Silicon nanowire oxidation: the influence of sidewall structure and gold distribution

The oxidation behavior of Si nanowires (SiNWs) grown by the gold (Au) catalyzed vapor-liquid-solid (VLS) growth process in an electron beam evaporation (EBE) reactor is studied. The VLS SiNWs exhibit hexagonal shape with essentially {112} facets where each facet shows a saw-tooth faceting itself, consisting of alternating {111} and {113} facets. Depending on growth temperatures (450-750 degrees C) and evaporation currents (40-80 mA) that determine the silicon vapor supply, this facet formation is more or less pronounced. The diffusion of Au atoms on the faceted SiNW surfaces and the formation of Au nanoparticles on the SiNW facets during growth and during ex situ annealing are studied. Upon diffusion, the Au atoms agglomerate and form Au nanoparticles that preferably arrange themselves on {113} facets. Upon annealing in air at temperatures between 800 and 950 degrees C the gold nanoparticles agglomerate further and form bigger particles of a few tens of nm in diameter that reside at the interface between the growing silica (SiO(2)) layer and the SiNW itself, which in turn shrinks during SiNW oxidation. The oxide layer thickness and the oxide appearance depend on the annealing conditions (time and temperature) and systematically varied oxidation processing is described in this paper as investigated by cross-sectional transmission electron microscopy (TEM) including high resolution studies as well as scanning electron microscopy (SEM) studies. Our results strongly suggest that the SiNWs can be fully oxidized, thus forming silica NWs that can either keep their initial shape or, under certain annealing conditions, do not keep their initial wire shape but assume a bamboo-like shape that forms most likely as a result of locally high stresses that are related to nanocrack formation. The nanocracks form in the growing oxide layer mediated by the presence of Au nanoparticles that yield gold-enhanced SiNW oxidation and thus a faster oxidation rate.