenched below the solidus temperature. The length that the liquid film spreads from the seed and its thickness can be explained by considering the spreading thermodynamics of droplets on cylinders.
Subject(s)
Crystallization/methods , Gold Compounds/chemistry , Models, Chemical , Nanostructures/chemistry , Nanostructures/ultrastructure , Nanotechnology/methods , Silicon/chemistry , Computer Simulation , Gases/chemistry , Macromolecular Substances/chemistry , Materials Testing , Molecular Conformation , Particle Size , Phase Transition , Solutions , Surface Properties , ThermodynamicsABSTRACT
Epitaxial Si nanowires grown from Au seeds using the vapor-liquid-solid method begin growing normal to the Si(111) substrate atop a tapered base. After a kinetically determined length, the NWs may kink away from [111] to another crystallographic direction. The smallest NWs prefer growth along 110 while larger Si NWs choose either 111 or 112 based on whether growth conditions favor Au-free sidewalls. "Vertical" growth normal to the Si(111) substrate is obtained only for slowly growing NWs with Au-decorated sidewalls. At the fastest growth rates, single-crystal Si NWs smoothly, continuously, and randomly vary their growth directions, producing a morphology that is qualitatively different than highly kinked growth.
