Characterization of Surfaces Relevant to Nanotechnology
Abstract: In this thesis investigations of the structure and dynamics of semiconductor surfaces relevant for the synthesis of nanostructures are presented. The studies were performed using Scanning Tunneling Microscopy (STM), Low Energy Electron Diffraction (LEED), and Spectroscopic PhotoEmission and Low Energy Electron Microcopy (SPELEEM). Nanostructures are defined as objects with one or more dimensions on the nanometer scale. Examples are the rod shaped nanowires synthesized by epitaxial crystal growth methods or the many different structures that can be fabricated by the Focused Ion Beam (FIB) technique. Nanostructures have many applications in devices for electronics, optoelectronics and life sciences. One part of the thesis is dedicated to studies of substrates used in nanowire growth. Diffusion of growth materials on the substrate and nanowire surfaces has a strong effect on the morphology of the resulting wires. Studies of the atomic scale structure of these surfaces are therefore important. The presence of gold particles used as seeds for nanowire growth may also in some cases alter the surface structure. In this thesis studies of the structures of GaAs(111)B, InAs(111)B, and GaP(111)B with and without gold particles and films are presented. A gold free growth method for III-V nanowires on Si have also been investigated. In this case an organic film is used to create an oxide template with patches on which the nanowires can nucleate and grow. Another part of the thesis is dedicated to the characterization of nanostructures. A method for studying the surfaces of III-V nanowires with STM was developed. Atomic scale images and Scanning Tunneling Spectroscopy (STS) measurements on surfaces of InAs/InP heterostructure nanowires are presented. Nanogrooves fabricated by FIB milling with Ga+ ions on Si were characterized with respect to their surface chemistry. It was found that a Ga layer forms in the grooves during milling. At temperatures as low as 150C Ga diffuses out of the grooves and spreads several micrometers on the substrate surface. Finally the thermal decomposition of the GaP(111)B surface has been investigated. Ga droplets form after annealing at a temperature above the maximum temperature for congruent evaporation. The droplets move across the surface, perpendicular to and uphill the steps introduced by the wafer miscut. When gold nanoparticles are deposited on the surface, the droplet movement become random instead of unidirectional.
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